WO2018057902A1

WO2018057902A1 - 3d volumetric display with improved brightness

Info

Publication number: WO2018057902A1
Application number: PCT/US2017/052972
Authority: WO
Inventors: Curtis J. BROADBENT; John C. Howell; Christopher MULLARKEY
Original assignee: University Of Rochester
Priority date: 2016-09-22
Filing date: 2017-09-22
Publication date: 2018-03-29

Abstract

A system for displaying one or more images in three dimensions. The system has a three dimensional illumination volume containing a gas that emits one or more types of visible light when at certain multi-photon excited states. The system includes lasers (e.g. lasers with beams outside of the visible wavelengths) that can be directed to intersect in the illumination volume to excite particles of the gas to a multi-photon excited state to emit visible light. Scanning the beam intersection (or multiple beam intersections) through the illumination volume generates three dimensional images. In some embodiments, the system is configured to increase the brightness of the displayed images compared to what has been possible with earlier systems.

Description

3D VOLUMETRIC DISPLAY WITH IMPROVED BRIGHTNESS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Patent Application 62/397,947 filed September 22, 2016, the contents of which are incorporated herein by reference in their entirety.

RELATED FIELDS

[0002] Systems and methods for generating real space three dimensional images

(including static and dynamic images), including laser systems and methods for producing real space three dimensional images using two (or more) photon absorption in gaseous particles.

BACKGROUND

[0003] Currently known three dimensional imaging devices often rely upon optical illusions in an effort to trick the eyes and brain so that the human observer experiences the perception of viewing a three dimensional image. For example, certain passive three dimensional projection techniques involve the use of a projector to project two orthogonally polarized images, and the images sent with each polarization are such that their separation gives the appearance of depth. In another example, certain active three dimensional projectors can operate to project back-to-back images, one for the left eye and one for the right eye. Specially made glasses then rapidly turn on and off the left and right lenses over those eyes, respectively. [0004] Although these and other three dimens

benefits, still further improvements would be desi

dimensional images.

BRIEF SUMMARY

[0005] This patent application describes several examples of systems and methods for displaying in three dimensions static or dynamic images using laser beam excitation of gaseous particles. These systems and methods may utilize a three dimensional illumination volume that includes gaseous particles that emit visible light following the absorption of excitation laser energy. These systems and methods may include at least a first laser generating a first laser beam and a second laser generating a second laser beam, and scanners for directing the first and second laser beams to intersect in the illumination volume and excite gaseous particles at the beam intersection to a multi-photon excited state, such that visible light is emitted by the particles at the beam intersection. The scanners can further operate to change the positions and/or orientations of the laser beams through the illumination volume so as to change a location of the laser beam intersection in three dimensions.

[0006] Light or electromagnetic radiation emitted from the excited gaseous particles at the beam intersections can be arranged and sequenced to generate static or dynamic images. In some cases, the gaseous particles are distributed in a transparent or semi- transparent medium. In some cases, one or more different types of particles can be used to emit light in various colors (e.g. red, green, yellow, blue). Software, hardware, and/or firmware can be used to control laser output and scanning so that light emits from addressable locations of the illumination volume, in a way that forms a static or dynamic three dimensional image that is perceptible to the eye of the viewer. [0007] In one example a system for displ

dimensions includes: a three dimensional illuminat

including at least one atomic or molecular vapor, the atomic or molecular vapor having a first ground state, a second ground state different from the first ground state, and a multi-photon excited state, the atomic or molecular vapor configured to emit at least a first type of visible light when at the multi-photon excited state; a first excitation laser configured to generate a first laser beam tuned to a transition terminating at the first ground state; a second excitation laser configured to generate a second laser beam, the system configured to direct the first and second laser beams into the illumination volume such that the first and second laser beams intersect in the illumination volume to excite at least some of the atomic or molecular vapor particles at the beam intersection to the multi-photon excited state; the system configured to pump at least a portion of the atomic or molecular vapor to the second ground state; and the system configured to pump at least a portion of the atomic or molecular vapor to the first ground state.

[0008] . In some instances, the system further includes a preparation laser configured to pump at least a portion of the atomic or molecular vapor to the second ground state.

[0009] In some instances, the preparation laser may be configured to flood illuminate the three dimensional illumination volume.

[0010] In some instances, the preparation laser may be configured to substantially co- propagate with the first laser beam.

[0011] In some instances, the system includes a first preparation laser and a second preparation laser, the first preparation laser configured to substantially co-propagate with the first laser beam, the second preparation laser configured to flood illuminate the three dimensional illumination volume. [0012] In some instances, the system includes

least a portion of the atomic or molecular vapor to the

[0013] In some instances, the system is configured to align a laser beam of the repump laser along a path coincident with the second laser beam.

[0014] In some instances, a cross-sectional area of the repump laser beam is larger than a cross-sectional area of the second laser beam.

[0015] In some instances, the at least one atomic or molecular vapor is Cesium.

[0016] In some instances, the at least one atomic or molecular vapor is Rubidium.

[0017] In some instances, the first ground state of the atomic or molecular vapor has a higher absorption than the second ground state for the first laser beam.

[0018] In some instances, the gas also includes a buffer gas.

[0019] In some instances, the first ground state of the at least one atomic or molecular vapor has a first absorption profile and the second ground state of the at least one atomic or molecular vapor has a second absorption profile, the first and second absorption profiles being non-overlapping profiles.

[0020] In some instances, the system is configured to scan the beam intersection of the first and second laser beams to generate an image.

[0021] In another example, a system for displaying one or more images in three dimensions includes: a three dimensional illumination volume including a gas, the gas being at least one atomic or molecular vapor, the atomic or molecular vapor having a first state, a second state different from the first state, and a third state different from the first and second states, the third state being a multi-photon excited state, the atomic or molecular vapor configured to emit at least a first type of visible light when at the third state; a first excitation laser configured to generate a first laser beam tuned to a transition including the first state; a second excitation laser configured to generate a second laser beam; a preparation laser tuned to a transition including the first state; and

including the second state.

[0022] The preparation laser may be configured to pump the atomic or molecular vapor to the second state throughout the three dimensional illumination volume.

[0023] The preparation laser may be configured to substantially co-propagate with the first excitation laser.

[0024] The preparation laser may be configured to substantially co-propagate with the repump laser.

[0025] The repump laser may be configured to generate a repump laser beam along a path of the second laser beam.

[0026] In another example, a system for displaying one or more images in three dimensions includes: a three dimensional illumination volume having at least one atomic or molecular vapor, the atomic or molecular vapor having a first ground state, a second ground state different from the first ground state, and a third multi-photon excited state different from the first and second ground states, the atomic or molecular vapor configured to emit at least a first type of visible light when at the third state; at least two excitation lasers configured to generate first and second laser beams, the system configured to direct the first and second laser beams into the illumination volume such that the first and second laser beams intersect in the illumination volume to excite at least some of the atomic or molecular vapor particles at the beam intersection to the third state; and the system configured to spatially control a ground state population of the gas.

[0027] The system may be configured to pump a region of the three dimensional illumination volume including a path of the first laser beam to the second ground state. [0028] The system may be configured to pu

illumination volume including the beam intersection o

first ground state.

[0029] In some instances, the first ground state has a first absorption efficiency for the first laser beam and the second ground state comprises a second absorption efficiency for the first laser beam, wherein the first absorption efficiency is greater than the second absorption efficiency.

[0030] In some instances, the first ground state has a first absorption profile and the second ground state comprises a second absorption profile, wherein the first and second absorption profiles do not substantially overlap.

[0031] In another example, a system for displaying one or more images in three dimensions includes: (a) a three dimensional illumination volume comprising a gas, the gas comprising at least one atomic or molecular vapor, the vapor comprising at least one ground state, at least one intermediate state, and at least one multi-photon excited state, the vapor configured to emit at least one type of visible light by decay from the multi-photon excited state; a plurality of lasers configured to generate at least a first laser beam, a second laser beam, and a third laser beam, wherein at least some of the laser beams comprise different wavelengths; and the system configured to direct the laser beams into the illumination volume such that the first, second, and third laser beams intersect at a beam intersection in the illumination volume to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state to the intermediate state, the emitted visible light having CIE 1931 xyY coordinates of x >0.60 and y >0.28.

[0032] In some instances, the vapor comprises at least a Cesium vapor. [0033] In some instances, the multi-photon ex

state above an n=5 energy level or a P state above an r

[0034] In some instances, the system excites the particles at the beam intersection to the multi-photon excited state by an excitation pathway that includes a 6D5/2 energy level or a 6D3/2 energy level.

[0035] In some instances, the multi-photon excited state comprises at least one of a

11F5/2 state or an 12F5/2 state.

[0036] In some instances, the Cesium vapor comprises a first ground state comprising a first absorption profile and a second ground state comprising a second absorption profile, wherein the first and second absorption profiles are at least partially overlapping.

[0037] In some instances, the system regulates the temperature of the gas such that the temperature is greater than 100° C.

[0038] In some instances, the first laser beam and the third laser beam propagate along a first beam path and the second laser beam propagates along a second beam path intersecting the first beam path at the beam intersection.

[0039] In some instances, the gas further comprises a buffer gas.

[0040] In some instances, the system is configured to scan the beam intersection to generate an image.

[0041] In another example, a system for displaying one or more images in three dimensions, the system includes: a three dimensional illumination volume comprising a gas, the gas comprising at least one atomic or molecular vapor, the vapor configured to emit at least one type of visible light by decay from a multi-photon excited state comprising at least one of a F state above an n=4 energy level or a P state above an n=7 energy level; a plurality of lasers configured to generate at least a first laser beam, a second laser beam, and a third laser beam, wherein at least some of the laser beams comprise different wavelengths; and the system configured to direct the laser beams into the i

second, and third laser beams intersect at a beam inti

excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light.

[0042] In some instances, the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light having CIE 1931 xyY coordinates of x >0.60 and y >0.28.

[0043] In some instances, the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light having CIE 1931 xyY coordinates of 0.4<x<0.56 and y>0.43.

[0044] In some instances, the multi-photon excited state comprises at least one P state above an n=8 energy level or F state above an n=10 level.

[0045] In some instances, at the beam intersection, the first laser beam excites the particles from a ground state to a first intermediate state, the second laser beam excites the particles from the first intermediate state to a second intermediate state, and the third laser beam excites the particles from the second intermediate state to the multi-photon excited state.

[0046] In some instances, at least some particles of the vapor at the beam intersection emit visible light by decay from the multi-photon excited state to a third intermediate state.

[0047] In some instances, the system regulates the temperature of the gas such that the temperature is greater than 100° C.

[0048] In some instances, the system regulates the temperature of the gas such that the temperature is greater than 125° C. [0049] In some instances, the gas further comp

[0050] In some instances, the system excites 1

the multi-photon excited state by an excitation pathway that includes a 6D5/2 energy level, a 6D3/2 energy level, a 5D3/2 energy level, or a 5D5/2 energy level.

In some instances, the particles at the multi-photon excited state comprises one or more decay pathways giving rise to a ground state filtered CIE 1931 xyY spectral luminance of 0.01 or greater.

[0051] In some instances, the particles at the multi-photon excited state comprises one or more decay pathways giving rise to a ground state filtered CIE 1931 xyY spectral luminance of 0.1 or greater.

[0052] In some instances, the system is configured to excite particles at the beam intersection to the multi-photon excited state via an excitation pathway including at least one intermediate state, the intermediate state comprising one or more decay pathways giving rise to a ground state filtered CIE 1931 xyY spectral luminance of 10^"4 or less.

[0053] In some instances, the system regulates the temperature of the gas such that the temperature is greater than 100° C, wherein the vapor comprises at least a Cesium vapor, wherein the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light from the particles having CIE 1931 xyY coordinates of x >0.60 and y >0.28, and wherein the multi -photon excited state comprises an F state above an n=5 energy level or a P state above an n=8 energy level.

[0054] In some instances, the system regulates the temperature of the gas such that the temperature is greater than 100° C, wherein the vapor comprises at least a Cesium vapor, wherein the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such tha

by decay from the multi-photon excited state, the e

having CIE 1931 xyY coordinates of 0.4<x<0.56 and y>0.43, and wherein the multi-photon excited state comprises a P state above an n=10 energy level.

[0055] In some instances, the system regulates the temperature of the gas such that the temperature is greater than 100° C, wherein the vapor comprises at least a Rubidium vapor, the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light from the particles having CIE 1931 xyY coordinates of x >0.60 and y >0.28, and the multi-photon excited state comprises an F state above an n=4 energy level or a P state above an n=7 energy level.

[0056] In some instances, the system regulates the temperature of the gas such that the temperature is greater than 100° C, wherein the vapor comprises at least a Rubidium vapor, wherein the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light from the particles having CIE 1931 xyY coordinates of 0.4<x<0.56 and y>0.43, and wherein the multi-photon excited state comprises an F state above an n=9 energy level or a P state above an n=8 energy level.

[0057] In another example, a system for displaying one or more images in three dimensions includes: a three dimensional illumination volume comprising a gas, the gas comprising at least one atomic or molecular vapor, the vapor configured to emit at least one type of visible light by decay from a multi-photon excited state; a plurality of lasers configured to generate at least a first laser beam, a second laser beam, and a third laser beam, wherein at least some of the laser beams comprise different wavelengths, wherein the first laser beam and the third laser beam propagate along

beam propagates along a second beam path intersei

intersection; and the system configured to direct the laser beams into the illumination volume such that the first, second, and third laser beams intersect at a beam intersection in the illumination volume to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light.

[0058] In some instances, the multi-photon excited state comprises at least one of a F state above an n=4 energy level or a P state above an n=7 energy level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figures 1 through 1(g) schematically illustrate non-limiting examples of a three-dimensional imaging system.

[0060] Figures 2 and 2(a) illustrate non-limiting examples of absorption and emission processes for a three-dimensional imaging system.

[0061] Figures 3 through 5 schematically illustrate additional non-limiting examples of three-dimensional imaging systems.

[0062] Figure 6 illustrates a non-limiting example of a three-dimensional imaging method.

[0063] Figure 7 illustrates an exemplary method for adjusting an angular emission pattern or intensity pattern of an illumination voxel according to some embodiments of the present invention.

[0064] Figure 8 illustrates a method of exciting particles to emit radiation according to some embodiments of the present invention.

[0065] Figure 9 illustrates an exemplary method for exciting particles to an upper auxiliary level according to some embodiments of the present invention. [0066] Figure 10 illustrates a method of exciti

that may decay to an intermediate level according

invention.

[0067] Figures l la-l lb illustrate an exemplary 3D display situation for the purpose of illustrating occlusion principles and methods and systems of the present invention.

[0068] Figure 12 illustrates an exemplary 2D cross-section through an illumination voxel according to some embodiments of the present invention.

[0069] Figure 13 illustrates a 3x3x3 cube of voxels surrounding an illumination voxel in some non-limiting examples of the present invention.

[0070] Figure 14 illustrates an example of ground state pumping.

[0071] Figure 15 illustrates IR scatter that may occur in some ground state pumping implementations.

[0072] Figure 16 illustrates an example of a three-dimensional imaging system including preparation and repump lasers in addition to lower and upper excitation lasers.

[0073] Figure 17 illustrates scattering-induced streaking.

[0074] Figure 18 illustrates dimmer streaking than Figure 17.

[0075] Figure 19 illustrates another example of a three-dimensional imaging system including preparation and repump lasers in addition to lower and upper excitation lasers.

[0076] Figure 20 illustrates another example of a three-dimensional imaging system including two preparation lasers and a repump laser in addition to lower and upper excitation lasers.

[0077] Figure 21 illustrates an example of three laser excitation.

[0078] Figure 22 illustrates an example of a three-dimensional imaging system including three lasers. [0079] Figure 23 is a table of destination ener^

the CIE 1931 xyY color coordinates, the intended prii

ns, and the laser wavelengths required to drive atoms to this level from the 6D3/2 and 6D5/2 energy levels. Blank entries indicate that the destination energy level cannot be reached from the given intermediate state by a single laser.

[0080] Figure 24 is a table of destination energy levels for Rubidium in a hot vapor cell, the CIE 1931 xyY color coordinates, the intended primary, the lifetime of the energy level in ns, the laser wavelengths required to drive atoms to this level from the 5D3/2 and 5D5/2 energy levels. Blank entries indicate that the destination energy level cannot be reached from the given intermediate state by a single laser.

DETAILED DESCRIPTION OF THE DRAWINGS

[0081] Figure. 1 depicts an example of a three-dimensional imaging system. As shown, the system 100 includes a three dimensional illumination volume 1 10 having at least one atomic or molecular gas. The atomic or molecular gas can include at least one type of atoms or molecules configured to emit a first type of visible light when at a two-photon excited state. In some cases, the system 100 can include a first laser 120 configured to generate a first laser beam 122 at a first wavelength λι and a second laser 130 configured to generate a second laser beam 132 at a second wavelength λ₂. The second wavelength λ2 can be different from the first wavelength λι.

[0082] The human eye has strong spectral sensitivity to light having wavelength values within a range from about 400 nm to about 700 nm. By using two-photon absorption, lasers producing light that is (e.g.) outside the spectral sensitivity of the eye, for example at a wavelength less than about 400 nm or greater than about 700 nm, can excite very small regions of the gas and make the gas emit light at visible wavelengths. Accordingly, the emission from the gas can be observed while the lasers exciting the gas are invisible. In other instances, lasers producing light that is within the :

utilized.

[0083] System 100 can be configured to direct the first and second laser beams 122,

132 to intersect in the illumination volume 1 10 to excite at least some of the first type of atoms or molecules at beam intersection 140 to the two-photon excited state, such that a first type of visible light 150 (e.g. a third wavelength λ₃) is emitted at the localized region or beam intersection 140. By changing (e.g. scanning) the location of laser beam intersection 140, 3- dimensional images can be produced in real space and, in some embodiments, changed in time to generate 3-dimensional videos.

Atomic or Molecular Gas

[0084] The illumination volume 110 has gaseous particles dispersed throughout it. In some cases, the particles may be present as a vapor, and may be atoms, molecules (elemental or compound), ions of atoms or molecules, or any combination thereof. In at least some embodiments, the gaseous particles have sufficient kinetic energy to move freely throughout the volume 110. When present within a container, gaseous particles can distribute so that the gas fills the volume of the container. In some cases, the gas within the illumination volume 110 is transparent when not undergoing an absorption/emission process. In some cases, gaseous particles of the illumination volume 1 10 can be specifically chosen based on their selective absorption of one or more laser wavelengths and emission of one or more visible wavelengths.

[0085] Figure 2 depicts an example of a particle excitation and emission process that may occur at the laser beam intersection 140 shown in Figure 1. As shown in this energy level diagram, a first photon 210 at a first wavelength λι or frequency in combination with a second photon 220 at a second wavelength λ2 or frequency can operate to excite a gaseous particle from a lower state (e.g. a first state or ground state) to a higher state (e.g. a second state or excited state). For example, the two photon

into a higher state (e.g. transitioning from one dis'

electron absorbs incident energy from the light photons. Following absorption of the two photons and elevation to the higher energy state, the excited electron decays to the lower state while also emitting a photon 230. The emitted light may be at a wavelength λ3 within the visible spectrum. Although Figure 2 depicts the lower to higher state transition occurring in a single step, in at least some embodiments, the transition will occur in multiple steps, such as by the first photon 210 causing a transition to an intermediate level and the second photon 220 causing a transition from the intermediate level to the higher level. Although Figure 2 depicts the higher to lower state transition occurring in a single step, in at least some embodiments, the transition will occur in multiple steps.

[0086] In some embodiments, the gas may include an atomic Rubidium (Rb) vapor.

Figure 2 depicts one example of a particle excitation and emission process for atomic Rubidium. In Figure 2a, a first laser beam at 780 nm excites a 5S i/2 to 5P3/2 transition, where it will remain for some period of time, and a second laser beam at 776 nm achieves the two- photon transition from the 5P3/2 to the 5Ds/2 states. As shown in Figure 2(a), when in this two-photon excited state, one spontaneous emission decay pathway emits a blue photon at 420 nm (in this particular case, infrared light is also emitted with the 420 nm light).

[0087] While not specifically shown in the figure, in this particular embodiment, the spontaneous emission pathway leading to the emission of 420 nm light proceeds from the 5D5/2 state to the 6P3/2 state emitting an infrared photon. From the 6P3/2 level the light is able to spontaneously emit a blue photon when it decays to the 5Si/₂ level. There are other decay pathways emitting other light, however, in at least some embodiments, none of those other pathways emit light in the visible range of wavelengths. [0088] In some embodiments, methods may b<

decay pathway (e.g. the emission of light at a desired

pathways. For example, additional lasers may be introduced to allow for the use of four- wave mixing to promote decay down the desired decay pathway. In some instances, however, four-wave mixing will not be suitable for a particular embodiment because typically, the phase-matching conditions restrict the angular emission partem of the emitted light to a very small solid-angle and in a precise and/or restricted angular direction.

[0089] In some, although not all, embodiments, the emission pathway depicted in

Figure 2a may be particularly desirable because the dipole matrix elements for these transitions is larger than some other transition pathways for Rb. Larger dipole matrix elements typically means, in at least some instances, that the transition is easier to pump or excite and often means that the particular decay pathway will occur with higher probability than other decay pathways. Larger dipole matrix elements also typically mean shorter excited state lifetimes. Since the number of times an atom can be excited and decay within the dwell time of the scanning lasers is directly related to the intensity of the emitted light, shorter excited state lifetimes can be very beneficial.

[0090] In some, although not all, embodiments, the emission pathways employed by the present system may be beneficial over other decay pathways that include decay through the 6P levels. In at least some instances, decay through the 6P level will mean that in addition to generating light at the desired wavelengths, such an approach will also generate light at 420 and 421 nm. Such approaches, in many instances, are unable to generate pure frequencies or wavelengths in the visible range, which may reduce the area of the color gamut which is accessible for a full color display, either RGB, CMYK, or other color mixing methodology.

[0091] The example of the excitation and emission process shown in Figure 2a uses two laser beams of infra-red light (e.g. having a wavelength of approximately 760 nm to 1000 μηι). More particularly, in this example, the two las

spectrum (e.g. having a wavelength of approximat

embodiments, other wavelengths outside of the spectrum of light visible to humans (e.g. outside of approximately 400 nm to 700 nm) may be employed. For example, in some embodiments, ultraviolet wavelengths may be employed.

[0092] Additional / other pathways than that shown in Figure 2a may be employed in some embodiments. Some non-limiting examples include pathways ending on the 6D5/2, 7D5/2, 8D5/2, I2D5/2 levels, which utilize the 5P3/2 intermediate level. Other examples include pathways ending on the 8S1/2, 9Si/₂, and 1 OS 1/2 levels, which utilize the 5Pi/₂ level. Still other examples include excitation pathways to the (5-12)D3/2 levels, the (9-l l)D5/2 levels, and the I I S1/2 level, which utilize either the 5Pi/₂ or 5P3/2 intermediate levels, all of which generate visible light when they decay. Some of these pathways may be preferable to other pathways in certain embodiments. For example, excitation pathways to the (9-l l)D5/2 levels may have a larger cross-section and branching ratio to the 5P3/2 level than the I2D5/2 level has to the 5P3/2 level. Broadly speaking, the P1/2 levels couple nearly as strong to the D3/2 levels as the P3/2 levels couple to the D5/2 levels (as measured by the transition matrix elements). Thus, the (5-12)D3/2 levels may be used with nearly the same effectiveness as the D5/2 levels in some embodiments. Additionally, the P3/2 levels appear to couple to S1/2 levels more strongly than at least some of the P1/2 levels (e.g. 8-IOS1/2 to 5Pi/₂). Levels above the U S and 12D levels may also be used, however both the cross-section and branching ratio to the 5P levels appear to decrease for higher levels. Since, in at least some embodiments, the design of a suitable display system will depend upon the availability of suitably configured lasers at the various transition wavelengths, identification of all levels which may be used may be an important consideration in constructing a suitable system in at least some instances. US 4,881,068 to Eric J. Korevaar and Brett Spivey identify other pathways that may be utilized in some embodiments, the disclosure of which is incorporated

Cesium vapors, the following transitions may be used:

• the 6S 1/2 level to the 6P3/2 level, then the 6P3/2 level to the 12-14D5/2 level;

• the 6S 1/2 level to the 6P1/2 level, then the 6P1/2 level to the 7-14D3/2 level;

• the 6S 1/2 level to the 6P1/2 level, then the 6P1/2 level to the 12-13Si/2 level;

• the 6S 1/2 level to the 6P3/2 level, then the 6P3/2 level to the 6D5/2 level which may decay to the 7P3/2 level via infrared radiation and subsequently to the 6S 1/2 level via 455 nm radiation;

• the 6S 1/2 level to the 6P1/2 level, then the 6P1/2 level to the 6D3/2 level which may decay to the 7Pi/2 level and the 7P3/2 level via infrared radiation and subsequently from these to the 6S1/2 level via radiation at 455 nm and 459 nm; or

• the 6S1/2 level to the 6P3/2 level, then the 6P3/2 level to the 8S1/2 level, which may decay to the 7Pi/2 level and the 7P3/2 level via infrared radiation and subsequently to the 6S 1/2 level via radiation at 455 nm and 459 nm.

[0093] In some, although not necessarily all, instances, one issue with excitation and decay pathways that are based on two-transition processes is that it may be difficult to find a scenario where the laser addressing the upper transition can be infrared but the decay pathway creating the desired visible light does not occur on the final decay to the ground state. In the scenario where the visible light is generated on the final transition to the ground state, one potential issue in some instances is a trade-off between having a sufficiently high density so that sufficient visible light is generated, but having a sufficiently low density so that the generated light is able to propagate out of the cell without being substantially rescattered. In some embodiments, this trade-off limits the density of the Rb atoms. In some embodiments, one solution to this problem is using a buffer gas, which is discussed in greater detail below. Another possible solution to this problem, whether alone or in combination with a buffer gas, is using ground-state pumping

population of the gas, which is also discussed in greats

solution, in scenarios where the laser addressing the upper transition is at a visible wavelength, the desired fluorescence may occur on the upper transition. Consequently the light is not resonant with the many ground state atoms in the gas and may propagate freely out of the volume. However, a visible laser which is very powerful (as is required to generate lots of fluorescence) can also create a lot of laser scatter that is hard to filter and eliminate. The laser scatter cannot necessarily be filtered easily because it is at nearly the same wavelength as the generated fluorescence. Any attempt to filter laser scatter will also filter the light emanating from the illumination voxel.

[0094] In some embodiments, this issue may be addressed by making use of an excitation pathway involving three infrared lasers and using a cascade processes to generate the visible light so that the visible light is created in an intermediate transition in the cascade process. One non-limiting example of this approach which can be used to generate red fluorescence is the excitation pathway: 5Si/₂ -> 5P3/2 -> 4Osn -> 8P3/2 with lasers at 780, 1530, and 953 nm. Decay pathways giving rise to significant amounts visible light in an intermediate transition are as follows: 630 nm light is created via 8P3/2 -> 6D5/2 -> 5P3/2 -> 5Si/2 and 8P3/2 -> 6D3/2 -> 5P3/2 -> 5Si/₂, 620 nm light is created via 8P3/2 -> 6P3/2 -> 5Pi/₂ -> 5Si/2, 616 nm is created via 8P3/2 -> 8S 1/2 -> 5P3/2 -> 5Si/2, and 607 nm light is created via 8P3/2 -> 8S1/2 -> 5Pi/2 -> 5Si/2. As with all other high-lying cascade processes, 420 and 421 nm light is still created from decay pathways that proceed though the 6P levels. Additionally, decay processes through the 7Si/₂ level will emit some radiation at 728 and 741 nm and decay from the 8P and 7P levels to the 5S level will generate ultraviolet radiation at 335 and 359 nm. The sum of the branching ratios through the five main visible decay pathways around 600 nm is about 25%, whereas the decay pathways giving rise to 420 and 421 nm light have a branching ratio sum of approximately 2%. With a twc

branching ratio to the 5P3/2 level which generates 63C

branching ratio sum generating 420 and 421 nm light as before. Thus a three-laser excitation process reduces the efficiency of the decay process branching ratios by only a factor of three, but completely eliminates visible laser scatter.

[0095] In some embodiments, this approach is used to generate other colors of visible fluorescent light. For example, the excitation pathway 5Si/2 -> 5P3/2 -> 4D5/2 -> 9P3/2 makes use of a 780, 1530, and 861 nm lasers. This transition will generate light decaying to the 9S, 8S, 7D, and 6D levels. Decay to the S-levels tends to favor the highest S-level, and decay to the D-levels tends to be equally distributed. Consequently, the emitted light will have frequency components at 557, 565, 572, 607, 616, 620, and 630 nm, with a heavier relative weighting of the green-yellow frequencies (557, 565, and 572). The perceived color is likely to be orange or yellow-orange. Some embodiments using this approach can also be used to generate predominantly green light by excitation up to the 10P, I IP, or 12P levels from the 4D5/2 level using lasers at 813, 784, and 764 nm, respectively. This approach can also be used to generate visible fluorescence without using visible lasers in different atomic species.

[0096] We note that if continuous wave lasers are used in a saturation condition, the total population in the 8P3/2 level will likely be reduced relative to the population which could be excited to the 6D5/2 level in a two laser configuration. If pulsed lasers are used, in principle, the entire population in the localized region could be excited to the desired level, either 8P3/2 in the three laser process, or the 6D5/2 in the two laser process. This can be done using so called "\pi pulses" to sequentially excite the atoms up to the desired excited state. A \pi pulse is a short laser pulse with a specific total area used to fully invert an atomic transition. By applying \pi pulses in sequence the population can be moved sequentially to the desired excited state before population decays significantly from any of the intermediate levels. In some instances, this approach requires prec

\pi pulse. Additionally, in some instances, level deg

Zeeman splitting tend to corrupt the process, and Doppler broadening can also reduce the efficiency of the excitation process.

[0097] Another alternate approach in some embodiments for efficiently exciting the atoms to the desired level is to use amplitude-modulated stimulated Raman adiabatic passage (AM-STIRAP). In this approach resonant pulses are used in sequence to coherently transfer the atoms between two final states without populating the intermediate state. This approach can be used for both ladder systems and lambda-type systems and can be applied to multilevel systems with more than three levels. The pulse lengths for this process should be much shorter than the decoherence time of the pairs of levels. In a ladder system the decoherence time between pairs of levels is exceedingly short, nevertheless it may be feasible if short laser pulses, including femptosecond, picosecond, or possibly, in some cases, few nanosecond pulses, are used. This approach tends to be robust to level degeneracies [Shore et al. Phys. Rev. A 45, 5297 (1992)].

[0098] Still other non-limiting examples of possible excitation pathways include excitation up to the 5F7/2 level: 5Si/2 -> 5P3/2 -> 4D5/2 -> 5F7/2. Atoms excited up to the 5F7/2 level will decay through the 4, 5, and 6D5/2 levels and subsequently through the 5, 6, and 7P3/2 levels, respectively, generating visible light at 630 nm and 420 and 421 nm. In this approach, only about 2% of the atoms will decay to the 6D5/2 level to emit 630 nm light but greater than 1% will decay through the 6P3/2 level to emit 420 nm light.

[0099] The approaches described above for generating localized visible fluorescence using two or more lasers can also be generalized to noble gases. Most noble gases can be excited with electronic excitation to the so-called metastable states. Metastable states have the property that they are long-lived states with decay lifetimes far exceeding other levels in the same atom. The metastable states exhibit incr

common ground state is forbidden by standard trans

can function like effective ground states for higher-lying levels above them. For example, in Argon, the are two metastable states, the .

configuration

state and the term J=0 state, using notation consistent

with the NIST Atomic Spectra Database [Kramida, A., Ralchenko, Yu., Reader, J. and NIST ASD Team (2014). NIST Atomic Spectra Database (version 5.2), [Online], Available: http://physics.nist.gov/asd [Tuesday, 17-Feb-2015]. National Institute of Standards and Technology, Gaithersberg, YID. | From the

configuration ¾ term

state a laser at 811.53 nm can excite the atom to the 3

configuration

term J=3 state. Then a visible laser of wavelength of 603 nm can excite the atom to the configuration state. It is important to note that metastable

states can be excited to states that are able to eventually decay in some instances to the Argon ground state via ultraviolet radiation, which may be undesirable in some, although not necessarily all, embodiments. Using levels that can decay to the ground state is not-preferred in some embodiments because energy is lost but visible light is not created. All of the levels listed above are forbidden from decaying to states which decay to the ground state. As such they constitute what we will call a metastable manifold of states. By this we mean that allowed decay pathways from these states always terminate on the lowest energy metastable state, in this case the

configuration

state. Other excitation pathways may also be envisioned in Argon. For example, instead of using the excited state with the

configuration, excitation to the

configuration term J=4 state is able to generate green light at 550 nm. Similarly, excitation to the

(7-12)D levels (same term and total electron angular momentum as the 4D and 6D states) emits (522, 506, 496, 489, 483, 480) nm light, respectively. This means that using the 5D, 7D, and 12D levels would allow for a full RGB col system. As above, these states are part of the metast; small amount of ultraviolet light will almost always be generated in these systems from the cascade decay of the excited d state to the 6-12P levels and subsequently to the 4s metastable state. This type of decay can be filtered by using coatings on the display window in addition to being naturally filtered by the display windows themselves.

[0100] The similarity of all noble gases, including Neon, Argon, Krypton, Xenon, and

Radon, means that if a sequence of levels can be found in one element, there is a nearly equivalent level structure in the other elements, albeit with different transition frequencies and different dipole transition matrix elements. This means, for example, that mixtures of noble gases can be used to generate multiple fully independent colors. In some cases it may be desirable to scan the red, green, and blue colored voxels independently. For this to be possible, in at least some embodiments, the laser driving the lower transition has to be different for each color. In some cases this may be possible with a single atomic species by utilizing different metastable states and intermediate transitions. In other cases it may be advantageous to mix atomic species so that each species creates one or more colors. For example, consider a set of levels in Krypton, with metastable state

configuration intermediate state configuration

and excited state configuration state. The lower

transition is accessed with 811.29 nm light, while the upper transition is accessed with and subsequently emits 646 nm light. The (7-12)D levels can be accessed with and emit (583, 552, 534, 522, 515, and 509) nm light, respectively.

[0101] Other levels in noble gases besides those mentioned above may be utilized in some embodiments. In some cases, additional decay pathways may be acceptable if the branching ratio through the primary pathway is large enough. Similar to the alkali vapors, excitation to the high-lying s levels can also be consic

or three-laser excitation with cascade emission of visi

gases similar to what is discussed above for alkali vapors.

[0102] In some embodiments utilizing noble gases, it may be challenging to create a very high density of metastable states without also creating large amounts of visible fluorescence from higher lying levels. In some embodiments, this problem can be surmounted by separating the metastable state creation region from the display volume with an opaque tube of sufficient length. Since higher-lying states decay very quickly, and the metastable states decay very slowly, atoms in higher lying states will decay before leaving the tube while the metastable states will not. In this way only ground state atoms and metastable state atoms will reach the display volume. One feature of using metastable atoms in at least some instances is that any atom in the ground state will act as a buffer gas to the metastable states. More details about buffer gases for some embodiments will be included below.

[0103] In some embodiments, metastable state densities close to those used in Alkali systems are possible. Typical methods for producing metastable states of noble gases have an efficiency in the range of 10^"5-10^"4. For Argon at a pressure of 10 Torr at room temperature, an efficiency of 10^"4 corresponds to a metastable state density of 3xl0^"13/cm³. This is roughly the same as the density of a Rb vapor heated to about 130 °C. The metastable states should be able to fill a large volume because effective lifetime of the metastable state (in the presence of collisions with ground state atoms) is estimated to be a few ms. At room temperature the Ar atoms have a mean velocity of about 400 m/s, so that a metastable state should be able to travel about 400-1200 mm before it relaxes to the ground state. We note that the intrinsic lifetime of the metastable state is actually 38 sec ; the effective lifetime includes collisions so the calculation does not appear to depend upon the mean free path of the metastable Ar states.

[0104] In at least some embodiments, the system may be configured to maintain the gas at a desired density in the illumination volume, si

to a desired temperature by using, for example, a heai

including atomic Rubidium can be heated anywhere from room temperature to approximately 150 degrees Celsius to maintain a target density of anywhere between 10¹⁰ to 10¹⁴ atoms/cm³. In other embodiments, including embodiments utilizing inert gasses, heating may be unnecessary to achieve target densities.

[0105] In some embodiments, the target density depends on the specific excitation and decay pathways as well as the composition of the atomic vapor. In some embodiments, an inert buffer gas may be used to collisionally broaden the energy levels. As noted above this has the effect in at least some embodiments of drastically improving the efficiency of the excitation and emission processes. Since in some embodiments the goal is to create a practical display that is easily visible in moderate ambient lighting, the target pressure may be reduced so the temperature of the vapor cell does not need to be so high and still allow for an acceptable production of visible fluorescence. In the case that the atomic species is primarily composed of inert gases and metastable states, the target density can be reached at room temperature simply by controlling the pressure relative to the production efficiency of the metastable states, as discussed above.

[0106] As discussed above, inert gases can be at room temperature and achieve the target densities. With inert gases, collisional energy transfer will tend to remove atoms from the metastable manifold of states. For this reason, target pressures of on the order of 10 Torr are preferred in some embodiments (this corresponds to a metastable density of about 3xl0¹³/cm³). Other embodiments may utilize a pressure in a range from 0.01 Torr to roughly 200 Torr.

[0107] For alkali atoms, the density is tied to the temperature of the gas. The relationship between density, pressure, and temperature may be calculated using the ideal gas law and species specific vapor pressure models (see, f

D Line Data," available online at http://steck.us/all;

2010)]). Using these models, the target densities listed above can be converted to target pressures, as well as target temperatures. For example, in Rubidium, 10¹⁰-10¹⁶ atoms/cm³ correspond to a temperature range from 22 °C to 270 °C. If the temperature of the Rb vapor is too high, then Rb-Rb molecules can be created - which may tend to corrupt the display. Consequently, temperatures above about 300 °C are not preferred in at least some embodiments.

[0108] The target density depends on a complex interplay of the excitation rate and the radiation trapping probability. This is discussed further below. If two alkali vapors are mixed in the display, they will each have a different density depending on the temperature of the display. For example, a mixture of Cesium and Rubidium will have partial pressures, and consequently densities, at a ratio from 3.5 to 2 over the temperature ranges listed above. Since the partial pressures of mixtures of inert gases can be controlled directly, any set of target densities can be produced without difficulty.

[0109] If the gas is too dense, several deleterious effects can be noted in at least some instances. First, the light which is resonant with a ground-state (or metastable state) transition can become radiation trapped. For example, in the Rb vapor the 780 nm laser will tend to excite atoms up to the intermediate level. Additionally, atoms that are further excited to a high lying D5/2 level, say, may decay back down to the 5P3/2 level. In both cases, the atom will decay back down to the ground state by emitting photons that are resonant with the 5Si/₂- 5P3/2 transition. If the vapor is too dense, this light will very quickly be reabsorbed. If the light is reabsorbed outside of the original beam of the 780 nm laser, it will mean that atoms outside the original 780 nm laser beam are able to absorb and emit the visible light. This will tend to lead to blurring and visual derealization of the illumination voxel, for very high densities. In a configuration where the visible emi

transition, the light will be absorbed and rescattered, 1

more moderate densities. In the extreme case, the light emitted from the illumination voxel will be completely blurred - all that will be observed is a haze of light at the visible wavelength; the illumination voxel will not be observed at all.

[0110] If the gas is not dense enough then the vapor or gas will not be able to create a sufficient amount of visible fluorescence for the display to be viewed in even low to moderate ambient light settings.

[0111] In some embodiments, the optimal target density will depend on many factors. For example, if the temperature and density is too high, then atoms excited to the intermediate level can decay emitting resonant light which will then be radiation trapped and will have the effect of increasing the voxel size. The density can be higher when the transition generating the visible light is not connected to the ground state because the visible light won't be absorbed and rescattered as it leaves the cell.

[0112] In some embodiments, using an inert buffer gas in the vapor cell can lead to several improvements. A buffer gas has the effect of causing collisional broadening which broadens the effective atomic linewidth, allowing many more velocity classes to absorb laser light and emit radiation. In a hot vapor, the motion of atoms relative to the incoming optical beams causes the photons to be red- or blue-shifted for each atom based upon its velocity. If the optical beams have a very small bandwidth, then generally speaking, only those atoms that are nearly stationary will experience correctly detuned light. (In some cases, a so-called Doppler-free configuration can be implemented by counter-propagating the lower and upper excitation lasers. This only works in at least some instances when the lasers have nearly the same wavelength as is the case for the 5Si/2-5P3/2 and 5P3/2-5D5/2 levels. Additionally, without complex frequency chirping techniques, counter-propagating beams cannot give rise to a well-defined voxel tightly localized in all three di

instances atoms having a large velocity will be less

Consequently, the density of atoms in the excited state will be much smaller than expected. This means that the emitted radiation will be much reduced. The effect can be significant for even moderate temperatures. A measure of the effect can be calculated by comparing the width of the Maxwell velocity distribution to the width of the excited level. For example, in Rb vapor the Doppler width at 120 °C is approximately 600 MHz (FWHM), whereas the natural linewidths (again, FWHM) of the 5P3/2 and 5D5/2 levels are approximately 6 MHz and 0.7 MHz, respectively. Consequently, only about 1 in every 1000 atoms will interact with light resonant with the two-photon transition, reducing by the same factor of 1000 the population density of atoms in the excited state. By including a buffer gas, the homogeneous linewidth of the atoms can be increased by collisional broadening with the buffer gas. With an increased homogeneous linewidth, effect of the Doppler broadening can be much reduced. For example with 20 Torr of Neon buffer gas, the homogeneous linewidth of both intermediate and excited levels increases to about 200 MHz (FWHM), so that roughly 1 in every three atoms will interact with light resonant with the two-photon transition. This represents an increase of a factor of about 300 over the non-buffer gas cell. The optimal pressure of the buffer gas should be chosen to give rise to collisional broadening of somewhere in the range of 0.1 to 2 times the Doppler width. Different inert gas species can be used. For example, at approximately 120 °C, Argon buffer gas imparts roughly 20 MHz/Torr of broadening, whereas Neon imparts roughly 10 MHz/Torr of broadening. One non-limiting embodiment may use 20 Torr of Neon buffer gas.

[0113] In some embodiments, the addition of buffer gas allows creation of a voxel that is easily viewed in normal room lighting with low power lasers (less than 30 mW power on target in each laser). [0114] Another advantage in some embodime

density of the atoms can be reduced and still be suffi

visible fluorescence. Reducing the density can drastically improve the problem of radiation trapping for visible light that is resonant with a ground state transition - this was mentioned briefly above. Since the total absorption (and subsequent reemission) of the visible fluorescence varies exponentially with the density, reducing the target density of the alkali vapor can drastically improve this problem in some instances.

[0115] Another advantage to including a buffer gas in some embodiments is that because the density can be reduced and still be sufficient to create an acceptable amount of visible fluorescence, the temperature can be reduced. This means that even the alkali vapor which requires heating can be considered viable in a practical implementation. Whereas temperatures of 160-180 °C appear to be optimal for the 5S-5P-5D based display, with a buffer gas temperatures of 40-100 °C may be acceptable. This drastically improves the electrical efficiency and reduces the possible danger of the 3D display.

[0116] In some embodiments, the illumination volume may include additional or alternative gasses or combinations of gasses. In some embodiments, multi-colored emissions may be achieved by using mixtures of different gases. For example, in some embodiments, for a red, green, and blue emission, three different gases may be included in the illumination volume / container, with different lasers driving those transitions.

Illumination Volume

[0117] In the example shown in Figure 1, the illumination volume 110 is the three dimensional space in which the first and second laser beams 122 and 132 may intersect in the atomic or molecular gas to form an image. The illumination volume 1 10 may be configured in a wide variety of geometries and sizes. In Figure 1 , the illumination volume 110 is a cube. In other embodiments, the illumination volume 110 may be cylindrical, spherical, or other shapes. The illumination volume 1 10 may have a vol

cubic meters, or larger.

[0118] The illumination volume 1 10 may be located in a container, such as a vapor cell. In at least some embodiments, the atomic or molecular gas is evenly distributed throughout the container. The container (or at least some surfaces of the container) may be transparent or semi-transparent to provide unimpeded or relatively unimpeded viewing of images formed in the viewing volume 1 10 from multiple vantage points. In some embodiments, the container may be glass. In some embodiments, for example some embodiments utilizing gases that are introduced into the container under high vacuum, the container may be constructed from materials and in geometries to withstand high internal vacuum. In other embodiments, less robust containers may be employed (e.g., in some embodiments utilizing noble gases (e.g. helium, neon, argon, krypton, xenon, or radon), it may be possible to have the noble gas in the container at lower pressure, without evacuating the container to so-called high-vacuum pressures.

[0119] Figure 3 shows one non-limiting example of a cylindrical container 1020. In

Figure 3, laser beam sources 1050, 1060 are positioned such that laser beams 1032, 1042 enter the container at points 1022, 1024, at a single side or face of the container (i.e., in this embodiment, a planar lower face of the cylinder). Cylindrical containers such as the one shown in Figure 3 may be advantageous in some instances, as the curved wall of the cylinder will present fewer edges or corners in the container to interfere with the viewer's view of the illumination volume and image formed therein or otherwise distract the viewer. Cylindrical containers may also be advantageous as being better able to withstand vacuum pressures that may be applied to them in some instances.

[0120] Other embodiments may use other types of containers. For example, hemispherical or partial sphere (e.g. a sphere that has been truncated by a plane - such as a spherical cap or spherical bowl or inverted spherical

Such forms may also be able to withstand a large p

glass. In some instances, the excitation lasers may enter the partial sphere through a flat surface in the same manner as which they enter a flat surface of a cylinder in some of the embodiments described above. Above the plane of the flat window of the partial sphere, no views of the fluorescence would be obstructed by glass corners. In some embodiments it may be advantageous to have two truncating planes and send one excitation through one plane and one laser through another plane. More generally, smooth glass surfaces, not necessarily spherical in shape, may be used above the flat entrance window or windows. As long as the glass above the flat window contains no sharp bends, it will induce minimal distortion to the emitted fluorescence. This freedom of the top surface above the flat window may enable designer shapes to be constructed. In still other embodiments, sharp bends or corners do not necessarily need to be avoided.

[0121] We describe further below methods for minimizing spurious intersections of the excitation lasers, which may be desirable in some, although not necessarily in all, embodiments. These techniques may or may not be employed with the additional use of dieletric coating and/or specially designed dichroic glass. For example, for a hemisphere container, a broadband antireflective coating can be given to the inside and outside of the hemisphere. This will permit the visible fluorescence to more easily be transmitted out of the container. Additionally, if the container is made from IR and UV absorptive glass, the excitation lasers that are infrared can be strongly absorbed by the glass with minimal reflections back into the main container. UV fluorescence generated by spurious decay pathways will also be absorbed by the glass. For example, Schott KG-1 Heat Absorbing Glass available from Edmund Optics strongly absorbs light below 300 nm and above 900 nm while transmitting visible wavelengths. Depending on the wavelengths of the excitation lasers, this glass could be very effective at reducing t

user to safe levels. The display could be made 01

commercially available. Additionally, the display container could be enclosed in additional filtering enclosures so that the container itself might not be absorptive, but the additional enclosures are absorptive of UV and/or infrared light. In this way, any light that is dangerous to the user can be strongly attenuated to a safe level. It is important to note that in many embodiments the fluorescence generated by the illumination voxel will never be of sufficient intensity to endanger display users, even if it also contains unwanted ultraviolet fluorescence from undesirable decay pathways.

[0122] In some cases an absorptive structure may partially enclose the display volume at some distance. This could be used to ensure that a user is never able to view the display from a direction that the excitation lasers are able to point. For example, in a cylinder container, if the excitation lasers are restricted so that they only exit the container through the top window, an absorbing surface such as a black velvet cloth (or similar absorber which is safe at the powers of the excitation lasers) could be used in addition to anti-reflective coatings to block the excitation laser. The absorbing material could be put at a distance from the display, depending on the display design. The primary purpose, as stated previously, would be to ensure that no one is able to view the display from a possibly dangerous viewing angle.

[0123] Figure 1 also shows an embodiment in which the laser beams 122, 132 can enter the illumination volume 110 through a single side or face (e.g. front face 1 1 1) of the illumination volume 1 10. By directing the laser beams through a single face, side or surface of the illumination volume 1 10, it is possible to construct a viewing display where the laser sources, scanning mechanisms, and other components of the display are situated out of view of the observer, for example in a cabinet under or behind the viewing volume. As shown here, the volume 110 also presents a top face 112, a bottom face 1 13, a right side face 1 14, a left side face 115, and a back face 1 16. As discus

configured to change orientations in at least two de₀

second laser beams 122, 132 in the illumination volume 1 10 to change a location of the laser beam intersection in three dimensions.

[0124] In some embodiments, the illumination volume 110 constitutes the entire (or substantially entire) internal volume of the container. In other embodiments, the illumination volume 110 may be a subset of the internal volume of the container, even though the gas is distributed throughout the entire internal volume of the container. In other words, in some embodiments, there may be regions within the internal volume of the container where the system is not configured to generate images (or configured to avoid generating images). Figure 1 (a) schematically illustrates an example of a container 102' and an illumination volume 1 10' in which the illumination volume 1 10' where images may be generated is smaller than the internal volume of the container 102', with outer boundaries of the illumination volume 1 10' being offset from the interior of the container 102' by one or more distances (e.g. distance "d" in Figure 1(a)).

[0125] Restricting the illumination volume can also be used in some embodiments to ensure the safety of the display users. For example, in some of the embodiments utilizing cylinder and hemispherical containers, a smaller illumination voxel means that the deviation angle of the scanning lasers will be smaller. This may make it easier to add protective absorptive materials in a visually appealing way. For example, in the cylindrical container, restricting the illumination volume so that the lasers only exit the container through the far flat window would make it possible to put absorptive material only within the cone defined by location of the scanning mirrors and the cylinder far window. If the top of the cylinder were as tall as a person, then the absorptive material can be put at a large stand-off distance, possibly attached to the ceiling of the room in which the display is located. This would improve the visual appeal of the display. Other emboc

be made to have this property by ensuring the inters

container window is not visually accessible to the viewer.

[0126] In some embodiments, the system may be configured to minimize, if not eliminate, certain reflections of the laser beams 122, 132. As discussed above, visible light may be generated in the illumination volume 110 where first and second laser beams 122, 132 intersect (e.g. beam intersection 140 in Figure 1). Reflections of one or both laser beams 122, 132 (such as by reflections off of surfaces of the container surrounding illumination volume 110) may result in laser beams 122, 132 following multiple trajectories within illumination volume 1 10 and potentially intersecting at more than location, potentially resulting in undesired or unintended light emissions within the illumination volume in addition to emissions at an intended location (e.g. other than light emission 150 in Figure 1). In some embodiments, such reflections may be minimized, if not eliminated, by associating the container with anti-reflective properties. For example, in some embodiments, an anti- reflective film or other anti-reflective coating may be applied to one or more surfaces of the container that will minimize, if not eliminate, reflections of laser beams 122, 132.

[0127] In some embodiments, the proper use of anti-reflective coatings will depend on the particular frequencies present both in the fluorescence and in the excitation laser beams. They also depend upon the wavelengths and powers used in lasers in the display. The powers of lasers used in the display will depend upon an optimization over detuning, buffer gas pressure, and temperature that will need to be performed for each display medium. If the class II lasers give acceptable fluorescence brightness then no precautions need to be taken apart from warning the users not to look into a stationary laser beam. In fact, the primary danger is that users will look into a stationary beam. When the system is operating, the beams will be scanning over the volume and will not be stationary. The only risk then is that the system might malfunction and leave an excitation

accessible direction. If the design of the system is s

stationary in a direction that is accessible by the viewers, then much brighter beams can be used without risk to users. This is predominantly an engineering problem, and could be done using absorptive enclosures in the directions that the lasers propagate, or by building active feedback into the intensity modulation controls. For example, a signal could be generated which switches off the intensity control module if the pointing control signal remains stationary for too long. Alternatively, the scanning device can be made so that the beam angle goes in a non-accessible direction whenever there is a stationary, i.e. DC signal, received by the scanning module.

[0128] In some embodiments the anti-reflective coating can be made so that it transmits visible light but reflects infrared and ultraviolet light. This could be used to ensure that the excitation laser beams do not reach the viewers, in the case that the excitation lasers have either ultraviolet or infrared wavelengths, but no visible wavelengths. This approach is not necessarily advantageous in all embodiments because of the possibility of creating spurious fluorescence when the reflections of the excitation lasers intersect. An alternate approach would be to manufacture the container out of a substance that is absorptive for UV and IR wavelengths, but transparent for visible wavelengths. In the case where one or more of the lasers have visible wavelengths, then the aforementioned methods won't work. In this case the visible laser beams may need to pass through and out of the container in such a way that they are reliably absorbed and that they cannot be viewed directly by the display users. This may involve the combined use of anti-reflection coatings and absorptive enclosures or beam blocks. More generally, the container could have dichroic or multichroic anti-reflection and/or reflection coatings and/or absorptive regions to safely guide the light to a location where it will be absorbed and not endanger the display users. [0129] In some embodiments, other aspects

alternatively be configured to minimize or elimina

illumination volume. For instance, by reducing the volume of the illumination volume relative to the container, and/or arranging the laser beams such that they enter the container from the same side or face of the container, the chance of laser beam reflections resulting in undesired secondary beam intersections can be reduced. Figures 1(b) - 1(e) show a top view of a three-dimensional imaging system in which the container 102', illumination volume 110', and laser beam sources 120' and 130' are sized and arranged to minimize secondary beam intersections due to reflections of those laser beams inside the container. In this particular, and non-limiting, example, and as shown in Figure 1(b), the container 102' is a cube and the illumination volume 110' is a smaller cube centered in the container (e.g. a cube occupying less than 50%, less than 25%, less than 10%, or other percentage of the total internal volume of the container). Laser beam sources 120' and 130' are arranged such that their beams will enter the container 102' through the same side and can cover the entire illumination volume 110' (in the top view) by scanning through 20 degree arcs (other arc ranges are also possible, depending on the scanning technology which is employed). In this non-limiting example, and as shown by the examples of possible laser beam reflection patterns in Figures 1(c) - (e), secondary intersections of the beams will not occur, at least prior to two or more reflections of one or both laser beams inside of the container.

[0130] In some instances, the container may be additionally or alternatively configured to minimize Fresnel reflections of laser beams as they pass through the container. Figure 1(f) shows an example of a Fresnel reflection of a laser beam 122' that may occur as it passes through the wall of a container 102'. Figure 1(g) shows an example of a container 102' that includes two spherically shaped windows 160' and 160" to suppress Fresnel reflections, with the spherical surfaces of the windows being arranged such that the laser beams are normal or approximately normal to the sp]

container. In other instances, planar windows could b

same effect (e.g. oriented to achieve nearly normal entry angles for the laser beams). In at least some embodiments utilizing entrance windows to minimize Fresnel reflections, dielectric coatings may be provided on the windows to decrease reflection loss at the entrance window.

[0131] In other embodiments, the configurations and features illustrated by Figures

1(a) - 1 (g) are unnecessary, and other mechanisms may be employed to address reflection of laser beams (e.g. through anti-reflective coatings as discussed above) or otherwise account for laser beam reflection.

Heating System

[0132] As mentioned above, some embodiments may include a heating system. The following is a non-limiting example of a heating system used with an experimental set up utilizing a cylindrical container embodiment. The cylinder may be mounted inside another glass cylinder that comprises the oven. In this non-limiting example, the oven cylinder has a diameter of 270 mm and a length of 10 inches, and the gas cylinder has a diameter of 200 mm and a length of 226mm (about 9 inches). The gas cylinder is mounted about ¾ of an inch off the side of the oven cylinder. Beneath the gas cylinder are 6 resistive heating rods, each 5 inches long. Around each of the gas cylinder windows resistive heating rope is wrapped. In the oven windows, two small holes are drilled to accommodate the electrical wires for one, and to accommodate a brass hot air blowing tube. The hot air blowing tube has a diameter of about 3/8" and blows super heated air into the oven. The super heated air goes down the tube and out small holes drilled at one inch spacing on the side of the brass tube. The little holes disperse the air so the heating is uniform. At each end of the brass tube are 4 holes drilled in the same position longitudinally which ensure that the gas cylinder windows are hotter than the sides of the gas cylinder. The super heated air is hi

blown using a small pump. The total electrical powe

can run from 0 to near 700W. The optimal electrical power, including the optimal ratio of electrical powers, has not been determined. The general principles guiding optimization are based upon the desired temperature and the requirement that the condensed Rubidium or other atomic or molecular vapor not obstruct the excitation lasers or the primary viewing angles. This means that the coldest part of the vapor cell needs to be as hot as the desired temperature and should be in a region that does not obstruct either the excitation lasers or the primary viewing angles. The heater rope ensures that the windows can be made hotter than other parts of the cell, and heating from above with super-heated air ensures that the coldest part of the vapor cell is on the bottom of the cell. The heater rods on the bottom of the cell ensure that we can achieve the target temperature of the coldest part of the cell.

Large Scale Displays

[0133] In some instances, scaling a 3D display up to larger sizes may create difficulties. For example, one difficulty is related to scaling the resolution of the display. Another difficulty is related to obtaining sufficient excited state atomic density in a large volume. We will first discuss the first problem.

[0134] The resolution problem with other 3-D systems has been noted elsewhere, for example, in Enhanced Visualization: Making Space for 3-D Images, by Barry G. Blundell [John Wiley and Sons, Hoboken, NJ, 2007]. This problem is somewhat independent of the absolute scale of the system. One difficulty is the amount of time available for a specific pair of excitation laser beams to visit all relevant voxels in the illumination region within the integration time scale of the eye. For example, for a frame rate of 24 Hz, each illuminated voxel in a frame should be visited once every 42 ms. If each voxel is illuminated for 250 ns then only about 168,000 individual voxels can be addressed in each frame. In a close-pack configuration, this would only correspond to roughly ί

[0135] In some non-limiting embodiments c

methods may incorporate 3D vector-scanning, which allows the effective resolution to be much larger. In some instances, for 3D vector-scanning the effective resolution is related to the total 2D surface area which can by drawn in the display. Since many 3D images are comprised of distinct surfaces separated by empty space, drawing only the surfaces can be a very efficient way of using the display because very little time is wasted directing the beams to voxels that are not illuminated.

[0136] In some non-limiting embodiments of the present invention, whether in combination with the vector-scanning technology discussed above or without that technology, buffer gas may be used to address the resolution issue. For example, assuming an optical pumping rate on the order of about 10 ns and a collisionally broadened lifetime of about 5 ns, dwell time may be reduced in some instances to about 20 ns with little to no reduction in brightness. For this dwell time, in some instances, we can address 2.1 million individual voxels. For a close-pack configuration this corresponds to about 128 pixels per side, or in a 3D vector-scanning approach, to a total surface area of 1449x1449 pixels². This corresponds roughly to the same area as a 1080p HD TV. In a 3D vector scanning approach, this means that the resolution of each surface could be at or nearly at full HD resolution. The 3D vector-scanning resolution (in terms of total pixels) can be increased by a factor of 2 or more by increasing the laser power and the collisional broadening so the optical pumping time and laser dwell-time can be decreased by a factor of two or more. This would correspond to a collisional broadening of about 400 MHz and a lifetime of about 2.5 ns. For collisional broadening much beyond this, we expect additional collisional broadening to begin to negatively affect the fraction of atoms that may be excited to the upper level due to the shortened lifetime of the atoms in the intermediate state. Nevertheless, in some non- limiting embodiments, as long as the collisional bro£

optical pumping time, a large fraction of atoms shouh

level. This means that, in some non-limiting embodiments, to reduce the cycle-time for the excitation decay process, one has to increase the optical pumping rate, which essentially means that the laser power should be increased. We expect that the additional cost associated with higher power lasers will put limits on how large the resolution may be scaled in some instances. Nevertheless, the continual progress in laser diodes, both in terms of availability, quality, and cost, suggest that this problem does not represent an insurmountable obstacle, but rather one that will be solved incrementally as laser diode technology continues to mature.

[0137] Another concern in some instances is obtaining sufficiently high atomic density so the display will be bright enough. For a display based upon a metal vapor such as Rubidium, one difficulty is to adequately heat the chamber and have it be safe for users. With the addition of buffer gas the heating requirement is drastically reduced in some instances. Additionally, in some embodiments, the vapor cell can be housed in transparent heater glass. Heater glass uses a 0.25 micron thick fluorine-doped tin oxide resistive coating which can be heated up to 176 °C. This represents one possible method for uniformly heating the surface of a large glass enclosure. Combined with an evacuated glass enclosure, we think even large scale implementations (linear dimensions of 1 -2 m) are possible.

[0138] With an inert gas, heating is not necessary in many embodiments though there is still difficulty in scaling to larger dimensions in some instances. For example, in some instances, one difficulty may be the effective lifetime of the metastable states in a low- pressure environment. Since the efficiency of creating metastable states by standard techniques is on the order of 1 : 10,000-100,000, the metastable states exist in an effective buffer gas of ground-state atoms. These ground state atoms lead to an increased quenching rate of the metastable states. The quenching rate depends on the pressure of the inert gas. Some sources list a few microseconds as a feasible

some non-limiting embodiments, as long as the metas

that short time to fill the display volume this method should be able to be used in larger volumes. An optimization can determine the trade-off between the density and the effective lifetime for each size of display. If the density must be reduced to fill the display, then the laser powers can increased to compensate.

Lasers

[0139] The laser sources 120, 130 of the system shown in Figure 1 may be selected based on the particular gas or gasses employed in the illumination volume 110. For example, in one embodiment that includes an atomic Rubidium gas in the illumination volume, lasers 120, 130 may include a laser configured to generate a 780 nm laser beam for exciting the 5Si/2 to the 5P3/2 transition and a laser configured to generate a 776 nm laser beam for exciting the 5P3/2 to the 5Ds/2 transition in order to stimulate emission of a blue light at 420 nm.

[0140] One non-limiting embodiment uses scientific grade narrowband cw lasers (~1 -

2 MHz bandwidth) with powers in the few tens of mW. In some instances, the fluorescence may be cleanest (in the sense of low blurring from fluorescence outside of the intersection volume) and brightest (for the level of voxel cleanliness) when the 780 nm laser is detuned away from the resonances of the D2 line. However, in some instances, we also find that due to the hyperfine splitting of the ground state, putting the 780 nm beam between the hyperfine resonances shows an improvement relative to putting it outside the resonances. This is because when the laser is between the resonances, it is equally likely to excite atoms out of either hyperfine state so that a preponderance of ground-state atoms do not develop in the ground state which is less likely to be excited. [0141] In one non-limiting embodiment, the

appears to be very close to or precisely at the two-phi

of both lasers add up to the energy difference between the top level and the bottom level.

[0142] In some non-limiting embodiments it will be the case that higher power lasers will produce better results, up until the saturation intensity is reached for a particular detuning. In these instances, additional power beyond that required for the saturation intensity doesn't contribute to the excitation process and is just wasted energy. There is the additional consideration of using as little light as possible so that the lasers pose less of a danger to the users. Finally, as the powers approach saturation, the fraction of atoms that may be excited relative to the increase in power decreases. Consequently, where possible, operating in the linear regime (below saturation) is relatively energy efficient. One difficulty in some instances of operating in a linear regime is that the power of the lower excitation laser is absorbed as it propagates through the vapor cell. This can mean, for example, that the intensity of the voxels at a distal location relative to the entrance window of the lasers can be reduced relative to proximate voxels. This may be corrected in some non-limiting embodiments by reducing the power of the upper excitation laser when addressing proximate voxels and increasing the power of the upper excitation laser when addressing distal voxels. The optimal power of the excitation laser for each voxel can be calibrated so that all voxels emit visible light with a uniform brightness or intensity. In some cases, the tradeoff between saving energy in the lower excitation laser by operating in the linear regime (below saturation) and having to attenuate the upper excitation laser to produce uniform brightness of the voxels may suggest that operating near or in the saturation regime for the lower excitation laser may be preferred. We discuss additional / alternative ways of addressing this issue further below. [0143] In some instances, the optimal beam dii

viewing distance. The resolution of the eye is roughly

foot [online: http://prometheus.med.utah.edu/~bwjone/2010/06/apple-retina-display]. In embodiments intended to be comfortably viewed at about 2-3 feet, the beams may have a diameter on the order of 300 microns so as to exceed the resolution of the human eye. This can easily be accommodated by optical beams focused by lenses that are required to be at a moderate stand-off distance from the intersection point. Larger displays will be viewed from further away in some instances and will therefore tolerate a larger voxel size, allowing larger beam diameters. Larger beam diameters, in turn, will accommodate larger stand-off distances between the illumination region and the focusing lens. In some instances, larger beam diameters will also likely require increased laser power to compensate for the decreased laser intensity.

[0144] The system may include alternative and/or additional lasers for use with different gases, to produce different colors, to produce multi-color images, and/or for other purposes.

[0145] Lasers 120, 130 may be continuous or pulsed. In some instances, pulsed lasers

(e.g. having a duration of milliseconds, microseconds, nanoseconds, picoseconds, or shorter or longer duration) may be utilized to enhance the absorption and visible emission and/or reduce the driving power of the laser.

[0146] In some instances, lasers may be intensity modulated to obtain intensity modulation (e.g., 8 bit gray scale) in the image or portions of the image.

[0147] With the inclusion of buffer gas, in some embodiments, lasers of moderate bandwidth may be employed. In some embodiments, the bandwidth of the laser diode should roughly match the collisional broadening width, or roughly 200-500 MHz. Diodes of this type may provide cost benefits. Additionally, in some non-limiting cases the bandwidth of the laser diodes can be increased beyond the requin

system is operated in a true two-photon regime (as

absorption regime), then each laser bandwidth can be increased beyond what is stated above. As long as the bandwidths of the lower and upper laser are matched and appropriately tuned relative to one another, each region of the lower excitation laser bandwidth will contribute with the complementary region of the upper excitation bandwidth to produce true two photon excitation. Even in a sequential two photon absorption regime, an increased bandwidth can still contribute, albeit with a reduced efficiency, to promoting the atomic population to the intermediate state.

[0148] In some embodiments, the lasers should be have a bandwidth equal to the homogeneous linewidth (collisional broadening is included in the homogeneous linewidth) with a frequency stability which is on the order or less than the homogeneous linewidth. In some cases active monitoring of the laser frequency and feedback will have to be used to ensure the laser frequencies do not drift over time. In other cases, larger bandwidths may be acceptable, and larger drifts may be tolerable, depending on the laser bandwidth and the size of the drift. In at least some implementations, these factors should be designed so as to reduce the variation of the brightness or intensity of the voxels over time to an acceptable level. Control System

[0149] The laser beam intersection 140 shown in Figure 1 can represent an addressable location or position within the illumination volume 1 10, such that selective excitation of a small region of the atomic or molecular gas at an addressable location within the volume 1 10 operates to produce an illumination at that specific location. In some cases, an individual illumination can form at least part of an image. In some cases, a first intersection can produce a first illumination or illumination region and a second intersection can produce a second illumination or illumination region, such that the first and second illuminations or illumination regions form at least part

[0150] According to some embodiments, look

correlate a desired xyz coordinate (or other addressable location) of the illumination volume with one or more angles (or other positioning or orienting information) for the laser beams. In some cases, xyz coordinates can be transformed into scan angles. For example, in the embodiment shown in Figure 1, a particular xyz coordinate can be transformed into a first and second scan angle for the first laser beam 122 (e.g. a first scan angle about a first degree of freedom and a second scan angle about a second degree of freedom that is perpendicular or otherwise transverse to the first degree of freedom) and third and fourth scan angles for the second laser beam 132 (e.g. with the third scan angle being about one degree of freedom and the fourth scan angle being about another degree of freedom). In some embodiments, look up tables or algorithms may include information or otherwise be configured to relate a particular xyz coordinate or other spatial coordinate to settings or adjustments for scanning mechanisms used to adjust the first and second laser beams in multiple degrees of freedom.

[0151] Figure 4 depicts aspects of a display system 1 100 according to another non- limiting embodiment of the present invention. As shown here, system 1 100 includes a laser source 1110, a scanning mechanism 1120, a display 1 130, and a control mechanism 1 140 such as a computer or other processing device or system. Although a single laser source 11 10 is shown in Figure 4 for simplicity, it should be understood that this embodiment and others may include multiple laser sources.

[0152] The scanning mechanism 1120 may provide for the controlled deflection of a laser beam 1 112 generated by the laser source 11 10. Scanning mechanism 1 120 may be one or more devices for scanning laser beam about one or more dimensions or degrees of freedom. According to some embodiments, the scanning mechanism 1 120 can include any suitable configuration of moveable mirrors or diffractive structures to direct or spatially displace one or more laser beams in various degrees o

mechanism 1 120 can direct a beam in one dimension

cases, the scanning mechanism 1130 can direct a beam in two dimensions or two degrees of freedom. Exemplary mirror control mechanisms may include electric motors, galvanometers, piezoelectric actuators, magnetostrictive actuators, mems scanners, and the like. In some cases, a scanning mechanism 1 120 can include acousto-optic deflectors and/or electro-optic deflectors. In some cases, a scanning mechanism may include a focus mechanism for adjusting the focal point of a beam along the beam path. In some cases, focusing can be implemented using an electrically-controlled variable-focus liquid lens. In some cases, focusing can be implemented using a servo-controlled lens. In some cases scanning technologies may be implemented sequentially, including a fast technology for small-scale deviations, and a large-scale scanning technology for large-scale deviations. In some cases this approach can increase the total deviation angle or arc without sacrificing scanning speed. An example of this type of embodiment would be an acousto-optical or electro-optical deflector followed by a galvanometer mirror scanner.

[0153] In some embodiments, the focus may be controlled with spatial light modulators as well. Additionally, one of the two laser beams may be made to be elliptical or elongated along the y-axis. When the beams intersect at the origin of the display volume they naturally define a coordinate system. The bisecting angle in the plane of the two beams we call the x-axis (we define positive x to be beyond the origin relative to the shared direction of propagation of the two beams), the right-handed cross-product between the two laser beam propagation directions we call the y axis, and the z-axis is defined by the right-handed cross product of the x- and y-axes. In this coordinate system, with the beams at the origin, we make the beam longer along the y-axis relative to the width in the direction perpendicular to this axis. For example, we might make the diameter of the beam in the vertical direction roughly 1 mm, whereas in the horizontal direction it would c

would be roughly 300 μηι by 300 μηι. Having one

direction makes it so that the system alignment is more robust with minimal effect on the voxel size. The voxel size is not increased because the voxel is controlled by the intersection of the two beams and this won't be strongly affected by lengthening one beam in the vertical direction. The system alignment is more robust because simpler transformations can be used to make the beams overlap. In practice determining the beam direction angles so that the laser beams overlap is a simple problem if the window through which they pass is not very thick. In some embodiments, because the window is quite thick it causes the beams to be translated slightly as they pass through the window. The translation depends upon the angle of incidence. Since the angle of incidence will be different for each beam, dynamically calculating a transformation can become quite complex. In contrast by simply lengthening one of the beams, a simple transformation can be used which gives rise to minimal image distortion. Lengthening the beam also means that steering overshoot cannot cause dimming of the voxels in some embodiments.

[0154] In some embodiments, the system may include one or more tunable lenses.

With a fixed focus approach the voxel size and brightness will naturally vary over the illumination region depending on the relative size of the beams at the beam intersection region. For example, when the intersection of the beams occurs away from the focus of either beam the voxel size will be increased and the brightness or intensity of the visible light may also be increased due to the increased number of atoms but slightly reduced laser intensity. When the intersection occurs near the focus of one beam the voxel can become elongated in one direction and have a reduced intensity or brightness, corresponding to a reduced number of atoms in the intersection which experience a slightly increased laser intensity. These effects also depend on other factors like whether the voxel is in the front or back of the vapor cell (relative to the side of the

Incorporating a tunable lens into each beam may be u

focused at the intersection region, which may be desirable in some, although not necessarily all, embodiments. Though the focus size will still vary slightly for near or far intersection locations, the change in the focus size can be drastically reduced, depending on the geometry of the non-tunable lens approach. For large illumination volumes, the stand-off distance of the final focusing optics from the illumination region may require the beam to have a sufficiently small divergence that a tunable lens will not offer a significant improvement in the variation of the focus size.

[0155] In some embodiments, intensity may be controlled with acousto-optical modulators. These may be fast enough to be used successfully with almost any scanning technology and exhibit high extinction ratios with relatively low loss. In other embodiments electro-optical modulators or other light modulating technology may be used.

[0156] In use, one or more scanning mechanisms can operate to create beam intersections within an illumination volume of the display 1130, such that the beam intersections occur at addressable locations of the illumination volume. By providing positional or direction control instructions from the processing device 1 140 to a laser source, a scanning mechanism, and/or a display, it is possible to position a beam intersection at variable locations in three dimensions throughout the space of an illumination volume.

[0157] In some cases, raster scanning can be used to create the beam intersections at the addressable locations. In some cases, instructions for the laser source 1 110, the scanning mechanism 1 120, and/or the display mechanism 1 130 can be provided via signals that are transmitted from a broadcasting entity, such as a television station, a cable service provider, an internet source or provider (e.g. via streaming media), or some other multimedia source. In other cases, information can be transmitted wirelessly from a processing device 1 140 or from the via the internet or internet cellular connection.

[0158] Computer 1 140 can be configured tc

scanning mechanism 1120. By changing the intensity and focal position (or beam-overlap position) of the light source, 3-dimensional color images can be produced in real space and changed in time. In this way, 3-dimensional videos can be generated.

[0159] Figure 5 depicts an example of a computer system or device 1200 (e.g., such as the computer or controller 1 140 of Figure 4) configured for use with a display system according to embodiments of the present invention. An example of a computer system or device 1200 may include an enterprise server, blade server, desktop computer, laptop computer, tablet computer, personal data assistant, smartphone, any combination thereof, and/or any other type of machine configured for performing calculations. The computer system or device 1200 may be configured to perform and/or include instructions that, when executed, instantiate and implement functionality of the laser source 11 10, the scanning mechanism 1120, and/or the display 1 130.

[0160] The computer 1200 of Figure 5 is shown comprising hardware elements that may be electrically coupled via a bus 1202 (or may otherwise be in communication, as appropriate). The hardware elements may include a processing unit with one or more processors 1204, including without limitation one or more general-purpose processors and/or one or more special-purpose processors (such as digital signal processing chips, graphics acceleration processors, and/or the like); one or more input devices 1206, which may include without limitation a remote control, a mouse, a keyboard, and/or the like; and one or more output devices 1208, which may include without limitation a presentation device (e.g., controller screen).

[0161] The computer system 1200 may further include (and/or be in communication with) one or more non-transitory storage devices 1210, which may comprise, without limitation, local and/or network accessible storage, ar

disk drive, a drive array, an optical storage device,

random access memory, and/or a read-only memory, which may be programmable, flash- updateable, and/or the like. Such storage devices may be configured to implement any appropriate data stores, including without limitation, various file systems, database structures, and/or the like.

[0162] The computer device 1200 can also include a communications subsystem

1212, which may include without limitation a modem, a network card (wireless and/or wired), an infrared communication device, a wireless communication device and/or a chipset such as a Bluetooth device, 802.11 device, WiFi device, WiMax device, cellular communication facilities such as GSM (Global System for Mobile Communications), W- CDMA (Wideband Code Division Multiple Access), LTE (Long Term Evolution), and the like. The communications subsystem 1212 may permit data to be exchanged with a network, other computer systems, controllers, and/or any other devices described herein. In at least some embodiments, the computer system 1200 can include a working memory 1214, which may include a random access memory and/or a read-only memory device, as described above.

[0163] The computer device 1200 also can include software elements, shown as being currently located within the working memory 1214, including an operating system 1216, device drivers, executable libraries, and/or other code, such as one or more application programs 1218, which may comprise computer programs provided by various embodiments, and/or may be designed to implement methods, and/or configure systems, provided by other embodiments, as described herein. By way of example, one or more system components might be implemented as code and/or instructions executable by a computer (and/or a processor, including an FPGA module, within a computer); in an aspect, then, such code and/or instructions may be used to configure and/or

other device) to perform one or more operations.

[0164] A set of these instructions and/or code can be stored on a non-transitory computer-readable storage medium, such as the storage device(s) 1210 described above. In some cases, the storage medium might be incorporated within a computer system, such as computer system 1200. In other embodiments, the storage medium might be separate from a computer system (e.g., a removable medium, such as flash memory), and/or provided in an installation package, such that the storage medium may be used to program, configure, and/or adapt a general purpose computer with the instructions/code stored thereon. These instructions might take the form of executable code, which is executable by the computer device 1200 and/or might take the form of source and/or installable code, which, upon compilation and/or installation on the computer system 1200 (e.g., using any of a variety of generally available compilers, installation programs, compression/decompression utilities, and the like), then takes the form of executable code.

[0165] It is apparent that substantial variations may be made in accordance with specific requirements. For example, customized hardware might also be used, and/or particular elements might be implemented in hardware, software (including portable software, such as applets, and the like), or both. Further, connection to other computing devices such as network input/output devices may be employed.

[0166] As mentioned above, in one aspect, some embodiments may employ a computer system (such as the computer device 1200) to perform methods in accordance with various embodiments of the disclosure. According to a set of embodiments, some or all of the procedures of such methods are performed by the computer system 1200 in response to processor 1204 executing one or more sequences of one or more instructions (which might be incorporated into the operating system 1216 and/or other code, such as an application program 1218) contained in the working memory 12]

the working memory 1214 from another computer-re£

the storage device(s) 1210. Merely by way of example, execution of the sequences of instructions contained in the working memory 1214 may cause the processor(s) 1204 to perform one or more procedures of the methods described herein.

[0167] The terms "machine-readable medium" and "computer-readable medium," as used herein, can refer to any non-transitory medium that participates in providing data that causes a machine to operate in a specific fashion. In an embodiment implemented using the computer device 1200, various computer-readable media might be involved in providing instructions/code to processor(s) 1204 for execution and/or might be used to store and/or carry such instructions/code. In many implementations, a computer-readable medium is a physical and/or tangible storage medium. Such a medium may take the form of a nonvolatile media or volatile media. Non-volatile media may include, for example, optical and/or magnetic disks, such as the storage device(s) 1210. Volatile media may include, without limitation, dynamic memory, such as the working memory 1214.

[0168] The communications subsystem 1212 (and/or components thereof) generally can receive signals, and the bus 1202 then can carry the signals (and/or the data, instructions, and the like, carried by the signals) to the working memory 1214, from which the processor(s) 1204 retrieves and executes the instructions. The instructions received by the working memory 1214 may optionally be stored on a non-transitory storage device 1210 either before or after execution by the processor(s) 1204.

[0169] It should further be understood that the components of computer device 1200 can be distributed across a network. For example, some processing may be performed in one location using a first processor while other processing may be performed by another processor remote from the first processor. Other components of computer system 1200 may be similarly distributed. As such, computer device 1:

computing system that performs processing in mi

computer system 1200 may be interpreted as a single computing device, such as a distinct laptop, desktop computer, or the like, depending on the context.

Method

[0170] Figure 6 depicts aspects of a display method 1100' according to embodiments of the present invention. Method 1100' may include generating a first laser beam at a first wavelength (e.g. using a first laser beam source), as depicted in step 1110' and generating a second laser beam at a second wavelength (e.g. using a second laser beam source), as depicted in step 1120'. The first wavelength can be different from the second wavelength. The method can also include directing the first and second beams to an intersection at an addressable location of an illumination volume, as depicted in step 1130'. The illumination volume can include gaseous particles excitable by the first and second laser beams. Further, the method may include scanning the first and second beams, for example in at least two degrees of freedom, so as to produce beam intersections throughout the a three-dimensional space of the illumination volume, as indicated by step 1140', so as to generate one or more static or dynamic images.

Controlling an Angular Intensity Partem of an Illumination Voxel

[0171] For convincing interpretation of three dimensionality in 3D displays, visual depth cues may be incorporated into the display in some non-limiting embodiments. These visual cues may include perspective, texture, lens accommodation, stereopsis, motion parallax, and many others. These cues may either be ignored or manufactured manually in 2D-projection based stereoscopic displays. In contrast, nearly all visual cues are naturally present in at least some implementations of the true-3D display with one exception - the absence of occlusion as a visual cue (e.g., the disappearance or brightness reduction of a light from a source when it passes through an opaque or

From a viewer's perspective, occlusion may be interj

light from a source when behind a foreground element. Occlusion is absent in volumetric 3D display systems because voxels are typically transparent. This means that light emanating from illumination voxels will pass through all foreground voxels, including illuminated foreground voxels. For example, with an image of a human head in a fluorescence-based volumetric 3D display, a person would be able to see the distant ear through the face when viewing the head at certain angles. In at least some implementations, it would be preferable to address the problem of occlusion in a volumetric 3D display system. Additionally, in a volumetric display different viewers may have different notions of which elements should be viewable and which should be occluded. For an implementation of occlusion to be complete, it may be preferable if it is correct for all viewing angles. Accordingly, some embodiments of the present invention are generally related to methods and systems for controlling an angular intensity pattern of an illumination voxel. As an example, controlling an angular intensity pattern of an illumination voxel may include controlling or adjusting emission angles of the illumination voxel and/or intensity of emitted radiation along certain trajectories.

[0172] Figure 7 illustrates an exemplary method 300 according to some embodiments of the present invention. At step 302, particles at a location may be excited to emit radiation in a plurality of directions. At step 304, an angular intensity partem of the radiation emitted may be controlled to reduce radiation emission in undesired directions.

[0173] In some embodiments of the present invention, the angular emission pattern or angular intensity pattern may be controlled 304 when the light is generated 302. This may be implemented, for example, by using a four-wave mixing process. Light will be emitted only in directions that are consistent with phase-matching conditions. In other embodiments, light may be emitted 302 in 4 pi steradian (4π sr) and then

absorbed 304 so that the light transmitted adhi

emission/intensity pattern.

[0174] The particles may be excited to emit radiation 302 using any of the methods and systems described herein. In a fluorescence-based 3D display, one implementation may be to illuminate the voxels of the volumetric medium sequentially in a 3D vector-scanning approach. This can be multiplexed on a small or large scale so that multiple, but not necessarily all, voxels are drawn at once. In some embodiments, a deformable mirror device (DMD) may be used to illuminate an entire plane of voxels. In some alternatives, sub- volumes of the total display volume may be specified and lasers may be dedicated to each of the sub-volumes. In this approach, each set of lasers for each sub-volume may perform a vector scanning or raster scanning of the sub-volume. In this way, the number of voxels that may be illuminated at one time can be reduced dependent on the scanning speed, illumination efficiency, and scanning path algorithm. The scanning path algorithm may determine an efficient vector scanning path through the 3D image to be presented in the display volume.

[0175] In some embodiments, a plurality of lasers may be intersected to excite particles (e.g., rubidium gas or the like) located at the intersection to a multi-photon state such that visible light is emitted from the beam intersection. The localized emission of radiation may have a ladder structure with a lower, intermediate, and upper level. One laser may promote the atoms from the lower level to the intermediate level and a second laser may promote the atoms from the intermediate level to the upper level. If the transition wavelength of the lower transition is in the infrared and the transition wavelength of the upper transition is in the visible, then the intersection of the two lasers will emit visible radiation into 4 pi steradians (4nsr) that propagates away from the illumination voxel. Figure 8 illustrates a specific implementation of step 302. Particles of rubidium may be excited from a 5S i/₂ state (lower state 400) to 5P3/2 level (an intermediate state <

The particles of rubidium at the 5P3/2 level 402 may

levels 406) using an upper excitation laser 408.

[0176] The visual cue of optical occlusion may be provided by controlling the angular emission/intensity of each voxel in the volumetric medium 304. By controlling the angular emission/intensity of each voxel in the volumetric medium 304, the light reaching the viewer can be made to conform to the principle of optical occlusion.

[0177] For fluorescence-based volumetric displays with a single subvolume, the volumetric medium may be enclosed in a box or enclosure which is able to locally control the intensity of the transmitted light, hereafter referred to as a light valve array (LVA). Accordingly, in some embodiments, an enclosure may be constructed out of liquid crystal light valve arrays (e.g., such as those found in standard liquid crystal displays). By controlling the transmissivity of the light valve arrays during the illumination of each voxel, one can control the angular emission/intensity partem 304 and thus implement optical occlusion. In some embodiments with a single subvolume there may only be one voxel illuminated at a single time. The light emitted by this voxel may be configured to conform to the principle of optical occlusion relative to the 3D image comprising the 3D video frame. The angular emission pattern can be controlled either locally, in the immediate vicinity of illuminated voxel, or where the light from the illuminated voxel leaves the illumination volume. In some embodiments, to conform an illumination voxel to the principle of optical occlusion, light emitted may be prevented from propagating to the viewing in a direction that is inconsistent with optical occlusion. In some embodiments, whether a direction is acceptable or not for a given illumination voxel may be determined ahead of time for each voxel in each frame of the 3D video or image. In some embodiments, when there is more than one subvolume, the emission pattern may be controlled in the local vicinity of each illumination voxel.

[0178] In further embodiments, occlusion v

created within the volumetric medium. This may be possible in a volumetric medium with multiple energy levels that are configured relative to the levels used to create fluorescence. Specifically, occlusion voxels may be generated by exciting particles to be resonant with the emitted radiation. A sufficiently high density of particles that are resonant with the emitted illumination radiation will cause the emitted illumination light to be absorbed and then remitted many times which will decrease the chances of the emitted illumination light propagating through the occlusion voxel. Accordingly, in some embodiments, one or more occlusion voxels may be generated adjacent illumination voxels and along undesired emission paths to modulate or adjust the emission angles and/or intensity of radiation from the illumination voxel. Strategic creation of the occlusion voxels about the illumination voxel may reduce light emission in undesired directions and/or intensities and may restrict light to propagate only in desired directions with the desired intensity.

[0179] The occlusion voxels may be created by promoting the particles in the vicinity of the illumination voxel and in the undesired direction up into the intermediate state. If the density of particles in the intermediate state is sufficiently high, the light will not be able to propagate in the forward direction through the occlusion voxel. A laser resonant, or nearly resonant, with the lower transition of a two-photon absorption may be used to push the particles up into the intermediate level in some embodiments. Using a laser resonant with the lower transition may promote all particles in the beam to the intermediate level. Alternatively, to make a localized region with a high density of atoms in the intermediate level, additional auxiliary levels may be used. The auxiliary levels may include an intermediate auxiliary level and an upper auxiliary level. As with the illumination voxel, to create the occlusion voxel, two auxiliary lasers may intersect and push atoms up to the upper auxiliary level. Optionally, in some embodiments, at<

may decay from the upper auxiliary level to the inte

exemplary method 500 of generating an occlusion voxel according to some embodiments. At step 502, particles adjacent an illumination voxel and in an undesired direction from the illumination voxel may be excited to an intermediate auxiliary state. At step 504, the particles at the intermediate auxiliary state at the location may be excited to an upper auxiliary state. In some embodiments, the particles may be excited to the intermediate auxiliary state by a first auxiliary laser. Particles at the intermediate auxiliary state may be excited to the upper auxiliary state by an upper auxiliary laser. Optionally, a one-step two- photon process using detuned lasers can promote atoms to the second auxiliary level without necessarily populating the first auxiliary level. This may be used for controlling the angular emission pattern by using detuning to keep some occlusion voxels transparent while neighboring occlusion voxels are made opaque.

[0180] Once in the upper auxiliary level, the particles may have a possibility of decaying to the intermediate level. Additionally, it may be preferable if the particles in the upper auxiliary level do not emit visible radiation when they decay. Additionally, a third laser nearly resonant with the transition between the intermediate level and the upper auxiliary level can be used in concert with the first two lasers to promote the transfer of population to the intermediate level. Accordingly, in some embodiments, two lasers (e.g., a lower auxiliary laser and an upper auxiliary laser), or more, may be used to excite particles adjacent an illumination voxel and in an undesired direction to the intermediate auxiliary level and the upper auxiliary level, respectively. Thus, in some embodiments, only atoms in the intersection of the auxiliary beams will be pushed into the upper auxiliary level and will possibly decay to the intermediate level. Consequently, in some embodiments, particles may be moved to the intermediate level in a localized manner. Once in the intermediate level, the atoms may absorb and reemit the radiation coming

sufficient density of particles in the intermediate

illumination voxel will be prevented from propagating in the undesired direction. Thus, one or a plurality of auxiliary lasers may be provided to produce occlusion/absorption voxels in the illumination volume for controlling light emission/intensity patterns according to some embodiments of the present invention.

[0181] Figure 10 illustrates a specific implementation of method 500. Particles of

Rubidium may be excited from a 5S i/₂ state (lower state 400) to 5Pi/2 level (an intermediate auxiliary state 410) using a lower auxiliary laser 412. The particles of rubidium at the 5Pi/₂ level 410 may be excited to an upper auxiliary level 4D3/2 (upper auxiliary level 414) using an upper auxiliary laser 416. When in this upper auxiliary level 414, the particles may have a chance (approximately 15%) of decaying 418 to the 5P3/2 level 402. Advantageously, the atoms in the upper levels 406 ((n>5)Ds/2) cannot decay to the P1/2 level 410. This may be beneficial because it means that the illumination lasers will not accidentally populate the intermediate auxiliary level 410.

[0182] Figure l la-l lb illustrate an exemplary 3D display state 600 for the purpose of illustrating occlusion principles and methods and systems of the present invention. Figure 1 l a shows a perspective view of the display of two opaque spheres 601, 602 of equal radius r. Figure l ib illustrates a side view of the exemplary situation 600. The centers 603, 604 of the spheres 601 , 602 are displaced by three times their radius, r, in a horizontal direction. The voxel 610 of sphere 601 may be the voxel of sphere 601 that is closest to sphere 602. The angular coordinates for this voxel 610 are defined with the zenith in the vertical direction and the direction corresponding to zero azimuthal angle defined by a line segment connecting the voxel 610 to the center 604 of the sphere 602. The polar angle is defined as theta (Θ) and the azimuthal angle is defined as phi (φ). From the side view in Figure 1 lb, in order to adhere to the principle of optical occlusion in this exemplary <

should only propagate in the angular region where the

than 90 degrees, with phi (φ) unconstrained. Accordingly, in the illustrated example, voxel 610 may emit radiation only into the shaded region 612. This specification of angles may be an angular emission partem for the specified voxel 610. In this way, light from the specified voxel 610 will never be perceived as transmitting through an opaque surface of sphere 601 or sphere 602. For a particular 3D image, each voxel may have a unique angular emission pattern that adheres to the principle of optical occlusion. Thus, a fluorescence-based 3D display that is able to control the angular emission pattern of each voxel may be able to fully implement optical occlusion. In the case that the foreground element (e.g., sphere 602 when viewed along axis 614) is semi-transparent as opposed to fully opaque, it may be sufficient to be able to control the angular intensity pattern of the illumination voxel. With a 3D display, many different viewers or view perspectives may be provided. For an implementation of occlusion to be complete, it may be preferable if occlusion is correct for all viewing angles. Accordingly, in some embodiments, it may be preferable to calculate and control angular emission intensity and/or angles for each of the illumination voxels defining sphere 601 in addition to the illumination voxels defining sphere 602.

[0183] To implement control over the full emission pattern, the transmission properties of each emission direction may be controlled independently. In some embodiments, the direction of the illumination laser nearly resonant with the upper transition may be ignored because, for laser safety reasons, the laser will not be along a viewing direction. To independently control all of the emission directions independently, spatial and frequency dependent multiplexing can be used. For simplicity Figure 12 illustrates a 2D cross-section 700 through the illumination voxel 710 that is perpendicular to one of the illumination lasers. Additionally, assume that the two illumination lasers for exciting the particles in the illumination voxel 710 are perpendici

700, voxels 701, 702, 703, 704, 706, 708, and 709 are

that lie in this cross-section 700. In the exemplary system illustrated, the lower auxiliary laser beams

propagate through the rows for the lower auxiliary transition (e.g., level 400 to level 410) and the upper auxiliary laser beams Ui, U2, U3, U₄, U₆, U7, Us, and U9 propagate down into the voxels 701, 702, 703, 704, 706, 708, and 709 for the upper auxiliary transition (e.g., level 410 to level 414). This means there may be three beams in the lower auxiliary laser and eight beams on the upper auxiliary laser. The three beams in the lower auxiliary laser may be L123, L456, and L789, where the indices indicate the beam path and the eight beams in the upper auxiliary laser may be Ui - Us>, omitting Us.

[0184] In the L123 and L789 beams, three distinct frequencies 001, ω₂, C03 of light nearly resonant with the lower auxiliary transition (e.g., lower auxiliary transition from level 400 to level 410) may be sent. In the L456 beam, two distinct frequencies coi and C03 may be sent. In Ui, U4 and U7, light with frequency vi may be sent, where is resonant or nearly

resonant with the energy difference between the ground state (e.g., state 400) and the upper auxiliary level (e.g. level 414) or the ground to upper auxiliary level two-photon transition. In U2 and Us light with frequency V2 may be sent, where is resonant or nearly resonant

with the ground to upper auxiliary level two-photon transition. Furthermore, in U3, U₆, and Us>, light with frequency V3 may be sent, where is resonant or nearly resonant with the ground to upper two-photon transition. With independent control over the power in each frequency and in each spatial mode, the voxels 701, 702, 703, 704, 706, 708, and 709 may be selectively made absorptive/occlusive. For example, to make voxel 701, 706, and 708 absorptive, laser powers corresponding to Lmcoi and Uivi; L456C03 and U6V3; and L789C02 and U8V2 may be turned on. Alternatively, to make voxel 701 and 703 and 706 absorptive, laser powers according to Lmcoi and Uivi; Li23C03 and U3V3; L456C03 and U6V3 may be turned on. With this understanding it should be clear that all vo

709 in the cross-section 700 can be independently cor

powers and lasers directed at the cross section 700.

[0185] This approach can be generalized to three dimensions. For example, Figure 13 illustrates a 3x3x3 cube of voxels surrounding an illumination voxel. In this case 9 lasers may be used in each beam which are labeled (L1-L9) and (U1-U9). The illumination lasers may copropagate with the L5 and U5 auxiliary lasers. Each beam may include three laser frequencies in three spatial groups, for example, Li, L4, and L7 may each carry the following three frequencies 001, ω₂, and 003. Additionally, L2, L5, and Ls may each carry 004, 005, and ωβ. L3, L₆, and L9 may each carry 007, ωβ, and 009. In contrast, the upper auxiliary laser may carry the corresponding frequencies to isolate the voxels in the orthogonal direction: Ui, Ut, and U7 may each carry vi, v₄, and V7; U2, Us, and Us may each carry v₂, vs, and vs; and U3, U₆, and U9 may each carry V3, V6, and vs>.

[0186] Similar to the above example, ωι + vi is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition; 002 + V2 is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition; C03 + V3 is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition; C04 + V4 is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition; cos + vs is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition; ωβ + V6 is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition; C07 + V7 is resonant or nearly resonant with the ground to upper auxiliary level two- photon transition; cos + V8 is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition; and C09 + V9 is resonant or nearly resonant with the ground to upper auxiliary level two-photon transition. [0187] Voxels may be identified, for example

those frequencies which complete the two-photon trar

the voxel at the intersection of Li and U2 may be made absorptive by turning on the Lico2 and U2V2 beam. The voxel at the intersection of L3 and U2 may be made absorptive by turning on the L3C08 and U2V8 beam. The voxel at the intersection of L2 and Ui may be made absorptive by turning on the L20M and U1V4 beam. The voxel at the intersection of L2 and U3 may be made absorptive by turning on the L2C06 and U3V6 beam. Notably, even with all of the voxels at the intersections of Li and U2; L3 and U2; L2 and Ui; and L2 and U3 absorptive, the voxel at the intersection of L2 and U2 may be left transmissive because none of the light flowing through that voxel can complete the two-photon transition. Specifically, the light propagating through the voxel at the intersection of L2 and U2 contains the following frequencies, aw, ωβ, V2, and V8 but the sum of any pair of these cannot complete the two-photon transition. In general, each occlusion voxel can be made absorptive or transmissive independently. In practice, the cos and V5 frequencies may not need to be turned on since they correspond to the position of the illumination voxel (at the center of the illustrated 3x3x3 volume). In some embodiments, they can be omitted from the system.

[0188] The previous example shows how the emission pattern can be controlled coarsely. By increasing the number of occlusion voxels from 3x3x3 to larger dimensions and/or by possibly including alternate geometry such as hexagonal packing of the beams, etc., the emission pattern can be controlled with improved angular resolution. Accordingly, systems and methods are provided herein for controlling an angular intensity pattern of an illumination voxel. Systems may utilize one or more auxiliary lasers for exciting particles to the intermediate level. For creating localized occlusion voxels, systems and methods may utilize a plurality of auxiliary lasers and the plurality of auxiliary lasers may be configured to send beams of varying frequency so as to selectively control whether voxels are absorptive or transmissive. Thus, emission angles/intensities for

volumetric 3D display may be calculated. Occh

generated using methods and systems disclosed herein to address the problem of optical occlusion in the volumetric 3D display.

[0189] In some embodiments, the angular intensity/emission partem may only be controlled in directions where the light may eventually be viewed by a user. For example, in some cases, for a given voxel there may be barriers restricting visual accessibility by a viewer in some directions. Accordingly, it may not be necessary to control the angular intensity/emission partem in these directions. For example, if the illumination volume is sitting on top of an opaque surface (e.g., table or stand or the like) so that the light propagating in the downward direction toward the surface is not viewable, a large region of the 4π steradians need not be calculated or controlled. This may drastically simplify some of the complexity, calculations, and processing needed to address the problem of optical occlusion in the volumetric 3D display.

Improving Brightness

[0190] As discussed above, for some particular implementations of three dimensional volumetric displays, it may be difficult to achieve bright images. In some of the above sections, we discussed some non-limiting examples of ways to increase image brightness. In this section, we discuss additional non-limiting examples of ways to increase image brightness.

[0191] One approach for increasing brightness is to solve the absorption-detuning dilemma experienced by the lower excitation laser as it propagates through the gas to the desired intersection point. The dilemma is this: in some implementations, if the lower excitation laser is tuned to resonance it is strongly absorbed before it gets to the desired intersection point, but if it is detuned away from resonance, it can propagate to the desired intersection point without being strongly absorbed,

absorbed once it gets there. Not only does this p

implementations of three dimensional volumetric displays, it makes scaling to large volumes very difficult in at least some instances. For larger volumes the longer propagation distance may necessitate a larger detuning and may result in even less efficient absorption at the illumination voxel. Even if the volume is not increased, the absorption-detuning dilemma may be seen when the density of the vapor cell is increased. A high density means that there are more atoms in the illumination voxel with the potential to emit visible radiation, but it also means there are more atoms along the beam path which will strongly absorb the excitation laser.

[0192] There is another dichotomy in some implementations, which arises relative to the visible light emitted from the illumination voxel. To maximize the light being emitted by the illumination voxel it is desirable to increase the density of the atomic vapor. A higher vapor density corresponds to more atoms within the illumination voxel which are able to emit visible light. But, in some implementations, since the emitted visible light is resonant with a ground-state transition, it has the potential of being absorbed and re-scattered as it propagates out of the vapor cell. Re-scattering of the emitted light may reduce the brightness of the illumination voxel, and lead to a hazy ambient background. Increasing the vapor density may result in an increased probability of scattering as the visible light leaves the cell. Thus, there can be a trade-off between an improved brightness by increasing the vapor density and a reduction in the image brightness and clarity due to absorption and rescattering.

[0193] There is a third problem which manifests itself in some configurations. When the lower-excitation laser is tuned to be resonant with the lower excitation transition, it is strongly absorbed and rescattered throughout the vapor cell. The rescattered light is naturally resonant with both of the hyperfine ground states and the rescattered light is scattered in all directions from the lower excitation laser beam. Some

path of the upper excitation laser. When this happens

state 'B' (see figure 15) and the upper excitation laser light can excite an atom in the correct ground state to the upper excitation level which will then fluoresce and emit visible light. The fluorescence generated in this way can occur anywhere along the upper excitation laser beam path and the intensity is observed to be roughly proportional to the distance from the lower excitation laser, for example, regions of the upper excitation laser beam path that are near the illumination voxel are brighter than regions of the upper excitation laser beam path that are far away from the illumination voxel, see figure 17. We call this effect scattering-induced streaking, or, for short, streaking. When the lasers are scanned and the illumination voxel moves throughout the vapor cell, the streaking may cause hazing in the final image.

[0194] We now describe a non-limiting example of an approach for circumventing these problems. In this approach, we use ground-state pumping to spatially control the ground-state population of the gas. Atomic vapors often have multiple ground states. For example, Rubidium and Cesium have two ground states. For clarity in the following discussion (and without in any way limiting the scope of this patent), let us refer to the two ground states as level A and level B. If the lower excitation laser is tuned to resonance with level A, under certain conditions it will be very likely to be absorbed by atoms in level A, but very unlikely to be absorbed by atoms in level B. By pumping the atoms in the beam path (e.g. in portions of the beam path leading up to the illumination voxel) into level B, the lower excitation laser can be made to propagate without absorption to the illumination voxel. However, if the atoms in the illumination voxel are pumped to level A, the laser will be efficiently absorbed there. In this way, the lower excitation laser can be made to propagate without absorption to the illumination voxel, but also be efficiently absorbed once it reaches the illumination voxel. This allows us to get around the first dilemma listed above. [0195] In some implementations, when thi

illumination voxel, roughly half of the light will be re;

A and half with a transition resonant with level B. If the atoms in the whole cell are pumped into level B (not just the atoms in the beam path of the lower excitation laser), then the emitted light which is resonant with a transition ending on level A will propagate freely out of the cell with no scattering or absorption. Consequently, no matter the size of the vapor cell or the atomic density, 50% of the emitted light will always exit the vapor cell. This allows us to get around the second dilemma listed above.

[0196] Using a repump laser beam allows us to get around the third problem listed above. With no repump laser, the ratio of atoms in each of the two ground states is roughly 50%. (More specifically, it goes as the ratio of the number of Zeeman sublevels for each state, 9:7 for Cesium.) Scattered light from the lower excitation laser is able to excite any atom in the correct ground state. For example scattered light resonant with the ground-state Ά' ('Β') to intermediate state transition can excite atoms from ground-state Ά' ('Β'). When the repump laser beam is on, the atoms are pumped out of ground-state 'A' and into the ground state 'Β' . Consequently scattered light with is resonant with the ground-state 'A' to intermediate state transition is substantially not able to excite atoms in the beam path of the upper-excitation laser into the intermediate state. One might expect the increased density of the atoms in ground-state 'B' (equal to the lost density in ground-state Ά') would make up for the lost fluorescence, however, since the amount of scattered light resonant with the ground-state 'B' to intermediate state transition is fixed and well below saturation, there is little to no improvement in the generated fluorescence. Consequently, the streaking is reduced by roughly the same amount as the density reduction of atoms in ground-state 'A' - see figure 18. [0197] Beyond reducing streaking, the repun

circumstances of increasing the brightness of the illui

voxel there are two competing effects. The first is that the repump laser is pumping atoms out of ground-state 'A' into ground-state 'Β' . The second effect is that the lower excitation laser has the effect of pumping atoms out of ground-state 'B' into ground-state Ά' . The pumping effect of the lower excitation laser comes both through the scattering of the lower excitation laser, as well as through the double excitation and fluorescence of atoms excited up to the upper excitation level by the combined effect of both the lower and upper excitation lasers. In steady-state, the ratios of atoms in the two ground-states, 'A' and 'B' is roughly proportional to the ratio of intensities of the lower excitation laser and the repump laser, i.e. atoms are more likely to be in the state with the lower intensity light. Consequently, brightness of the illumination voxel is increased when the intensity of the repump laser at the illumination voxel is much larger than the intensity of the lower excitation laser. Additionally, by making the repump laser beam profile slightly larger in diameter than the illumination voxel, atoms traveling toward the illumination voxel can be sure to be pumped into ground-state 'B' by the time they reach the illumination voxel - see figure 18. The precise values of the repump beam profile and relative intensities of the lower excitation and repump lasers can be optimized experimentally. The improvement in brightness can be quite significant for the following reasons. Without the repump laser, atoms in ground-state 'B' entering the illumination voxel would tend to get quickly pumped into ground-state Ά', all but eliminating their ability to contribute to the visible fluorescence while they are in the illumination voxel. A precise calculation of this effect depends on many factors and so will not be attempted, but it is easy to see that if the transit time through the illumination voxel is much larger than the optical pumping time, this effect can drastically reduce the achievable brightness of the display. Additionally, roughly half of the atoms entering the illumination voxels are in ground-state 'A' and therefore never pai

the visible fluorescence.

[0198] Finally, we discuss the conditions under which the preparation laser can be used to improve the brightness of the display. The purpose of the preparation laser is to pump atoms along the lower excitation laser into ground state 'A' so that the lower excitation laser may propagate with reduced absorption to the illumination voxel. In the illumination voxel, the presence of the repump laser at a suitable intensity ensures through optical pumping that there is a sufficient population of atoms in ground-state 'B' to participate in the excitation and fluorescence process. If the density of atoms is such that the lower excitation laser is able to propagate through and exit the vapor cell at its largest width and still retain an intensity far above saturation of the ground-state 'B' to intermediate energy level transition, then the preparation laser is redundant in terms of assisting the lower-excitation laser; at each location in the vapor cell there will be more than enough lower excitation laser light to saturate the double excitation and fluorescence process. However, in settings where the lower excitation laser intensity is restricted for some reason, or in settings where it is desirable to use a reduced lower excitation laser intensity, the preparation laser can be used to ensure that a sufficient amount of power in the lower excitation laser beam is able to reach the illumination voxel. Additionally, wherever the preparation laser intensity is too small to overcome the thermalizing effect of collisions off of the walls and the pumping effect of the other lasers in the vapor cell, it cannot sufficiently pump the atoms into ground-state 'A' thereby decreasing the absorption and rescattering of the blue fluorescence as it propagates out of the vapor cell, or reduce the absorption of the lower excitation laser as it travels to the illumination voxel. Consequently, there is a minimum required intensity for the preparation laser to be useful for reducing scattering of the blue light and aiding in the lower excitation laser transmission. Finally, we note that there may be scenarios where it is advantageous to have the preparation laser largely copropagate with the lower excitation las

insufficient intensity in the lower excitation laser, bv

laser to achieve the minimum required intensity to pump the atoms in the vapor cell. By allowing the preparation laser to largely co-propagate with the lower excitation laser, the preparation laser will be able to aid with reducing absorption of the lower excitation laser, even though it won't be able to reduce the absorption of the visible as it propagates out of the vapor cell. One can also imagine scenarios where both largely co-propagating and flooding preparation beams might be useful. This configuration would allow better control over the relative intensities to achieve the right population balance in different regions of the vapor cell. See figures 15, 19 and 20.

[0199] The following describes how the above scheme may work in one non-limiting example of a volumetric display utilizing a buffer gas. If the atoms are hot and there is a buffer gas present, the absorption profile of each transition will be broadened. If the width of the absorption profile of one atomic transition (ending on ground state A, say) begins to overlap another atomic transition (ending on ground state B), the effect described above deteriorates because the atoms cannot be pumped cleanly into either of the required ground states. Thus, it is critical to ensure that the broadening is small enough so there is very little overlap of the absorption profiles of the different atomic transitions. A precise determination of the optimal broadening may readily be determined experimentally. In some embodiments, the resonance width should be smaller than the separation of the hyper-fine ground state resonances, and should be larger than the Doppler broadening of the atoms as noted below. In some instances, the optimal collisional broadening width will match the Doppler broadening width for a given temperature. The collisional and Doppler broadening may be characterized experimentally by fitting the absorption profile generated by measuring the transmission of a resonant wavelength-scanning laser with a Voigt profile. [0200] Then the atoms can be cleanly pumpe

described above is possible. In some implementations ,

overlap or significantly overlap may amount to controlling the temperature and buffer gas pressure relative to the frequency difference between the ground state transition frequencies, the so-called hyperfine splitting. In some implementations, the buffer gas pressure could be large enough to circumvent Doppler broadening for a given temperature but small enough so the broadening is less than the hyperfine splitting.

[0201] In some implementations, to optimize the trade-offs, it may be preferable to utilize an atomic species which has heavy atoms and a large hyperfine splitting. For example, while naturally abundant Rb has an atomic mass of 85, Cs has an atomic mass of 133. The increased mass means that the Doppler profile increases more slowly with temperature so that higher temperatures (and corresponding densities) may be reached before the absorption profiles of the ground-state transitions begin to overlap. Cesium also has the advantage that the hyperfine splitting is 9.2 GHz, much larger than the 6.8 GHz splitting of Rb87 or the 3.2 GHz of Rb85. In other embodiments, it is acceptable to use atomic species with lighter atoms and a smaller hyperfine splitting.

[0202] Beyond implementation of ground-state pumping, changing the gas vapor to

Cesium along with the associated lasers may further improve the brightness of the display. In Rubidium, the wavelength of the emitted light is 420 nm, but in Cesium, the wavelength of the emitted light will be 455 nm and 459 nm. In some implementations, this change in wavelength will increase the apparent brightness by a factor of roughly 5 due to the increased sensitivity of our eyes to 455 nm and 459 nm light. Additionally, the same atomic density can be achieved with a roughly 20°C decrease in the oven temperature, reducing the heating load on the system. Finally, Cesium has another key property in that the absorption cross-section of atoms to the visible fluorescence is much smaller than the absorption cross-section of the lower excitation laser. This means that the atoms ;

excitation laser, but have a diminished ability to absoi

as it propagates out of the vapor cell.

[0203] Figures 14 and 16 illustrate one non-limiting example of a system implementing the ground-state pumping scheme described above, in which two additional lasers will be added to the system. These lasers will be resonant with a different ground-state transition than the lower excitation laser as shown in Fig. 14. For example, in Cesium atoms, the lower excitation laser might be resonant with the D2 transition. The two new lasers will be resonant with the Dl transition. The first of the two new lasers, called the preparation laser, will flood illuminate the cell and be tuned to pump the atomic population into one of the ground states (the 6S 1/2 F=3 state), as illustrated in Figs. 14 and 16. In this example, the lower excitation laser is tuned to be resonant or nearly resonant with the D2 transition terminating on the other state (6S 1/2 F=4 state), and will therefore propagate with very little absorption through the cell. As shown in Figure 16, the second of the two new lasers copropagates with the upper excitation laser and is tuned to be resonant with the Dl transition terminating on the lower ground state (6S 1/2 F=3 state). This repump laser acts to push the population back into the 6S 1/2, F=4 level along the upper excitation laser beam path. The diameter of the repump laser will be larger than the upper excitation laser, ensuring that a majority of atoms addressed by the upper excitation laser have been pumped back into the F=4 ground state. In this way we reduce the absorption of the lower excitation laser along its beam path, but ensure there is sufficient absorption within the illumination voxel.

[0204] We have implemented the hyperfine ground-state pumping protocol in Cs and have found that compared with no pumping it is able to increase the brightness of the illumination voxel by a factor of roughly 8. Combined with the other advantages of using Cs, including a change to the emitted wavelength, and with an increase to the power of the lasers and optimizing the cell temperature, we have achieved

perceived brightness over Rb without pumping. W

darkened room, the illumination voxel is bright enough to induce a slight reflex to look away. This compares very favorably to the estimate of Korevaar that the perceived brightness of an illumination voxel using slightly different transitions in Rb should be similar to that of Sirius, the brightest star in the night sky. Notably, Korevaar's estimate depends on using much more powerful lasers.

Alternate Embodiments for Improving Brightness

[0205] Above, we have describe a dichotomy between relatively high temperatures in the vapor cell that increase the density of the atomic gas(es) to improve the brightness of the display, and relatively low temperatures in the vapor cell so that the generated visible light emitted by the voxel doesn't get trapped and blurred by repeated scattering events before it exits the illumination volume (the so-called radiation trapping effect). We have also discussed examples of systems and methods that improve display brightness including: a repump laser and a preparation laser which either co-propagates with the lower excitation laser or floods the vapor cell or both; and the inclusion of a buffer gas. The following is a discussion of additional strategies that may be employed to improve brightness (in conjunction with or as an alternative to the above described examples).

[0206] In some applications, it is acceptable for the temperature of the vapor cell to be relatively high (e.g. higher than 70° C, such as 100° C or higher, 125° C or higher, or 150° C or higher), providing a boost in brightness of the display, although, in at least some of these applications, it will be desirable to ensure that the radiation trapping effect on the visible light and/or streaking issues are effectively mitigated as well.

[0207] By way of example, some systems that operate at a vapor cell temperature of about 70° C correspond to an atomic vapor density of about 2 x 10⁹ / mm³ for a Cesium vapor. Increasing the temperature of such a system to

about 2 x 10¹¹ / mm³. This is a factor of about 1 12 i

temperature of such a system further to about 180° C would increase density to about 8.2 x 10¹¹ / mm³, for a total improvement factor of about 420 over the density at 70° C.

[0208] As discussed above, increasing temperature (and therefore density) of the atomic vapor can exacerbate the radiation-trapping effect. We describe above some methods and systems for mitigating this effect, as well as the streaking effect. We describe in more detail below other methods and systems for mitigating these effects.

[0209] Figures 21 and 22 illustrate one example of a three laser system. In this example, the lasers propagate into the vapor cell along two beam paths, Bl and B2. In this example, we describe our implementation using energy levels of Cesium to illustrate the role that each laser and energy level would play. The first laser Ql propagates along Bl and drives the D2 transition of Cesium connecting the 6S 1/2 and 6P3/2 levels using a 852 nm laser. In this example, the temperature of the vapor cell, at 150-180° C is sufficiently high such that the two hyperfine ground levels of Cesium are thermally broadened enough that both ground levels are addressed. Consequently, provided that the Ql laser is sufficiently powerful, the atoms in this column will be completely saturated, so that approximately ½ of the atoms in the column are in the 6P3/2 energy level. The second laser Q2 propagating along B2 addresses the 6P3/2 to 6D5/2 transition using a 917 nm laser. This will drive the intersection of the Ql and Q2 beams (the intended voxel region) into the 6D5/2 level. It will also drive many of the atoms along the Q2 beam into the 6D5/2 level because of scatter from the Ql laser. In the configuration described here, the decay from the 6D5/2 level that occurs along transitions which emit visible light amounts to only about 0.5% of all possible decay paths. The emitted color is entirely at the 455 nm wavelength. [0210] This decay from the 6D5/2 level can

coordinates. The spectrum emitted by the 6D5/2 le\

atomic vapor, essentially re-scattering all ground-state resonant light. The remaining spectrum can be used to calculate the CIE 1931 xyY coordinates. Of these coordinates, the Y coordinate is the luminance, which is a measure of the perceived brightness of the color by a human viewer. The ground state filtered CIE 1931 xyY spectral luminance is the CIE 1931 xy Y spectral luminance calculated on the spectrum of light emitted by the voxel and exiting the display which has had all the ground-state resonant spectral components filtered out by re-scattering as the light transmits through the hot vapor. It can be measured experimentally by imaging the voxel onto a standard power meter and into a standard spectrometer and then calculating the CIE 1931 xyY spectral luminance, renormalizing to match the units used in this application. The spectra used in this application to calculate the CIE 1931 xyY coordinates are derived solely from atomic branching ratios. An expert in the art of spectroscopy is able to make this unit renormalization to compare measured and calculated spectra. We note that the measure spectra will include broadening, dispersion and other effects which cannot be calculated using only the branching ratios. Nevertheless, the CIE 1931 xyY coordinates can be calculated from both spectra and may be compared when properly normalized.

[0211] For the 6D5/2 and 6D3/2 energy levels the luminance of the remaining spectrum is zero. For Rubidium, the luminance of the 5D5/2 and 5D3/2 energy levels are 0 and 6χ10^Λ(-5) respectively. By comparison, the luminance of the destination energy level filtered by the hot vapor can be as high as 0.41 in some cases. By performing a color analysis on potential intermediate energy levels, we can select intermediate energy levels that definitively mitigate streaking by the Ql and Q2 lasers. [0212] Returning to Figure 22, a third laser ⁽

atoms in the 6D5/2 level up to one of many possibl

which path to drive is influenced by the desired color the voxel is intended to have. When a collection of atoms are excited to a specific energy level and allowed to decay, the possible decay paths from that energy level and the associated wavelengths of the emitted light will give rise to a characteristic emission spectrum. The emission spectrum can be used to determine the perceived color of the fluorescence. In general, it may be desirable in some implementations to create three or more primary colors, typically red, green, and blue, and then, by controlling the relative intensity of each primary color, any color in the associated color gamut can be reproduced.

[0213] Further analysis yields quite a few candidates which would likely make for a very strong red primary in at least some embodiments, including the 11F5/2 and 12F5/2 energy levels. These are well outside the sRGB color gamut with coordinates of roughly (0.70,0.30,0.27) and (0.69,0.31 ,0.32) respectively. Other choices also exist and can be found by application of the above mentioned methods.

[0214] The wavelength of the Q3 laser should be chosen to be resonant with the transition from 6D5/2 (or other intermediate level) to the desired destination energy level (e.g. either 11F5/2 or 12F5/2, or 1272 nm or 1249 nm respectively).

[0215] Another feature of these destination energy levels described above is that they have a very high fraction of decay through visible wavelengths. This can be quantified by examining the luminance of the emitted spectrum. The luminance gives a quantitative measure of perceived brightness of the emitted spectrum by a human viewer.

[0216] We now deal with the question of whether it is possible to create visible streaking from the 11F5/2 energy level. This would occur in beam path Bl when the Q3 laser inadvertently excites atoms from the 6D5/2 energy level up to the 11F5/2 energy level. An atom in the B l path can only be excited to the 6D5/2 1

atoms in B2 column. The atoms in the B2 beam path (

first excited to the 6P3/2 level by 852 nm light scattered from atoms in the Bl beam path. So two scattering events (Bl scatters 852 nm light to B2, and B2 scatters 917 nm light back to Bl) have to occur to drive Bl atoms into the 6D5/2 energy level. Since the amount of scattered light is reduced with each scattering event, it is very unlikely for atoms in the B2 column to be excited to the 6D5/2 level. The amount of scattering is reduced with each scattering event because the scattering goes in all directions, thus, most of the time the scattered light does not go in the direction of the other beam, Bl -> B2, or vice-versa. Thus, the probability of creating visible streaking in the B l beam path is very small.

[0217] In this example, because the decay channels which generate the desired color

(e.g. in this example red) are not resonant with the ground state, there is no radiation trapping effect in this example.

[0218] The net gain of the approaches described in this section to the overall brightness can be estimated in the following way. Compared to some implementations in which atoms are promoted from the 6S 1/2 -> 6P3/2 -> 8S 1/2 level, the addition of an additional level means that we should expect that only roughly ½ of that atoms that would have been promoted to the 8S 1/2 level will make it all the way up to the 11F5/2 energy level. This assumes that the column of atoms in the B l beam is fully saturated in the 6P3/2 level, the atoms in the 6P3/2 level in the voxel are saturated in the 6D5/2 level, and those atoms are further saturated into the 11F5/2 level. Though only about ½ of the atoms can be expected to make it up to the 11F5/2 energy level, the luminance of light emitted by decay from that level is 67.5 times the fraction of visible light emitted by decay from the 8S 1/2 level in a warm but not hot vapor cell (no filtering of ground-state resonant wavelengths), consequently, the net increase in perceived brightness is more than the just the increase in the number of atoms due to the increased density. In reality, complicated eqi

decay must be used to estimate the actual number of

Such methods are known to experts in the art of optical pumping. Since the total number of atoms can be up to 210 time more at 180 degrees C than at 70 degrees C we estimate that this approach will give rise to a brightness which is improved by a factor of roughly 1.4xl 0⁴.

[0219] An additional consideration which can be taken into account when selecting the destination energy level is the lifetime of that energy level. While the 11F5/2 and 12F5/2 states are good choices in terms of the generated color, their lifetimes are fairly long, roughly 767 ns and 989 ns respectively. This limits the number of times a single atom can be driven up to the destination excited state and decay producing visible light. For comparison, the 8S 1/2 level has a lifetime of approximately 92 ns. Thus, the expected improvement for the 11F5/2 should be reduced a factor of 8.3 to 1.7xl 0³, still a significant improvement. Nevertheless, the above approximations neglect many other physical effects. In the end, the brightness of light emitted by various destination energy levels described here can easily be evaluated experimentally using a broadly tunable near IR continuous-wave laser for the Q3 laser, but standard current- and temperature-tunable DBR lasers for the Ql and Q2 lasers. For clarity, all of the energy levels given in the table are acceptable choices for embodiments of the present invention.

[0220] We note that the power of the Ql laser can be chosen so that most of the power is absorbed and rescattered by the atoms in the vapor cell. This means that the high- power required to achieve saturation along the beam is reduced in power by the time it exits the vapor cell. Consequently, the potential safety concern from using a higher power laser is eliminated as long as the beam cannot travel in a direction without being absorbed by the hot atoms. (The laser on/off switch will have to be triggered by a sensor based on the temperature of the atoms and the frequency of the laser). The o

powers; it is sufficient for them to saturate only the vo

[0221] In some implementations, the following considerations can be used to determine if a particular energy level is a good candidate for a color primary using a 3-laser excitation scheme. First, the energy level must be reached using 3-lasers. This rules out (for Cesium and/or Rubidium based systems) the S, D, and G states that can only be reached using 2 or 4 lasers. Second, for very hot vapor cell based systems, the color emitted by the energy level should be calculated assuming that all light emitted on a transition ending on the ground-state is nearly completely filtered by radiation trapping and rescattering, since that is what will occur in a very hot vapor cell (e.g. 100° C or greater). In Figures 23 and 24, we calculate the CIE 1931 xyY coordinates on the emitted spectrum with all ground-state resonant wavelengths removed. Third, the color emitted by the energy level should be as close as possible to the pure wavelength boundary and/or near well-established color primaries, such as RGB or CMYK.

[0222] Energy levels satisfying these criteria can then be evaluated in terms of the

CIE 1931 xyY luminance, Y, which gives a representation of the perceived brightness of the emitted spectrum. They can also be analyzed in terms of the lifetime of the energy state, or by a combination of those two variables.

[0223] It turns out that for Cesium all of the F states above the n=5 state and all of the

P states above the n=8 state are candidate states satisfying the above criteria and having non- negligible visible emission from non-ground state transitions. Some of these states are closer to traditional primary regions of the CIE 1931 xyY graph, but since all have coordinates very near to the pure wavelength values they are all candidates. A similar analysis in Rb reveals a similar result; all of the F states above the n=4 state, and all of the P states above the n=7 state are candidates. [0224] However, while all of these are candic

the lifetime reveal that some of those candidates may

least in certain implementations of the system.

[0225] Additionally, it should be noted that whereas we have explained the 3-laser excitation approach in Cs as going through the 6D5/2 (5D5/2 for Rb) energy level, the 6D3/2 (5D3/2 for Rb) energy level also has a very small fraction of visible light emission on the ground state transition and is therefore an excellent candidate for an intermediate energy level from which to reach some destination energy levels.

[0226] Figures 23 and 24 list some of these states which are closest to the red and yellow primaries. We group energy levels by primary and then order them within each primary by the ratio of the luminance Y to the lifetime x, Y/x. This ratio gives a rough heuristic for ordering the perceived brightness of fluorescence emitted by candidate states. As a rough metric we have quantified the red primary as x>0.6 and y>0.28, and the yellow primary as 0.4<x<0.56 and y>0.43.

[0227] The systems and methods described herein may be performed using a computer or other processor having hardware, software, and/or firmware. The various method steps may be performed by modules, and the modules may comprise any of a wide variety of digital and/or analog data processing hardware and/or software arranged to perform the method steps described herein. The modules optionally comprising data processing hardware adapted to perform one or more of these steps by having appropriate machine programming code associated therewith, the modules for two or more steps (or portions of two or more steps) being integrated into a single processor board or separated into different processor boards in any of a wide variety of integrated and/or distributed processing architectures. These methods and systems will often employ a tangible media embodying machine-readable code with instructions for performing the method steps described above. Suitable tangible media may comprise a memory (inc

volatile memory), a storage media (such as a magne

disk, a tape, or the like; on an optical memory such as a CD, a CD-R/W, a CD-ROM, a DVD, or the like; or any other digital or analog storage media), or the like.

[0228] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

CLAIMS:

1. A system for displaying one or more images in three dimensions, the system comprising:

(a) a three dimensional illumination volume comprising a gas, the gas comprising at least one atomic or molecular vapor, the vapor comprising at least one ground state, at least one intermediate state, and at least one multi-photon excited state, the vapor configured to emit at least one type of visible light by decay from the multi-photon excited state;

(b) a plurality of lasers configured to generate at least a first laser beam, a second laser beam, and a third laser beam, wherein at least some of the laser beams comprise different wavelengths; and

(c) the system configured to direct the laser beams into the illumination volume such that the first, second, and third laser beams intersect at a beam intersection in the illumination volume to excite at least some particles of the vapor at the beam intersection to the multi- photon excited state such that the excited particles emit visible light by decay from the multi- photon excited state to the intermediate state, the emitted visible light having CIE 1931 xyY coordinates of x >0.60 and y >0.28.

2. The system of claim 1, wherein the vapor comprises at least a Cesium vapor.

3. The system of claim 2, wherein the multi-photon excited state comprises at least one of a F state above an n=5 energy level or a P state above an n=8 energy level.

4. The system of claim 3, wherein the syste

intersection to the multi-photon excited state by an e¾

energy level or a 6D3/2 energy level.

5. The system of claim 3, wherein the multi-photon excited state comprises at least one of a 11F5/2 state or an 12F5/2 state.

6. The system of claim 2, wherein the Cesium vapor comprises a first ground state comprising a first absorption profile and a second ground state comprising a second absorption profile, wherein the first and second absorption profiles are at least partially overlapping.

7. The system of claim 2, wherein the system regulates the temperature of the gas such that the temperature is greater than 100° C.

8. The system of claim 1, wherein the first laser beam and the third laser beam propagate along a first beam path and the second laser beam propagates along a second beam path intersecting the first beam path at the beam intersection.

9. The system of claim 1, wherein the gas further comprises a buffer gas.

10. The system of claim 1, wherein the system is configured to scan the beam intersection to generate an image.

11. A system for displaying one or more ima

comprising:

(a) a three dimensional illumination volume comprising a gas, the gas comprising at least one atomic or molecular vapor, the vapor configured to emit at least one type of visible light by decay from a multi-photon excited state comprising at least one of a F state above an n=4 energy level or a P state above an n=7 energy level;

(c) the system configured to direct the laser beams into the illumination volume such that the first, second, and third laser beams intersect at a beam intersection in the illumination volume to excite at least some particles of the vapor at the beam intersection to the multi- photon excited state such that the excited particles emit visible light.

12. The system of claim 11 , wherein the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light having CIE 1931 xyY coordinates of x >0.60 and y >0.28.

13. The system of claim 11 , wherein the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light having CIE 1931 xyY coordinates of 0.4<x<0.56 and y>0.43.

14. The system of claim 13, wherein the multi-phc

P state above an n=8 energy level or F state above an r

15. The system of claim 11, wherein, at the beam intersection, the first laser beam excites the particles from a ground state to a first intermediate state, the second laser beam excites the particles from the first intermediate state to a second intermediate state, and the third laser beam excites the particles from the second intermediate state to the multi-photon excited state.

16. The system of claim 15, wherein at least some particles of the vapor at the beam intersection emit visible light by decay from the multi-photon excited state to a third intermediate state.

17. The system of claim 11, wherein the system regulates the temperature of the gas such that the temperature is greater than 100° C.

18. The system of claim 11, wherein the system regulates the temperature of the gas such that the temperature is greater than 125° C.

19. The system of claim 17, wherein the gas further comprises a buffer gas.

20. The system of claim 17, wherein the system excites the particles at the beam intersection to the multi-photon excited state by an excitation pathway that includes a 6D5/2 energy level, a 6D3/2 energy level, a 5D3/2 energy level, or a 5D5/2 energy level.

21. The system of claim 17, wherein the parti<

comprises one or more decay pathways giving rise tc

spectral luminance of 0.01 or greater.

22. The system of claim 17, wherein the particles at the multi-photon excited state comprises one or more decay pathways giving rise to a ground state filtered CIE 1931 xyY spectral luminance of 0.1 or greater.

23. The system of claim 17, wherein the system is configured to excite particles at the beam intersection to the multi-photon excited state via an excitation pathway including at least one intermediate state, the intermediate state comprising one or more decay pathways giving rise to a ground state filtered CIE 1931 xyY spectral luminance of 10^"4 or less.

24. The system of claim 11 ,

wherein the system regulates the temperature of the gas such that the temperature is greater than 100° C,

wherein the vapor comprises at least a Cesium vapor,

wherein the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light from the particles having CIE 1931 xyY coordinates of x >0.60 and y >0.28, and

wherein the multi-photon excited state comprises an F state above an n=5 energy level or a P state above an n=8 energy level.

The system of claim 11 , wherein the system regulates the temperature

greater than 100° C,

wherein the vapor comprises at least a Cesium vapor,

wherein the system is configured to excite at least some particles of the vapor at the beam intersection to the multi-photon excited state such that the excited particles emit visible light by decay from the multi-photon excited state, the emitted visible light from the particles having CIE 1931 xyY coordinates of 0.4<x<0.56 and y>0.43, and

wherein the multi -photon excited state comprises a P state above an n=10 energy level.

26. The system of claim 11 ,

wherein the vapor comprises at least a Rubidium vapor,

wherein the multi-photon excited state comprises an F state above an n=4 energy level or a P state above an n=7 energy level.

27. The system of claim 11 ,

wherein the vapor comprises at least a Rubidium vapor, wherein the system is configured to excite at

beam intersection to the multi-photon excited state su<

light by decay from the multi-photon excited state, the emitted visible light from the particles having CIE 1931 xyY coordinates of 0.4<x<0.56 and y>0.43, and

wherein the multi-photon excited state comprises an F state above an n=9 energy level or a P state above an n=8 energy level.

28. A system for displaying one or more images in three dimensions, the system comprising:

(a) a three dimensional illumination volume comprising a gas, the gas comprising at least one atomic or molecular vapor, the vapor configured to emit at least one type of visible light by decay from a multi-photon excited state;

(b) a plurality of lasers configured to generate at least a first laser beam, a second laser beam, and a third laser beam, wherein at least some of the laser beams comprise different wavelengths, wherein the first laser beam and the third laser beam propagate along a first beam path and the second laser beam propagates along a second beam path intersecting the first beam path at the beam intersection; and

29. The system of claim 28, wherein the multi-photon excited state comprises at least one of a F state above an n=4 energy level or a P state above an n=7 energy level.