WO2007002756A2

WO2007002756A2 - Laser tweezer actuated micro-photonic devices

Info

Publication number: WO2007002756A2
Application number: PCT/US2006/025184
Authority: WO
Inventors: Aaron L. Birkbeck; Sadik C. Esener
Original assignee: The Regents Of The University Of California
Priority date: 2005-06-27
Filing date: 2006-06-27
Publication date: 2007-01-04
Also published as: WO2007002756A3; WO2007002756A9

Abstract

Optical devices that optically trap a lens suspended in a fluid for various applications, including but not limited to, optical filtering, optical imaging, optical focusing, optically trapping and manipulating of particles; examples for fabrication techniques for making suitable lenses including solid immersion lenses; and other optical devices such as micro optical resonators that are fabricated using the fabrication techniques for making the lenses.

Description

OiSER^" TWEEZER ACTUATED MICRO-PHOTONIC DEVICES

[0001] This application claims the benefit of U.S. Provisional Application No. 60/694,482 entitled "LASER TWEEZER-BASED DEVICES USING NEAR-FIELD LENS AND SELF-ALIGNED SPATIAL FILTER" and filed June 27, 2005, the entire disclosure of which is incorporated by reference as part of the specification of this application.

Background [0002] This application relates to optical devices for various optical applications including, among others, optical filtering, optical imaging, optical focusing, optical sensing, optical trapping and manipulating of particles, and techniques for fabrication of such optical devices.

[0003] Light, like other forms of electromagnetic energy, caries momentum and thus can exert a force on an object that is illuminated by the light. FIG. 1 illustrates light- induced forces on a transparent sphere 102 due to optical reflection and refraction at the sphere surfaces under illumination of an incident light beam 101 such as a laser beam. Subscripts "D" and "R" represent the diffractive force and refractive force, respectively. In a tightly focused beam, the largest incident angle photons provide the greatest axial restoring forces and can be used to form a three-dimensional optical trap to force the sphere 102 to align itself along the optical axis of the beam 101. Laser tweezers are optical devices that use focused laser light to optically trap microparticles suspended in liquids based on the principal illustrated in FIG. 1. U.S. Patent No. 6,778,724 assigned to the Regents of University of California, for example, describes laser tweezers based on vertical cavity surface emitting lasers and micro-fiuidic devices.

Summary

[0004] This application describes, among others, examples of optical devices that optically trap a lens suspended in a fluid for various applications, including but not limited to, optical filtering, optical imaging, optical focusing, optically trapping and manipulating of particles; examples for fabrication techniques for making suitable lenses including solid immersion lenses; and other optical devices such as micro optical resonators that are fabricated using the fabrication techniques for making the lenses. [0005] In one example, an optical device is described to include a lens module operable to receive a laser beam and to focus a laser beam to a first position away from the lens module; and a solid immersion lens having a spherical surface and a flat surface opposing the spherical surface and being suspended in a liquid at a position between the lens module and the first position to focus the laser beam at a second position which is between the first position and the lens module and is at or near the flat surface

[0006] .In another example, an optical method is described to include using a lens module to focus a laser beam into a body of a liquid sample in which a solid immersion lens is suspended; adjusting a relative position of the lens module and the liquid sample to place the suspended solid immersion lens in the focused laser beam at a position to optically trap the suspended solid immersion lens which further focuses the focused laser beam; and controlling the focusing of the focused laser beam by the lens module and the position of the laser beam entering the liquid sample to move and control a position of the suspended solid immersion lens in the liquid sample. [0007] In another example, a method is described to include forming a metal layer over a substrate; forming an optically transparent dielectric layer on the metal layer; placing at least one dielectric microsphere on the dielectric layer; applying heat to the substrate to raise a temperature of the dielectric microsphere above a glass transition temperature of the dielectric microphere and to reshape the dielectric microsphere into a solid immersion lens that has a flat surface in contact with and attached to the dielectric layer and a top spherical surface; removing the heat to cool the substrate to solidify the solid immersion lens; and removing the metal layer between the dielectric layer and the substrate to remove the solid immersion lens and the dielectric layer to which the solid immersion lens is attached. [0008] In another example, a method is described to include forming an optical waveguide structure over a substrate; placing a dielectric microsphere on the optical waveguide structure; and applying heat to the substrate to raise a temperature of the dielectric microsphere above a glass transition temperature of the dielectric microphere and to reshape the dielectric microsphere into an optical resonator that has a flat surface in contact with and attached to the optical waveguide structure and a top spherical surface.

[0009] In another example, a device is described to include a substrate; a vertical cavity surface emitting laser structure formed on the substrate, the vertical cavity surface emitting laser structure operable to emit laser light; an optical waveguide structure on the vertical cavity surface emitting laser structure and comprising a region to allow the laser light pass through vertically; and an optical resonator having a bottom flat surface in contact with a top surface of the optical waveguide structure and a top spherical surface. The optical resonator is placed above the region of the optical waveguide structure to receive the laser light and doped with active ions to produce light under optical pumping by the laser light.

[00010] In another example, a device is described to include a substrate; an optical waveguide structure on the substrate; an optical resonator having a bottom flat surface in contact with a top surface of the optical waveguide structure and a top spherical surface, the optical resonator doped to produce light under optical pumping by laser light; and a vertical cavity surface emitting laser structure formed on a separate substrate to produce the laser light. The vertical cavity surface emitting laser structure is engaged to the substrate in a position to direct the laser light to the optical resonator.

[00011] In another example, an optical device is described to include a container to enclose a liquid; and a solid immersion lens suspended in the liquid and having a spherical surface and a flat surface opposing the spherical surface. [00012] In another example, a method is described to include directing a beam through a focusing lens to produce a focused beam; placing a container that encloses a liquid and a lens suspended in the liquid along an optical path of the focused beam, wherein the lens is attached to an optical spatial filter with a pinhole to focus light through the pinhole; and adjusting the position of the lens to be at or near a focal point of the focused beam to optically trap the lens, thus automatically aligning the pinhole to the focused beam for spatial filtering. [00013] In another example, a device is described to include a container to enclose a liquid; and a device suspended in the liquid and comprising a lens and an optical spatial filter having an opaque portion and a central hole in the opaque portion. The lens is placed in and attached to a center of the central hole through which the lens directs an optical beam. [00014] In another example, a method is described to include forming a metal layer over a substrate; forming an optically transparent dielectric layer on the metal layer; forming at least one optical lens attached on the dielectric layer; forming a metal ring layer around the lens; and removing the metal layer between the dielectric layer and the substrate to remove the lens, the metal ring layer and the dielectric layer. [00015] In yet another example, a method is described to include placing polymer microspheres coated with an alcohol medium over a layer of a metal formed on a substrate; heating the substrate to evaporate the alcohol medium on each microsphere; raising the temperature of the substrate above a glass transition temperature of the polymer microspheres for a selected duration to shape the microspheres; cooling the substrate to solidify the newly shaped microspheres; and performing a lift-off etch on the layer of the metal to remove the shaped microspheres.

[00016] In yet another example, a method is described to include patterning a plurality of films deposited on a substrate into a pattern with a plurality of positions for holding microspheres, where a bottom film in contact with the substrate is a metal layer. This method further includes placing microspheres in the patterned positions; heating the substrate to attach the spheres to respective patterned positions; etching the films to separate the films into separated segments respectively holding the substrates; and removing the separated segments.

[00017] The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

Brief Description of the Drawings

[00018] FIG. 1 illustrates optical trapping of a particle by a focused optical beam.

[00019] FIG. 2 shows an example of a device using a solid immersion lens suspended in a liquid.

[00020] FIG. 3 shows an operation of the device in FIG. 2 for imaging the liquid. [00021] FIG. 4 shows a device using an array of lasers based on the design in

FIG. 2.

[00022] FIG. 5 shows an example of a solid immersion lens for use in FIGS. 2 and 4.

[00023] FIGS. 6A and 6B show photographs of exemplary solid immersion lenses made of a polymer.

[00024] FIG. 7 show one exemplary fabrication process for making the lens in

FIG. 5.

[00025] FIGS. 8, 9, 10, 11, 12, 13, and 14 show devices that use micrresonators. [00026] FIGS. 15, 16A and 16B illustrate an optical spatial filter and its operations.

[00027] FIGS. 17 and 18 show two lenses suitable for use in a self-aligned spatial filter. [00028] FIGS. 19 and 20 show two fabrication processes for making the lenses in FIGS. 18 and 17, respectively.

Detailed Description

[00029] Laser tweezers for trapping and moving particles suspended in flowing or still fluids generally direct focused light to one or more particles in a liquid sample and to create an optical trap to hold the particles. The focusing of the light is usually achieved by using one or more lenses in a lens module to focus the light to a target location in the liquid sample. For example, microscope object lens is used in some laser tweezers to produce the desired focused light for creating an optical trap. A lens module with a large numerical aperture (NA) is desirable to produce a tightly focused beam for efficient optical trapping.

[00030] Solid immersion lenses (SILs) are known to have large NA values and have been used in various application that require focusing light into a small beam spot, including near field scanning optical microscope incorporating an optically actuated solid immersion lens immersed in a liquid and optical storage devices using solid immersion lenses for reading and writing data on optical disks. An SIL is usually held in a lens holder and is aligned with other optical elements in a particular optical device. For example, various solid immersion lens imaging systems use mechanical cantilevers to hold SILs at a desired position in the optical train of the imaging systems. Such cantilever-mounted SILs can be difficult to integrate into microfluidic systems and may requrie an extra alignment step with external optics. [00031] FIG. 2 illustrates an example of a SIL optical device 200 that integrates a laser tweezer to optically trap and control an SIL 202 and to optically trap and manipulate objects in three-dimensions with the resolution-enhanced imaging capabilities of the SIL 202. Different other SIL devices and systems that use a cantilever or other lens holding mechanism to hold the SIL 202, the device 200 suspends the SIL 202 in a liquid 203 in a liquid container 230 without directly attaching any lens holding element to the SIL 202. This device 200 uses a lens module 201, which may include one or more lenses to produce a large NA, to receive and focus an optical beam 210 to a focal point 212 in the liquid 203 in absence of the SIL 202. A microscope objective lens, for example, can be this lens module 201. A laser or other light source can be used to generate the optical beam 210. The container 230 may be a part of a fluid system.

[00032] The SIL 202 suspends in the liquid 203 and is located in the optical path of the beam 210 between the lens module 201 and the focal point 212 to further focus the beam 210 along a beam path 220 to new focal point 222 between the lens module 201 and the focal point 212. Due to the optical forces exerted on the SIL 202, an optical trap can be formed by the beam 210 when the SIL 202 is placed at a proper position in the path of the beam 210. Once the SIL 202 is trapped by the beam 210, the forces of the beam 210 automatically force the SIL 202 to align along the optic axis of the beam 210 and hold the SIL 202 at the trapped position along the optic axis. Under this optically trapped condition, the SIL 202 is fixed in space by the invisible "optical cantilever" formed by the beam 210. Due to the nature of the optical trapping, any deviation in the position of the SIL 202 from the trapped position can be automatically corrected by the optical trap to pull the SIL 202 back to the trapped position. As such, this configuration provides an automatic and self-aligned mechanism to hold the SIL 202 in place relative to the lens module 201 without any mechanical lens holder and without needing any alignment operation or device to ensure the proper alignment between the lens module 201 and the SIL 202. In addition, the use of the liquid 203 for suspending the SIL 202 in the device 200 allows easy and direct integration of the device 200 into microfluidic and biological samples. [00033] The SIL 202 can be a truncated spherical lens having a spherical surface on one side and a flat surface on the opposite side. As well known in the near-field optics, the SIL 202 can increase the numerical aperture of the optical system by up to n², where n is the index of refraction of the lens material. The SIL 202 may be a free-floating polymer SIL with a diameter ranging from 1 μmlOO μm, (e.g., between 5 μm and 80 μm). The laser tweezer in FIG. 2 can be created by focusing the laser beam 210 through a high numerical aperture microscope objective as the lens module 201. Notably, the laser beam 210 in the device 200 in FIG. 2 acts in a two-fold manner: both as a trapping beam for the positioning and alignment of the SIL 202 and as an imaging beam to image or scan the liquid sample 203 through the SIL 202. Combining the alignment, positioning, and imaging functions into a single device allows for the direct integration of a high resolution imaging system into microfluidic and biological environments. [00034] The SIL 202 can be made light and compact for being properly suspended in various liquids. In some implementations, the SIL diameters can rang from 1 μmlOO μm, (e.g., between 5 μm and 80 μm). The SIL 202 can be designed to have a height, r(l+l/n), where r is the radius of the spherical surface. The height can be altered through the fabrication process to accommodate for different polymer materials with different refractive indices.

[00035] In operation, the SIL 202 is first optically actuated by focusing the laser beam 210 through the high numerical aperture lens (NA > 0.6) in the lens module 201 to produces a three-dimensional optical trapping force that arises from the interaction of the photons with the relative index of refraction of the SIL lens. The largest incident angle photons provide the greatest axial restoring force to the three- dimensional optical trap causing the SIL to align itself along the beam's optical axis. Once the solid immersion lens is trapped by the laser beam, actuation occurs by moving the sample to be imaged or scanned relative to the optically trapped SIL. [00036] The device 200 can be used for near field imaging of the liquid sample

203 by either scanning the beam 210 via moving the lens module 201 relative to the container 230 or moving the container 230 relative the lens module 201 so that different parts of the liquid sample 203 are imaged. When optically trapping the SIL202 in three dimensions, the SIL 202 becomes self-aligned to the trapping beam which is also the same beam that is focused by the SIL 202 for near field imaging. This design can essentially avoid extra alignment steps via external optics and any misalignment of the SIL caused b the alignment via the external optics. [00037] When the beam 210 for trapping and imaging is too energetic for the sample to be imaged, a two-beam configuration may be used. In general, short wavelength laser beams can be used to produce high imaging resolutions but also increase the beam energy due to high energy photons at short wavelengths. In one implementation of the two-beam scheme, a trapping beam can be shifted to a longer wavelength with lower energy to trap the SIL 202 and a separate but spatially overlapping imaging beam at a short wavelength is used to obtain the image of the sample and can be set at a a lower beam intensity.

[00038] FIG. 3 shows me operation of the device 200 in FIG. 2 for imaging a sample. The device 200 in FIG. 2 can be modified to include multiple beams for a highly parallel SIL based near field scanning optical microscopy simultaneously using multiple trapping/imaging beams, each corresponding to a different SIL. FIG. 4 shows one example where a laser array 410 with multiple lasers are used to produce multiple laser beams; a lens head 412 is used to focus the laser beams onto multiple SILs 202 in the liquid sample 203 in the container 230. The lens head 412 may include a single objective lens or multiple lenses.

[00039] Other applications for the device 200 in FIG. 2 are possible. For example, the device 200 in FIG. 2 can be used to effectuate evanescent wave coupling using the free-floating SIL 202. The free-floating SILs can be used as wavelength couplers that are positioned by a laser optical tweezer. The SIL 202 can also be used, with a proper light coupling mechanism, to support whispering gallery modes which are dependent upon the radius of the SIL 202. SILs can be fabricated with different diameters to support different whispering gallery modes and the optical coupling of a particular wavelength out of a waveguide or fiber can be controlled by altering the diameter of the SIL. A polymer SILs can be directly fabricated on, or an optical tweezer can precisely place the SIL, on a waveguide or optical fiber core as evanescent field wavelength couplers. [00040] In evanescent wavelength coupling in optical resonators that support whispering gallery modes, one technical issue is the shape of the cavity resonator and the accurate position of the resonator with respect to the waveguide or optical fiber. For Gaussian beams in free space, the ideal cavity resonator is a perfect sphere, but in the case of waveguides and optical fibers, the very geometry of the sphere inhibits very efficient coupling. The geometry of the SIL in this case is ideal for this purpose since it retains its spherical shape and has a surface flat that allows a much closer interaction distance with the waveguide. With respect to the problem of positioning the resonator, it is quite simple to use a laser tweezer to very accurately place the SIL resonator to maximize evanescent coupling. Once the desired placement of the SIL resonator is achieved using a laser tweezer, the fluid can be drained from the system so that the evanescent wavelength coupler can function correctly. Of course this whole process can be expanded to include multiple SIL resonators and optical trapping beams that can work in a parallel fashion. A wide range of applications may employ the above SIL-based techniques. Examples of the applications include but are not limited to: optical scanning microscopy, optical data storage, optical inspection systems and optical wavelength sampling and routing. [00041] FIG. 5 shows one example of a SIL mounted on an optically transparent substrate 510. The substrate 510 may be made of an oxide or other dielectric material. The geometry of the substrate 510 can be a disk to provide rotational stability to the SIL 202 when it is spinning under the control of the focused beam 210 in the device 200 in FIG. 2 and other devices using the free-floating SIL 202.

[00042] One way to fabricate the SIL 202 is to reshape a sphere at a high temperature above the glass transition temperature of the sphere material, e.g., to melt, at at least partially, the sphere place on a substrate. The bottom of the sphere during this processing becomes a flat surface while the top surface is spherical due to the surface tension and interfacing between and the respective wetting properties of the sphere material and the underlying surface. The processing time at the high temperature can be adjusted to achieve different shapes in the final SILs. FIG. 6A shows photographs of different shapes of a polystyrene sphere under this thermal processing at different times. FIG. 6B further shows Microscope image of a single 20μm polystyrene solid immersion lens fabricated using a thermal melting process. Other polymer or non-polymer materials may be used to construct the SILs. The material for the sphere to form the SILs and microreosnators and other structures is selected to have a sufficiently low glass transition temperature so that the high temperature used in the fabrication does not adversely affected the structures already formed on the substrate.

[00043] FIG. 7 illustrates processing steps A through L of an exemplary fabrication process. The substrate used in this example is silicon and it is understood that other substrates can also be used, such as a glass. First, a metal layer is formed over the substrate and next an optically transparent dielectric layer is formed on the metal layer. See steps A, B, C, D, E and F. At steps G and H, at least one dielectric microsphere is placed on the dielectric layer. Many spheres can be placed at different locations on the dielectric layer to form multiple SILs at the same time. Step I shows the process of applying heat to the substrate to raise a temperature of the dielectric microsphere above a glass transition temperature of the dielectric microphere. This is to reshape the dielectric microsphere into a solid immersion lens that has a flat surface in contact with and attached to the dielectric layer and a top spherical surface. The heat is then removed to cool the substrate to solidify the solid immersion lens. The metal layer between the dielectric layer and the substrate is subsequently removed by a "lift-off procecs to remove the solid immersion lens and the dielectric layer to which the solid immersion lens is attached. This is shown in Step J and K. The SIL is then placed in a liquid within a container for use in various devices such as the example in FIG. 2. [00044] The above process can be used to fabricate many SILs, e.g., tens of thousands of highly uniform SILs, in a single fabrication process at a low cost in materials and labor. In implementing the above process, the following may be performed. To begin with, an aluminum or a different metal layer is thermally evaporated on a polished silicon substrate to act as a sacrificial layer for detaching the SILs from the substrate. Next a layer of SiO₂ is deposited on the aluminum using plasma enhanced chemical vapor deposition (PECVD) where the thickness is constrained by the near-field evanescent coupling distance of the SIL. The SiO₂ layer is then patterned using a photoresist mask and a CF4 plasma etch into circular disks that are twice the diameter of the SILs to be fabricated. Once the SiO₂ layer is patterned into circular discs, the exposed regions of the Aluminum are removed using a wet acid etch, which assists with the photomask alignment in the next step. Next a photoresist or photo polymer layer is applied and patterned into well structures that are centered on the SiO₂ disks and have a diameter slightly larger than that of the SIL. A suspension of highly uniform polystyrene divinylbenzene microspheres (Bangs Labs, Inc) with a refractive index of ?? = 1.59, is spread over the surface and the substrate is agitated until all the photoresist wells are filled with microspheres. Excess microspheres are then rinsed off and the photoresist layer is carefully dissolved in acetone leaving the microspheres centered on the SiO₂ disks. The next step involves melting the polymer microspheres by placing the substrate in direct contact with a highly uniform temperature heat source whose temperature is raised well above the glass transition temperature of the polymer. Optimizing the melting time and temperature allows for the transformation of the microsphere into the form of a solid immersion lens. After the correct form of the lens is achieved, the substrate is allowed to cool and the SILs are removed from the substrate by performing a liftoff etch of the Aluminum sacrificial layer. The SILs are then washed in a cleaning solution, centrifuged down and suspended in a predetermined liquid medium. [00045] Alternatively, polymer microspheres suspended in an alcohol medium can be spin-coated on the substrate at a speed and concentration to maximize the number of microspheres on the substrate yet minimize microsphere aggregation. The coated substrate is then heated in order to allow the alcohol to evaporate leaving the microspheres on the substrate's surface. The next step involves melting the polymer microspheres by placing the substrate in direct contact with a hotplate whose temperature is well above the glass transition temperature for the for a specified amount of time. [00046] Devices based on Optical Resonators such as vertical cavity surface emitting lasers (VCSELs), Vertical-Cavity Semiconductor Optical Amplifiers (VCSOA's) and High-Q micro-ring/disk resonators can be integrated with silicon CMOS circuitryto form adaptive photonic circuits. The above fabrication technique based on thermal heating can be used to form microresonators for photonic circuits that incorporate several types of micro-resonators for optical amplification, light regeneration, switching, and chemical sensing. One of the potential limitations of implementing a silicon based photonics technology is the difficulty of fabricating area and power efficient light modulators on silicon. Hybrid integration of vertical cavity resonators on GaAs with in-plane devices on silicon via flip chip bonding may be used to mitigate some of these limitations.

[00047] The following sections describe various technical features to realize the potential of these hybrid circuits and to achieve the integration of high quality factor Q micro-resonators on silicon CMOS chips that can be mated by flip-chip bonding to GaAs chips supporting VCSELs and VCSOAs enabling area efficient low cost adaptive photonic circuits. Examples include the fabrication and characterization of a hemispherical high Q micro-resonator that can be integrated on silicon CMOS using, e.g., glass and polymer based resonators; sensitive chemical sensors based on the above high Q resonator and incorporation of surface plasmon coupling to further increase sensitivity and selectivity of such chemical sensors; and resonators doped with active ions to provide optical gain under optical pumping (e.g., an Erbium doped HIGH Q micro-resonator pumped by a 980nm VCSEL for area efficient amplification and light generation at 1550nm. [00048] The micro resonators can be formed by a similar heating process described above in FIG. 7. First, an optical waveguide structure is formed over a substrate. A dielectric microsphere is then placed on the optical waveguide structure. Heat is applied to the substrate to raise a temperature of the dielectric microsphere above a glass transition temperature of the dielectric microphere and to reshape the dielectric microsphere into an optical resonator that has a flat surface in contact with and attached to the optical waveguide structure and a top spherical surface. This fabrication process allows the hemispherical High-Q micro-resonator to be integrated on CMOS and MEMS devices. Such structures can be designed in full compatibility with CMOS and MEMS devices and to disperse the resonator with suitable dopants such as Erbium and various quantum dots for achieving gain. The present fabrication process can provide manufacturing of solid immersion lenses (SIL) in large quantities and integrated on glass, silicon dioxide, silicon or metal surfaces in any desired pattern. These hemispheric devices are fabricated by surface tension manipulation taking advantage of hydrophobic/hydrophilic selectivity of suitably patterned substrates. They can be made of Glass, Sol gel and Polymers. The fabrication of these devices requires a planarized surface that is not larger than the equatorial area of SIL. The surface smoothness of these devices is remarkable in comparison with similar semispherical structures fabricated using other methods such as polishing. Therefore, when used as a resonator, the resonator can exhibit a high quality factor Q. [00049] FIG. 8 shows an example of high Q semispherical resonator 801 formed over a waveguide structure on a silicon substrate using the above thermal heating method. The waveguide structure includes two waveguides (WGs) 810 and 820 that are optically coupled to the resonator 801 via evanescent fields. A waveguide gap 830 is formed between the waveguides 810 and 820. This structure can be used to form various devices. FIG. 9 shows a sensor based on the structure in FIG. 8 where the exterior surface of the resonator 801 is coated with biological or chemical polymer strands that can be used to selectively attract or attach targeted molecules to be sensed. Attached molecules typically modify the optical path length in the resonator for example by increasing the effective index of refraction and therefore result in a shift of the resonance frequency of the resonator. For example, streptavidin can be coated to later detect avidin concentrations. FIG. 10 further shows another example sensor where the top surface of the semisphereical resonator is flattened and is coated with a metal structure to create a dielectric-metal interface at the flattened surface for surface plasmon coupling with the resonator. On top of the metal structure, a biological or chemical coating can be further formed for detection of selected molecules.

[00050] The major advantages of these photonic chemical sensors include: a) their ultra high sensitivity for small reagent volume resulting from the high quality factor Q; b) their selectivity resulting from the chemical nature of the detection, c) their on-chip integration in array format; and their ultra low cost, of these sensors fabricated on a silicon chip with each resonator sensitized to a different chemical and each tuned to operate at slightly different wavelength. If each resonator/sensor on the chip is then probed (via the waveguides) with a wide band optical probe, the frequency spectrum of the probe will be modified in the event of detection of a given chemical of interest appropriately to reveal the detection event as well as the type of chemical detected. [00051] FIGS. 11, 12, 13 and 14 show examples of integrated photonic devices having doped resonators formed based on the above thermal heating process. As an example, an array of Erbium doped micro resonators optically pumped by 980nm VCSEL array for area efficient amplification and lasing at 1550nm can be produced. The resonators with optical gain may also be formed by including quantum dots for operation at wavelengths shorter than 1550nm. For Er doped resonators, the doping concentration may be less than 1% by weight. Alternatively, the micro-spheres are coated with Erbium doped Sol gel layer to provide the necessary gain. The lasing thresholds of these micro-resonators are typically on the order of tens of μW of absorbed pump power. The coupling of the pump to the lasing whispering gallery modes is a problematic area and actual pump powers in the order of l-2mW are required for lasing. It becomes therefore possible to pump an HIGH Q micro- resonator with a VCSEL for optical amplification. This design can be used to achieve compact optical amplifiers as well as optical circuits capable of performing signal processing functions to be tightly integrated. The resonators incorporating Er dopants can be shaped for efficient optical pumping and use a Laguerre-Gaussian mode VCSELs emitting at 980nm as a pump source.

[00052] More specifically, FIG. 11 shows the structure in FIG. 8 with a resonator 801 coated with a gain coating integrated on a VCSEL chip. The VCSEL directs pump light vertically through the waveguide gap 830 to be coupled into the resonator 801. FIG. 12 shows that the structure in FIG. 8 and the VCSEL chip are separated formed but are packaged together to place the VCSEL chip over the top of the resonator 801. An opening is formed on the top portion of the resonator 801 to receive the pump from the VCSEL. FIG. 13 further shows a design that is integrated via flip-chip bonding with CMOS circuits. The substrate for the resonator includes a Si photodetector and CMOS circuitry and the two chips are connected via a flip-chip conductive bump. FIG. 14 shows an example of a hybrid intelligent photonic circuit with a silicon chip for the resonator, the Si detector and CMOS circuitry and a GaAS chip with a VCSEL and a VCSOA to provide optical pumping and optical amplification. Optical couplers are used on the Si chip to couple light between the two chips. The laser resonator in this example is a Fabry-Perot resonator formed by the VCSEL and VCSOA as two end reflectors and the high Q resonator with gain is placed inside the FP resonator via the waveguide underneath the resonator. Such a circuit can be used to provide functions such as all optical switching, logic, and signal processing with the area and power efficiency of surface normal devices and the ease of integration of in plane devices. Special attention will be put to adjust the resonance wavelengths of the HIGH Q resonator and that of the VCSOAs to operate the VCSOA in their bistable region of operation. By coupling two VCSOAs via HIGH Q resonators it becomes possible to build a variety of circuits including flip flops. During the latter part of the program we will also investigate direct coupling of VCSELs with HIGH Q resonators to achieve phase locked VCSEL arrays. [00053] Referring back to FIG. 2, the self-alignment of the free-floating SIL

202 with respect to the trapping beam 210 can be used to build devices that often require precision optical alignment. Alignment of optical spatial filters is one example and the tself-alignment of the free-floating SIL in FIG. 2 can be used to provide a self-aligned spatial optical filter integrated into a microfluidic system. . [00054] The concept of spatial filtering a laser beam in optics is synonymous with the low-pass filtering technique used in analog and digital signal processing. In time-based signal analysis, a low-pass filtering operation is performed to either selectively sample low frequencies from a modulated channel or to attenuate unwanted high frequency noise components that are added to the signal as it propagates through a circuit or transmission line. In optics, the noise sources can be derived from multi-order energy peaks from the source laser, the diffraction of the laser beam from dust, imperfections in optical components such as lenses, beam splitters, and mirrors, etc. that are usually present in any optical system. The resulting interference patterns produced by these diffractive noise sources degrade the quality of the laser beam by producing phase, amplitude, and / or modulation variations which show up as Fresnel zone patterns in the imaging plane of the detector. [00055] For a Gaussian laser beam profile, the spatial filter consists of a circular aperture, or pinhole, that is placed at the focus of a relatively high numerical aperture lens. The optical system used for spatial filtering is depicted in FIG. 15. The actual spatial filtering of the laser beam can be explained by taking into account the focusing properties of sources at different points of origin. The beam output of a laser can be described as a point source originating at infinity, whereas the optical noise sources will have a point of origin at a finite distance from the spatial filter. This difference in the points of origin directly affects the focusing ability of the lens to produce a single spot. At the focal plane of the lens, the Gaussian laser beam will be tightly focused around the optical axis and the noise sources will experience less of a focusing effect resulting in the formation of a ring or annulus profile that encircles the focused beam. [00056] FIG. 16A and 16B illustrate the energy distributions of a Gaussian laser beam in the spatial frequency with noise interference (FIG. 16A) and the separation of frequencies that occurs at the focal point of the lens (FIG. 16B). In terms of spatial frequencies, the act of focusing of the laser beam produces a Fourier decomposition of the spatial frequencies at the focal point of the lens where the higher frequency noise components are separated from the lower frequency, narrow spectral width of the laser. Placing a circular aperture or pinhole at the focal plane of the lens and adjusting its size to be slightly larger than the envelope of the focused beam, allows the laser beam to pass through while at the same time severely attenuating, or even eliminating the noise sources.

[00057] The optimal diameter of the spatial filter's circular aperture is a function of the Gaussian beam waste at the focused spot (coo), which is shown in FIGS. 16A and 16B as the distance (r = a ), where the intensity Io falls off by a factor of 1/e². Assuming that the beam diameter (D) is at least twice the focused spot size, the focused spot size can be calculated as follows:

_4

2m « — 2 F F = ^- π D

where/is the focal length of the lens and a>o = a shown in FIGS. 16A and 16B. For most spatial filtering systems including ours, a microscope objective is used as the focusing lens. In general, as a compromise between ease of alignment, optical power loss in the beam, and the completeness of the spatial filtering, the diameter of the circular aperture of the spatial filter is increased to a factor of two times the 1/e² contour at the focus, resulting in the final equation:

[00058] When a lens is attached to the central hole of a spatial filter as a single device floating in a liquid, focusing a laser beam through the this lens with a high numerical aperture lens (NA > 0.6) can be used to produce a three-dimensional optical trapping force to trap the refractive lens at the center of the micro-optic spatial filter. In a tightly focused beam, the largest incident angle photons provide the greatest axial restoring force to the three-dimensional optical trap causing the micro-optic spatial filter to align itself along the beam's optical axis. Once the micro-optic spatial filter is trapped by the laser beam, actuation occurs by moving the sample to be imaged or scamied relative to the optically trapped spatial filter. Unlike a conventional spatial filter, the present free-floating spatial filter with a lens can automatically align itself in three-dimensions to the focus of the laser beam where it can perform its primary function of filtering out higher frequency additive noise components. The self- alignment capability of this spatial filter is made possible through the attachment of a refractive optical element directly over the circular aperture or pinhole of the spatial filter. The refractive properties of the optical element allow the light from the focused laser beam to exert a trapping force on the micro-optical spatial filter in the direction of highest intensity or toward the focus of the laser beam. [00059] Two examples of a suitable refractive lens for this design are a hemispherical lens shown in FIG. 18 and a spherical lens shown in FIG. 17. [00060] FIG. 19 shows an example fabrication process for fabricating the photoresist reflow lens (PRL) spatial filter in FIG. 18. This process uses photolithographically defined features and allows for the creation of many PRL spatial filters simultaneously through the same process. Initially, a metal layer is thermally evaporated on polished substrate to a predetermined thickness. Next a layer of silicon dioxide (oxide) is deposit on top of the metal layer using a plasma enhanced chemical vapor deposition (PECVD) process (Step A). Next, a layer of photoresist is spin coated on the substrate, patterned using a circular mask and UV light, and developed yielding cylindrical structures (Steps B and C). The substrate is then heated so that the photoresist cylinders liquefy and reflow producing hemispherical shapes whose radius of curvature is governed by the surface tension between the photoresist and the oxide layer. In addition to forming the hemispherical lens shapes of interest, the reflow process also induces a cross-linking in the photoresist as it is cooled that produces a rigid structure that has a high solvent resistance. Following the photoresist reflow step, the hemispherical lenses are only semi-transparent and need to undergo a flood exposure under UV light to induce photo bleaching in the photoresist and make the hemispherical lenses transparent (Step D). Next another layer of photoresist is spun on over the reflow lenses and patterned using a dark-field mask consisting of two concentric circles where the inner circle is opaque (Steps E and F). After the photoresist is developed, the sample is placed in an e-beam evaporator where a thin layer of metal is deposited in the exposed circular regions. (Step G). Once the metal evaporation is complete, the sample is placed in acetone to dissolve the photoresist and remove any metal that does not reside directly on the oxide layer (Step H). The next step involves patterning the oxide layer using a plasma etcher that removes any exposed region of oxide that is not masked by the reflow lenses or the deposited metal (Step I). Now that the oxide layer has been patterned, the final form of the PRL spatial filter is complete and the only remaining step is to perform an metal acid etch to release the spatial filters from their substrate where they can be washed in an alcohol to get rid of any remaining acid, centrifuged down, and placed in a liquid buffer solution (Steps J and L). [00061] FIG. 20 shows an exemplary fabrication process for fabricating the microsphere lens (ML) spatial filter shown in FIG. 17. This process follows the same general processing procedure as the PRL spatial filters in that they can be mass produced in an array form and fabricated on the same base substrate. In addition, different lenses are released from the substrate using the same acid etch lift-off process. The process in FIG. 20 differs from the process in FIG. 19 in the type of lens used and how the lens is integrated onto the spatial filter.

[00062] The ML spatial filter procedure begins by depositing a metal layer directly onto the oxide-metal-polished substrate (Steps A and B). Next a thick layer of photoresist is spin coated on the substrate and patterned using a bright field mask with the concentric circle pattern where this time, the inner circle is transparent. The end result of this lithographic patterning step is the creation of a photoresist ring structure (Steps C and D). The following two steps utilize the photoresist ring structure as a mask so that both the top metal layer and the subsequent oxide layer can be patterned using a wet acid etch and a plasma etch, respectively (Steps E and F). Now that the spatial filtering part of the device has been fabricated, the spherical lenses can be incorporated into the device structure by pipetting a concentrated solution of appropriately sized glass microspheres onto the sample and agitating the substrate until a majority of the spatial filter holes have been filled with the microspheres (Step G). Once the desired fill factor has been achieved, the remaining microspheres can be washed off by using the photoresist spinner and a Di-H₂O rinse. In order to make sure that the microsphere lens remains fixed in place when the spatial filter is released from the substrate, the sample is placed on a hot-plate where the temperature is increased until the photoresist reflows against the sides of the microsphere, attaching the lens to the spatial filter substrate (Step H). The remaining steps in the fabrication procedure follow exactly like the PRL spatial filter in that the ML spatial filter is released from the substrate using an metal acid etch, washed in an alcohol to remove the excess acid, and re-suspended in a buffer solution (Steps I and L).

[00063] The above described micro-optic spatial filters can be fabricated in a uniform, low cost, parallel process through a simple photolithographic process. The micro-optical spatial filters can be fabricated with an aperture or pinhole ranging from less than 1 μm to greater than 20μm. The micro-optic spatial filters are suspended in a fluid and can be directly integrated into microfluidic and biological samples. At the onset of the spatial filtering process, the micro-optic spatial filter is self-aligned in three-dimensions to the center of the focused laser beam, eliminating the need for mechanical alignment. A parallel spatial filtering system with multiple such filters can be used to filter multiple trapping beams, each corresponding to a different micro- optic spatial filter. The described spatial filers can be used in a number of applications, including laser beam spatial filtering and optical inspection systems. [00064] While this specification contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. [00065] Thus, particular embodiments have been described. Other embodiments are within the scope of the following claims.

Claims

ClaimsWhat is claimed is:

1. An optical device, comprising: a lens module operable to receive a laser beam and to focus a laser beam to a first position away from the lens module; and a solid immersion lens having a spherical surface and a flat surface opposing the spherical surface and being suspended in a liquid at a position between the lens module and the first position to focus the laser beam at a second position which is between the first position and the lens module and is at or near the flat surface.

2. The device as in claim 1, further comprising: a transparent disk attached to the flat surface of the solid immersion lens.

3. The device as in claim 1, further comprising: a container operable to encloses the liquid and the solid immersionlens.

4. The device as in claim 1, wherein: the lens module comprises a microscope objective lens.

5. The device as in claim 1, wherein: the solid immersion lens has a dimension between 5 micro and 80 microns.

6. The device as in claim 1, wherein: the solid immersion lens is made of a polymer material.

7. An optical method, comprising: using a lens module to focus a laser beam into a body of a liquid sample in which a solid immersion lens is suspended; adjusting a relative position of the lens module and the liquid sample to place the suspended solid immersion lens in the focused laser beam at a position to optically trap the suspended solid immersion lens which further focuses the focused laser beam; and controlling the focusing of the focused laser beam by the lens module and the position of the laser beam entering the liquid sample to move and control a position of the suspended solid immersion lens in the liquid sample.

8. The method as in claim 7, further comprising: after the suspended solid immersion lens is optically trapped in the focused laser beam, using the focused laser beam as an optical probe to obtain an image of a portion of the liquid sample at which the suspended solid immersion lens focuses the focused laser beam.

9. The method as in claim 8, further comprising: after using the focused laser beam to place the suspended solid immersion lens at a first location in the liquid sample and obtaining a first image at the first location, using the focused laser beam to move the optically trapped and suspended solid immersion lens to at least one different location in the liquid sample and to obtain another image at the different location.

10. The method as in claim 9, further comprising: moving the lens module relative to the liquid sample to move the optically trapped and suspended solid immersion lens to different locations in the liquid sample.

11. The method as in claim 9, further comprising: moving the liquid sample relative to the lens module to move the optically trapped and suspended solid immersion lens to different locations in the liquid sample.

12. The method as in claim 7, wherein: using a microfluidic system to supply the liquid sample.

13. A method, comprising: foπning a metal layer over a substrate; forming an optically transparent dielectric layer on the metal layer; placing at least one dielectric microsphere on the dielectric layer; applying heat to the substrate to raise a temperature of the dielectric microsphere above a glass transition temperature of the dielectric microphere and to reshape the dielectric microsphere into a solid immersion lens that has a flat surface in contact with and attached to the dielectric layer and a top spherical surface; removing the heat to cool the substrate to solidify the solid immersion lens; and removing the metal layer between the dielectric layer and the substrate to remove the solid immersion lens and the dielectric layer to which the solid immersion lens is attached.

14. The method as in claim 13, further comprising: prior to placing the dielectric microsphere on the dielectric layer, patterning the dielectric layer and the metal layer to form at least one island area with the dielectric layer and the metal layer over which the dielectric microsphere is subsequently placed.

15. The method as in claim 14, further comprising: suspending the solid immersion lens and the attached dielectric layer in a liquid.

16. The method as in claim 15, further comprising: directing a laser beam to the suspended the solid immersion lens to optically trap the suspended the solid immersion lens in the liquid.

17. The method as in claim 14, further comprising: after patterning the dielectric layer and the metal layer to form the at least one island area, forming a structure on the dielectric layer in the island area for defining a location of the dielectric microsphere on the dielectric layer; and placing the dielectric microsphere at the location on the dielectric layer defined by the structure; and removing the structure on the dielectric layer before applying the heat.

18. The method as in claim 17, wherein: the structure is a cylindrically shaped well around the location where the dielectric microsphere is placed on the dielectric layer.

19. The method as in claim 17, wherein: the structure is made of a photoresist or photo polymer.

20. A method, comprising: forming an optical waveguide structure over a substrate; placing a dielectric microsphere on the optical waveguide structure; and applying heat to the substrate to raise a temperature of the dielectric microsphere above a glass transition temperature of the dielectric microphere and to reshape the dielectric microsphere into an optical resonator that has a flat surface in contact with and attached to the optical waveguide structure and a top spherical surface.

21. The method as in claim 20, further comprising: forming a biological or chemical coating over at least a portion of the spherical surface.

22. The method as in claim 20, further comprising: flattening a top portion of the spherical surface of the optical resonator; forming a structure on the flattened top portion to provide surface plasmon coupling with the optical resonator.

23. The method as in claim 22, further comprising: forming a biological or chemical coating over at least a portion of the structure for surface plasmon coupling.

24. The method as in claim 20, further comprising: before forming the optical waveguide structure over the substrate, forming a vertical cavity surface emitting laser structure on the substrate, the vertical cavity surface emitting laser structure operable to emit laser light directed to the optical resonator; forming the optical waveguide structure on the vertical cavity surface emitting laser structure; and in forming the optical resonator, doping the optical resonator with active ions to provide an optical gain when pumped by the laser light.

25. A device, comprising: a substrate; a vertical cavity surface emitting laser structure formed on the substrate, the vertical cavity surface emitting laser structure operable to emit laser light; an optical waveguide structure on the vertical cavity surface emitting laser structure and comprising a region to allow the laser light pass through vertically; and an optical resonator having a bottom flat surface in contact with a top surface of the optical waveguide structure and a top spherical surface, the optical resonator placed above the region of the optical waveguide structure to receive the laser light and doped with active ions to produce light under optical pumping by the laser light.

26. A device, comprising: a substrate; an optical waveguide structure on the substrate; an optical resonator having a bottom flat surface in contact with a top surface of the optical waveguide structure and a top spherical surface, the optical resonator doped to produce light under optical pumping by laser light; and a vertical cavity surface emitting laser structure formed on a separate substrate to produce the laser light, the vertical cavity surface emitting laser structure engaged to the substrate in a position to direct the laser light to the optical resonator.

27. The device as in claim 26, further comprising: a photodetector formed on the substrate and coupled to receive light from the optical waveguide structure.

28. An optical device, comprising: a container to enclose a liquid; and a solid immersion lens suspended in the liquid and having a spherical surface and a flat surface opposing the spherical surface.

29. A method, comprising: directing a beam through a focusing lens to produce a focused beam; placing a container that encloses a liquid and a lens suspended in the liquid along an optical path of the focused beam, wherein the lens is attached to an optical spatial filter with a pinhole to focus light through the pinhole; and adjusting the position of the lens to be at or near a focal point of the focused beam to optically trap the lens, thus automatically aligning the pinhole to the focused beam for spatial filtering.

30. A device, comprising: a container to enclose a liquid; and a device suspended in the liquid and comprising a lens and an optical spatial filter having an opaque portion and a central hole in the opaque portion, wherein the lens is placed in and attached to a center of the central hole through which the lens directs an optical beam.

31. A method, comprising: forming a metal layer over a substrate; forming an optically transparent dielectric layer on the metal layer; forming at least one optical lens attached on the dielectric layer; forming a metal ring layer around the lens; removing the metal layer between the dielectric layer and the substrate to remove the lens, the metal ring layer and the dielectric layer.

32. The method as in claim 31, further comprising: using a photoresist material to form the lens.

33. A method, comprising: placing polymer microspheres coated with an alcohol medium over a layer of a metal foπned on a substrate; heating the substrate to evaporate the alcohol medium on each microsphere; raising the temperature of the substrate above a glass transition temperature of the polymer microspheres for a selected duration to shape the microspheres; cooling the substrate to solidify the newly shaped microspheres; and performing a lift-off etch on the layer of the metal to remove the shaped microspheres.

34. A method, comprising: patterning a plurality of films deposited on a substrate into a pattern with a plurality of positions for holding microspheres, where a bottom film in contact with the substrate is a metal layer; placing microspheres in the patterned positions; heating the substrate to attach the spheres to respective patterned positions; etching the films to separate the films into separated segments respectively holding the substrates; and removing the separated segments.