WO2007013648A1

WO2007013648A1 - Light vortex generator, microobject operating unit, astronomical probing equipment, and polarization vortex transformation element

Info

Publication number: WO2007013648A1
Application number: PCT/JP2006/315092
Authority: WO
Inventors: Kazuhiko Oka; Ryuji Morita; Satoshi Tanda; Norihiko Nishiguchi; Atsushi Taniguchi; Taisuke Ogoshi
Original assignee: National University Corporation Hokkaido University
Priority date: 2005-07-26
Filing date: 2006-07-25
Publication date: 2007-02-01
Also published as: JP5017659B2; JPWO2007013648A1

Abstract

A light vortex generator comprising a first optical system for transforming coherent light into circularly polarized light, a polarization vortex transformation element arranged to receive circularly polarized light exiting from the first optical system, and a second optical system arranged to receive light exiting from the polarization vortex transformation element. The polarization vortex transformation element has linear birefringence and/or linear dichroism, its polarization characteristics are constant at each point on the same radius from the center of coordinate excluding the azimuth of main axis, and the azimuth of main axis at each point thereon is proportional to the azimuth of coordinate. The second optical system extracts the circularly polarized light component of reverse direction to that of the above-mentioned circularly polarized light from the light exiting from the polarization vortex transformation element.

Description

Technical Document Optical Vortex Generator, Micro Object Manipulator, Astronomical Exploration Device, and Polarized Vortex Transformer Technical Field

The present invention relates to an optical vortex generating device, a minute object manipulating device using an optical vortex, an astronomical exploration device, and a polarization vortex conversion element suitable for use in the optical vortex generating device. Background art

An optical vortex is light (light flux) having a spiral wavefront (phase distribution). In the optical vortex, when a circle centered on the optical axis is considered in a plane perpendicular to the optical axis (light traveling axis) (Fig. 1A), the phase of the light at each point on this circle is the azimuth angle Θ (Fig. 1B). The optical vortex is characterized by having an orbital angular momentum (torque) quantized around the optical axis and a dark line (a line where the light intensity becomes 0) along the optical axis. This orbital angular momentum is L = 1 (h / 2 π) per photon (where h is Planck's constant, 1 is a quantum number and an arbitrary integer). As an example, the optical vortex with 1 = _ 1 is shown in Fig. 2 (see, for example, Miles Padgett, Johannes Courtial, and Les Allen, May 2004 Physics Today 35). The arrows in Fig. 2 indicate the pointing vector.

Using the unique properties of the optical vortex as described above, for example, rotation of microscopically wrapped fine particles, photoexcitation of nanomaterials with ring / cylindrical structures such as ring crystals and carbon nanotubes, and exploration of extrasolar planets Corona graph for faint stellar light that is off the optical axis Detecting starlight). For this reason, in various fields, attempts to positively use the properties of this optical vortex are being made energetically.

Conventionally, several methods have been proposed as methods for generating optical vortices. The first method uses a pair of cylindrical lenses (see, for example, Miles Padgett, Johannes Courtial, and Les Allen, May 2004 Physics Today 35). In this method, as shown in Fig. 3, high-order mode (Hermite-Gaussian (HG) mode) laser light 10 2 is incident on the cylindrical lens pair 10 1 constituting the κ / 1 mode converter. To a low-order mode (Laguerre-Gaussian (LG) mode), which is then incident on the cylindrical lens pair 10 3 constituting the redundant mode converter and converted to the inverse LG mode. A vortex 1 0 4 is generated. Practically, it is convenient to use a Dove prism as shown in Fig. 4 instead of the cylindrical lens pair 103.

The second method uses a glass plate having a spiral thickness distribution. In this method, as shown in FIG. 5, the laser beam 1 1 1 emitted from the laser device 1 1 1 is passed through a collimator 1 1 3 consisting of two lenses 1 1 3 a and 1 1 3 b in parallel. A light vortex 1 15 is generated by passing the light through a glass plate 1 1 4 having a spiral thickness distribution. FIG. 6 shows an example of the glass plate 1 1 4. As shown in Fig. 6, in this glass plate 1 1 4, the thickness and hence the light transmission distance is from ho to h with respect to the azimuth angle Θ. It has a spiral distribution that changes to + h _s. Reflecting this, the phase distribution of transmitted light is also spiral.

The third method uses a reflector having a spiral thickness distribution. In this method, this reflector is used instead of the glass plate 1 1 4 in FIG. In this reflector, the phase distribution of the reflected light is also spiraled, reflecting the spiral distribution of this mirror. The fourth method uses holography (see, for example, Miles Padgett, Johannes Courtial, and Les Allen, May 2004 Physics Today 35). This method uses a hologram in which the refractive index distribution is helical, and the phase distribution of transmitted light is also helical to reflect this. As this hologram, for example, a triple dislocation hologram is used. Then, by making a plane wave incident on this hologram, an optical vortex is generated as first-order diffracted light. .

Gabriel Biener, Avi Niv, Vladimir Kleiner, and Erez Hasman, OPTICS LETTERS, 27, 1875 (2002) proposed a method of generating a spiral beam, that is, an optical vortex, using a Pancharatnam-Berry phase optical element. However, the configuration of the optical vortex generator according to the present invention is not disclosed.

In the conventional optical vortex generation method described above, phase discontinuities almost always exist, and this causes noise generation, which is one of the obstacles to the application of optical vortices. For example, when a glass plate 1 1 4 having a thickness distribution as shown in Fig. 6 is used, the phase change width when making a round around the central axis is TL [(n-1) h _s / λ] (where η is the refractive index of the glass plate 1 1 4 and λ is the wavelength of the laser beam 1 1 2). To accurately match this value to an integer multiple of 2 redundant, it is essential to process the glass plate with nm-class accuracy. This is extremely difficult, and as a result, the existence of phase discontinuities is inevitable.

In addition, the conventional optical vortex generation method described above has a drawback that the orbital angular momentum of the obtained optical vortex varies greatly depending on the wavelength. For example, when a glass plate 1 14 having a thickness distribution as shown in FIG. 6 is used, the phase change width when making a round around the central axis is 2; [(n− 1) h _s / e] Yes, even if the wavelength dependence of n is ignored, it varies greatly in inverse proportion to the wavelength. Ingredients Specifically, for example, when the light of 60 nm (red) is transmitted through the glass plate 1 14, and the phase change width after one round is 2 X 1.2 When light of λ = 4500 nm (blue) is transmitted, the phase change width after one round becomes 2 ^ X 1.6, and the rainy person has a different phase change width. This means that the orbital angular momentums of both optical vortices are very different from each other. This drawback has become a major obstacle to the application of optical vortices. For example, even if an optical vortex obtained by an ultrashort pulse laser with a high peak intensity is used for optical excitation of fine particles, the optical spectrum obtained by a conventional pulse laser is wide. The orbital angular momentum is blurred, and the efficiency of photoexcitation is unavoidable. Also, when using optical vortices for exploring extrasolar planets in astronomy, it is required to use light in a wide wavelength band in order to effectively use light from weak planets. Dependency causes a decrease in exploration accuracy. .

Furthermore, the fact that the orbital angular momentum, that is, the change width of the phase of the optical vortex depends strongly in inverse proportion to the wavelength, makes the above-mentioned phase discontinuity problem more remarkable. Since the amount of phase change when making a round around the central axis is roughly inversely proportional to the wavelength, even if this phase change is an integer multiple of 2 at a specific wavelength, it is an integer multiple of 2 at other wavelengths. And sometimes can have very large phase discontinuities.

Therefore, the problem to be solved by the present invention is based on a novel operating principle, which is achromatic and has no phase discontinuity line regardless of the wavelength of the light source. An optical vortex generator that can easily generate an optical vortex with extremely small wavelength dependency of the step, and an astronomical exploration device such as a micro object manipulation device and a planetary exploration device to which this optical vortex generator is applied. That is. Another problem to be solved by the present invention is to provide a polarization vortex conversion element suitable for use in the above-described optical vortex generator. Disclosure of the invention

In order to solve the above problem, the first invention is:

A first optical system for converting coherent light into circularly polarized light;

A polarization vortex conversion element arranged so that circularly polarized light emitted from the first optical system is incident, and has linear birefringence and / or linear dichroism, and the polarization characteristic is the center of coordinates. Each point on the same radius is constant except for the main axis azimuth, and the main axis azimuth of each point above it is proportional to the coordinate azimuth.

A second optical system arranged so that light emitted from the polarization vortex conversion element is incident thereon, wherein the circular polarization component has a direction opposite to the circular polarization from the light emitted from the polarization vortex conversion element. It is an optical vortex generator characterized by having an extractor.

This optical vortex generator does not require a light source when generating optical vortices using coherent light incident from the outside of the device, for example, when inputting stellar light and planetary light as coherent light in planetary exploration. Otherwise, it further has a light source that generates coherent light. A laser light source is typically used as this light source, but is not limited to this.

The polarization vortex conversion element described above has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the main axis at each point on the same radius from the coordinate center. The azimuth angle of the principal axis of each point above it is proportional to the azimuth angle of the coordinates. This is a normal polarizer that converts the incident light into linear polarization in one direction, and the long axis of the incident light. Elliptical polarization in one direction It is a special one that is different from ordinary phase shifters that convert to. Here, the polarization vortex is elliptically polarized light (including linearly polarized light and circularly polarized light) having the same ellipticity on the same coordinate radius, and the major axis azimuth of the polarized ellipse is the coordinate. It is proportional to the azimuth angle. Having linear birefringence and / or linear dichroism means having at least one of linear birefringence or linear dichroism, but circular birefringence or circular dichroism. May further be included. In addition, the fact that the azimuth angle of the principal axis of each point is proportional to the azimuth angle of the coordinate means that the direction of the coordinate axis for describing the polarization characteristics is different for each point. Φ = η θ / 2 (where η is an integer other than 0), where the azimuth of the principal axis of each point on this polarization vortex transducer is ^ and the azimuth of the coordinates is 6.

In this optical vortex generator, the light transmitted through the polarization vortex conversion element is swirled around the optical axis, that is, a polarization vortex (the optical axis becomes a singular point at this time). Then, when this light passes through the second optical system, the wavefront becomes a vortex around the optical axis, and an optical vortex is obtained. The phase of each point of the optical vortex generated in this way is determined by the azimuth angle of each point of the polarization vortex converter, and does not depend on the wavelength of the light source. Depending on the polarization vortex conversion element used, phase discontinuity lines may not be generated as in the conventional optical vortex generation method using a glass plate with a spiral thickness distribution. Even so, the wavelength dependence of the phase difference in the discontinuous line is extremely small. .

Various types of polarization vortex conversion elements can be used. Specifically, for example, one using a photoelastic material, one using a birefringent medium such as a liquid crystal, one having a plurality of wedge-shaped polarizing plates arranged radially, and having an interval smaller than the wavelength of coherent light. It has a periodic structure with radial orientation. Here, a polarization vortex conversion element using a birefringent medium such as a photoelastic material or liquid crystal has linear birefringence, and has a plurality of A polarization vortex conversion element in which wedge-shaped polarizing plates are arranged radially has a linear dichroism (however, one component is completely attenuated), and a polarization vortex having a periodic structure with radial azimuth smaller than the wavelength of coherent light. The conversion element has linear birefringence and linear dichroism.

The first optical system may have any configuration as long as it can convert coherent light into circularly polarized light. To give a specific example, a polarizer that converts coherent light into linearly polarized light in one direction and the polarizer One with a quarter-wave plate in the latter stage and one with a circular dichroic material. The second optical system is not limited in its configuration as long as it can extract a circularly polarized component in the direction opposite to the circularly polarized light from the light emitted from the polarization vortex conversion element. For example, it has a quarter-wave plate on which light emitted from the polarization vortex conversion element is incident and an analyzer at the subsequent stage of the quarter-wave plate. .

The second invention is

A second optical system arranged so that light emitted from the polarization vortex conversion element is incident thereon, wherein a circular polarization component in a direction opposite to the circular polarization is obtained from the light emitted from the polarization vortex conversion element; A device for manipulating a micro object characterized by having an extractor.

In this micro object operating device, an optical vortex is generated by the same configuration as the optical vortex generator according to the first invention, and a micro object, for example, a micro Particles (such as atoms) can be trapped and rotated.

In the second invention, what has been described in relation to the first invention is valid.

The third invention is

A second optical system arranged so that light emitted from the polarization vortex conversion element is incident thereon, wherein a circular polarization component in a direction opposite to the circular polarization is obtained from the light emitted from the polarization vortex conversion element; An astronomical exploration device characterized by having an extractor.

In this astronomical exploration device, for example, the same configuration as the optical vortex generator according to the first invention, that is, the optical axes of the first optical system, the polarization vortex conversion element, and the second optical system are made to coincide with the stellar light. By using the property that the optical vortex has a dark line along the optical axis, strong star light can be extinguished, so weak planetary light can be detected and exoplanet exploration can be performed with high accuracy. Can be done. In addition to exploring extrasolar planets, this astronomical exploration device can also detect, for example, binary stars (multiple stars that are coupled by gravity and are orbiting) (called high-contrast imaging). Can be done.

In the third invention, what has been described in relation to the first invention is valid.

The fourth invention is: It has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the principal axis at each point on the same radius from the center of the coordinates, and the azimuth angle of the principal axis of each point is A polarization vortex conversion element proportional to the azimuth angle of coordinates,

A photoelastic material is used.

The fifth invention is:

It has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the principal axis at each point on the same radius from the center of the coordinates, and the azimuth angle of the principal axis of each point is A polarization vortex conversion element proportional to the azimuth angle of coordinates,

This is characterized in that a medium having birefringence is used.

The sixth invention is:

It has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the main axis at each point on the same radius from the coordinate center, and the azimuth angle of the main axis of each point Is a polarization vortex transducer that is proportional to the azimuth angle of the coordinates,

A plurality of wedge-shaped polarizing plates are arranged radially.

The seventh invention

It is characterized by having a periodic structure with a radial orientation at intervals smaller than the wavelength of coherent light.

In the fourth to seventh inventions, the first invention is provided unless it is contrary to its nature. What has been explained in connection with Ming holds. Brief Description of Drawings

1A and 1B are schematic diagrams for explaining optical vortices, FIG. 1 is a schematic diagram showing an example of optical vortices, and FIG. 3 is a schematic diagram of a conventional optical vortex generation method. FIG. 4 is a schematic diagram illustrating a double prism used in place of the cylindrical lens pair in the conventional optical vortex generator shown in FIG. 3, and FIG. 5 is a schematic diagram illustrating the example of 1. Fig. 6 is a schematic diagram for explaining a second example of the conventional optical vortex generation method. Fig. 6 shows the spiral thickness distribution used in the second example of the conventional optical vortex generation method. FIG. 7 is a schematic diagram showing an optical vortex generator according to the first embodiment of the present invention, and FIG. 8 is an optical vortex generation according to the first embodiment of the present invention. FIG. 9 is a schematic diagram showing a polarization vortex conversion element used in the apparatus. FIG. 9 is a diagram used in the optical vortex generator according to the first embodiment of the present invention. FIG. 10 is a schematic diagram showing a first example of a polarization vortex conversion element to be manufactured, and FIG. 10 is a diagram of a first example of a polarization vortex conversion element used in the optical vortex generator according to the first embodiment of the present invention. FIG. 11 and FIG. 12 are schematic diagrams for explaining a manufacturing method. FIG. 11 shows a second example of the polarization vortex conversion element used in the optical vortex generator according to the first embodiment of the present invention. FIGS. 13 and 14 are schematic diagrams illustrating a third example of a polarization vortex conversion element used in the optical vortex generator according to the first embodiment of the present invention. FIG. FIGS. 15 and 16 are schematic diagrams showing a fourth example of the polarization vortex conversion element used in the optical vortex generator according to the first embodiment of the present invention, and FIG. FIG. 4 is a schematic diagram for explaining the principle of a fourth example of the polarization vortex conversion element used in the optical vortex generator according to the first embodiment of the invention; FIG. 18 shows circularly polarized light incident on the polarization vortex conversion element in the optical vortex generator according to the first embodiment of the present invention. FIG. 19 is a schematic diagram for explaining the function of the polarization vortex conversion element used in the optical vortex generator according to the first embodiment of the present invention, and FIG. FIG. 2 is a schematic diagram for explaining the function of a polarization vortex conversion element used in the optical vortex generator according to the first embodiment of the present invention; FIG. 21 shows an optical vortex according to the first embodiment of the present invention; Fig. 22 is a schematic diagram for explaining an example in which the generator is applied to the operation of fine particles. Fig. 22 is a schematic diagram showing the experimental system used in Example 1 of the present invention. Fig. 24 is a schematic diagram showing the interference pattern between the optical vortex and the plane wave obtained in Example 1 of the present invention. Fig. 24 shows the interference between the optical vortex and the spherical wave obtained in Example 1 of the present invention. FIG. 25 is a schematic diagram showing the polarization vortex conversion element used in Example 1 of the present invention. FIG. 16 is a schematic diagram showing an experimental system used in Example 2 of the present invention. FIGS. 27A and 27B are optical vortices obtained in Example 2 of the present invention. Fig. 28A and Fig. 28B are schematic diagrams showing the interference pattern between the optical vortex and the spherical wave obtained in Example 1 of the present invention. FIG. 29 is a schematic diagram showing an optical vortex generator according to a second embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 7 shows an optical vortex generator according to the first embodiment of the present invention. As shown in FIG. 7, this optical vortex generator includes a coherent light source 1, an optical system 3 that converts coherent light 2 emitted from the coherent light source 1 into circularly polarized light, and a linear compound. Refractive and / or linear dichroism, polarization characteristics are constant at each point on the same radius from the center of the coordinate except for the main axis azimuth, and the main axis azimuth of each point is the coordinate Proportional to azimuth A polarization vortex conversion element 4 and an optical system 5 that extracts a circular polarization component in a direction opposite to the circular polarization emitted from the optical system 3 from the light emitted from the polarization vortex conversion element 4. A light vortex 6 is emitted from 5.

The coherent light source 1 includes a laser device 1 1 and a collimator 1 2 composed of two lenses 1 2 a and 1 2 b. The optical system 3 includes a polarizer 3 1 that converts coherent light 2 into linearly polarized light in one direction, and a quarter-wave plate 3 2 at the subsequent stage. The optical system 5 includes a 1/4 wavelength plate 5 1 and an analyzer 5 2 at the subsequent stage.

From the optical system 5, a circularly polarized component in the direction opposite to the circularly polarized light emitted from the optical system 3 is extracted, but this is to make it completely dark when the polarization vortex conversion element 4 is extracted. That means dark field.

Figure 8 shows the azimuth angle φ of the principal axis of each point and the azimuth angle S of the coordinate in the polarization vortex transducer 4. In order to return to the main axis after one round, it must be an integral multiple of Φ # (Θ / 2), ie ^ = η θ / 2 (where η is an integer other than 0).

Specifically, for example, the following is used as the polarization vortex conversion element 4. FIG. 9 shows the first example, which is a polarization vortex conversion element 4 made of a photoelastic material. As shown in FIG. 10, this polarization vortex conversion element 4 can be manufactured by preparing a disk-like photoelastic material 71 and applying uniform compression in the radial direction from the outside. . As the photoelastic material 71, conventionally known materials such as glass, epoxy resin, and optical crystal can be used.

In the second example, a polarization vortex conversion element 4 is artificially made using a birefringent medium such as a liquid crystal or a crystal, and the radial main axis is formed by combining the birefringent medium in a radiant form. Have a distribution. Specifically, the first A polarization vortex conversion element 4 shown in FIG. 1 is a nematic liquid crystal cell in which liquid crystal molecules 72 are aligned using a radially rubbed substrate. The polarization vortex conversion element 4 shown in FIG. 11 has an azimuth angle (change of ^ is one revolution of four, but may be of any number of revolutions (however, two redundant integer multiples), for example, FIG. As shown in Fig. 4, the change in azimuth may be 8 π.

In the third example, as shown in FIG. 13, a polarization vortex conversion element 4 having a radial orientation distribution is configured by arranging a plurality of wedge-shaped polarizing plates 73 in a radial pattern so as to form a circular shape as a whole. As shown in FIG. 14, a polarization vortex conversion element 4 having any number of laps (however, two redundant integer multiples) can be configured. The outer shape of the polarization vortex conversion element 4 is not limited to a circle, and may be, for example, a square.

In the fourth example, the polarization vortex conversion element 4 is configured by a sub-wavelength periodic structure having a radial direction with an interval smaller than the wavelength of the coherent light 2. For example, as shown in FIG. 15, by preparing a disk-shaped transparent substrate and irradiating it with an electron beam, etc., by forming grooves 4 1 at intervals smaller than the wavelength of the coherent light 2, A polarization vortex conversion element 4 having a principal axis (one of them) in the direction of the groove 41 can be produced. As shown in Fig. 16, a polarization vortex transducer 4 with any number of laps (however, two redundant integers) can be fabricated. The reason why the polarization vortex conversion element 4 can be obtained by these subwavelength periodic structures is as follows. That is, the subwavelength periodic structure can be considered as a diffraction grating, but as shown in Fig. 17, when light of wavelength; L is incident on the diffraction grating of period Λ, A sin 0 = m; L ( m is an integer). In order for this equation to hold, the condition Λ≥ 义 is necessary. In other words, if Λ; L, no diffracted light is emitted. At this time, the complex refractive index for the electric field of light differs between the direction parallel to the grating and the direction perpendicular to the grating. The This fact means that this diffraction grating, that is, the sub-wavelength periodic structure, works as a polarization vortex conversion element. As this sub-wavelength periodic structure, a ring-like crystal (see, for example, Japanese Patent Laid-Open No. 2000-0175 579) can also be used.

Next, the operating principle of this optical vortex generator will be described.

The Jones vector of the light emitted from the optical system 3 is the clockwise circularly polarized light.

The counterclockwise circular polarization is

It is expressed.

The function to extract clockwise circularly polarized light of optical system 5 is

To extract counterclockwise circularly polarized light.

It is expressed as multiplying from the left.

The Jones matrix at the point where the principal axis of the polarization vortex transducer 4 is ø is

R (-^)-J (r,) -R (^)

It is expressed. However, J is the Jones matrix of the polarization vortex transducer 4 when the coordinate axis is taken in the principal axis direction. Here, the polarization vortex conversion element 4 is J can depend on the radius (r in Fig. 8) and wavelength; L because it has the same polarization characteristics except for the principal axis direction on the same radius of the mark. However, R (Φ) does not depend on the radius or wavelength.

)) In this Jones matrix

J (r,)

), The polarization vortex conversion element 4 according to the first and second examples described above has linear birefringence,

One

Ten

0 e— ^(r) (5 (Γ, λ) is retardation), and in the polarization vortex conversion element 4 in the third example, one linear polarization component disappears due to linear dichroism.

The polarization vortex conversion element '4 according to the fourth example can have both linear birefringence and linear dichroism.

0 t _y {r,) e ⁱ³ ^.

Based on the above assumptions, the operation of this optical vortex generator is described by Jones calculation. As an example, consider the case where the circularly polarized light incident on the polarization vortex conversion element 4 is a clockwise circularly polarized light and the optical system 5 extracts a counterclockwise circularly polarized light component. The In this case, the electric field of the light emitted from the analyzer 5 2 of the optical system 5 is

()-^ ()]-, + (/% Λ)]}. (Γ) ^.β- '( ^{ί + 2 (ί} ).

If the azimuth angle of the main axis is (ηΖ2) times the azimuth angle 0 of the coordinate, that is, 0/2, that is, k ^,)-, ^Λ )] —,) + ^) — ' You can see that it is a light vortex. In order for the underlined part of this expression not to be 0,. ≠ or just _Jvx ≠ is necessary. This conditional expression is valid if the polarization vortex conversion element 4 has at least one of linear birefringence and linear dichroism.

In addition, the handling of the phase in the course of the above Jones calculation is shown

E _iC ^† R (-^). J (r, λ) R (Me- _RC \ a ₀ (r) e

become that way. From the double underlined part and the single underlined part of this formula, depending on the principal axis direction at the time of incidence and at the time of emission, respectively. It can be seen that a phase is given (by coordinate rotation).

Next, the operating principle of this optical vortex generator will be described qualitatively.

As an example, let us consider a case where the circularly polarized light incident on the polarization vortex conversion element 4 is clockwise circularly polarized light and the optical system 5 extracts a counterclockwise circularly polarized light component.

Figure 18 shows right-handed circularly polarized light incident on the polarization vortex transducer 4. The X component of this clockwise circularly polarized electric field is expressed as a c 0 s (ω t), and the y component is expressed as a c o s (ω t + 7/2). Where a is amplitude, ω is angular frequency, and t is time. The y component has a phase advance of / 2 relative to the X component. If the coordinates are (^ rotated for this clockwise circularly polarized light, the phase shifts. For the counterclockwise circularly polarized light, the phase shifts.

Now, as shown in Fig. 19, we consider two points P ₂ on the polarization vortex conversion element 4 on the same radius, and compare the phases of the counterclockwise circularly polarized components after passing through these two points. Is a point on the horizontal axis, azimuth angle Θ 0, P ₂ is azimuth angle S 0). The azimuth angle of the main axis of polarization is = n 0/2. 'At the point of incidence on the polarization vortex transducer 4, the points P i and P ₂ are the same clockwise circularly polarized light. However, comparing the phase of the light in each principal axis direction, it can be seen that the point P ₂ is more advanced than the point P i than the above fact. When this light passes through the polarization vortex conversion element 4, the light transmitted through the points P i and P ₂ becomes elliptically polarized light having the same ellipticity and different major axis directions by ζ. Again, when comparing the phase of the respective main axis direction, towards the point [rho ₂ is advanced + against the point.

Now, the light emitted from the polarization vortex conversion element 4 can be viewed as a superposition of a clockwise circularly polarized component and a counterclockwise circularly polarized component. Among them, towards the right-handed circularly polarized light component, the point at P ₂ have the same initial phase. This is because the phase shift in the principal axis direction between the point P ₂ and the point described above is canceled by the coordinate rotation when converting to clockwise circularly polarized light. on the other hand, Towards the left-handed circularly polarized light component, because the reverse phase stick during coordinate rotation, the result towards the point P ₂ is the sub-routine proceeds + 2 with respect to the point P i as. This is the reason why the phase of the counterclockwise circularly polarized component of the light emitted from the polarization vortex conversion element 4 is an optical vortex.

As is apparent from the above, the phase of the optical vortex generated by this optical vortex generator does not depend on the wavelength of the coherent light 2 in principle. That is, according to this optical vortex generator, an agromatic optical vortex can be generated. Actually, there may be some wavelength dependence due to the dispersion of the quarter-wave plates 3 2 and 5 1, etc., but at least the wavelength dependence is significantly higher than that of conventional optical vortex generators. In addition, the wavelength dependence can be further reduced by using an achromatic material such as the quarter-wave plate 3 2 or 5 1. Note that the light intensity of the optical vortex itself is affected by the dispersion of the polarization vortex conversion element 4. However, if the polarization vortex conversion element 4 is not dependent on the wavelength; L, in other words, if the matrix J is independent of the wavelength, an achromatic optical vortex including the light intensity can be created. As an example of such a polarization vortex conversion element 4, there can be mentioned one in which a plurality of wedge-shaped polarizing plates 73 as shown in FIG. 13 are arranged radially. The polarization vortex conversion element 4 has an azimuth angle (^ direction is continuous everywhere except for a plurality of wedge-shaped polarizing plates 7 3 arranged in a radial pattern. It is also advantageous in that there is no phase discontinuity line and no noise is generated due to this, and even if there is a phase discontinuity line, the phase step at this discontinuity line Since the wavelength dependence of the phase difference is extremely small, it is possible to minimize the phase step at the discontinuous line, especially in applications that use wavelength scanning and broadband light, and the noise caused by this phase step is sufficiently low. It can be suppressed to. This optical vortex generator can generate a good optical vortex even when, for example, a laser device 11 having a wide band (such as a Super Continuum light source) is used.

This optical vortex generator can be used, for example, as a minute object manipulation device (manipulator). That is, as shown in Fig. 21, the micro-particles 74 are optically trapped by the optical vortex 6 generated by this optical vortex generator (optical tweezers), and the orbital angular momentum ( Torque) can be transferred to the microparticles 74 and rotated.

An embodiment of this optical vortex generator will be described.

Example 1

As the polarization vortex conversion element 4, the first example made of a photoelastic material was used. Prepare a disk-shaped photoelastic material 7 1 as a disk-shaped epoxy resin with a diameter of 48 mm and a thickness of 3 mm, and support it at four points 90 ° apart from each other. A polarization vortex conversion element 4 as shown in Fig. 9 was produced by compressing in the radial direction.

In order to confirm that the optical vortex was generated by the optical vortex generator using the polarization vortex conversion element 4 thus fabricated, an interference experiment between the optical vortex and the plane wave and spherical wave was performed. Figure 12 shows the experimental system. As shown in Fig. 22, a He-Ne laser is used as the laser device 1 1 and is generated by this He-Ne laser; L = 6 3 2.8 nm (red) laser light As a coherent light 2, it passes through a collimator 1 2 consisting of two lenses 1 2 a and 1 2 b to form a parallel light beam, which is incident on a beam splitter 8 1 and divided into two. One of the coherent lights 2 divided by the beam splitter 8 1 is reflected by the reflecting mirror 82 and sequentially passed through the polarizer 3 1 and the quarter-wave plate 3 2 of the optical system 3 as described above. The light is incident on the produced polarization vortex conversion element 4 and emitted from the polarization vortex conversion element 4. The coherent light 2 is sequentially passed through the quarter-wave plate 5 1 and the analyzer 5 2 of the optical system 5 and then extracted through the beam splitter 8 3. On the other hand, the other coherent light 2 divided into two by the beam split 8 1 is sequentially reflected by the reflecting mirror 84 4 and the beam splitter 8 3 and taken out. In this way, two coherent lights 2 that have been extracted from the Beam Split 83 and passed through different paths were made incident on a CCD (Charge Coupled Device), and interference fringes were observed. The experiment system shown in Fig. 22 is used when conducting an interference experiment between an optical vortex and a plane wave. However, when conducting an interference experiment between an optical vortex and a spherical wave, a beam splitter 8 1 and a reflecting mirror are used. Insert convex lens 8 5 between 8 4 and 8 4.

Figure 23 shows an image of the interference pattern between the optical vortex and the plane wave obtained by the above experimental system. As can be seen from Fig. 23, an inverted Y-shaped fringe characteristic of the interference fringe between the optical vortex and the plane wave was observed near the center of the image. Figure 24 shows an image of the interference between the optical vortex and the spherical wave obtained by the above experimental system. As can be seen from Fig. 24, a vortex interference fringe, which is a feature of the interference fringe between the optical vortex and the spherical wave, was observed near the center of the image.

Example 2

As the polarization vortex conversion element 4, as shown in Fig. 25, in the third example, eight right-angled isosceles triangular wedge-shaped polarizing plates 73 are arranged radially to form a square as a whole. Was used. Each wedge-shaped polarizing plate 73 has a bottom length of 15 mm and a thickness of about 0.3 mm, and was formed by cutting a sheet polarizer.

In order to confirm that the optical vortex was generated by the optical vortex generator using the polarization vortex conversion element 4 thus fabricated, an interference experiment between the optical vortex and a plane wave and a spherical wave was conducted. Figure 26 shows the experimental system. As shown in Fig. 26, a He-Ne laser was used as the laser device 1 1 and this He-N e Laser light generated by the laser is made coherent light 2 and incident on the beam splitter 8 1 to be split into two. One beam of light 2 divided into two by this beam split 8 1 is reflected by a reflecting mirror 8 2 and is sequentially passed through the polarizer 3 1 and the quarter wave plate 3 2 of the optical system 3. The coherent light 2 incident on the polarization vortex conversion element 4 manufactured as described above and emitted from the polarization vortex conversion element 4 is applied to the quarter wave plate 5 1 and the analyzer 5 2 of the optical system 5. After passing sequentially and further passing through the spatial frequency filtering device 86, the convex lens 87 and the beam splitter 83 are sequentially passed through and taken out. The spatial frequency filtering device 8 6 includes a convex lens 8 6 a and a pinhole 8 6 b. On the other hand, the other coherent light 2 divided into two by the beam splitter 8 1 is sequentially reflected by the reflecting mirror 8 4 and the beam splitter 8 3 and extracted. In this way, two coherent lights 2 that have been extracted from the beam splitter 83 and passed through different paths were incident on the CCD and the interference fringes were observed. The experiment system shown in Fig. 26 is used when conducting an interference experiment between an optical vortex and a spherical wave. However, when performing an interference experiment between an optical vortex and a plane wave, the beam splitter 8 1 and the reflector 8 4 Insert a convex lens 8 8 between them. There are two types of laser light generated by the He-N Θ laser: a laser beam with a = 6 32.8 nm (red) and a laser beam with L = 5 43.5 nm (green). Using.

Figure 27 A and Figure 27 B show the following: when L = 6 3 2.8 nm red laser light and I = 5 4 3.5 nm green laser light are used, respectively. An image of the interference pattern between the optical vortex and the plane wave obtained by the above experimental system is shown. As can be seen from Fig. 27A and Fig. 27B, as the interference fringes are traced, a place where the number of fringes changes near the center of the image can be seen. This is the interference fringe between the optical vortex and the plane wave, the red wavelength and the green wavelength It was observable in the rainy day.

Figures 28A and 18B show the following: red light with I = 6 3 2.8 nm and green light with green = 5 4 3.5 nm Figure 6 shows an image of the interference pattern between the optical vortex and the spherical wave obtained by the above experimental system. As can be seen from Fig. 28A and Fig. 28B, vortex interference fringes are visible near the center of the image. This is an interference fringe between an optical vortex and a spherical wave, and was observed at both red and green wavelengths.

In this experiment, as can be seen from Fig. 27 A and Fig. 27 B and Fig. 28 A and Fig. 28 B, the results of red laser light and green laser light are similar. For example, the fact that the number of interference fringes in Fig. 27A and Fig. 27B is the same is related to the generation of optical vortices independent of wavelength in this method. The difference in the number of interference fringes between the red laser beam and the green laser beam is not due to the nature of the optical vortex itself, but, for example, due to imperfect adjustment of the convex lens 88.

FIG. 29 shows an optical vortex generator according to a second embodiment of the present invention. As shown in FIG. 29, this optical vortex generator has the same configuration as the optical vortex generator according to the first embodiment, except that it does not have a coherent light source 1. In this case, the coherent light 2 enters the optical system 3 from the outside of the optical vortex generator.

This optical vortex generator can be used, for example, for astronomical exploration, specifically for exploring extrasolar planets. In other words, powerful star light can be extinguished by aligning it with the optical axis of this optical vortex generator, so it is possible to detect faint planetary light that deviates from the optical axis with high accuracy and to perform planetary exploration with high accuracy. Can be done.

The embodiment of the present invention has been specifically described above, but the present invention is not limited to the above-described embodiment, and the technical idea of the present invention. Various modifications based on the idea are possible. '

For example, the numerical values, configurations, arrangements, materials, and the like given in the above-described embodiments are merely examples, and different numerical values, configurations, arrangements, materials, and the like may be used as necessary.

As described above, the polarization vortex conversion element has linear birefringence and / or linear dichroism, and the polarization characteristic is the azimuth angle of the principal axis at each point on the same radius from the center of the coordinates. The azimuth angle of the principal axis of each point above it is proportional to the azimuth angle of the coordinates, while the phase shifter is an element having linear birefringence, and the polarizer is a straight line Extremely dichroic, that is, an element that almost completely eliminates one polarization component, but the phase shifter and polarizer itself do not use linear birefringence or linear dichroism. Since it can be created by utilizing the characteristics, the condition of having linear birefringence and / or linear dichroism is that the amplitude ratio between two linearly polarized light components orthogonal to the transmitted light and / Or the condition of having the property of changing the phase difference It is also possible to extend. As described above, according to the present invention, there is no achromatic discontinuity line regardless of the wavelength of the light source, or even if it exists, the wavelength dependence of the phase step at the discontinuous line exists. Therefore, it is possible to easily generate an optical vortex that can minimize the phase difference in the discontinuous line, particularly in applications using wavelength scanning or broadband light. By using this optical vortex generation principle, it is possible to realize a high-performance micro-object manipulation device or an astronomical exploration device such as an extrasolar planet exploration device.

Claims

1. a first optical system that converts coherent light into circularly polarized light;

A polarization vortex conversion element arranged so that circularly polarized light emitted from the first optical system is incident, and has linear birefringence and / or linear dichroism, and the polarization characteristic is the center of coordinates. Each point on the same radius is a fixed number except for the azimuth angle of the main axis, and the azimuth angle of the main axis of each point above it is proportional to the azimuth angle of the coordinates.

A second optical system arranged so that light emitted from the polarization vortex conversion element is incident thereon, wherein the circular polarization component has a direction opposite to the circular polarization from the light emitted from the polarization vortex conversion element. An optical vortex generator characterized by having an enclosure for extracting the light.

2. When the azimuth angle of the main axis of the polarization vortex transducer is φ and the azimuth angle of the coordinates is S, φ = η Θ / 2 (where n is an integer other than 0) The optical vortex generator according to claim 1.

3. The optical vortex generator according to claim 1, further comprising a light source that generates the coherent light.

4. The optical vortex generator according to claim 1, wherein the polarization vortex conversion element is manufactured using a photoelastic material.

5. The optical vortex generator according to claim 1, wherein the polarization vortex conversion element is manufactured using a birefringent medium.

6. The optical vortex generator according to claim 1, wherein the polarization vortex conversion element comprises a plurality of wedge-shaped polarizing plates arranged radially.

7. The optical vortex generator according to claim 1, wherein the polarization vortex conversion element has a periodic structure with a radial orientation at intervals smaller than the wavelength of the coherent light.

8. The first optical system includes: a polarizer that converts the coherent light into linearly polarized light in one direction; and a quarter-wave plate at the subsequent stage of the polarizer. The optical vortex generator according to item 1.

9. The optical vortex generator according to claim 1, wherein the first optical system is made of a material having circular dichroism.

10. The second optical system is characterized in that it has a quarter-wave plate on which light emitted from the polarization vortex conversion element is incident and an analyzer subsequent to the quarter-wave plate. The optical vortex generator according to claim 1.

1 1. A first optical system that converts coherent light into circularly polarized light;

1 2. A first optical system for converting coherent light into circularly polarized light;

A second optical system arranged so that light emitted from the polarization vortex conversion element is incident on the second optical system; A celestial body exploration device characterized by having a device for extracting circularly polarized light components in the direction opposite to the circularly polarized light.

1 3. It has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the principal axis at each point on the same radius from the center of the coordinate, and the principal axis of each point. Is a polarization vortex transformation whose azimuth is proportional to the azimuth of coordinates,

A polarization vortex conversion element characterized by using a photoelastic material.

1 4. It has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the principal axis at each point on the same radius from the center of the coordinate, and the principal axis of each point. Is a polarization vortex transducer whose azimuth is proportional to the azimuth of coordinates,

A polarization vortex conversion element using a birefringent medium.

1 5. It has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the principal axis at each point on the same radius from the center of the coordinate, and the principal axis of each point. Is a polarization vortex transducer whose azimuth is proportional to the azimuth of coordinates,

A polarization vortex conversion element comprising a plurality of wedge-shaped polarizing plates arranged radially.

1 6. It has linear birefringence and / or linear dichroism, and its polarization characteristic is constant except for the azimuth angle of the principal axis at each point on the same radius from the center of the coordinate, and the principal axis of each point. Is a polarization vortex transducer whose azimuth is proportional to the azimuth of coordinates,

A polarization vortex conversion element characterized by having a periodic structure with a radial orientation at intervals smaller than the wavelength of coherent light.