JP2018138985A - Optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element that has circular dichroism with a relatively simple configuration.SOLUTION: An optical element 10 has a base material 13 and structures 14. The base material 13 is a plate-like transparent dielectric substance. The structures 14 are two-dimensionally arranged on the base material 14. The structures 14 have a non-twice rotationally symmetric shape on a plane of the two-dimensional arrangement. The structures 14 have a shape having non-reflection symmetry in a direction perpendicular to a first direction on the plane of the two-dimensional arrangement. The structures 14 are made of metal. A light beam is made incident on the optical element 10 from a second direction inclined with respect to the first direction as an axis.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical element.

  In recent years, optical elements having circular dichroism having different transmittance and absorptivity for clockwise and counterclockwise circularly polarized light have been developed. For example, a circular dichroic element having a laminated structure of a 1 / wavelength plate and an anisotropic transmission plate and having a high degree of circular dichroism has been proposed (see Patent Document 1).

JP 2012-123327 A

  In the circular dichroic element proposed in Patent Document 1, the size and arrangement of the negative dielectric constant member and the electromagnetic wave transmission member forming the quarter wavelength plate and the anisotropic transmission plate are complicated, and with high accuracy. It was difficult to manufacture.

  Accordingly, an object of the present disclosure made in view of the above-described problems of the prior art is to provide an optical element having circular dichroism with a relatively simple configuration.

In order to solve the problems described above, the optical element according to the first aspect is
A substrate that is a flat transparent dielectric,
A shape two-dimensionally arranged on the substrate and having a non-rotational symmetry in the plane of the two-dimensional array and a non-mirror symmetry in a direction perpendicular to the first direction on the plane of the two-dimensional array A metal structure, and
Light rays are incident from a second direction inclined about the first direction.

  The optical element according to the present disclosure configured as described above can have circular dichroism with a relatively simple configuration.

It is a perspective view of a light source unit containing an optical element concerning one embodiment of this indication. It is an expanded sectional view along the II-II line of FIG. It is a partial top view of an optical element which shows the shape and arrangement | positioning of the structure of FIG. It is a graph which shows the difference of the Joule loss of right circularly polarized light and left circularly polarized light with respect to the frequency of the light incident on the optical element of FIG. It is a fragmentary sectional view of the optical element designed in the Example. It is a fragmentary top view which shows the dimension of the structure designed in the Example. It is a graph which shows the transmittance | permeability of right circularly polarized light and left circularly polarized light with respect to the wavelength of the light ray which injects into the optical element of an Example.

  Embodiments of an optical element to which the present invention is applied will be described below with reference to the drawings.

  As illustrated in FIG. 1, a light source unit 11 including an optical element 10 according to an embodiment of the present disclosure includes an optical element 10 and a light source 12.

  As shown in FIG. 2, the optical element 10 includes a base material 13 and a structure body 14.

  The base material 13 is formed in a flat plate shape by a member that is transparent to visible light and has dielectric properties. The member applied to the base material 13 is, for example, a dielectric whose imaginary part of the complex refractive index in the entire visible light region is substantially zero, such as an acrylic resin and a polycarbonate resin that are transparent to visible light. It may be a transparent resin and a glass material such as silicon dioxide. The imaginary part of the complex refractive index is an extinction coefficient representing light absorption. If the value of the imaginary part of the complex refractive index is substantially zero, even if it is not exactly zero, the base material 13 hardly absorbs light, and if it is recognized that the substrate 13 has transparency according to its use, it is not. It means that zero extinction coefficient is allowed.

  The thickness of the substrate 13 can be arbitrarily selected as long as the incident light has a sufficient thickness that does not cause interference due to reflection by the boundary surface. For example, if the thickness of the base material 13 is about several hundred μm, it is easy to create.

  As shown in FIG. 1, the plurality of structures 14 are two-dimensionally arranged on the base material 13. As will be described later, the size of the structure body 14 is very small and it is difficult for the naked eye to visually recognize the structure body. However, in FIG.

  The plurality of structures 14 are, for example, two-dimensionally arranged on the plate surface of the base material 13 or embedded in the base material 13. The plurality of structures 14 may be two-dimensionally arranged in a plane parallel or inclined with respect to the plate surface in the configuration embedded in the base material 13.

  In the configuration in which the plurality of structures 14 are embedded in the base material 13, the depth at which the structure 14 is embedded from the light incident side is sufficient to prevent the incident light from causing interference due to reflection by the boundary surface. Any thickness can be selected. For example, if the thickness of the base material 13 is about several hundred μm, it is easy to create.

As shown in FIG. 3, the structures 14 are arranged at predetermined intervals d in two different directions on a two-dimensionally arranged plane, for example. The predetermined distance d satisfies Equation (1), where λ is the wavelength of the light beam emitted from the light source 12, N is the refractive index of the base material 13, and θ _in is the incident angle of the light beam on the surface of the optical element 10. It is preferable. In the present embodiment, the plurality of structures 14 are arranged at predetermined intervals d in the two different directions, but may be arranged at different predetermined intervals in every two directions.

  The structure 14 is formed of a conductor whose plasma frequency is shorter than the wavelength λ of the light emitted from the light source 12, for example, a metal such as gold, silver, copper, or aluminum. The thickness of the structure 14 is preferably 10 nm or more and less than 100 nm.

  The size of the structure 14 is smaller than the wavelength of visible light. The dimension of the structure 14 is the length in the direction in which the length in the two-dimensionally arranged plane is the longest. Moreover, being smaller than the wavelength of visible light means smaller than the wavelength λ of the light emitted from the light source 12 and further smaller than about 360 nm which is the lower limit of the visible light band.

  The structure 14 has non-two-fold rotational symmetry in a two-dimensionally arranged plane. In other words, as shown in FIG. 1, in the structure 14, the shape rotated by 180 ° with respect to the perpendicular pl of the two-dimensionally arranged plane does not match the original shape. The structure 14 has non-mirror symmetry in a direction perpendicular to the first direction, which is any direction on a two-dimensionally arranged plane. In other words, the structure 14 does not have an axis that is line-symmetric and perpendicular to the first direction in a two-dimensionally arranged plane. Examples of the shape of the structure 14 include P1, Pm, Pg, Cm, P3, P3m1, and P31m having non-two-fold rotational symmetry in 17 types of two-dimensional space groups.

  In addition, as shown in FIG. 3, the structure 14 is preferably formed with a notch 15 on the first direction side from the center of gravity cg of the two-dimensionally arranged plane. Moreover, as shown in FIG. 1, it is preferable that the structure 14 has mirror symmetry with respect to a plane p1 perpendicular to the two-dimensionally arranged plane and parallel to the first direction. The shape of the structure 14 is mirror-symmetric with respect to a plane p1 that is perpendicular to the plane arranged two-dimensionally and parallel to the first direction, among the 17 types of two-dimensional space groups listed. Pm, Cm, P3m1, and P31m are preferable. Moreover, it is more preferable that the shape of the structure 14 is Pm and Cm that can be manufactured only with a line that is orthogonal from the viewpoint of manufacturing.

  As shown in FIG. 3, for example, the structure 14 has a rectangular tubular shape, in other words, a substantially annular shape, with one side cut out, in a two-dimensionally arranged plane. The direction from the center of gravity cg of the structure 14 toward the notch is determined to be the first direction.

  The structure 14 can be formed on the base material 13 by using general EB (Electron Beam) lithography. EB lithography is a process using vapor deposition, sputtering, etching, and the like.

  As shown in FIG. 1, the light source 12 emits a beam of light toward the optical element 10. The light source 12 is arranged such that light rays are incident on a plate surface of the optical element 10 from a second direction inclined with the first direction as an axis.

  In the optical element 10 of the present embodiment configured as described above, the structure 14 having a shape having non-rotational symmetry and non-mirror symmetry in a direction perpendicular to the first direction is the base material 13. Are incident in a second direction inclined about the first direction. By having the above-described configuration, the optical element 10 has circular dichroism as described below.

  In the optical element 10 of the present embodiment, the structure 14 has a non-rotational symmetry on a two-dimensionally arranged plane, and thus does not have an inversion center. The relationship between the two-fold rotational symmetry and the inversion center will be described below.

  A coordinate axis orthogonal to a two-dimensional plane on which a general structure is arranged is an x-axis and a y-axis, the plane is the origin, and a coordinate axis perpendicular to the plane is a z-axis. (2) holds for the structure.

  Note that f (x, y, z) is a function representing the trajectory of the structure. For example, 1 is returned when the structure exists at the coordinates, and 0 is returned when the structure does not exist.

  When a structure satisfying equation (2) is rotated at an angle θ about the x axis as a rotation axis, equation (2) is transformed into equation (3).

  Equation (3) indicates that the structure has inversion symmetry, and a shape having two-fold rotational symmetry is synonymous with having an inversion center. Accordingly, a shape having non-two-fold rotational symmetry does not have inversion symmetry.

  Further, in the optical element 10 of the present embodiment, since light rays are incident from the second direction, the structure 14 does not have a mirror surface perpendicular to the incident light.

  In the optical element 10 of the present embodiment, the structure 14 has a non-mirror symmetry in a direction perpendicular to the first direction, and thus the structure 14 does not have a mirror surface parallel to the incident light. The relationship between the non-mirror symmetry in the direction perpendicular to the first direction and the mirror plane parallel to the incident light will be described.

  In the above xyz coordinate system, when having a mirror symmetry with the first direction parallel to the x-axis and the yz plane perpendicular to the first direction as the mirror plane, (4) The formula holds.

  When a structure satisfying equation (4) is rotated at an angle θ about the x axis as a rotation axis, equation (4) is transformed into equation (5).

  Equation (5) indicates that the structure has the yz plane as a mirror surface. Since the incident light is incident from a second direction inclined with respect to the first direction as an axis, the second direction is parallel to the yz plane. Therefore, having the yz plane as a mirror surface of the structure is synonymous with having a mirror surface parallel to the incident light. Therefore, a shape having non-mirror symmetry in a direction perpendicular to the first direction does not have a mirror surface parallel to the incident light.

  As a condition of a structure for expressing circular dichroism, in general, from the viewpoint of symmetry, the structure does not have an n-th reflection axis, or has no inversion center, and is perpendicular or parallel to incident light. It is known that there is no special mirror surface. As described above, the optical element 10 of the present embodiment has a circular dichroism because there is no inversion center and there is no mirror surface perpendicular or parallel to the incident light.

  In the optical element 10 according to the present embodiment, the structure 14 has an annular shape in which a part on the first direction side is cut away from the center of gravity cg. By having such a configuration, as will be described below, the optical element 10 can further improve the circular dichroism with respect to light of a specific wavelength by the structure 14 functioning as a resonator.

  When light is incident on the metal structure from an oblique direction, an electromagnetic wave component exists in the light in a direction parallel to and perpendicular to a two-dimensionally arranged plane. When considering the response of a minute metal structure to electromagnetic waves, the metal structure can be regarded as a circuit having an inductance L, a capacitance C, and a resistance R determined by the shape. Since the metal structure behaves as an LC resonance circuit, the behavior of the metal structure can be represented by the circuit equation (6).

  In the equation (6), V is an electromotive force given to the circuit from the outside, and is expressed by the equation (7).

In Equation (7), + is left circularly polarized light,-is right circularly polarized light, E ₁ is an electric field component parallel to a plane in which the metal structure is two-dimensionally arranged, and B ₂ is two-dimensionally arranged in the metal structure. The magnetic field component perpendicular to the plane, S is the area where the voltage is induced by the magnetic field change.

  The plane wave solution of equation (6) is equation (8).

である。 However, in equation (8)

It is.

  When the current component is calculated by differentiating the equation (8), the electromagnetic wave component is expressed by the equation (9).

Therefore, loss IR ² is expressed by the equation (10).

(10) from the equation, B ₂ is increased by incident obliquely to the plane of arrangement of light two-dimensionally. As the component increases, the loss increases with respect to the left circularly polarized component, and the loss decreases with respect to the right circularly polarized component. The loss corresponds to the light absorptance, and when the structure 14 functions as a resonator, the transmittance is small for the left circularly polarized light and large for the right circularly polarized light.

When the relationship of the difference in Joule loss due to left and right circularly polarized light is obtained as the frequency of light incident on the horizontal axis based on the equation (10), the current denominator is small in the vicinity of the resonance frequency ω ₀ as shown in FIG. Therefore, the loss difference relatively increases. The loss difference becomes maximum at the resonance frequency ω ₀ , and the loss difference becomes smaller as the distance from the resonance frequency ω ₀ is increased. Therefore, the optical element 10 can increase the difference in loss by entering electromagnetic waves (light) having a frequency near the resonance frequency, and as a result, can further improve the circular dichroism.

  In the optical element 10 of the present embodiment, the structure 14 has a shape having mirror symmetry with respect to the plane p1 parallel to the first direction. With such a configuration, as will be described below, the light transmittance of right circularly polarized light and left circularly polarized light is the same regardless of the direction of the light incident from the direction of the plate surface of the optical element 10.

  As described above, in the above-described xyz coordinate system, when the first body is parallel to the x axis and has a mirror symmetry with the xz plane parallel to the first direction as a mirror surface, the structure has On the other hand, Expression (11) is established.

  When a structure satisfying equation (11) is rotated at an angle θ about the x axis as a rotation axis, equation (11) is transformed into equation (12).

  Expression (12) indicates that the system does not change even if coordinate conversion of (x, y, z) to (x, -y, -z) is performed. Since the system is not changed by this coordinate conversion, the transmittance of each incident light from both sides of the flat plate with respect to each circularly polarized light is equal. Therefore, the light transmittance of the right circularly polarized light and the left circularly polarized light is equal even if the light beam is incident from any direction of the plate surface of the optical element 10.

  In the optical element 10, a shape having mirror symmetry with respect to the plane p <b> 1 parallel to the first direction is formed in the structure 14, so that one surface and the other surface of the flat plate of the optical element 10 are formed. It is also possible to provide a difference in the transmittance of each of the right circularly polarized light and the left circularly polarized light when light is incident.

Moreover, the optical element 10 of this embodiment has a substantially annular shape. The loss at the resonance frequency ω ₀ is expressed by equation (13), and the difference between the loss of right circularly polarized light and left circularly polarized light is expressed by equation (14).

  As can be seen from the equation (14), in order to increase the loss difference, it is preferable that the circuit area S is large. The annular shape maximizes the area within the same circumference. Therefore, since the optical element 10 of the present embodiment has a substantially annular shape, the circular dichroism can be further improved.

  In the optical element 10 of the present embodiment, the size of the structure 14 is smaller than the wavelength of visible light. With such a configuration, the optical element 10 can reduce the occurrence of diffraction of transmitted light.

  In the optical element 10 of the present embodiment, the structure 14 is formed of a conductor whose plasma frequency is shorter than the wavelength λ of the light beam emitted from the light source 12. With such a configuration, an absorption action of one-side circularly polarized light by plasmon resonance can be generated, and the structure 14 can function as a resonator as described above.

  Moreover, in the optical element 10 of this embodiment, the thickness of the structure 14 is 10 nm or more. If the thickness of the structure 14 is 10 nm or more, the characteristics as a metal can be exhibited, and the optical element 10 can exhibit the circular dichroism described above. Moreover, since the thickness of the structure 14 is less than 100 nm, the thickness of the structure 14 can be suppressed to be less than the width of the structure 14, and the optical element 10 can be easily manufactured.

  EXAMPLES Hereinafter, although an Example is given and embodiment of this invention is described, this invention is not limited to these Examples.

  As shown in FIG. 5, an optical element 10 is designed in which a plurality of aluminum structures 14 are embedded in a flat PMMA base material 13 so as to be two-dimensionally arranged in a plane parallel to the plane of the flat plate. did.

  As shown in FIG. 6, the structure 14 in the design was formed in an annular shape in which one side of a rectangle was cut out on a two-dimensionally arranged plane. The long side of the rectangle was 350 nm. The short side of the rectangle was 320 nm. The width of each side of the rectangle was 100 nm. As shown in FIG. 5, the height of the structure 14 with respect to the two-dimensionally arranged plane was 65 nm.

  In the design, the plurality of structures 14 are 815 nm in a first direction which is the direction of the notch 15 from the center of gravity cg of the rectangle in the plane of the two-dimensional array and a direction perpendicular to the first direction on the plane. Arranged with a period of.

  The light source 12 was set to be disposed with respect to the optical element 10 so as to emit light from a direction inclined by 4 ° with respect to a direction perpendicular to the plate surface of the optical element 10. The transmission characteristics of the optical element 10 were simulated for the above assumed conditions. For the simulation, Synopsis Inc. The option of DiffractMOD using the RCWA method was applied using the simulation software “Rsoft (registered trademark)”. The result of the simulation is shown in FIG. One direction of right circularly polarized light and left circularly polarized light was represented by 90 °, and the other direction was represented by 270 °.

  As shown in FIG. 7, the circularly polarized light oriented at 270 ° has a transmittance of about 90% with respect to light having a wavelength of 1310 nm. Further, in the case of circularly polarized light oriented in the 90 ° direction, the transmittance of about 10% is obtained for light having a wavelength of 1310 nm. Therefore, in the assumed optical element 10, the light is separated to about −10 dB. In order to obtain higher transmittance, for example, by overlapping the two optical elements 10 described above, a transmittance of about 80% with respect to light having a wavelength of 1310 nm with circularly polarized light of 270 ° and the reverse direction It is also possible to obtain a transmittance of about 1% with respect to light having a wavelength of 1310 nm with circularly polarized light of 90 °.

  Although the present invention has been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various variations and modifications based on the present disclosure. Therefore, it should be noted that these variations and modifications are included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Optical element 11 Light source unit 12 Light source 13 Base material 14 Structure 15 Notch cg Center of gravity pl Perpendicular p1 A plane perpendicular to the two-dimensionally arranged plane and parallel to the first direction

Claims (3)

A substrate that is a flat transparent dielectric,
A shape two-dimensionally arranged on the substrate and having a non-rotational symmetry in the plane of the two-dimensional array and a non-mirror symmetry in a direction perpendicular to the first direction on the plane of the two-dimensional array A metal structure, and
An optical element that receives light from a second direction inclined with respect to the first direction. The optical element according to claim 1,
The structure has an annular shape in which a part on the first direction side is cut away from a center of gravity. The optical element according to claim 1 or 2,
The structure has a shape having mirror symmetry with respect to a plane parallel to the first direction.

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