Optical antenna, spectroscopic analyzer for chiral molecule, light radiation device, and design method for optical antenna

Abstract

To provide an optical antenna capable of improving accuracy of chiral molecule identification.SOLUTION: An optical antenna includes: a plurality of structures respectively including a continuous hole for irradiating or emitting light in a predetermined polarization state, surrounded by a twisted structure; and a nano gap disposed between the plurality of structures and communicating with a continuous hole.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical antenna, a spectroscopic analyzer for chiral molecules, an optical radiation device, and a method for designing an optical antenna.

Background Art

[0002] Previously, it has been known that many molecules have chirality, and molecules with different chirality (enantiomers) can exhibit different effects on biological activities in vivo. For example, in the pharmaceutical field, enantiomers may exhibit different metabolic reactions in vivo, interactions with different receptors, and different toxicities. Also, when using chiral reactants or catalysts in chemical reactions, enantiomers can result in differences in reaction rate and yield. From these facts, it is known that identifying chirality is important not only in the fields of pharmaceutical development and organic synthesis described above, but also in fields such as the design of optical sensors.

[0003] As a method for identifying chirality, circular dichroism spectroscopy is widely used. In this method, a target molecule is irradiated with right-circularly polarized light and left-circularly polarized light, and the difference in absorbance (CD) of the molecule with respect to the direction of each circularly polarized light is measured. However, since the size of the molecule is extremely small compared to the wavelength of light, it has been difficult to detect a signal related to CD (hereinafter referred to as a CD signal), and it has been difficult to identify chirality.

[0004] By the way, in recent years, a physical quantity called optical chirality related to an electromagnetic field has been found to be related to the magnitude of a CD signal, and has attracted attention as an index for the discrimination accuracy of chiral molecules (Non-Patent Document 1). In Non-Patent Document 1, it is stated that the larger the value of the optical chirality represented by C = ωIm[E * ·B], the more enhanced the CD signal is (in the above formula, ω is the angular frequency, E and B are the electric field and magnetic field, respectively, and Im represents the imaginary part of a complex number). Regarding the above optical chirality, for example, the optical chirality of a circularly polarized plane wave (plane wave circular polarization) propagating in free space is C = ±2|E| 2It is represented by ω / c (where c is the speed of light, with a positive sign representing left-handed circularly polarized light and a negative sign representing right-handed circularly polarized light, respectively).

[0005] Therefore, if the value of optical chirality can be made larger than the value of optical chirality of plane-wave circularly polarized light, the CD signal can be enhanced, and the discrimination accuracy of chiral molecules can be increased dramatically. Thus, in recent years, nanostructures (optical antennas) for making the value of optical chirality larger than the value of optical chirality of plane-wave circularly polarized light and design methods for the above nanostructures have been proposed (for example, Non-Patent Documents 2 to 4).

Prior Art Documents

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] Although nanostructures (optical antennas) for increasing the value of optical chirality are known as in Non-Patent Document 2 and Non-Patent Document 3, higher discrimination accuracy of chiral molecules is required.

[0008] In addition, Non-Patent Document 4 discloses a method for designing a structure capable of enhancing optical chirality by utilizing deep learning. However, in this method, a large amount of teacher data linking the structure and optical chirality is required. Therefore, a desired structure could not be designed simply.

[0009] Therefore, there is a demand for an optical antenna capable of further enhancing the discrimination accuracy of chiral molecules and a method for simply designing the optical antenna.

[0010] By the way, devices such as three-dimensional displays, next-generation information communication devices, quantum computers, and quantum cryptographic communication devices use devices that emit circularly polarized light. It is desirable that the above device be capable of generating circularly polarized light having high anisotropy and controlling the polarization state.

[0011] An object of the present invention is to provide an optical antenna capable of further enhancing the discrimination accuracy of chiral molecules, and a spectroscopic analyzer for chiral molecules including the above optical antenna. Another object of the present invention is to provide a light emitting device including the above optical antenna, capable of generating circularly polarized light having high anisotropy and controlling the polarization state. A further object of the present invention is to provide a method for designing an optical antenna that can easily design the above optical antenna.

Means for Solving the Problems

[0012] One aspect of the present invention for solving the above problems relates to the optical antennas of the following [1] to [5]. [1] A plurality of structures each including a communication hole for incident or emitting light in a predetermined polarization state, surrounded by a twisted structure, A nanogap disposed between the plurality of structures and communicating with the communication hole, An optical antenna having. [2] The optical antenna according to [1], wherein the plurality of structures include two structures whose central axes are arranged on the same straight line. [3] The optical antenna according to [2], wherein the number of the plurality of structures is two. [4] The optical antenna according to [3], wherein the twisting directions of the two structures with respect to the nanogap of the twisted structure are opposite to each other. [5] The optical antenna according to [3], wherein the shapes of the two structures are mirror-symmetric to each other.

[0013] One aspect of the present invention for solving the above problems relates to the spectroscopic analyzers for chiral molecules of the following [6] and [7]. [6] A spectroscopic analyzer for chiral molecules including the optical antenna according to any one of [1] to [5]. [7] The spectroscopic analyzer for chiral molecules according to [6], further having a flow path portion for feeding a chiral molecule to be analyzed into the nanogap of the optical antenna.

[0014] One aspect of the present invention for solving the above problems relates to the optical radiation device of the following [8]. [8] The optical antenna according to any one of [1] to [5], A light source disposed in the nano gap, An optical radiation device having

[0015] One aspect of the present invention for solving the above problems relates to a method for designing an optical antenna according to the following [9] to

[13] . [9] A method for designing an optical antenna performed using a computer, A step of setting a design region, A step of setting a distribution of dielectric functions in the design region, A step of calculating an electromagnetic field in a region including the design region when circularly polarized light or an optical vortex is incident from a plurality of directions on the design region based on the distribution of the dielectric function, A step of obtaining a predetermined physical quantity at a predetermined position in the design region based on the electromagnetic field, A step of making a first determination as to whether or not the physical quantity is equal to or greater than a threshold value, Including When it is determined in the step of making the first determination that the physical quantity is less than the threshold value, a step of changing the distribution of the dielectric function in the design region is further performed using a mathematical programming method, and then, after the step of changing, the step of calculating, the step of obtaining, and the step of making the first determination are performed, When it is determined in the step of making the first determination that the physical quantity is equal to or greater than the threshold value, the step of changing is not performed. A method for designing an optical antenna.

[10] When it is determined in the step of making the first determination that the physical quantity is less than the threshold value, before the step of changing, a step of obtaining a gradient of the physical quantity with respect to the distribution of the dielectric function in the design region is performed, In the step of changing, the distribution of the dielectric function in the design region is changed based on the gradient of the physical quantity. The method for designing an optical antenna according to [9].

[11] When the steps of calculating and obtaining are performed multiple times, before the step of making the first determination, a second determination step is performed to determine whether the rate of change between the obtained physical quantity and the physical quantity for which the first determination was made immediately before is less than a threshold value. When it is determined in the second determination step that the rate of change is greater than or equal to the threshold value, the first determination step is performed. When it is determined in the second determination step that the rate of change is less than the threshold value, the first determination step is not performed. The method for designing an optical antenna according to [9] or

[10] .

[12] In the step of calculating, circularly polarized light or an optical vortex is incident on the design region from two opposing directions. The method for designing an optical antenna according to any one of [9] to

[11] .

[13] The physical quantity is a physical quantity selected from the group consisting of electromagnetic field strength, optical chirality, and angular momentum of light. The method for designing an optical antenna according to any one of [9] to

[12] .

Advantages of the Invention

[0016] According to the present invention, it is possible to provide an optical antenna capable of further improving the discrimination accuracy of chiral molecules, and a spectroscopic analysis apparatus for chiral molecules including the above optical antenna. Further, according to the present invention, it is possible to provide a light emitting device including the above optical antenna, capable of generating circularly polarized light having high anisotropy and controlling the polarization state. Furthermore, according to the present invention, it is possible to provide a method for designing an optical antenna that can easily design the above optical antenna.

Brief Description of the Drawings

[0017]

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Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments.

[0019] 1. Optical Antenna FIG. 1 is a diagram showing an example of the appearance of the optical antenna 100 according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing a cross section of the optical antenna 100 at a predetermined height. Further, FIG. 3A is a cross-sectional view showing cross sections at each height of the first structure of the optical antenna. FIG. 3B is a cross-sectional view showing cross sections at each height of the second structure of the optical antenna. The optical antenna 100 includes a plurality of structures 110 each including a communication hole 112 for incident or emitting light in a predetermined polarization state surrounded by a twisted structure 111, and a nanogap 120 disposed between the plurality of structures 110 and communicating with the communication hole 112. Note that FIGS. 1 to 3 illustrate an example in which the optical antenna 100 includes two structures 110, but the structure of the optical antenna 100 is not limited to that shown in FIGS. 1 to 3. Further, in FIGS. 3A and 3B, a cross-sectional view is shown in which the cross section is taken every 4 nm from the upper end to the lower end of the optical antenna 100 with the length in the central axis Ax direction of the optical antenna 100 as the height. And zi = 1 to 50 in FIG. 3A shows the cross section of the first structure 110, and zi = 51 to 100 in FIG. 3B shows the cross section of the second structure 110.

[0020] When analyzing chiral molecules using the optical antenna 100, when light in a predetermined polarization state is incident on the communication hole 112, a predetermined physical quantity (for example, electromagnetic field intensity, optical chirality, and angular momentum of light) that is an index of the discrimination accuracy of chiral molecules can be sufficiently increased in the nanogap 120. Therefore, by arranging the target molecules in the nanogap 120 and analyzing the chiral molecules, the discrimination accuracy of the chiral molecules can be further increased.

[0021] Each of the plurality of structures 110 includes a twisted structure 111 and a communication hole 112 surrounded by the twisted structure 111. In the present specification, the “twisted structure” refers to a structure that extends in a direction rotating with respect to the central axis Ax along the central axis Ax direction of the plurality of structures 110.

[0022] The twisting direction (the direction of rotation with respect to the central axis Ax) of the twisted structure 111 is not particularly limited, and may be clockwise or counterclockwise with respect to the central axis Ax when viewed from the outside of the optical antenna 100. In the example of FIG. 1, the twisting direction is counterclockwise with respect to the central axis Ax when viewed from the outside of the optical antenna 100. The number of twisted structures 111 included in one structure 110 is also not particularly limited. In the example of FIG. 1, the number of twisted structures 111 included in one structure 110 is one.

[0023] The communication hole 112 is a communication hole for allowing light in a predetermined polarization state to enter or exit, and communicates with a nanogap 120 described later. The communication hole 112 surrounded by the twisted structure 111 is formed along the twisting direction (the direction of rotation with respect to the central axis Ax) of the twisted structure 111.

[0024] The communication hole 112 may be a communication hole for allowing light in a predetermined polarization state to enter, or a communication hole for allowing the above light to exit. However, when analyzing chiral molecules using the optical antenna 100, it is a communication hole for allowing the above light to enter. When the optical antenna 100 is used in a light emitting device described later, the communication hole 112 is a communication hole for allowing the above light to exit. The width (the length in a direction intersecting and perpendicular to the central axis) of the communication hole 112 may be constant or not constant. The maximum width of the communication hole 112 is, for example, 50 nm or more and 2000 nm or less, and preferably 20 nm or more and 500 nm or less. Note that the communication hole 112 may branch or merge in the process of moving away from the nanogap 120 in the direction of the central axis Ax. When a spectroscopic analyzer 200 for chiral molecules, which will be described later, has a flow path portion 230, the communication hole 112 can also function as a flow path for sending molecules to be analyzed to the nanogap 120.

[0025] The light in a predetermined polarization state is not particularly limited, but is preferably circularly polarized light or an optical vortex, and more preferably circularly polarized light. In the example of FIG. 1, the above light is circularly polarized light.

[0026] In addition to the twisted structure 111, the plurality of structures 110 may include a non-twisted structure 113. At this time, the communication hole 112 may include a portion surrounded by the non-twisted structure 113.

[0027] The plurality of structures 110 preferably includes two structures whose central axes are arranged on the same straight line. That is, the optical antenna 100 preferably includes two structures 110 whose central axes are arranged on the same straight line. In this case, the nanogap 120 is located between the two structures 110. Thereby, a predetermined physical quantity serving as an index of the discrimination accuracy of chiral molecules in the nanogap 120 can be increased more sufficiently. As a result, the discrimination accuracy of chiral molecules can be increased more sufficiently.

[0028] The number of the structures 110 is not particularly limited, but is preferably two. At this time, the nanogap 120 is located between the two structures 110. At this time, it is preferable that the two structures 110 have opposite twisting directions with respect to the nanogap 120, and it is more preferable that the shapes are mirror-symmetrical to each other. Thereby, a predetermined physical quantity serving as an index of the discrimination accuracy of chiral molecules can be increased more sufficiently.

[0029] The nanogap 120 is disposed between the plurality of structures 110 and communicates with the communication hole 112. In this specification, the "nanogap" refers to a gap having the shortest width and a width on the nanometer order among the voids included in the optical antenna 100. The width of the nanogap 120 is not particularly limited. For example, it is 1 nm or more and 20 nm or less, but is preferably a length of 1 / 10 or less with respect to the wavelength of light that can be incident or emitted.

[0030] The length in the central axis Ax direction of the optical antenna 100 (the length of the arrow A in FIG. 1) is not particularly limited and is appropriately adjusted according to the wavelength of light that can be incident or emitted. The length in the central axis Ax direction is, for example, 20 nm or more and 10,000 nm or less.

[0031] The material of the optical antenna 100 is not particularly limited. For example, materials with high refractive indices such as titanium dioxide, zirconium dioxide, silicon, germanium, zinc selenide, zinc sulfide, etc., and metals such as gold, silver, aluminum, indium, etc. Among these, titanium dioxide, zirconium dioxide, and silver are preferred. Also, by using metals such as gold, silver, and aluminum as the above material, surface plasmon resonance can be generated to more sufficiently enhance the optical chirality in the nanogap 120. The above-described effect of the metal is more significantly exerted on light having wavelengths from the ultraviolet region to the infrared region. In FIGS. 1, 2, 3A, and 3B, the above material is titanium dioxide.

[0032] The manufacturing method of the optical antenna 100 is not particularly limited. For example, the optical antenna 100 can be manufactured using an electron beam lithography method, a laser multiphoton polymerization processing method (for example, the methods described in Non-Patent Document 5 and Patent Document 1), etc. [Non-Patent Document 5] Taguchi, A., Nakayama, A., Oketani, R., Kawata, S. & Fujita, “K. Multiphoton-Excited Deep-Ultraviolet Photolithography for 3D Nanofabrication.” ACS Appl. Nano Mater Vol.3,pp11434-11441. [Patent Document 1] International Publication No. 2019 / 155933

[0033] When manufacturing the optical antenna 100 using the electron beam lithography method, the optical antenna 100 can be manufactured by stepwise fabricating a plurality of layers. Specifically, a resist pattern is formed on a substrate by the electron beam lithography method, and after forming a film of the material of the optical antenna by evaporation or sputtering, the residual resist is removed, thereby completing one layer. By repeating this process, an optical antenna 100 is manufactured in which a plurality of layers each having a desired pattern are laminated.

[0034] The laser multi-photon polymerization processing method is a method in which femtosecond pulsed laser light is focused on a liquid raw material monomer to cause a photopolymerization reaction only at the focal point of the laser light, thereby curing the raw material monomer. When manufacturing the optical antenna 100 using the laser multi-photon polymerization processing method, an optical antenna 100 having a desired shape can be manufactured by three-dimensionally scanning the focal point of the laser light.

[0035] The optical antenna 100 can also be manufactured using manufacturing methods other than the above. For example, it may be manufactured using a known 3D printer, or a mold may be prepared in advance and manufactured by mold molding.

[0036] 2. Spectroscopic Analyzer for Chiral Molecules FIG. 4 is a schematic diagram showing the configuration of a spectroscopic analyzer 200 for chiral molecules according to an embodiment of the present invention. As shown in FIG. 4, the spectroscopic analyzer 200 includes the above-described optical antenna 100, a plurality of light sources 210, a plurality of polarization units 220, a flow path unit 230, and a detection unit 240. Note that the light source 210, the polarization unit 220, the flow path unit 230, and the detection unit 240 are optional components. In the spectroscopic analyzer 200, the chiral molecule to be analyzed is placed in the nanogap 120 of the optical antenna 100 for analysis.

[0037] The plurality of light sources 210 are members that emit predetermined light. The number of the light sources 210 is appropriately adjusted according to the number of the structures 110 included in the optical antenna 100. For example, when the optical antenna 100 has two structures 110, the number of the light sources 210 is two. The type of light emitted by the light source 210 is not particularly limited, and examples thereof include natural light, linearly polarized light, circularly polarized light, and optical vortices. The wavelength is not particularly limited, and for example, it is 120 nm or more and 25000 nm or less.

[0038] The plurality of polarization units 220 convert the light emitted from the light source 210 into a predetermined polarization. For example, when the light source 210 emits natural light, the polarization unit 220 converts the light emitted from the light source 210 into linearly polarized light, circularly polarized light, or an optical vortex. The number of polarization units 220 is appropriately adjusted according to the number of light sources 210. For example, when the number of light sources 210 is two, the number of polarization units 220 is two.

[0039] The flow path unit 230 is a flow path for sending chiral molecules to be analyzed into the nanogap 120 through the communication hole 112 of the optical antenna 100. The flow path unit 230 is, for example, a micro flow path. The flow path unit 230 may be provided with a pump or the like for sending chiral molecules to be analyzed into the nanogap 120 together with a fluid (gas or liquid).

[0040] The detection unit 240 detects a predetermined signal regarding the chiral molecules to be analyzed disposed in the nanogap 120 of the optical antenna 100 when light in a predetermined polarization state is incident thereon. For example, when circularly polarized light or an optical vortex is incident on the optical antenna 100, the detection unit 240 detects a CD signal regarding the chiral molecules. Then, based on the detected signal, the chiral molecules can be identified.

[0041] (Effect) FIG. 5A is a cross-sectional view showing the electric field enhancement intensity at the nano-gap 120 (Z = 0 nm in FIG. 2) when right-handed circularly polarized light is incident on the optical antenna 100 simulated in a computer. FIG. 5B is a graph showing the electric field enhancement intensity on the X-axis in FIG. 5A. As shown in FIGS. 5A and 5B, when right-handed circularly polarized light is incident on the optical antenna 100, it was confirmed that the electric field is greatly enhanced and localized at the nano-gap 120. And the electric field enhancement intensity of the gap mode is about 35 times that of the electric field amplitude of the right-handed circularly polarized plane wave, and it was found that a high enhancement intensity can be obtained. Also, FIG. 5D is a cross-sectional view showing the electric field enhancement intensity at the nano-gap 120 (Z = 0 nm in FIG. 2) when left-handed circularly polarized light is incident on the optical antenna 100. In this case, the electric field enhancement intensity was about 10 times that of the electric field amplitude of the plane wave. Thus, when the incident circularly polarized light is left-handed, although the enhancement of the electric field at the nano-gap 120 could be confirmed, it was found that a high enhancement intensity cannot be obtained compared to when right-handed circularly polarized light is incident. That is, the optical antenna 100 of the present embodiment shows a particularly high enhancement intensity when right-handed circularly polarized light is incident, and it was found that the enhancement intensity with respect to the direction of circularly polarized light is asymmetric. FIG. 5C is a distribution of the enhancement intensity of optical chirality (ratio C / C 0 ) of the circularly polarized plane wave. It was also found that the optical chirality is enhanced about 55 times at the nano-gap 120 as well.

[0042] Thus, the optical antenna 100 can sufficiently increase physical quantities (such as electromagnetic field intensity and optical chirality) that serve as indices for the discrimination accuracy of chiral molecules at the nano-gap 120 when light in a predetermined polarization state is incident. Therefore, the optical antenna 100 and the chiral molecule spectroscopic analyzer 200 including the same can sufficiently increase the discrimination accuracy of chiral molecules. For example, the reason why the optical chirality at the nano-gap 120 is sufficiently increased can be considered as follows.

[0043] First, when right-handed circularly polarized light is incident on the optical antenna 100 from both ends (the +Z and -Z directions in FIG. 2) in opposite directions, the spins of the circularly polarized light cancel each other out throughout the optical antenna 100, resulting in linearly polarized light. FIG. 6A is a diagram showing the distribution of the spin angular momentum (SAM) density vector for the cross-section at Z = 0 nm in FIG. 2. FIG. 6C is a diagram showing the distribution of the spin angular momentum density vector for the cross-section at Y = 0 nm in FIG. 2. SAM is an axial vector (pseudo-vector) defined by s = 1 / (4ω)Im[E * ×E + H * ×H], and its sign corresponds to the rotation direction of the circularly polarized light. In FIGS. 6A and 6C, the state where SAM converges to the nanogap 120 is visualized. Furthermore, when taking the divergence of the vector field of SAM, as shown in FIGS. 6B and 6D, the state where a large suction is formed in the nanogap 120 is visualized. This means that when SAM gathers in the nanogap 120, the opposing spins have opposite signs, so they cancel each other out and the spin disappears.

[0044] Optical chirality is a conserved quantity of the electromagnetic field, and it is known that a continuity equation dC / dt + ∇·F = 0 holds between the optical chirality C and the chirality flux F (Non-Patent Document 3). Here, the chirality flux F = ω / 4Im[E*×E + H*×H] is expressed, and since it is in a proportional relationship with the aforementioned SAM, it is understood that a field where the optical chirality is enhanced is formed at that location due to the suction of SAM.

[0045] In FIGS. 5A to 5D and FIGS. 6A to 6D, the results when the material of the optical antenna 100 is titanium dioxide are shown. However, as described above, the material of the optical antenna 100 is not limited to titanium dioxide. For example, FIG. 7 is an example of a cross-sectional view showing the cross-section at each height of one structure of the optical antenna 100 when the material of the optical antenna 100 is silver. Further, FIG. 8 is a cross-sectional view showing the electric field enhancement intensity at each height of one structure of the optical antenna 100 when right-handed circularly polarized light is incident on the optical antenna 100 when the material of the optical antenna 100 is silver. In FIGS. 7 and 8, the position in the direction of the central axis Ax of the optical antenna 100 is indicated by the value of Z, and the position of the nanogap 120 is set to Z = 0 nm.

[0046] As shown in FIGS. 7 and 8, also in the optical antenna 100 using silver as the material, the electric field enhancement intensity in the nanogap 120 is about 125 times the electric field amplitude of the right-handed circularly polarized plane wave, and it was found that a high enhancement intensity can be obtained.

[0047] 3. Light emitting device FIG. 9A is a schematic diagram showing the configuration of a light emitting device 300 according to an embodiment of the present invention. As shown in FIG. 9A, the light emitting device 300 includes the above-described optical antenna 100 and a light source 310 disposed in the above-described nanogap 120.

[0048] When the above-described optical antenna 100 disposes the light source 310 in the nanogap 120 and generates predetermined light from the light source 310, light (including circularly polarized light) in a predetermined polarization state can be emitted to the outside through the communication hole 112 communicating with the nanogap 120. FIG. 9B is a schematic diagram showing a state in which circularly polarized light is emitted from the light source 310 disposed in the nanogap 120.

[0049] Heretofore, materials and devices for generating circularly polarized light have been known (for example, Patent Documents 2 to 6 and Non-Patent Document 6). However, the organic molecular-based circularly polarized light emitters described in Patent Documents 1 and 2 and Non-Patent Document 6 have g = ΔI / I = 2 (I L -IR )) / (I L -I R ) anisotropy factor (I that indicates the left and right polarization biases of circularly polarized light represented by L , I R represents the left-handed circularly polarized light emission intensity and the right-handed circularly polarized light emission intensity, respectively, and the g value takes a value between -2 and +2), where g = 10 -3 to 10 -2 There was a problem that it was extremely small, on the order of. The reason is that the size of the molecule is too small compared to the wavelength of light, so the phase difference (ideally π / 2) of the orthogonal electric field vectors required for the electromagnetic wave to be converted into circularly polarized light cannot be obtained. [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-109707 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-107542 [Patent Document 4] Japanese Patent Application Laid-Open No. 2020-121928 [Patent Document 5] Japanese Patent Application Laid-Open No. 2020-136576 [Patent Document 6] Japanese Patent Application Laid-Open No. 2003-507915 [Non-Patent Document 6] Zhao, WL et al. “Advances in helicene derivatives with circularly polarized luminescence,” Chem Comm 2019, Vol.55, 13793.

[0050] In Patent Document 4 using a rare earth complex, a relatively large value of about g = 1.5 is obtained, but the problems are that the luminescence efficiency is as low as about 10% and the wavelength band of the emitted light is limited to the wavelength band where rare earths emit fluorescence (usually from 300 nm to 1600 nm). Also, the circularly polarized LED of Patent Document 5 is a light-emitting device that directly extracts highly pure circularly polarized light from the electron spin of the material, but there are problems such as the wavelength of the emitted light being limited due to the constraints of the LED material and the yield of the device occurring. Also, the circularly polarized antenna of Patent Document 6 is used in the radio wave region and cannot be used in the optical wavelength region.

[0051] Furthermore, the materials and devices described in Patent Documents 2 to 6 and Non-Patent Document 6 were unable to control the polarization states of light in a wide range from circular polarization to linear polarization.

[0052] In contrast, by using the above-described optical antenna 100, the optical radiation device 300 according to the present embodiment can emit highly anisotropic circular polarization and can control the polarization state of the emitted light. Also, in the optical radiation device 300, the wavelength of the emitted light is not limited either.

[0053] When a light source 310 that generates linearly polarized light is disposed in the nanogap 120 to emit light, the near-field of the light source 310 is coupled to the nanogap 120 and converted into an electromagnetic field that propagates within the optical antenna 100 or on the surface of the optical antenna 100, and is radiated far away from the openings at both ends. Here, the two-pole gap structure is a suitable structure for coupling with an electromagnetic wave component that is in the middle of the gap and parallel to the symmetry axis connecting the two poles of the gap (see Non-Patent Document 7). The electromagnetic wave coupled to the nanogap is propagated through the twisted structure above and below it, and its polarization is converted from linear polarization to circular polarization and radiated from the openings at both ends. Here, by changing the azimuth angle of the light source 310, the polarization of the radiation can be changed from circular polarization to linear polarization according to the value of the azimuth angle. [Non-Patent Document 7]Novotny, L. & Hulst, N. van. “Antennas for light.” Nature Photonics Vol.5, 83-90.

[0054] The type of the light source 310 is not particularly limited as long as it can generate linearly polarized light. For example, it can be a fluorescent molecule, a semiconductor quantum dot, a perovskite quantum dot, a fluorescent nanodiamond, a quantum well structure, or the like. Since the optical axes of the fluorescent molecule, the semiconductor quantum dot, the perovskite quantum dot, and the fluorescent nanodiamond can be specifically found by observing the image of the fluorescence emission pattern with a microscope (see Non-Patent Document 8), these axial directions can be controlled and coupled to the antenna. [Non-Patent Document 8] Dolan, P. R., Li, X., Storteboom, J. & Gu, M. “Complete determination of the orientation of NV centers with radially polarized beams.” Opt. Express 22, 4379.

[0055] Regarding the configuration of the optical antenna 100, since it can be the same as that described above, a detailed description thereof will be omitted.

[0056] (Effect) FIG. 10A is a diagram showing the result of simulation of the scattered field and polarization analysis of the far field when the light source is placed such that the direction of linearly polarized light is parallel to the gap axis without using the optical antenna 100. FIG. 10B is a diagram showing the result of simulation of the scattered field and polarization analysis of the far field when the light source is placed such that the direction of linearly polarized light is parallel to the gap axis using the optical antenna 100. When the optical antenna 100 is used, since the intensity of light is increased, it was found that the near field of the light source is strongly coupled to the antenna and the scattering efficiency is increased. Next, the polarization of the far field becomes elliptical when the optical antenna 100 is used, and becomes linearly polarized when the optical antenna 100 is not used. From this, it was found that the optical antenna 100 has the effect of converting the linearly polarized light emitted from the light source 310 into circularly polarized light. The asymmetry factor in the case where there is an antenna is g = 1.69, and a very large value was obtained even when compared with a conventional circularly polarized light emitting device.

[0057] FIG. 11 is a diagram showing the polarization state of the far field of the electromagnetic field radiated from the optical antenna 100 on the Poincaré sphere when the in-plane azimuth angle of the linearly polarized light emitted from the light source 310 is changed from 0 degrees to 180 degrees. Here, the Poincaré sphere is a polarization display method in which the Stokes parameters representing polarization are plotted on the sphere, and the plot is the north pole (S 3When it is at the [axis], it indicates right-handed circular polarization; when the plot is at the South Pole, it indicates left-handed circular polarization; and when the plot is on the equator, it indicates linear polarization. As shown in Fig. 11, when the azimuth angle φ of the linearly polarized light emitted from the light source 310 is rotated by 180 degrees, the plot makes one full circle on the Poincaré sphere. Specifically, when the azimuth angle φ = 0 degrees, it is left-handed circular polarization, and it follows a trajectory such that it passes through linear polarization at an azimuth angle of 85 degrees, right-handed circular polarization at 87 degrees, linear polarization at 90 degrees, and returns to the original left-handed circular polarization. From this, it is shown that various polarization states can be created depending on the direction of the linearly polarized light emitted from the light source 310.

[0058] From these facts, the light emitting device 300 using the above-described optical antenna 100 can emit highly anisotropic circular polarization and can control the polarization state of the emitted light. The reason is considered as follows. For the sake of simplicity in explanation, the light receiving process, which is the reverse process of antenna radiation, will be described as an example, but from the reciprocity theorem of antennas, the following explanation also holds for the time-reversed radiation process.

[0059] When right-handed circular polarization is incident on the optical antenna 100 from both ends (the +Z and -Z directions in Fig. 2), as shown in Figs. 5A and 5B, the electric field is strongly enhanced and localized in the nanogap 120. Fig. 12 is a three-dimensional display of the electric field intensity distribution localized in the nanogap 120 in Fig. 5A. From this, it can be seen that the energy of the circular polarization incident on the optical antenna 100 is efficiently transmitted to the central nanogap 120. Furthermore, when examining the polarization state of the nanogap 120 portion, as shown in Figs. 13A and 13B, it can be seen that it is linearly polarized in the direction along the target axis ax of the nanogap 120. Therefore, considering the reverse process, if the light source 310 is installed in the nanogap 120, the electromagnetic field energy of the light source 310 is highly efficiently coupled to the nanogap 120 and is highly efficiently radiated far away from both ends of the optical antenna 100. Furthermore, when the light source 310 is installed in the direction along the polarization direction of the nanogap 120 portion shown in Figs. 13A and 13B, it is converted into circular polarization and radiated most efficiently. Fig. 10B shows that state.

[0060] Next, similarly, the distributions of the spin angular momentum (SAM) density vectors when right-handed circularly polarized light is incident from both ends of the optical antenna 100 are shown for the cross-sections of Z = 0 nm and Y = 0 nm in FIG. 2 (FIGS. 6A and 6C). Here, SAM is an axial vector (pseudo-vector) defined by s = 1 / (4ω)Im[E * ×E + H * ×H], and its sign corresponds to the rotation direction of the circularly polarized light. Looking at FIGS. 6A and 6C, it can be visualized that the SAM flowing in from both ends of the optical antenna 100 converges to the nanogap 120. Here, FIGS. 6B and 6D are diagrams showing the results of calculating the divergence of the SAM vector field. As shown in these figures, it can be visualized that a large suction of SAM is formed in the gap portion. This means that after the SAM gathers in the nanogap 120, the spins facing each other from above and below the antenna have opposite signs, so they cancel each other out in the nanogap 120 and the spin disappears. Considering these reverse processes, it can be deduced that the nanogap 120 functions as a boiling out of SAM, and as a result, the optical antenna 100 functions effectively for the SAM to be radiated as circularly polarized light to the far field.

[0061] 4. Design method of optical antenna FIG. 14 is a flowchart showing a method for designing an optical antenna according to an embodiment of the present invention. By this design method, the above-described optical antenna 100 can be designed. As shown in FIG. 14, the design method of the optical antenna 100 includes a step of setting a design region (step S10), a step of setting a distribution of dielectric functions within the design region (step S20), and a step of calculating an electromagnetic field within a region including the design region when circularly polarized light or an optical vortex is incident from a plurality of directions on the design region based on the distribution of the dielectric functions (step S30), a step of obtaining a predetermined physical quantity at a predetermined position within the design region based on the electromagnetic field (step S40), and a step of making a first determination as to whether or not the physical quantity is equal to or greater than a threshold value (step S50). In the present embodiment, the design method may include a step of obtaining a gradient of the physical quantity with respect to the distribution of the dielectric functions within the design region, which is an arbitrary step (step S60), a step of changing the distribution of the dielectric functions within the design region (step S70), and a step of making a second determination (step S80).

[0062] The method for designing an optical antenna in the present embodiment is performed using a computer. The computer is, for example, a computer.

[0063] 4-1. Step of setting a design region (step S10) In this step, a design region is set. The shape and size of the design region are not particularly limited. FIG. 15 is a schematic diagram showing an example of the design region 1 and a state in which circularly polarized light L is incident on the design region 1. As shown in FIG. 15, in the present embodiment, the design region 1 is a cube of 400 nm × 400 nm × 400 nm.

[0064] 4-2. Step of setting a distribution of dielectric functions (step S20) In this step, a distribution of dielectric functions within the design region set in step S10 is set.

[0065] The method for setting the distribution of dielectric functions is not particularly limited. For example, a random number (existence probability) ρ whose median value is 0.5, a dielectric function ε in the design region m , and a dielectric function ε of the surrounding medium0 Using the value of m ε = ρε 0 +(1 - ρ)ε

[0066] The dielectric function is ε m The materials in the design region where it becomes are not particularly limited. For example, materials with high refractive indices such as titanium dioxide, zirconium dioxide, silicon, germanium, zinc selenide, zinc sulfide, etc., and metals such as gold, silver, aluminum, indium, etc. In the example of FIG. 16, the above material is titanium dioxide. The type of the peripheral medium is not particularly limited, but for example, it is air.

[0067] 4-3. Step of calculating the electromagnetic field (Step S30) In this step, the electromagnetic field in the region including the design region when circularly polarized light or an optical vortex is incident from a plurality of directions on the design region is calculated based on the distribution of the dielectric function set in Step S20.

[0068] The number of directions for incident circularly polarized light or an optical vortex is not particularly limited, but is preferably two. At this time, in this step, it is preferable to calculate the electromagnetic field when circularly polarized light or an optical vortex is incident from two opposite directions.

[0069] The wavelength of the circularly polarized light or the optical vortex is not particularly limited. For example, it is 120 nm or more and 25000 nm or less. Also, the direction of rotation of the circularly polarized light or the optical vortex may be clockwise or counterclockwise. When a plurality of circularly polarized lights or optical vortices are incident, the wavelengths of each may be the same or different, but it is preferable that they are the same. Also, when a plurality of circularly polarized lights or optical vortices are incident, it is preferable that all are circularly polarized lights or all are optical vortices. In this embodiment, for the design region 1, the electromagnetic field when right-handed circularly polarized light L with a wavelength of 532 nm is incident from two opposite directions using an objective lens with a numerical aperture NA = 0.25 is calculated.

[0070] The size of the region including the above design region is not particularly limited. In the present embodiment, the region including the above design region is a rectangular parallelepiped of 1.1 μm × 1.1 μm × 1.0 μm as shown in FIG. 15. In FIG. 15, the above design region is arranged at the center of the rectangular parallelepiped. The method for calculating the above electromagnetic field is not particularly limited. For example, the above electromagnetic field can be calculated by solving the Maxwell equations based on the distribution of the above dielectric function. When solving the Maxwell equations, any method such as the finite difference frequency domain method (FDFD method), the finite difference time domain method (FDTD method), and the finite element method (FEM method) can be used.

[0071] 4-4. Step of obtaining a predetermined physical quantity (Step S40) In this step, based on the electromagnetic field calculated in Step S30, a predetermined physical quantity at a predetermined position within the above design region is obtained. The above predetermined position is not particularly limited, but is preferably the center (center of gravity) within the above design region. In the present embodiment, the above predetermined position corresponds to the nanogap 120 of the optical antenna 100.

[0072] The above predetermined physical quantity may be a physical quantity that serves as an index for the discrimination accuracy of chiral molecules. For example, it is the electromagnetic field strength, optical chirality, and angular momentum of light.

[0073] The method for obtaining the predetermined physical quantity is appropriately selected according to the type of the above physical quantity. At the above desired position, a method of minimizing the value obtained by multiplying the negative sign to the overlap integral of the electromagnetic field distribution (gap mode) with the minimum mode volume and the physical field is preferable. For example, when the predetermined physical quantity is the electromagnetic field strength (energy), the electromagnetic field strength can be obtained by solving the following formula (1). In formula (1), P is the electromagnetic field strength (energy), E is the electric field, H is the magnetic field, E gap is the electric field distribution in the gap mode, and H gap represents the magnetic field distribution in the gap mode.

Equation

[0074] 4 - 5. Step of making the first determination (Step S50) In this step, it is determined whether the physical quantity obtained in Step S40 is greater than or equal to the threshold value. If it is determined in this step that the physical quantity is less than the threshold value, a step of changing the distribution of the dielectric function (Step S70), which will be described later, is further performed. Then, after Step S70, the above - mentioned calculating step (Step S30), the above - mentioned obtaining step (Step S40), and this step are performed. If it is determined in this step that the physical quantity is greater than or equal to the threshold value, the above - mentioned changing step (Step S70) is not performed, and the design of the optical antenna is completed.

[0075] 4 - 6. Step of obtaining the gradient of the physical quantity (Step S60) In the method for designing an optical antenna according to this embodiment, before the step of changing (Step S70) to be described later, a step of obtaining the gradient of the physical quantity with respect to the distribution of the dielectric function in the design region may be performed. The method for obtaining the gradient is not particularly limited. For example, the gradient can be obtained by the differential method.

[0076] 4 - 7. Step of changing the distribution of the dielectric function (Step S70) In this step, the distribution of the dielectric function in the design region is changed using a mathematical programming method. When the method for designing an optical antenna according to this embodiment includes Step S60, in this step, the distribution of the dielectric function in the design region is changed based on the gradient. Examples of the mathematical programming method include the optimality criterion method, the sequential linear programming method, the sequential convex function approximation method, etc. When the method for designing an optical antenna according to this embodiment does not include Step S60, the distribution of the dielectric function in the design region is changed based on the magnitude of the physical quantity obtained in Step S40.

[0077] After this process, the above-described calculating process (process S30), the above-described obtaining process (process S40), and this process are performed again. In this way, by repeatedly performing these processes, it is possible to obtain the distribution of the optimal dielectric function in which the above physical quantity becomes a desired value. As a result, the structure of the optical antenna corresponding to the distribution of the dielectric function can be determined, and an optimal optical antenna can be designed. In these processes, since it is not necessary to use a huge amount of data required for the method described in Non-Patent Document 4, the optical antenna 100 can be easily designed by the optical antenna design method according to the present embodiment.

[0078] FIG. 16 is a graph showing the relationship between the number of repetitions of the set of process S70, process S30, and process S40 and a predetermined physical quantity to be obtained. In FIG. 16, the influence of the number of repetitions on the value of -P is shown when the absolute value of the physical quantity is P. As shown in FIG. 16, as the number of repetitions increases, the value of the above physical quantity (-P) decreases. That is, as the number of repetitions increases, the absolute value P of the physical quantity increases.

[0079] 4-8. Step of making a second determination (step S80) In the optical antenna design method according to the present embodiment, when the above-described calculating process (process S30) and the above-described obtaining process (process S40) are performed a plurality of times, before the above-described first determination process (process S50), a second determination process may be performed as to whether or not the change rate between the obtained physical quantity and the physical quantity for which the first determination was made immediately before is equal to or greater than a threshold value.

[0080] If it is determined in this process that the change rate is equal to or greater than the threshold value, the above-described first determination process (process S50) is performed. If it is determined in this process that the change rate is less than the threshold value, the above-described first determination process (process S50) is not performed. Specifically, the change rate is the ratio of the difference between the obtained physical quantity and the physical quantity for which the first determination was made immediately before to the physical quantity for which the first determination was made immediately before.

[0081] With this process, when the acquired physical quantity is close to the previously determined physical quantity even though the physical quantity for which the first determination was made immediately before was less than the threshold value, the implementation of the optical antenna design method can be terminated. As a result, the conditions in each process can be reviewed, and the design of the optical antenna can be performed again starting from the implementation of step S10.

[0082] In the above embodiment, a cube of 400 nm × 400 nm × 400 nm was used as the design region, but it is not limited to this. For example, the thickness of the design region (the length in the Z-axis direction in FIG. 15) can be reduced, and a structure can be designed (2D optimization) such that the structure changes in the plane direction (the X-axis and Y-axis directions in FIG. 15) but does not change in the thickness direction (the Z-axis direction in FIG. 15).

[0083] (Effect) With the optical antenna design method according to this embodiment, the optical antenna 100 can be easily designed.

Industrial Applicability

[0084] The optical antenna and chiral molecule spectroscopic analyzer of the present invention can sufficiently improve the discrimination accuracy of chiral molecules. In addition, the optical antenna design method of the present invention can easily design the above optical antenna. Furthermore, the light emitting device of the present invention can emit highly anisotropic circularly polarized light and can control the polarization state of the emitted light. Therefore, the present invention is useful in fields such as highly sensitive chirality analysis of drugs, proteins, and DNA molecules utilizing optical chirality enhancement, circularly polarized light emitting elements, displays, and quantum computers.

Explanation of Signs

[0085] 1 Design region 100 Optical antenna 110 Structure 120 Nanogap 200 Chiral molecule spectroscopic analyzer 210 Light source 220 Polarizing section 230 Flow path section 240 Detection unit 300 Light emission device 310 Light source

Claims

1. a plurality of structures each including a through hole for allowing light having a predetermined polarization state to enter or exit the structure, the through hole being surrounded by the twisted structure; a nanogap disposed between the plurality of structures and communicating with the communicating hole; An optical antenna having

2. The optical antenna according to claim 1 , wherein the plurality of structures includes two structures whose central axes are arranged on the same straight line.

3. The optical antenna of claim 2 , wherein the number of said plurality of structures is two.

4. The optical antenna according to claim 3 , wherein the two structures have twist structures with respect to the nanogap in opposite directions.

5. The optical antenna of claim 3 , wherein the shapes of the two structures are mirror images of each other.

6. An apparatus for spectroscopic analysis of chiral molecules, comprising an optical antenna according to any one of claims 1 to 5.

7. 7. The spectroscopic analysis device for chiral molecules according to claim 6, further comprising a flow path for feeding the chiral molecules to be analyzed into the nanogap of the optical antenna.

8. An optical antenna according to any one of claims 1 to 5; a light source disposed in the nanogap; 1. A light emitting device comprising:

9. A method for designing an optical antenna using a computer, comprising the steps of: defining a design space; Setting a distribution of the dielectric function within the design domain; Calculating an electromagnetic field in a region including the design region when circularly polarized light or an optical vortex is incident on the design region from a plurality of directions based on the distribution of the dielectric function; acquiring a predetermined physical quantity at a predetermined position within the design domain based on the electromagnetic field; making a first determination as to whether the physical quantity is equal to or greater than a threshold; Including, When it is determined in the first determination step that the physical quantity is less than a threshold value, a step of changing a distribution of the dielectric function in the design region using a mathematical programming method is further performed, and then, after the changing step, the calculating step, the obtaining step, and the first determination step are performed; When it is determined in the first determination step that the physical quantity is equal to or greater than a threshold value, the changing step is not performed. How to design an optical antenna.

10. when it is determined in the first determination step that the physical quantity is less than a threshold value, a step of determining a gradient of the physical quantity with respect to a distribution of a dielectric function within the design domain is performed before the change step; In the changing step, a distribution of the dielectric function within the design domain is changed based on a gradient of the physical quantity. The method for designing an optical antenna according to claim 9.

11. When the calculating step and the acquiring step are performed a plurality of times, a second determining step is performed before the first determining step as to whether or not a rate of change between the acquired physical quantity and a physical quantity for which a first determination was performed immediately before is equal to or greater than a threshold value; When it is determined in the second determination step that the rate of change is equal to or greater than a threshold value, the first determination step is performed; When it is determined in the second determination step that the rate of change is less than a threshold value, the first determination step is not performed. The method for designing an optical antenna according to claim 9.

12. In the calculating step, circularly polarized light or an optical vortex is incident on the design area from two opposing directions. The method for designing an optical antenna according to claim 9.

13. The method for designing an optical antenna according to claim 9 , wherein the physical quantity is selected from the group consisting of electromagnetic field strength, optical chirality, and angular momentum of light.

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