Topological Optical Switch

Abstract

To provide a topological optical switch capable of switching an output direction while maintaining an optical vortex mode.SOLUTION: A topological optical switch 100 includes: an X region 150 arranged at an intersection position of a first topological photonic structure 151 and a second topological photonic structure 152 and composed of a photonic structure of a No bandgap; a first topological edge 171 that expresses a topological edge state capable of performing optical vortex transmission at a boundary among the first topological photonic structure 151, a first photonic structure 161, and a second photonic structure 162; and a second topological edge 172 that expresses the topological edge state capable of performing optical vortex transmission at the boundary among the first topological photonic structure 151, the first photonic structure 161, and the X region 150.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a topological optical switch that utilizes topological photonics. [Background technology]

[0002] The various optical elements used in optical networks include lasers, modulators, multiplexing elements, optical switches, etc. Optical integrated circuits integrate various functions essential for optical communications on a single chip. Compared to single-function optical devices, they can reduce implementation costs, power consumption, size, etc., and therefore numerous modules have been put into practical use, dominating the current optical market. In general optical integrated circuits, the mode of propagating light is fixed to either the TE mode (Transverse Electric mode) or the TM mode (Transverse Magnetic mode). Therefore, various devices integrated on the optical circuit are designed to operate in these modes.

[0003] In recent years, there has been growing interest in communication methods that actively utilize the degrees of freedom of light. In particular, research into optical vortices (the orbital angular momentum of light) has been active, as there are still many unexplored areas. Optical vortices theoretically enable infinite channel multiplexing by encoding information onto the spiral period of the wavefront, and are considered to be extremely compatible with optical communications.

[0004] Patent Document 1 describes an optical element that includes a first layer and a second layer facing the first layer, the first layer including a plurality of first structures each having optical anisotropy, and the second layer reflecting light incident from the first layer while maintaining the polarization state of the light when the light is incident and when it is reflected. Paragraph

[0258] of Patent Document 1 states, "Light LT2 is emitted as an optical vortex. An optical vortex is light that has a singularity and whose equiphase surface forms a helical surface." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-84679 Summary of the Invention [Problem to be solved by the invention]

[0006] By replacing part of conventional optical circuits with topological photonics systems, we aim to perform various controls within optical circuits, including optical vortex propagation (TPICs: Topological Photonic Integrated Circuits). However, in a topological photonics system, a technology for switching the output direction while maintaining the optical vortex mode has not yet been realized.

[0007] The present invention has been made in view of the above circumstances, and aims to provide a topological optical switch that can switch the output direction while maintaining the optical vortex mode. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the topological optical switch according to the present invention comprises: a first topological photonic structure whose interior is an insulator having an energy gap and whose edges are in a gapless metallic state; a second topological photonic structure whose interior is an insulator having an energy gap and whose edges are in a gapless metallic state, which is arranged so as to intersect with the first topological photonic structure; and a second topological photonic structure whose interior is an insulator having an energy gap and whose edges are in a gapless metallic state, which is arranged at the intersection of the first topological photonic structure and the second topological photonic structure, an X region consisting of a photonic structure with a bandgap; a first photonic structure arranged between the second topological photonic structure and one side of the X region and the first topological photonic structure, the bulk of which is an insulator having an energy gap; a second photonic structure arranged between the second topological photonic structure and the other side of the X region and the first topological photonic structure, the bulk of which is an insulator having an energy gap; a first topological edge that exhibits a topological edge state allowing optical vortex propagation at the boundary between the first topological photonic structure and the first photonic structure and the second photonic structure; and a second topological edge that exhibits a topological edge state allowing optical vortex propagation at the boundary between the first topological photonic structure and the first photonic structure and the X region. and a third topological edge that exhibits a topological edge state in which optical vortex propagation is possible at the boundary between the second topological photonic structure and the second photonic structure, and the X region switches the output direction of signal light according to a phase difference between control light input to the third topological edge and propagating in an optical vortex, and signal light input to the first topological edge and propagating in an optical vortex. It is characterized by: [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a topological optical switch that can switch the output direction while maintaining the optical vortex mode. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating the structure of a topological edge state waveguide used in the topological optical switch of the present invention. [Figure 2] FIG. 1 is a Brillouin zone diagram showing the structure of a nanohole having C6v symmetry in the topological optical switch of the present invention. [Figure 3] FIG. 1 is a diagram showing the structure of a nanohole having C6v symmetry for explaining the characteristics of the topological photonic crystal in the topological optical switch of the present invention. [Figure 4] 4A and 4B are photonic band diagrams of a photonic structure of a topological edge transmission line using the topological photonic crystal shown in FIG. 3 and of the topological photonic structure in a trivial state, a no bandgap state, and a topological state. [Figure 5] FIG. 1 is a diagram illustrating optical vortex propagation using a topological photonic crystal of the topological optical switch of the present invention. [Figure 6A] FIG. 1 is a diagram illustrating optical vortex propagation using a topological photonic crystal of the topological optical switch of the present invention. [Figure 6B] FIG. 1 is a diagram illustrating optical vortex propagation using a topological photonic crystal of the topological optical switch of the present invention. [Figure 6C] FIG. 1 is a diagram illustrating optical vortex propagation using a topological photonic crystal of the topological optical switch of the present invention. [Figure 6D] FIG. 1 is a diagram illustrating optical vortex propagation using a topological photonic crystal of the topological optical switch of the present invention. [Figure 7A] FIG. 1 is a conceptual diagram of a topological optical switch using the extrinsic technique of the topological optical switch of the present invention. [Figure 7B] FIG. 1 is a conceptual diagram of a topological optical switch using the intrinsic technique of the topological optical switch of the present invention. [Figure 8] 1 is a diagram showing an element structure of a topological optical switch according to a first embodiment of the present invention. [Figure 9] FIG. 2 is a diagram showing a phase distribution in a topological waveguide calculated by the FDTD method of the topological optical switch according to the first embodiment of the present invention. [Figure 10]1 is a diagram showing an element structure of a topological optical switch according to a first embodiment of the present invention. FIG. [Figure 11] FIG. 1 shows the propagation mode distribution of the topological optical switch according to the first embodiment of the present invention, calculated by the FDTD method. The left figure shows the propagation mode distribution when the control light is in phase with the signal light (phase difference of 0°), and the right figure shows the propagation mode distribution when the control light is in antiphase with the signal light (phase difference of 180°). [Figure 12] FIG. 10 is a diagram showing the element structure of a topological optical switch according to a second embodiment of the present invention. [Figure 13] FIG. 10 is a diagram showing an example of a unit cell of a topological crystal (PhC) loaded with a phase-change material of the topological optical switch according to the second embodiment of the present invention. [Figure 14] FIG. 10 is a diagram showing the simulation results of the transition of the photonic band gap width when the refractive index at a specific location in the unit cell of the photonic crystal (PhC) of the topological optical switch according to the second embodiment of the present invention is changed. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. (Explanation of the principle) Attempts to trace the topology of electron systems in topological insulators and Weyl semimetals to photon systems are called topological photonics and have been making rapid progress in recent years. Topological insulators are materials that are insulating with an energy gap in the bulk, but have gapless metallic states at their edges (edges in two-dimensional systems, surfaces in three-dimensional systems).

[0012] In particular, C 6vThe emergence of Z2 topology (a class in the classification of topological structures of electronic wave functions) in a structure in which dielectrics with symmetry (which overlaps when rotated by 60°) are arranged in a honeycomb lattice pattern makes it possible to realize topological edge states capable of transmitting optical vortices.

[0013] Topological photonics is an attempt to trace the topological concepts of electronic systems to photon systems, and in particular, C 6v In a structure in which symmetric dielectrics are arranged in a honeycomb lattice pattern, it is possible to create two types of photonic crystals (PhCs) with different topologies by changing the "dielectric arrangement."

[0014] There are two types of photonic crystals (PhCs): Trivial Photonic Crystals and Topological Photonic Crystals. A trivial photonic crystal is a "trivial" (ordinary) photonic structure whose bulk is an insulator with an energy gap, while a topological photonic crystal is a photonic structure whose interior is an insulator with an energy gap but whose edges are in a bandgapless metallic state.

[0015] It is known that optical vortex modes propagate at the interface between these two photonic crystals (PhCs) (topological transmission line), making this a promising method for controlling optical vortices within optical circuits.

[0016] <Design of topological edge transmission lines> FIG. 1 is a diagram illustrating the structure of a topological edge state waveguide 20 used in a topological optical switch according to a first embodiment of the present invention (Si-based topological edge state waveguide used in simulation). As shown in Figure 1, the topological edge transmission line 20 has a photonic structure (Trivial photonic structure) 11 whose bulk is an insulator with an energy gap, a topological photonic structure (Topological photonic structure) 12 whose interior is an insulator with an energy gap and whose edge is in a gapless metallic state, and a topological edge (Topological edge) 13 that exhibits a topological edge state at the boundary between the photonic structure 11 and the topological photonic structure 12, allowing optical vortex propagation.

[0017] <Photonic Structure> Photonic Structure (Trivial PhC) 11 is C 6v The structure is such that first dielectrics 111 having symmetry are arranged in a honeycomb lattice pattern (for example, period a=800 nm). Topological Photonic Structures (Topological PhC) 12 are C 6v The structure is such that the second dielectric 112 having symmetry is arranged in a honeycomb lattice pattern (for example, period a=800 nm).

[0018] The first dielectric 111 is formed on a silicon-on-insulator (SOI) wafer. 6v Symmetrical nanoholes 111a are arranged in a honeycomb lattice (period a = 800 nm) cell (unit cell) 121 (nanometer, 1 nm = 10 -9 m) Use structure. The second dielectric 112 is formed by depositing C on an SOI wafer (for example, a Si film thickness of 220 nm) 131. 6v A nanostructure is used in which symmetrical nanoholes 112a are arranged in cells 122 (unit cells) in a honeycomb lattice pattern (period a=800 nm). Nanoholes 111a of first dielectric 111 and nanoholes 112a of second dielectric 112 have different parameters, namely, distance R from the center of honeycomb lattice cells 121, 122 to the center of nanoholes 111a, 112a and length L of one side of the nanohole (described later).

[0019] In a topological photonics system, a topological edge state occurs at the interface between two types of photonic structures where the length of one side of the triangle in the honeycomb dielectric part and the distance from the center of the cell are slightly different, and light propagates through it. Depending on the direction of rotation of the optical vortex, light propagates in only one direction, either left or right. In Figure 1, a topological edge mode is manifested at the topological edge 13 at the boundary between photonic structure 11 and topological photonic structure 12. The optical state (topological edge state) that arises at the interface between the two regions created by the topological photonic structure 12 only allows light with specific polarization and optical vortices, and these specific polarization and optical vortices have propagation direction dependence.

[0020] <C 6v Symmetrical nanohole structure Figure 2 shows the C 6v 1 is a Brillouin Zone diagram (schematic image of a unit cell in the reciprocal space) showing the structure of a symmetric nanohole. Take the nanohole 111a in the first dielectric 111 of the photonic structure 11 as an example. The nanohole 112a in the second dielectric 112 of the topological photonic structure 12 also has a similar structure. As shown in the right diagram of Figure 2, the center of the honeycomb lattice cell 121 is taken as the Γ point, which is the center (origin) of the Brillouin zone. The highly symmetrical points of the Brillouin zone are point M (the center of a rectangular surface), point K (the center of an edge connecting two rectangular surfaces), point A (the center of a hexagonal surface), point H (an end point), and point L (the center of an edge connecting a hexagonal surface and a rectangular surface).

[0021] As shown in the left diagram of FIG. 2, the Si film 133 is made up of honeycomb lattice cells 121 and C 6vSymmetrical nanoholes 111a are formed. A photonic structure consisting of the remaining Si film 133 and the nanoholes 111a opened in the Si film 133 forms the first dielectric 111 of the photonic structure 11. The left diagram in Fig. 2 is a diagram of the cell 121 of the photonic structure 11 viewed obliquely from above in front of the top surface, with the SiO2 insulating film 132 below the opened nanoholes 111a exposed. 2, the parameters of the nanohole 111a are the distance R from the center of a cell 121 (Γ point) of the honeycomb lattice to the center of the nanohole 111a and the length L of one side of the nanohole 111a. The central angle between the cells 121 of adjacent nanoholes 111a is π / 3. In the case of the nanohole 111a of the photonic structure 11, for example, R=240 nm and L=240 nm. In addition, in the case of the nanohole 112a of the topological photonic structure 12, for example, R=290 nm and L=250 nm. Furthermore, as shown in the left diagram of FIG. 2, the distances a1 and a2 between the centers (Γ points) of adjacent honeycomb lattice cells 121 are the same (here, a1=a2=800 nm≡a).

[0022] <Characteristics of topological photonic crystals> Figure 3 shows the C diagram to explain the properties of topological photonic crystals. 6v 1 is a diagram showing the structure of a symmetrical nanohole, in which the same components as in FIG. 2 are assigned the same reference numerals. 3, the nanoholes 111a have parameters R, which is the distance from the center (Γ point) of a cell 121 (unit cell) of the honeycomb lattice to the center (center of gravity) of the triangular nanohole 111a, and L, which is the length of one side of the nanohole 111a. The central angle between the cells 121 of adjacent nanoholes 111a is π / 3. In the case of the nanohole 111a of the photonic structure 11, for example, R=213 nm and L=281 nm. In addition, in the case of the nanohole 112a of the topological photonic structure 12, for example, R=264 nm and L=284 nm. Furthermore, the distance a (period) between the centers (Γ points) of adjacent cells 121 of the honeycomb lattice is the same (here, a=730 nm for two-dimensional analysis).

[0023] <Photonic band diagram of a topological photonic crystal> Figure 4 shows typical photonic bands for (left) trivial and (right) topological photonic crystals in the trivial, no bandgap, and topological states of the photonic structure 11 and the topological photonic structure 12 in the topological edge transmission line using the topological photonic crystal shown in Figure 3. In each photonic band diagram in Figure 4, the wavelength λ = 1.55 μm is the bandgap center (see the dashed lines in Figure 4). The horizontal axis of each photonic band diagram in Figure 4 shows the wave vector (2 π / a ) and the vertical axis is the normalized frequency (ωa / 2 π c= a / λ) is taken. The horizontal axis of each band diagram in Figure 4 is the wave vector (2 π / a ) is the center of the Brillouin zone of the honeycomb lattice cell 121 (see FIG. 3), point K is the center of the edge connecting two rectangular faces, and point M is the center of the rectangular face (see FIG. 3).

[0024] Fig. 4 also shows the transition of the photonic band of the topological photonic crystal shown in Fig. 3. The photonic band can be shifted by the variables: the distance R from the center (Γ point) of the cell 121 (unit cell) of the honeycomb lattice to the center (center of gravity) of the triangular nanohole 111a, and the length L of one side of the nanohole 111a.

[0025] The variable R mainly controls the transition from the Trivial state to the Topological state. When R < Period / 3, the topological photonic crystal shown in Fig. 3 is in the trivial state (see the two band diagrams in the trivial state of Fig. 4. In Fig. 4, "Period" is denoted as "a"). When R = Period / 3, it is in the no bandgap state (see the band diagram in the no bandgap state of Fig. 4). The "no bandgap" structure is used in the X region (see Figs. 8 and 10) described later. When R > Period / 3, it is in the topological state (see the two band diagrams in the topological state of Fig. 4).

[0026] The variable L mainly moves the entire band up and down and controls the center of the bandgap. For example, by changing the variable L, the entire band can be moved up and down without changing the center of the bandgap (band shape).

[0027] <Optical Vortex Propagation Using Topological Photonic Crystal> Figs. 5 and 6A - D are diagrams for explaining optical vortex propagation using a topological photonic crystal. The same reference numerals are given to the same components as in Figs. 1 - 4. The lower diagram in Fig. 5 is a diagram (Calculated magnetic field Hz) showing the magnetic field distribution (Hz) near the topological edge transmission path (the boundary (Interface) between the photonic structure (Trivial PhC) 11 and the topological photonic structure (Topological PhC) 12) calculated by the FDTD method (Finite - difference time - domain method). The x - axis (x axis) (μm) is taken on the horizontal axis and the y - axis (y axis) (μm) is taken on the vertical axis. The shading in Figs. 5 and 6A - D represents the intensity of the magnetic field distribution (Hz) (the darker the stronger the intensity). As shown in the lower diagram of Fig. 5, the electromagnetic field is localized at the topological edge 13 of the boundary between the photonic structure 11 and the topological photonic structure 12.

[0028] The upper diagram in Fig. 5 is a photonic band diagram corresponding to the magnetic field distribution (Hz) near the topological edge transmission line (the interface between the photonic structure (Trivial PhC) 11 and the topological photonic structure (Topological PhC) 12) in the lower diagram in Fig. 5. The upper diagram in Fig. 5 corresponds to Fig. 4.

[0029] Figure 6A is a diagram showing the band gap magnetic field distribution (Hz) of d± (electromagnetic mode) in the trivial state shown by symbol A in Figure 5, Figure 6B is a diagram showing the band gap magnetic field distribution (Hz) of p± (electromagnetic mode) in the trivial state shown by symbol B in Figure 5, Figure 6C is a diagram showing the band gap magnetic field distribution (Hz) of d± and p± in the topological state shown by symbol C in Figure 5, and Figure 6D is a diagram showing the band gap magnetic field distribution (Hz) of d± and p± in the topological state shown by symbol D in Figure 5. In Figures 6A-D, the horizontal axis is the x-axis (μm) and the vertical axis is the z-axis (μm).

[0030] As shown in the upper diagram of Figure 5, the band shapes of the photonic structure (Trivial PhC) 11 and the topological photonic structure (Topological PhC) 12 are almost identical. However, the electromagnetic mode at the Γ point is inverted (band inversion), making them physically different.

[0031] As shown in the lower diagram of Figure 5, at the PhC interface between the photonic structure (Trivial PhC) 11 and the topological photonic structure (Topological PhC) 12, i.e., the interface where the bands close, a topological transmission path 13 is manifested and an optical vortex mode is propagated.

[0032] <Two types of topological optical switch approaches> We propose two approaches to realize a topological optical switch. The switching control of optical vortex propagation has not been reported before, and we are the first to disclose it. 7A and 7B are conceptual diagrams of two types of topological optical switches. FIG. 7A is a conceptual diagram of a topological optical switch using an extrinsic technique, and FIG. 7B is a conceptual diagram of a topological optical switch using an intrinsic technique.

[0033] The extrinsic technique shown in Figure 7A is a technique for controlling outside the photonic crystal (PhC), which is a component of the device. A topological optical switch (extrinsic optical vortex switch) 100 using the extrinsic technique utilizes phase interference in the topological transmission line. In other words, the extrinsic technique switches the output to Port1 or Port2 by causing phase interference at the input. The topological optical switch 100 using the extrinsic method will be described in the first embodiment (see FIGS. 8 to 11).

[0034] Incidentally, in a Mach-Zehnder interferometer used in a conventional optical circuit, a light beam propagating through an optical waveguide is split into two optical waveguides, and then merged after a phase difference is given. A Mach-Zehnder interferometer switches the output by changing the phase difference of the light in the two transmission paths. The extrinsic method shown in Figure 7A realizes the same phenomenon as a Mach-Zehnder interferometer in a topological transmission path.

[0035] The intrinsic technique shown in Figure 7B is a technique for applying control to the photonic crystal (PhC) itself, which is a component of the device. A topological optical switch (intrinsic optical vortex switch) 200 using the intrinsic technique controls the photonic band with a phase change material and utilizes band engineering, which changes the topology of the photonic crystal (PhC). In other words, the intrinsic technique uses band engineering to convert one input into two outputs. The topological optical switch 200 using the intrinsic method will be described in the second embodiment (see FIGS. 12 to 14).

[0036] (First embodiment) 8 to 11 are diagrams showing the element structure of the topological optical switch 100 according to the first embodiment of the present invention (schematic images of a proposed device). The topological optical switch 100 in Fig. 8 is a topological optical switch (extrinsic optical vortex switch) that uses the extrinsic method shown in Fig. 7A.

[0037] [Element structure] The topological optical switch 100 comprises a first topological photonic structure 151, the interior of which is an insulator having an energy gap and whose edges are in a gapless metallic state; a second topological photonic structure 152, which is arranged so as to intersect with the first topological photonic structure 151 and is an insulator having an energy gap and whose edges are in a gapless metallic state; an X region 150, which is arranged at the intersection of the first topological photonic structure 151 and the second topological photonic structure 152 and is made of a no-bandgap photonic structure that switches the output direction of signal light depending on the phase difference between input control light and signal light; and a first photonic structure, the bulk of which is an insulator having an energy gap and which is arranged on one side of the first topological photonic structure 151, the second topological photonic structure 152, and the X region 150. a second photonic structure 162 arranged on the other side of the first topological photonic structure 151, the second topological photonic structure 152, and the X region 150 and having a bulk that is an insulator with an energy gap; a first topological edge 171 that exhibits a topological edge state that allows optical vortex propagation at the boundary between the first topological photonic structure 151, the first photonic structure 161, and the second photonic structure 162; and a second topological edge 172 that exhibits a topological edge state that allows optical vortex propagation at the boundary between the first topological photonic structure 151, the first photonic structure 161, and the X region 150.

[0038] The topological optical switch 100 has a third topological edge 173 that exhibits a topological edge state in which optical vortex propagation is possible at the boundary between the second topological photonic structure 152 and the second photonic structure 162. The X region 150 switches the output direction of the signal light so that when the control light input to the third topological edge 173 and propagated through the optical vortex and the signal light input to the first topological edge 171 and propagated through the optical vortex are in phase, the X region 150 outputs the signal light to Port 1 (first port) of the first topological edge 171, and when they are in opposite phases, the X region 150 outputs the signal light to Port 2 (second port) of the second topological edge 172.

[0039] The X region 150 is a no-bandgap photonic structure (photonic crystal (PhC)) that closes the photonic bandgap width. In other words, the X region 150 is a photonic structure (photonic crystal (PhC)) when R=Period / 3 as shown in Fig. 4. For example, the X region 150 can use photonic graphene (no bandgap) for the photonic crystal (PhC), and when R=Period / 3 as shown in Fig. 4, it has a size of, for example, (@1550 nm).

[0040] The photonic structure in the X region 150 is an intermediate structure between the topological photonic structures 151, 152 and the trivial photonic structures 161, 162. That is, the topological photonic structures 151, 152 and the trivial photonic structures 161, 162 differ, for example, in the distance from the center of the unit cell to the center of the triangular hole. The photonic structure in the X region 150 differs from both of the above-mentioned distances of the topological photonic structures 151, 152 and the trivial photonic structures 161, 162, and has an intermediate distance therebetween.

[0041] As shown in FIG. 8, the photonic structure in the X region 150 has four cells (4×4) in both the vertical and horizontal directions of the honeycomb structure cells. Here, if the cells in the X region 150 are made into two cells (2×2) in both the vertical and horizontal directions, the width of the photonic band gap cannot be completely closed, and switching cannot be successfully performed. If the cells in the X region 150 are made more than four cells (4×4) in both the vertical and horizontal directions, propagation loss occurs in the topological transmission path (optical vortex transmission path).

[0042] The first topological photonic structure 151 and the second topological photonic structure 152 are, for example, the topological photonic structures (Topological PhC) 12 shown in FIGS. 1 and 3. The first topological photonic structure 151 and the second topological photonic structure 152 are shown in the band diagram in the Topological state when R>Period / 3 shown in FIG. 4.

[0043] The first photonic structure 161 and the second photonic structure 162 are, for example, the photonic structures (Trivial PhC) 11 shown in FIGS. 1 and 3. The first photonic structure 161 and the second photonic structure 162 are shown in the band diagram in the Trivial state when R<Period / 3 shown in FIG. 4.

[0044] The first topological edge 171, the second topological edge 172, and the third topological edge 173 are, for example, the topological edges 13 that exhibit a topological edge state in which optical vortex propagation is possible at the boundary between the photonic structure 11 and the topological photonic structure 12 shown in FIGS. 1 and 3.

[0045] <Method for reproducing phase difference of light> A method for reproducing the phase difference of light will be described. There are a method using the delay waveguide 30 and a method using the phase shifter 40. Both are external methods (methods of performing control outside the PhC which is a component of the element) shown in FIG. 7A, and utilize phase coherence in the topological transmission path.

[0046] First, the method using the delay waveguide 30 will be described. Figure 9 shows the phase distribution in a topological waveguide calculated using the FDTD method. The horizontal axis is the x-axis (μm) and the vertical axis is the y-axis (μm). The same components as in Figure 8 are designated by the same reference numerals. An optical vortex (see the rotating arrow in the cell in Figure 9) propagates in the optical vortex propagation direction indicated by arrow a in Figure 9. By using delay waveguide 30, a phase delay can be generated in the topological waveguide. There are eight cells between the cell surrounded by symbol b in Figure 9 and the cell surrounded by symbol c in Figure 9, and the phase delay in the topological waveguide is accumulated, resulting in a phase difference of 180° (i.e., opposite phase) for the eight cells (180° phase difference occurs in about 8 cells). Here, it is known that the phase difference for the eight cells is 180°. The method of using delay waveguide 30 in Figure 9 is a method of structurally creating a phase difference. The phase difference of the light is reproduced by using the delay waveguide 30 shown in FIG. 9 in one path when multiplexing in the X region 150 (see FIG. 8).

[0047] Next, a method using the phase shifter 40 will be described. 10 is a diagram showing the device structure of the topological optical switch 100. The same components as in FIG. 8 are denoted by the same reference numerals. As shown in FIG. 10, the topological optical switch 100 has a phase shifter 40 provided on the path of the control light portion outside the photonic crystal (PhC) that is a component of the device. The phase shifter 40 changes the phase difference of the control light by using the electro-thermo-optic effect (for example, by applying / not applying a bias to the pn junction of the optical waveguide). Here, the phase shifter 40 can dynamically apply a phase difference by changing the phase difference of the control light to 180° (opposite phase) or 0° by using the electro-thermo-optic effect.

[0048] [Switching control of optical vortex propagation] Fig. 11 shows the propagation mode distribution (magnetic field distribution (Hz)) of the topological optical switch 100 calculated by the FDTD method. The left diagram in Fig. 11 shows the propagation mode distribution when the control light is in phase with the signal light (phase difference 0°), and the right diagram in Fig. 11 shows the propagation mode distribution when the control light is in antiphase with the signal light (phase difference 180°). The horizontal axis is the x-axis (μm) and the vertical axis is the y-axis (μm). In the left diagram of Figure 11, symbol e (●) schematically indicates the phase of the signal light, and symbol f (●) schematically indicates the phase of the control light. The control light is in phase with the signal light (● marks). When the signal light and control light overlap in phase upon reaching area X150, one topological transmission line 171 becomes dominant, and the signal light is output to Port 1.

[0049] On the other hand, as shown by symbol g in the right diagram of Figure 11, if the signal light and control light are in opposite phase (circle) when they reach area X150, the other topological transmission path 172 becomes dominant and the signal light is output to Port2.

[0050] In this way, in topological optical switch 100, if the signal light and control light are in phase when they reach region X150 (see FIGS. 8 and 10), the signal light is output to Port 1 (see FIGS. 8 and 10), and if they are out of phase, the signal light is output to Port 2 (see FIGS. 8 and 10). For example, by changing the phase difference of the control light using phase shifter 40 (see FIG. 10) that uses the electro-thermo-optic effect, it is possible to switch the output direction while maintaining the optical vortex mode.

[0051] As described above, the topological optical switch 100 according to the first embodiment includes a first topological photonic structure 151, the interior of which is an insulator having an energy gap and whose edges are in a gapless metallic state, a second topological photonic structure 152, the interior of which is an insulator having an energy gap and whose edges are in a gapless metallic state, and a third topological photonic structure 153, which is disposed so as to intersect with the first topological photonic structure 151 and is disposed at the intersection of the first topological photonic structure 151 and the second topological photonic structure 152, and The optical fiber has an X region (150) consisting of a bandgap photonic structure, a first photonic structure (161) arranged between the first topological photonic structure (151) and one side of the X region (150) and a second topological photonic structure (152), the first photonic structure (161) having a bulk that is an insulator with an energy gap, a second photonic structure (162) arranged between the second topological photonic structure (152) and the other side of the X region (150) and the first topological photonic structure (151), the second photonic structure (162) having a bulk that is an insulator with an energy gap, a first topological edge (171) that exhibits a topological edge state that allows optical vortex propagation at the boundary between the first topological photonic structure (151) and the first photonic structure (161) and the second photonic structure (162), and a second topological edge (172) that exhibits a topological edge state that allows optical vortex propagation at the boundary between the first topological photonic structure (151) and the first photonic structure (161) and the X region (150).

[0052] This configuration makes it possible to provide a topological optical switch (extrinsic optical vortex switch) that can switch the output direction while maintaining the optical vortex mode. This makes it possible to realize a topological optical switch that can be integrated on topological silicon optical circuits (Topological photonic integrated circuits: TPICs). Optical vortices are theoretically free from multiplexing constraints, making them a promising elemental technology for optical vortex communication technology aimed at large-capacity transmission. The topological optical switch 100 can be applied to optical circuits with topological properties. A topological optical switch using optical vortex transmission can be formed on a semiconductor substrate (silicon, InP, etc.) and is highly compatible with semiconductor lasers and multicore fibers, which are key components for large-capacity transmission, and is therefore expected to improve compatibility with optical communications.

[0053] In the topological optical switch 100, the X region 150 switches the output direction of the signal light based on the phase interference between the input control light and the signal light.

[0054] This configuration allows the output direction of the signal light to be switched based on phase interference, and the output direction can be switched while maintaining the optical vortex mode.

[0055] In the topological optical switch 100, the X region 150 switches the output direction of the signal light depending on the phase difference between the input control light and the signal light.

[0056] This phase difference can be achieved, for example, by providing a phase shifter 40 in the path of the control light outside the photonic crystal (PhC), a component of the device. The phase shifter 40 uses the electro-thermo-optic effect to change the phase difference of the control light to 180° (opposite phase) or 0°, thereby dynamically imparting a phase difference and dynamically switching the output direction while maintaining the optical vortex mode. Furthermore, by using the delay waveguide 30, which accumulates the phase delay in the topological waveguide, a phase delay can be generated in the topological waveguide. A phase difference can be provided structurally, and the phase shifter 40 is not required.

[0057] The topological optical switch 100 has a third topological edge 173 that exhibits a topological edge state in which optical vortex propagation is possible at the boundary between the second topological photonic structure 152 and the second photonic structure 162, and the X region 150 switches the output direction of the signal light according to the phase difference between the control light input to the third topological edge 173 and propagating in the optical vortex, and the signal light input to the first topological edge 171 and propagating in the optical vortex.

[0058] With this configuration, by changing the phase of the control light input to the third topological edge 173 and propagated through the optical vortex, it is possible to switch the output direction while maintaining the optical vortex mode.

[0059] In topological optical switch 100, X region 150 switches the output direction of the signal light so that when the control light input to third topological edge 173 and propagated in the optical vortex and the signal light input to first topological edge 171 and propagated in the optical vortex are in phase, this signal light is output to Port 1 (first port) of first topological edge 171, and when they are in opposite phase, this signal light is output to Port 2 (second port) of second topological edge 172.

[0060] This configuration makes it possible to provide a topological optical switch that can switch the output direction of signal light depending on the phase difference, and can switch the output direction while maintaining the optical vortex mode.

[0061] In the topological optical switch 100, the topological photonic structure or photonic structure is a honeycomb lattice of cells 121 and C 6v The photonic crystal has symmetric nanoholes 111a, and when the distance R from the center (Γ point) of a cell 121 (unit cell) of the honeycomb lattice to the center of the nanohole 111a and the length L of one side of the nanohole 111a are set, the distance R is a variable that controls the transition from the trivial state to the topological state, and the length L of one side is a variable that moves the entire band up and down and controls the band gap center, the photonic crystal is When R < Period / 3, it is in the Trivial state. When R = Period / 3, it is in the No bandgap state. When R > Period / 3, it is in the Topological state.

[0062] The X region 150 is a photonic crystal with R = Period / 3.

[0063] With this configuration, since the X region 150 is a photonic crystal with R = Period / 3, a photonic structure with no bandgap can be arranged.

[0064] (Second Embodiment) Figs. 12 to 14 are diagrams showing the element structure of the topological optical switch 200 according to the second embodiment of the present invention. The topological optical switch 200 in Fig. 12 is a topological optical switch (intrinsic optical vortex switch) using the intrinsic method shown in Fig. 7B. The same reference numerals are given to the same components as in Fig. 8.

[0065] [Element Structure] The topological optical switch 200 includes a first topological photonic structure 151, the interior of which is an insulator having an energy gap and whose edges are in a gapless metallic state; a second topological photonic structure 152, which is arranged to intersect with the first topological photonic structure 151 and is an insulator having an energy gap and whose edges are in a gapless metallic state; an X region 250, which is arranged at the intersection of the first topological photonic structure 151 and the second topological photonic structure 152 and is made of a photonic structure loaded with a phase change material 250a that changes the phase state of a photonic crystal (PhC); and a region of the first topological photonic structure 151, the second topological photonic structure 152, and the X region 250. a first photonic structure 161 arranged on one side of the first topological photonic structure 151, the second topological photonic structure 152 and the X region 250, and a second photonic structure 162 arranged on the other side of the first topological photonic structure 151, the second topological photonic structure 152 and the X region 250, and a bulk of the second photonic structure 162 having an energy gap; a first topological edge 171 which realizes a topological edge state allowing optical vortex propagation at the boundary between the first topological photonic structure 151, the first photonic structure 161 and the second photonic structure 162; and a second topological edge 172 which realizes a topological edge state allowing optical vortex propagation at the boundary between the first topological photonic structure 151, the first photonic structure 161 and the X region 250.

[0066] The X region 250 is a photonic structure with no bandgap, for example, photonic graphene (no bandgap) (@1550 nm) when R=Period / 3 as shown in FIG. The photonic structure in the X region 250 is an intermediate structure between the topological photonic structures 151, 152 and the trivial photonic structures 161, 162. That is, the topological photonic structures 151, 152 and the trivial photonic structures 161, 162 differ, for example, in the distance from the center of the unit cell to the center of the triangular hole, but the photonic structure in the X region 250 differs from both of the above-mentioned distances of the topological photonic structures 151, 152 and the trivial photonic structures 161, 162, and has an intermediate distance therebetween.

[0067] A phase change material 250a (functional material) that changes the phase state of the photonic crystal (PhC) is loaded on the upper part of the photonic structure in the X region 250. FIG. 13 is a diagram showing an example of a unit cell of a topological crystal (PhC) loaded with a phase change material 250a. The phase-change material 250a is a functional material that reversibly changes the physical properties of a photonic crystal (PhC). Examples of phase-change materials include GeSbTe-based chalcogenide compounds (GSTs), such as Ge2Sb2Te5. GSTs undergo reversible phase changes when subjected to Joule heating through the application of voltage pulses of different magnitudes. GSTs crystallize with a pulse voltage of 7 to 8 V, resulting in a low resistance reset state of approximately 3 kΩ, and an amorphous, high resistance set state of 105 Ω with the application of a pulse voltage of 2 to 4 V. The advantage of GST is its high-speed phase change (several tens of nanoseconds).

[0068] The first topological photonic structure 151 and the second topological photonic structure 152 are, for example, the topological photonic structures (Topological PhCs) 12 shown in Figures 1 and 3. The first topological photonic structure 151 and the second topological photonic structure 152 are shown in a band diagram of a topological state when R>Period / 3 as shown in Figure 4.

[0069] The first photonic structure 161 and the second photonic structure 162 are, for example, the photonic structures (Trivial PhC) 11 shown in FIGS. 1 and 3. The first photonic structure 161 and the second photonic structure 162 are shown in a band diagram in the Trivial state when R < Period / 3 as shown in FIG. 4.

[0070] The first topological edge 171, the second topological edge 172, and the third topological edge 173 are, for example, topological edges 13 that exhibit a topological edge state in which optical vortex propagation is possible at the boundary between the photonic structure 11 and the topological photonic structure 12 shown in FIGS. 1 and 3.

[0071] FIG. 14 is a diagram showing the simulation results of the transition of the photonic bandgap width when the refractive index at a specific location in the unit cell of a photonic crystal (PhC) is changed. The horizontal axis represents the refractive index, and the vertical axis represents the photonic bandgap width (Photonic bandgap width (△a / λ)). As shown in FIG. 14, the refractive index around 4.40 to 5.18 at a specific location in the unit cell of the photonic crystal (PhC) has a photonic bandgap width in the Topological state (see Topological state in FIG. 14). The bandgap width gradually narrows from the state where the refractive index is 4.40 (see Topological state in FIG. 14), and the bandgap disappears around a refractive index of 5.1 to 5.4 (see Band inversion in FIG. 14). A band inversion occurs at a refractive index of 5.4, and the bandgap width expands again as the refractive index increases from 5.4 (see Trivial state in FIG. 14). Here, it is in the Trivial state. For example, when a phase change material represented by a GeSbTe-based chalcogenide compound (GST) is subjected to photoinduced phase transition, the refractive index reversibly changes from about 4.4 (amorphous state) to about 7.2 (crystalline state). Therefore, when this phase change material 250a is used, it becomes possible to switch the topology of the photonic crystal (PhC) itself.

[0072] The topological optical switch 200 loads a phase-change material 250a as a functional material into the photonic crystal (PhC) of the X region 250 (see FIG. 12). When using a GeSbTe-based chalcogenide (GST), applying voltage pulses of different magnitudes to Joule heat the GST reversibly changes its phase, causing a change in the photonic crystal (PhC) itself. For example, it is possible to change the X region 250 (see FIG. 12) from a trivial to a topological structure.

[0073] In the topological optical switch 200, one topological transmission path becomes dominant as the phase state of the phase change material 250a of the photonic crystal (PhC) in the X region 250 (see Figure 12) switches, and an optical vortex signal is transmitted to the corresponding path. For example, in the X region 250, an optical vortex signal is transmitted from Trivial (see the Trivial state of refractive index 5.4 to 7.20 in FIG. 14) Input to Port 2. X region 250 is a no bandgap (see band inversion of refractive index 5.18 to 5.20 in Figure 14): An optical vortex signal is transmitted from Input to Port 1 and Port 2. X region 250 is topological (see the topological state of refractive index 4.40 to 5.18 in Figure 14): An optical vortex signal is transmitted from Input to Port 1.

[0074] As described above, the topological optical switch 200 according to the second embodiment includes the first topological photonic structure 151, the inside of which is an insulator having an energy gap and whose edges are in a gapless metallic state, the second topological photonic structure 152, which is arranged so as to intersect with the first topological photonic structure 151 and is an insulator having an energy gap and whose edges are in a gapless metallic state, the X region 250, which is arranged at the intersection of the first topological photonic structure 151 and the second topological photonic structure 152 and is made of a photonic structure loaded with a phase change material 250a that changes the phase state of a photonic crystal (PhC), and the second topological photonic structure 152, one side of the X region 250, and the first topological photonic structure 152. a first photonic structure 161 disposed between the first topological photonic structure 151 and the second topological photonic structure 152, a bulk of which is an insulator with an energy gap; a second photonic structure 162 disposed between the other side of the X region 250 and the first topological photonic structure 151, and a bulk of which is an insulator with an energy gap; a first topological edge 171 which exhibits a topological edge state allowing optical vortex propagation at the boundary between the first topological photonic structure 151 and the first photonic structure 161 and the second photonic structure 162; and a second topological edge 172 which exhibits a topological edge state allowing optical vortex propagation at the boundary between the first topological photonic structure 151 and the first photonic structure 161 and the X region 250.

[0075] This configuration provides a topological optical switch (intrinsic optical vortex switch) that can switch the output direction while maintaining the optical vortex mode, thereby realizing a topological optical switch that can be integrated on topological silicon optical circuits (TPICs).

[0076] In the topological optical switch 200, the X region 250 switches the output direction of the input signal light by using band engineering that changes the topology of the photonic crystal (PhC).

[0077] This configuration allows us to change the topology of the photonic crystal (PhC) using band engineering, and switch the output direction while maintaining the optical vortex mode.

[0078] In the topological optical switch 200, the phase change material 250a is a functional material that reversibly changes the physical properties of a photonic crystal (PhC).

[0079] This configuration allows the phase-change material 250a to reversibly change its physical properties as a photonic crystal (PhC) itself, enabling the output direction to be switched while maintaining the optical vortex mode. For example, by using a GeSbTe-based chalcogenide (GST), the phase change can be performed quickly (within tens of nanoseconds), enabling high-speed switching.

[0080] In the topological optical switch 200, when the phase change material 250a undergoes a photoinduced phase transition, the refractive index at a specific point in the unit cell of the photonic crystal (PhC) changes to one of a topological state, a band inversion state in which the band gap disappears, or a trivial state.

[0081] With this configuration, by changing the phase state of the photonic crystal (PhC), the refractive index at a specific point in the unit cell can be changed, which changes the photonic crystal (PhC) to one of the topological, band inversion, or trivial states. This makes it possible to switch the output direction while maintaining the optical vortex mode.

[0082] The present invention is not limited to the above-described embodiments, but includes other modifications and applications within the scope of the claims. [Explanation of symbols]

[0083] 11 Trivial photonic structure 12 Topological photonic structure 13 Topological edge 20 Topological Edge Transmission Line 30 Delay waveguide 40 Phase Shifter 100 Topological Optical Switch (Extrinsic Optical Vortex Switch) 111 First Dielectric 111a,112a Nanoholes 112 Second Dielectric 121 Photonic Structure Cell (Unit Cell) 122 Cells (unit cells) of topological photonic structures 151 First Topological Photonic Structure 152 Second Topological Photonic Structure 150,250 X area 161 First Photonic Structure 162 Second Photonic Structure 171 First Topological Edge 172 Second Topological Edge 173 Third Topological Edge 200 Topological Optical Switch (Intrinsic Optical Vortex Switch) 250a Phase change materials (functional materials)

Claims

1. a first topological photonic structure whose interior is an insulator having an energy gap and whose edges are in a gapless metallic state; a second topological photonic structure arranged to intersect with the first topological photonic structure, the second topological photonic structure having an interior that is an insulator having an energy gap and an edge that is in a gapless metallic state; an X region that is arranged at an intersection of the first topological photonic structure and the second topological photonic structure and is made of a no-bandgap photonic structure; a first photonic structure (Trivial photonic structure) disposed between the second topological photonic structure and one side of the X region and the first topological photonic structure, the bulk of which is an insulator having an energy gap; a second photonic structure disposed between the second topological photonic structure and the other side of the X region and the first topological photonic structure, the second photonic structure having a bulk that is an insulator having an energy gap; a first topological edge that exhibits a topological edge state in which optical vortex propagation is possible at a boundary between the first topological photonic structure and the first and second photonic structures; a second topological edge that exhibits a topological edge state in which optical vortex propagation is possible at a boundary between the first topological photonic structure and the first photonic structure and the X region; a third topological edge that exhibits a topological edge state in which optical vortex propagation is possible at the boundary between the second topological photonic structure and the second photonic structure; The X region switches the output direction of the signal light according to the phase difference between the control light input to the third topological edge and propagated in the optical vortex and the signal light input to the first topological edge and propagated in the optical vortex. A topological optical switch characterized by:

2. a first topological photonic structure whose interior is an insulator having an energy gap and whose edges are in a gapless metallic state; a second topological photonic structure arranged to intersect with the first topological photonic structure, the second topological photonic structure having an interior that is an insulator having an energy gap and an edge that is in a gapless metallic state; an X region arranged at an intersection of the first topological photonic structure and the second topological photonic structure, and consisting of a photonic structure loaded with a phase change material that changes the phase state of a photonic crystal; a first photonic structure (Trivial photonic structure) disposed between the second topological photonic structure and one side of the X region and the first topological photonic structure, the bulk of which is an insulator having an energy gap; a second photonic structure disposed between the second topological photonic structure and the other side of the X region and the first topological photonic structure, the second photonic structure having a bulk that is an insulator having an energy gap; a first topological edge that exhibits a topological edge state in which optical vortex propagation is possible at a boundary between the first topological photonic structure and the first and second photonic structures; the first topological photonic structure and a second topological edge that exhibits a topological edge state in which optical vortex propagation is possible at the boundary between the first photonic structure and the X region. A topological optical switch characterized by:

3. The X region switches the output direction of the signal light based on the phase interference between the input control light and the signal light.

2. The topological optical switch according to claim 1 .

4. The X region switches the output direction of the signal light depending on the phase difference between the input control light and the signal light.

2. The topological optical switch according to claim 1 .

5. The X region switches the output direction of the signal light so that, when the control light input to the third topological edge and propagated in an optical vortex and the signal light input to the first topological edge and propagated in an optical vortex are in phase, the signal light is output to a first port of the first topological edge, and when the control light is in phase with the first topological edge and propagated in an optical vortex, the signal light is output to a second port of the second topological edge.

2. The topological optical switch according to claim 1 .

6. A photonic structure, comprising a first dielectric material having C 6v symmetry arranged in a honeycomb lattice pattern with a period a; The topological photonic structure has a structure in which a second dielectric material having C 6v symmetry is arranged in a honeycomb lattice pattern with a period a, The topological photonic structure or the photonic structure has honeycomb lattice cells and C arranged in the cells. ６ｖ and a photonic crystal having a symmetric nanohole, When the distance R from the center of a cell of the honeycomb lattice to the center of a nanohole and the length L of one side of the nanohole are set, and the period a is represented as Period, The distance R is a variable that governs the transition from the Trivial state to the Topological state, The length L of one side is a variable that moves the entire band up and down and controls the band gap center. The photonic crystal is When R<Period / 3, it is in the Trivial state. When R = Period / 3, there is no bandgap. When R > Period / 3, it is in a topological state.

2. The topological optical switch according to claim 1 .

7. The X region is a photonic crystal with R=Period / 3.

7. The topological optical switch according to claim 6.

8. The X region switches the output direction of the input signal light by using band engineering that changes the topology of the photonic crystal.

3. The topological optical switch according to claim 2.

9. The phase change material is a functional material that reversibly changes the physical properties of the photonic crystal.

3. The topological optical switch according to claim 2.

10. When the phase change material undergoes a photoinduced phase transition, the refractive index at a specific point in the unit cell of the photonic crystal becomes Topological condition, Band inversion state where the band gap disappears The phase state of the photonic crystal changes to one of the trivial states.

3. The topological optical switch according to claim 2.

11. The topological photonic structure comprises a dielectric material with symmetry within the arranged cells.

3. The topological optical switch according to claim 1 or 2.

12. The topological photonic structure is ６ｖ The structure is such that dielectrics containing symmetric nanoholes are arranged in honeycomb lattice cells.

3. The topological optical switch according to claim 1 or 2.

13. The topological photonic structure is Z ２ It is a topological structure expressed by topology.

3. The topological optical switch according to claim 1 or 2.

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