JP2021193437A - Nonmagnetic waveguide type isolator - Google Patents

Abstract

To provide a nonmagnetic waveguide type isolator capable of integrating on a silicon optical circuit.SOLUTION: A nonmagnetic waveguide type isolator 1 includes: a topological photonic structure 12 which is an insulator having an energy gap inside and of which edge 13 is in a gapless metal state; and a chiral antenna 20 that is arranged on a surface of the topological photonic structure 12 and that is formed of metal or a dielectric which is a metamaterial having chirality.SELECTED DRAWING: Figure 3

Description

The present invention relates to a non-magnetic waveguide isolator that can be integrated on a silicon optical circuit.

Various optical elements used in optical networks include lasers, modulators, multiplexing elements, optical switches, and the like. An optical integrated circuit is a circuit in which various modules such as a laser, a modulator, and a duplexer are integrated on one chip without using an optical fiber. The advantage of optical integrated circuits is that power consumption and manufacturing costs can be reduced by realizing various functions in optical communication with a single-chip small module.

Materials for the optical integrated circuit include indium phosphide (InP) or Si. The material of the multiplexing element is SiO ₂ . _{The materials for the modulator include lithium niobate (LiNbO 3} ), which exhibits a large EO (Electro-optic) effect, and lead zirconate titanate ((Pb, La) (Zr, Ti)) with lanthanum added. Inorganic optical crystals such as O _{3) are widely used.}

The current optical communication system is realized by a combination of individual optical element components. In order to make these systems more sophisticated, it is essential to integrate and integrate various optical elements (optical circuit) and to highly stabilize the intensity, frequency, and phase of the laser beam that is the light source. .. Since the oscillating state of a laser is easily destabilized by the influence of return light due to reflection from the outside, an optical isolator (optical) is a non-reciprocal element that transmits the output light of the laser but blocks the return light in the opposite direction. isolator) is indispensable.

Optical isolators are classified into Polarization Dependent Isolators, which correspond to the case where the incident light is linearly polarized light, and Polarization Independent Isolators, which do not depend on the polarization state of the incident light. Each optical isolator uses a magneto-optical effect (Faraday effect) in which the polarization state of light is rotated by a magnetic field. The Faraday rotator has a function of rotating a plane of polarization of linearly polarized light, and for communication, a ferromagnetic crystal such as bismuth / iron / garnet (BIG) and yttrium / iron / garnet (YIG) is used.

In an optical transmitter using silicon photonics, a DFB (Distributed Feed-Back) laser or the like is used as a light source. In the optical transmitter, the light emitted from the DFB laser is incident on an optical waveguide or the like formed on a silicon substrate, and such light is incident as reflected return light from the emission end of the DFB laser. In some cases. When the reflected return light is incident on the DFB laser, the laser oscillation becomes unstable and causes noise.

As a countermeasure against such reflected return light, there is a method of inserting an optical isolator between the DFB laser and the optical waveguide to suppress the incident of the reflected return light from the outside of the DFB laser (for example, patent). See Document 1).

Japanese Unexamined Patent Publication No. 2019-160842

The optical isolators currently in practical use are so-called bulk type optical isolators in which polarizing elements are arranged on both sides of a magnetic garnet crystal. Magnetic garnet crystals are difficult to form on semiconductor substrates (silicon, InP, etc.) that are generally used in optical circuits, and because they require a decoder, they cannot be integrated with other devices. It is said to be extremely difficult.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a non-magnetic waveguide type isolator that can be integrated on a silicon optical circuit.

In order to solve the above-mentioned problems, the non-magnetic waveguide isolator according to the present invention includes a topological photonic structure in which the inside is an insulator having an energy gap and the edges thereof are in a gapless metallic state, and the topological photonic structure. It is characterized by having a metamaterial having a chiral property, which is arranged in a photonic structure.
Other means will be described in the form for carrying out the invention.

According to the present invention, it is possible to provide a non-magnetic waveguide type isolator that can be integrated on a silicon optical circuit.

It is a perspective view which shows the structure of the non-magnetic waveguide type isolator which concerns on embodiment of this invention. FIG. 1 is a cross-sectional view taken along the line AA'in FIG. It is a top view of the non-magnetic waveguide type isolator which concerns on embodiment of this invention. It is a figure of the Brillouin Zone which shows the structure of the nanohole having _C6v symmetry in the non-magnetic waveguide type isolator which concerns on embodiment of this invention. It is a band diagram of the photonic structure and the topological photonic structure used in the topological edge transmission line in the non-magnetic waveguide type isolator according to the embodiment of the present invention. It is a schematic diagram of the band diagram of the photonic structure and the topological photonic structure used in the topological edge transmission line in the non-magnetic waveguide type isolator according to the embodiment of the present invention. It is a figure which shows the magnetic field distribution (Hy) near the boundary of the topological photonic structure in the non-magnetic waveguide type isolator which concerns on embodiment of this invention. It is a figure explaining the reduction of the return light on the optical integrated circuit which has the topological photonic structure of the non-magnetic waveguide type isolator which concerns on embodiment of this invention. It is a figure explaining the unidirectional light propagation in the optical integrated circuit which has the topological photonic structure of the non-magnetic waveguide type isolator which concerns on embodiment of this invention. It is a figure explaining the unidirectional light propagation in the optical integrated circuit which has the topological photonic structure of the non-magnetic waveguide type isolator which concerns on embodiment of this invention. It is a figure explaining the chiral antenna of the non-magnetic waveguide type isolator which concerns on embodiment of this invention. It is a figure which shows the structure which arranged the chiral antenna on the topological photonic structure of FIG. This is a simulation model in which a monitor is mounted on the optical input / output side of the optical integrated circuit by the topological edge transmission line of FIG. This is a simulation model in which two chiral antennas are arranged on the topological photonic structure of FIG. 12 and a monitor is placed on the optical input / output side of an optical integrated circuit. This is a simulation model in which two chiral antennas 20R are arranged on the topological photonic structure of FIG. 12 and a monitor is placed on the optical input / output side of an optical integrated circuit. In the non-magnetic waveguide type isolator according to the embodiment of the present invention, there is no chiral antenna, one chiral antenna 20, one chiral antenna 20R, two chiral antennas 20R, and two chiral antennas 20R. It is a graph which compares and shows the Loss of the light propagation from left to right and the Loss of light propagation from right to left. It is a figure which shows the simulation result of the simulation model of FIG. 16 in a table. It is a figure which shows the magnetic field distribution (Hy) when two chiral antennas 20 are arranged on the topological photonic structure shown in FIG. It is a figure which shows the magnetic field distribution (Hy) when two chiral antennas 20R are arranged on the topological photonic structure shown in FIG. It is a top view of the non-magnetic waveguide type isolator of the modification which concerns on embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(Embodiment)
FIG. 1 is a perspective view showing the structure of a non-magnetic waveguide type isolator according to an embodiment of the present invention. FIG. 2 is a cross section (Cross section of topological edge state waveguide) of FIG. FIG. 3 is a top view (Si-based topological edge state waveguide we used in simulation) of the non-magnetic waveguide type isolator of FIG.

Attempts to trace the topology of electronic systems such as topological insulators and Weyl Semimetal to photon systems are called topological photonics and have been rapidly progressing in recent years. A topological insulator is a substance in which a bulk is an insulator having an energy gap, but a gapless metallic state is generated at an edge (an edge in a two-dimensional system and a surface in a three-dimensional system).

In particular, C _6v symmetry (one class in the field of topological structures with electron wave function) Z ₂ topologies in the structure in which a dielectric is arranged in a honeycomb lattice shape having a (60 ° Rotation overlap symmetry) The manifestation allows the realization of a topological edge state in which optical vortices can be transmitted.

As shown in FIGS. 1 to 3, the non-magnetic waveguide isolator 1 is an insulator having an energy gap inside, and its edge is a gapless metallic state. Topological photonic structure. 12 and a chiral metamaterial 20 arranged on the surface of the topological photonic structure 12.
In the present embodiment, the non-magnetic waveguide type isolator 1 includes a photonic structure 11 in which the bulk is an insulator having an energy gap, a topological photonic structure 12, and a photonic structure 11. And a topological edge 13 that expresses a topological edge state capable of propagating an optical vortex at the boundary of the topological photonic structure 12, and a metamaterial 20 arranged in the vicinity of the topological edge 13.

<Metamaterial 20>
The metamaterial 20 interacts with light having a polarization or an optical vortex in one of the rotation directions of a specific polarized light or an optical vortex having a direction dependence in which the rotation direction is reversed depending on the transmission direction. ..
The metamaterial 20 is a nanoscale metal structure having chiral symmetry with respect to circularly polarized light.
The metamaterial 20 may be a dielectric having chiral symmetry with respect to circularly polarized light.
The metamaterial 20 is an antenna having a structure that satisfies a resonance condition of interacting in a predetermined wavelength band including a 1.55 μm wavelength band.
The metamaterial 20 is a swastika-shaped or inverted swastika-shaped chiral antenna when viewed from the surface of the topological photonic structure 12.
The metamaterial 20 is arranged in one or more in the topological photonic structure 12.

<Topological photonic structure 12>
The light state (topological edge state) generated at the interface between the two regions created by the topological photonic structure 12 allows only light having a specific polarization / optical vortex, and these specific polarization / optical vortex are in the propagation direction. Has a dependency.

The photonic structure 11 is a structure in which _{the first dielectric 111 having C 6v} symmetry is arranged in a honeycomb lattice pattern (for example, period a = 800 nm).
The topological photonic structure 12 is a structure in which _{the second dielectric 112 having C 6v} symmetry is arranged in a honeycomb lattice pattern (for example, period a = 800 nm).

_{The first dielectric 111 is a nano (nm) in which nanoholes 111a having C 6v} symmetry are arranged in a honeycomb lattice-like (period a = 800 nm) cell (unit cell) 121 on an SOI (Silicon-On-Insulator) wafer. , 1 nm = ^10-9 m) structure is used.
_{The second dielectric 112 uses a nanostructure in} which nanoholes 112a having C 6v symmetry are arranged in a honeycomb lattice-shaped (period a = 800 nm) cell 122 on an SOI wafer (for example, Si film thickness 220 nm) 131. The nanohole 111a of the first dielectric 111 and the nanohole 112a of the second dielectric 112 have parameters of the distance r from the center of the honeycomb lattice cells 121 and 122 to the center of the nanoholes 111a and 112a and the length l of one side of the nanohole, respectively. Different (see below).

As shown in FIG. 2, the topological edge transmission line consists of a SiO ₂ insulating film 132 having a film thickness of 1.0 μm, a Si film 133 having a film thickness of 220 nm, and Au having a film thickness of 20 nm on a Si substrate 131. The metamaterial 20 made of the material 20 is laminated. The air gap on the Si film 133 is 1.0 μm or more.

In the Si film 133, nanoholes 111a and nanoholes 112a having C6v symmetry are opened toward the Si substrate 131, and a photonic structure composed of the residual Si film 133 and the nanoholes 111a _{opened in the Si film 133.} Form the photonic structure 11. Further, the photonic structure composed of the residual Si film 133 and the nanohole 112a opened in the Si film 133 forms the topological photonic structure 12.
A topological edge mode is expressed in the topological edge 13 at the boundary between the photonic structure 11 and the topological photonic structure 12.

FIG. 4 is a Schematic image of a unit cell showing the structure of a nanohole having _{C 6v symmetry.} Taking the nanohole 111a of the first dielectric 111 of the photonic structure 11 as an example. The structure is the same for the nanohole 112a of the second dielectric 112 of the topological photonic structure 12.
As shown in the right figure of FIG. 4, the center of the cell 121 of the honeycomb lattice is set as the center (origin) Γ point of the Brillouin zone. The high symmetry points of the Brill en zone are point M (center of rectangular surface), point K (center of side connecting two rectangular surfaces), point A (center of hexagonal surface), point H (end point), and L. There is a point (the center of the side connecting the hexagonal surface and the rectangular surface).

As shown in the left figure of FIG. 4, the Si film 133 (see FIG. 3) is formed with honeycomb lattice-shaped cells 121 and nanoholes 111a having _{C6v symmetry arranged in the cells 121.} The photonic structure composed of the residual 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 figure of FIG. 4 is a view of the cell 121 of the photonic structure 11 viewed from diagonally above in front of the upper surface, and the SiO ₂ insulating film 132 under the opened nanohole 111a is exposed.
The nanohole 111a in the left figure of FIG. 4 has parameters of the distance r from the center (Γ point) of the cell 121 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 of the cells 121 of the 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.
Further, in the case of the nanohole 112a of the topological photonic structure 12, for example, r = 290 nm and l = 250 nm.
Further, as shown in the left figure of FIG. 4, the distances a1 and a2 between the centers (Γ points) of the cells 121 of the adjacent honeycomb lattices are the same (here, a1 = a2 = 800 nm≡a).

FIG. 5 is a band diagram (Typical photonic bands for (left) trivial and) of the photonic structure 11 and the topological photonic structure 12 used in the topological edge transmission line in the non-magnetic waveguide type isolator according to the embodiment of the present invention. (right) topological photonic crystals). The horizontal axis of FIG. 5 is the Wave vector (2 ■ / ■), and the vertical axis is the Normalized frequency (ωa / 2 ■ c ■ / λ). The Γ point of the Wave vector (2 ■ / ■) on the horizontal axis is the center of the Brillouin zone of the honeycomb lattice-shaped cell 121 (see Fig. 2), the K point is the center of the side connecting the two rectangular surfaces, and the M point is a rectangle. It is the center of the surface (see the right figure in FIG. 4). FIG. 6 is a schematic diagram (Band diagram for optical vortex propagation) of the photonic structure 11 and the topological photonic structure 12 used in the topological edge transmission line in the non-magnetic waveguide type isolator according to the embodiment of the present invention. with charge number of ± 1).

FIG. 7 shows the magnetic field distribution (Hz) near the topological edge transmission path (boundary between the photonic structure 11 and the topological photonic structure 12) calculated by the FDTD method (Finite-difference time-domain method). ) Is shown in the figure (Calculated magnetic field Hy). The horizontal axis is x-axis (μm), and the vertical axis is y-axis (μm).
The shades in FIG. 7 represent the intensity of the magnetic field distribution (Hz) (the darker the intensity, the greater the intensity). As shown in FIG. 7, the electromagnetic field is localized at the topological edge 13 at the boundary between the photonic structure 11 and the topological photonic structure 12.

Here, the topological edge transmission line may be any one having a topological edge 13 that expresses a topological edge state at the boundary between the photonic structure 11 and the topological photonic structure 12, and the photonic structure is not limited. For example, the dielectrics are _{not limited to being arranged in a honeycomb lattice with C 6v} symmetry, and the obvious photonic structure is the first dielectric with symmetry within the arranged cells. The topological photonic structure may include a second dielectric having symmetry within the arranged cells.

Further, the method for forming the dielectric is not limited to the nanoholes described above, and for example, a configuration in which a dielectric pillar is provided may be used. Furthermore, the number of nanoholes is not limited as well as the parameters of the nanoholes. However, when the cells are arranged in a honeycomb lattice, the arrangement of the nanoholes naturally has a structure having _{C 6v symmetry.} Similarly, the shape of the cell is not limited to the honeycomb grid pattern.

<Reduction of return light>
FIG. 8 is a diagram illustrating reduction of return light on an optical integrated circuit having a topological edge 13.
The topological edge transmission line 40 using the topological photonic structure 12 is a Si-based waveguide (Si waveguide) 41 for Input that transmits light in TE (Transverse Electric) / TM (Transverse Magnetic) mode, and a TE / TM mode. It has a Si-based waveguide (Si waveguide) 42 for monitoring that transmits light, and a topological edge 13 that converts TE / TM mode transmission to optical vortex transmission.

The Si-based waveguide 41 for Input and the Si-based waveguide 42 for Monitor are, for example, a c-Si waveguide made of c-Si (crystal silicon) or a-Si: H made of a-Si (amorphous silicon): H. It is a waveguide. The material of the Si-based waveguide 41 for Input and the Si-based waveguide 42 for Monitor is not limited to silicon (Si) and may be any material. For example, the material of the waveguide may be a compound semiconductor (for example, InP).
The Si-based waveguide 41 for Input and the Si-based waveguide 42 for Monitor have tapers 41a and 42a whose width narrows toward the tip. The optical axis extending from the head of the taper 41a, 42a of the Si-based waveguide 41 for Input and the Si-based waveguide 42 for Monitor is the topological edge of the boundary between the tapered photonic structure 11 and the topological photonic structure 12. I'm heading for 13.

As shown by the white arrow in FIG. 8, the input light 51 in the TE / TM mode inserted in the Si system waveguide 41 for input is the boundary between the photonic structure 11 and the topological photonic structure 12. It is input to the topological edge 13 in the topological edge state, and is transmitted by an optical vortex (not shown) on the topological transmission path. That is, the TE / TM mode input light 51 inserted in the input Si system waveguide 41 is converted into an optical vortex with high efficiency in the topological edge state. Further, the optical vortex transmitted on the topological transmission path is guided to the Si-based waveguide 42 for monitoring and converted into the light 51 in the TE / TM mode with high efficiency.

The light emitted from the Si-based waveguide 42 for a monitor may be incident as reflected return light from an emission end that emits light in, for example, a DFB laser. As shown by the shaded arrow in FIG. 8, the return light from Si waveguide 52 from the monitor Si-based waveguide 42 is undesired light propagation on the optical integrated circuit.

As shown in FIG. 8, the light state (topological edge state) generated at the interface between the two regions created by the photonic structure 11 and the topological photonic structure 12 allows only light having a specific polarized light / optical vortex. However, they are directionally dependent. Specifically, the rotation direction of the polarized light / optical vortex is reversed depending on the transmission direction.

As described above, in the optical integrated circuit having the photonic structure 11 and the topological photonic structure 12 of FIG. 8, the following (1) and (2) are required.
(1) The topological edge 13 propagates a specific polarized light or optical vortex in a single direction.
(2) Reduce the return light on the optical integrated circuit.

9 and 10 are diagrams illustrating unidirectional light propagation in an optical integrated circuit with the topological photonic structure 12 of FIG. FIG. 9 shows an example in which a chiral antenna (LCP) having a counterclockwise chiral property (see the left figure of FIG. 11) is arranged above the surface of the topological edge 13 of the topological photonic structure 12 of FIG. FIG. 10 shows an example in which a chiral antenna (RCP) (see the right figure of FIG. 11), which also has a clockwise chiral property, is arranged.

Chiral antennas LCP and RCP, which absorb and emit large amounts of light, are arranged on the optical integrated circuit for each polarized light.
As shown by the arrow in FIG. 9, by arranging the chiral antenna LCP at the topological edge 13 portion, unidirectional light propagation depending on the LCP (leftward light propagation in FIG. 9) can be realized.
Further, as shown by the arrow in FIG. 10, by arranging the chiral antenna RCP at the topological edge 13 portion, unidirectional light propagation depending on the RCP (rightward light propagation in FIG. 10) can be realized.

In this way, by arranging a chiral antenna having a large absorption / emission for each polarized light on the device, light propagation in the corresponding direction can be suppressed.

FIG. 11 is a diagram illustrating a chiral antenna. The left figure of FIG. 11 is a diagram showing a chiral antenna (LCP) 20 having a counterclockwise chiral property, and the right figure of FIG. 11 is a diagram showing a chiral antenna (RCP) 20R having a clockwise chiral property. ..

The chiral antennas 20 and 20R have a chiral property with respect to circularly polarized light, and interact with light having either polarized light or an optical vortex in one of the rotational directions to radiate (loss) light. Any structure (including appearance shape) and material may be used. For example, the chiral antennas 20 and 20R have a nanoscale metal structure having chiral properties with respect to circularly polarized light. Further, it may be a dielectric having chiral properties (not shown).

The chiral antennas 20 and 20R are antennas having a structure that satisfies the resonance condition of interacting in a predetermined wavelength band including the 1.55 μm wavelength band. The chiral antennas 20, 20R interact with and emit light having a polarization or an optical vortex in one of the rotational directions of a specific polarized light or an optical vortex having a direction dependence. Specifically, when an electromagnetic wave enters the chiral antennas 20 and 20R, the electrons resonate and vibrate in the chiral antenna, and some of them are lost (absorbed) due to heat or the like, and the electrons in the chiral antenna are generated by the induced current. Light is emitted above the chiral antenna in an electromagnetic field via (radiation).

The chiral antenna (LCP) 20 and the chiral antenna (RCP) 20R are mirror image symmetric with respect to the broken line in the left figure of FIG.
The chiral antenna (LCP) 20 has, for example, a swastika shape when viewed from the surface of the topological photonic region, and the chiral antenna (LCP) 20R has an inverted swastika shape.

For the chiral antennas 20 and 20R, the material is Au, the length of one side when divided into four equal parts from the center point is 200 nm, the width Side of the metamaterial is 90 nm, and the thickness Height of the metamaterial is 20 nm.
The materials of the metamaterials constituting the chiral antennas 20 and 20R and the dimensions of each part are examples and are not limited. Further, the shape of the chiral antenna is not limited, and the chiral antenna 20A having the shape shown in FIG. 20 described later may be used.

<Structural example of non-magnetic waveguide type isolator>
FIG. 12 is a diagram showing a non-magnetic waveguide type isolator 1 in which a chiral antenna (LCP) 20 is arranged on the topological photonic structure 12 of FIG.
As shown in FIG. 12, in the non-magnetic waveguide type isolator 1, the chiral antenna (LCP) 20 shown in the left figure of FIG. 11 is arranged on the topological photonic structure 12 shown in FIG.

By arranging the chiral antenna (LCP) 20 having a large absorption / emission with respect to polarized light on the topological photonic structure 12, light propagation in the corresponding direction can be suppressed. That is, the chiral antenna (LCP) 20 has a chiral property with respect to circularly polarized light, and is a Si-based waveguide for monitoring due to an interaction with light having either polarized light in the rotation direction or an optical vortex. Since the return light 52 from 42 is radiated (loss), the optical integrated circuit operates as a unidirectional isolator.

[simulation]
In order to realize the non-magnetic waveguide type isolator 1 according to the present embodiment, two techniques, "a technique for forming a hole on a silicon optical circuit" and "a technique for arranging a nanoscale metal structure", are required. However, both can be achieved using standard silicon photonics processes. Simulation models (conditions) are shown in FIGS. 12 to 15, and simulation results are shown in FIGS. 16 and 17.

FIG. 13 is a simulation model in which monitors 61 and 62 are mounted on the optical input / output side of the optical integrated circuit by the topological edge 13 of FIG. 8, and the left figure of FIG. 13 is shown by the arrow of the left figure of FIG. As shown in the example (propagation of light from left to right) in which the Incident light 51 from the left side is detected by the monitors 61 and 62, the figure on the right in FIG. 13 is shown by the arrow in the figure on the right in FIG. This is an example (propagation of light from right to left) in which the input light 51 from the right side is detected by the monitors 62 and 61. The simulation results of the simulation model of FIG. 13 are shown in "WG" of FIGS. 16 and 17.

FIG. 14 shows a simulation in which two chiral antennas 20 (see the left figure of FIG. 11) are arranged on the topological photonic structure 12 of FIG. 12, and monitors 61 and 62 are placed on the optical input / output side of an optical integrated circuit. It is a model. As shown by the arrow in the left figure of FIG. 14, the left figure of FIG. 14 shows an example in which the input light 51 from the left side is detected by the monitors 61 and 62 (propagation of light from left to right), and the right figure of FIG. 14 is a diagram. 14 As shown by the arrow in the right figure, this is an example (propagation of light from right to left) in which the input light 51 from the right side is detected by the monitors 62 and 61. The simulation results of the simulation model of FIG. 14 are shown in “swastika × 2” of FIGS. 16 and 17.

FIG. 15 shows a simulation in which two chiral antennas 20R (see the right figure of FIG. 11) are arranged on the topological photonic structure 12 of FIG. 12, and monitors 61 and 62 are placed on the optical input / output side of an optical integrated circuit. It is a model. As shown by the arrow in the left figure of FIG. 15, the left figure of FIG. 15 shows an example in which the input light 51 from the left side is detected by the monitors 61 and 62 (propagation from left to right), and the right figure of FIG. 15 shows the right side of FIG. As shown by the arrow in the figure, this is an example (propagation from right to left) in which the input light 51 from the right side is detected by the monitors 62 and 61. The simulation results of the simulation model of FIG. 15 are shown in “reverse swastika × 2” of FIGS. 16 and 17.

The relationship between the rotation direction and the propagation direction of the chiral antenna will be described.
The chiral antenna interacts with and absorbs circularly polarized light with light having either polarized light or an optical vortex in one of the rotational directions, and emits (losses) the light having this polarized light or an optical vortex. The absorption / radiation amount absorbed / emitted by the chiral antenna for each polarized light is the propagation loss Loss [dB], which is expressed by the following equation (1). The larger the Loss, the larger the amount of absorption and radiation in the antenna. There is a propagation loss loss even in the forward direction (see No chiral antenna (WG) in FIGS. 16 and 17).

FIG. 16 shows no chiral antenna (WG), one chiral antenna 20 (swastika x 1), one chiral antenna 20R (reverse swastika x 1), two chiral antennas 20 (swastika x 2), and chiral. A graph showing a comparison between the loss of light propagation from left to right and the loss of light propagation from right to left in each of two antennas 20R (reverse swastika × 2). Is.

FIG. 17 is a diagram showing the simulation results of the simulation model of FIG. 16 in a table.
As shown in FIGS. 16 and 17, there is no significant difference in Loss in the case of no chiral antenna (WG) with no chiral antenna on topological photonic structure 12.

As shown in FIGS. 16 and 17, in the simulation model (swastika × 1) in which one chiral antenna 20 is arranged, the light propagation from left to right is small loss, and the light propagation from right to left is large loss. Is. In addition, the simulation model (swastika x 2) in which two chiral antennas 20 are arranged has a difference in loss of light propagation (difference in absorption amount between left and right) as compared with the case where one chiral antenna 20 and 20R is arranged. , Remarkably large.

Similarly, in the simulation model (reverse swastika × 1) in which one chiral antenna 20 is arranged, the light propagation from left to right has a large loss, and the light propagation from right to left has a small loss. In the simulation model (reverse swastika x 2) in which two chiral antennas 20R are arranged, the difference in loss of light propagation (difference in absorption amount between left and right) is significantly larger than that in the case where one chiral antenna 20R is arranged. big.

In this way, in the simulation model (swastika x 1, swastika x 2) in which the chiral antenna 20 is arranged on the topological photonic structure 12, the light propagation from left to right is small loss, and the light propagates from right to left. Propagation is lossy. Further, it was confirmed that when two chiral antennas 20 were arranged, the difference in loss of light propagation was larger than when one chiral antenna 20 was arranged. For example, when two chiral antennas 20 are arranged, the difference in absorption amount between the left and right is 5 [dB].
Similarly, in the simulation model (swastika x 1, swastika x 2) in which the chiral antenna 20R is arranged on the topological photonic structure 12, the light propagation from left to right is large loss, and the light propagation from right to left is large. Is small loss. Further, it was confirmed that when two chiral antennas 20R were arranged, the difference in loss of light propagation was larger than that when one chiral antenna 20R was arranged.

[Actual measurement]
FIG. 18 is a diagram (Calculated magnetic field Hy) showing a magnetic field distribution (Hz) when two chiral antennas 20 are arranged on the topological photonic structure 12 shown in FIG. The vertical axis is the z-axis (μm), and the horizontal axis is the y-axis (μm). The shades in FIG. 18 represent the intensity of the magnetic field distribution (Hz) (the darker the intensity, the greater the intensity).

As shown in the left figure of FIG. 18, in the propagation of light from left to right, the light propagates to the second chiral antenna 20 on the right side without loss without being affected by the first chiral antenna 20 on the left side. It was confirmed that the light propagation from left to right had a small loss. Further, as shown in the right figure of FIG. 18, in the propagation of light from right to left, the light that interacts with the first chiral antenna 20 on the right side and is radiated (loss) is on the left side. It was confirmed that the light propagating to the chiral antenna 20, that is, the light propagating from right to left has a large loss.

FIG. 19 is a diagram (Calculated magnetic field Hz) showing the magnetic field distribution (Hy) when two chiral antennas 20R are arranged on the topological photonic structure 12 shown in FIG. The vertical axis is y-axis (μm), and the horizontal axis is x-axis (μm). The shades in FIG. 18 represent the intensity of the magnetic field distribution (Hz) (the darker the intensity, the greater the intensity).

As shown in the left figure of FIG. 19, the propagation from left to right interacts with the first chiral antenna 20R on the left side, and the emitted (loss) light is transferred to the chiral antenna 20 on the right side. It was confirmed that the light propagating, that is, the light propagating from left to right has a large loss. On the other hand, as shown in the right figure of FIG. 19, in the propagation from right to left, the chiral antenna 20R on the second left side is not affected by the first chiral antenna 20R on the right side, and there is no loss on the chiral antenna 20R on the second left side. It was confirmed that the light propagated and the light propagation from right to left was small loss.

[Modification example]
FIG. 20 is a top view of a non-magnetic waveguide type isolator of a modified example according to the embodiment of the present invention. The same components as those in FIG. 3 are designated by the same reference numerals.
The non-magnetic waveguide isolator 1 of the modification has a chiral antenna 20A on the topological photonic structure 12.

The chiral antenna 20A is a nanoscale metal structure having chiral properties. The chiral antenna 20A has a four-wing windmill shape when viewed from above, and is arranged along the topological edge 13 so that the direction of the wings is horizontal and perpendicular to the optical vortex propagation direction.

As described above, the non-magnetic waveguide type isolator 1 (see FIG. 3) according to the present embodiment is a topological photonic structure in which the inside is an insulator having an energy gap and the edge thereof is a gapless metallic state. It has a body 12 and chiral antennas 20 and 20R (metamaterials) arranged on the surface of the topological photonic structure 12.

With this configuration, by arranging the chiral antennas 20 and 20R having a large absorption / emission for each polarization on the device, light having either polarization / optical vortex is emitted (loss). That is, as shown in FIG. 3, in the non-magnetic waveguide type isolator 1, the input light 51 propagates in an optical vortex in a topological edge state. Further, the return light (Counter propagation) 52 to the non-magnetic waveguide type isolator 1 interacts with the chiral antenna 20 to become a Radiation of counter propagating light 53 and radiate (loss). As a result, the return light 52 becomes the radiated (loss) return light 54. As a result, light propagation in the corresponding direction can be suppressed, and the optical integrated device operates as a one-way isolator. It is possible to realize a non-magnetic waveguide type isolator that can be integrated on topological photonic integrated circuits (T-PICs).

The non-magnetic waveguide type isolator 1 according to the present embodiment can easily monolithically integrate with other elements formed on a silicon optical circuit, and can be made of another material system (magnetic material system) such as a garnet crystal on a silicon platform. There is no need to introduce it to. It is promising as a solution to problems with existing technologies.

In the present embodiment, the non-magnetic waveguide type isolator 1 includes a photonic structure 11, a topological photonic structure 12, a photonic structure 11, and a topological photonic structure in which the bulk is an insulator having an energy gap. It has a topological edge 13 that expresses a topological edge state capable of propagating an optical vortex at the boundary of the body 12, and 20, 20R arranged in the vicinity of the topological edge 13.

With this configuration, it is possible to realize a non-magnetic waveguide type isolator capable of transmitting an optical vortex in a topological edge state capable of special light propagation.

The non-magnetic waveguide type isolator 10 can be applied to an optical circuit having topological characteristics. Non-magnetic waveguide isolators that use optical vortex transmission can be formed on semiconductor substrates (silicon, InP, etc.) for consistency with semiconductor lasers and multi-core fibers, which are key components of large-capacity transmission. Since it is also excellent, it can be expected to improve its affinity with optical communication.

The present invention is not limited to the above embodiment, and includes other modifications and applications as long as it does not deviate from the gist of the present invention described in the claims.
For example, the chiral metamaterial may be arranged in any arrangement as long as it is arranged in the topological photonic structure. Here, the arrangement of the topological photonic structure includes the case where the topological photonic structure is arranged in the vicinity of the topological edge (the vicinity of the topological edge also includes the photonic structure).

Further, the structure, arrangement, dimensions, number, and material of the metamaterial are not limited to the embodiments / modifications. The number of metamaterials may be three or more, but keep in mind that the forward propagation loss increases. Further, since the metamaterial has a nanoscale metal structure, it has an advantage that it is easy to create a chiral antenna. However, when metamaterial metal is used, the interaction is large and the forward propagation loss is large. The metamaterial may be manufactured using a dielectric, and there is a possibility that the interaction can be reduced by using a dielectric, but several metamaterials are arranged for use as an isolator.

Further, in the above embodiment, the name of the non-magnetic waveguide type isolator is used, but this is for convenience of explanation, and may be an optical isolator or the like.

1 Non-magnetic waveguide type isolator 11 Photonic structure (Trivial photonic structure)
12 Topological photonic structure
13 Topological edge
20,20R, 20A Chiral antenna (Chiral metamaterial)
40 Topological edge transmission line 41 Si-based waveguide for Input 42 Si-based waveguide for Monitor 51 Incident light
52 Return light (Counter propagation)
53 Radiation of counter propagating light
54 Radiated (loss) return light 111 1st dielectric 112 2nd dielectric 121 Photonic structure cell 122 Topological photonic structure cell

Claims (14)

A topological photonic structure, which is an insulator with an energy gap inside and whose edges are in a gapless metallic state,
A non-magnetic waveguide type isolator characterized by having a chiral metamaterial arranged in the topological photonic structure. The metamaterial interacts with light having the polarized light or the optical vortex in one of the rotational directions of a specific polarized light or an optical vortex having a direction dependence in which the rotation direction is reversed depending on the transmission direction. The non-magnetic waveguide type isolator according to claim 1, wherein the light is emitted. The non-magnetic waveguide type isolator according to claim 1, wherein the metamaterial has a nanoscale metal structure having chiral symmetry with respect to circularly polarized light. The non-magnetic waveguide type isolator according to claim 1, wherein the metamaterial is a dielectric having chiral symmetry with respect to circularly polarized light. The non-magnetic waveguide type isolator according to claim 1, wherein the metamaterial is an antenna having a structure that satisfies a resonance condition that interacts in a predetermined wavelength band including a 1.55 μm wavelength band. The non-magnetic waveguide type isolator according to claim 1, wherein the metamaterial is a swastika-shaped or inverted swastika-shaped chiral antenna when viewed from the surface of the topological photonic structure. The non-magnetic waveguide type isolator according to claim 1, wherein the metamaterial is arranged one or more in the topological photonic structure. The Trivial photonic structure, in which the bulk is an insulator with an energy gap,
The non-magnetism according to claim 1, wherein the photonic structure has a topological edge that expresses a topological edge state capable of propagating an optical vortex at the boundary between the photonic structure and the topological photonic structure. Waveguide isolator. The non-magnetic waveguide type isolator according to claim 8, wherein the topological edge allows light having a specific polarized light or an optical vortex. The non-magnetic waveguide type isolator according to claim 9, wherein the specific polarized light or the optical vortex has a direction dependence in which the rotation direction of the polarized light or the optical vortex is reversed depending on the transmission direction. The non-magnetic waveguide type isolator according to claim 8, wherein the metamaterial is arranged in the vicinity of the topological edge. The non-magnetic waveguide type isolator according to claim 1, wherein the topological photonic structure includes a dielectric having symmetry in the arranged cells. The non-magnetic waveguide isolator according to claim 1, wherein the topological photonic structure comprises a structure in which _{dielectrics including nanoholes having C 6v symmetry are arranged in honeycomb lattice cells.} The topological photonic structure, non-magnetic waveguide isolator of claim 1, characterized in that the topological structure represented by Z ₂ topology.

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