Energy valley photonic crystal structure, optical waveguide and optical beam splitter

Abstract

The invention relates to an energy valley photonic crystal structure, an optical waveguide and an optical beam splitter. According to the energy valley photonic crystal structure, the time reversal symmetry of lattice space is broken by changing the ratio of the silicon dielectric cylinders to air in the photonic crystal, namely the duty ratio of lattices, and the energy valley photonic crystal (VPC) with C3 symmetry is obtained, so that topological phase change is achieved, and higher stability is achieved. The optical waveguide provided by the invention has a good unidirectional transmission characteristic, and has good robustness for defects such as introduced cavities, disorder, sharp turns and the like. The light beam splitter has two different boundary states, light splitting can be achieved at multiple frequencies, the beam splitting ratio of the light splitter fluctuates within the range of 0.2-4.9 along with the change of the incident light frequency, and equal power light beam splitting can be achieved at multiple frequencies.

Description

Technical Field

[0001] The invention relates to the technical field of photonic crystals, and in particular to a valley photonic crystal structure, an optical waveguide and an optical beam splitter. Background Art

[0002] In the study of topological photonics, photonic crystals, as an artificial periodic dielectric structure, provide an important platform for achieving efficient optical signal transmission. However, traditional photonic crystals are prone to scattering losses due to manufacturing defects and structural disorder during the transmission process, resulting in reduced transmission efficiency. To overcome this problem, the field of topological photonics has introduced topologically protected edge states, which enable optical signals to be transmitted unidirectionally in edge channels, have anti-scattering robustness, and maintain good transmission performance even in the presence of defects or tortuous paths.

[0003] So far, the realization of topological boundary states is mostly achieved by splicing different topological photonic crystals, but different splicing methods will also have different effects on the transmission effect: the first is the Bread-type splicing method, in which the photonic crystals at the boundary are arranged relatively up and down to obtain a bread-like boundary; the second is the Ziazag-type splicing method, in which the photonic crystals at the boundary are staggered like a zipper; the third is composed of three photonic crystals, and different boundary states are obtained by changing the positions of the upper and lower photonic crystals. The above-mentioned splicing methods are widely used, that is, they only use one boundary state to study the boundary transmission effect of the system. Since one boundary state can only support one boundary transmission mode, the single boundary splicing method makes the research on the boundary state and its constructed devices relatively limited, so it is very necessary to study the impact of different boundary state structures on the working performance of the device. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a valley photonic crystal structure, an optical waveguide and an optical beam splitter.

[0005] A valley photonic crystal structure is formed by a plurality of regular hexagonal unit cells which diverge from the center of their cross sections in six directions around them and are arranged in an array, and the distance between the centers of adjacent regular hexagonal unit cells is a lattice constant a; a single regular hexagonal unit cell is formed by first-type silicon dielectric columns and second-type silicon dielectric columns being evenly placed in the air, and the first-type silicon dielectric columns and the second-type silicon dielectric columns are respectively arranged in a triangular lattice and staggered to form a regular hexagonal basic unit, that is, the first-type silicon dielectric columns and the second-type silicon dielectric columns are alternately arranged at the six vertices of the regular hexagon; The cross section of the first type of silicon dielectric column is an equilateral triangle; the cross section of the second type of silicon dielectric column is an equilateral triangle ring with an equilateral triangle hole, and the size of the equilateral triangle hole is the same as that of the first type of silicon dielectric column.

[0006] Preferably, the unit cell when the upper vertex of the regular hexagon is a first type silicon dielectric column is recorded as VPC1, and the unit cell when the upper vertex of the regular hexagon is a second type silicon dielectric column is recorded as VPC2.

[0007] Preferably, the distance from the center to the vertex of the first type of silicon dielectric column is , the distance from the center to the vertex of the second type of silicon dielectric column is r2=2r1.

[0008] Preferably, the lattice constant a=346nm.

[0009] An optical waveguide comprises a waveguide interface and the above-mentioned valley photonic crystal structure, wherein the valley photonic crystal structure comprises a plurality of unit cells VPC1 and a plurality of unit cells VPC2; the plurality of unit cells VPC1 and the plurality of unit cells VPC2 are respectively distributed and arranged on both sides of the waveguide interface.

[0010] Preferably, when the unit cell VPC1 and the unit cell VPC2 are symmetrically distributed with the waveguide interface as the symmetry axis, the optical waveguide has a “Bridge” type boundary.

[0011] Preferably, when the second type of silicon dielectric pillars of the unit cell VPC1 and the second type of silicon dielectric pillars of the unit cell VPC2 are staggeredly distributed on the waveguide interface, the optical waveguide has a "zig-zag" type boundary.

[0012] A beam splitter, the beam splitter is divided into a first region, a second region, a third region and a fourth region, the first region and the fourth region are composed of a plurality of unit cells VPC1, the second region and the third region are composed of a plurality of unit cells VPC2; the interface between the first region and the second region, the interface between the first region and the third region, and the interface between the second region and the fourth region respectively form the optical waveguide as described above; Among them, the optical waveguide formed at the interface between the first area and the second area is a light input waveguide, the optical waveguide formed at the interface between the first area and the third area is a first light output waveguide, and the optical waveguide formed at the interface between the second area and the fourth area is a second light output waveguide; the light input waveguide and the first light output waveguide adopt a "Bridge" type boundary; the second light output waveguide adopts a "zig-zag" type boundary.

[0013] The above technical solution of the present invention has the following advantages compared with the prior art: 1. The valley photonic crystal structure of the present invention breaks the time reversal symmetry of the lattice space by changing the ratio of silicon dielectric columns to air in the photonic crystal, that is, the duty cycle of the lattice, to obtain a valley photonic crystal (VPC) with C3 symmetry, thereby achieving topological phase transition and having higher stability.

[0014] 2. The optical waveguide described in the present invention has good unidirectional transmission characteristics and has good robustness against defects such as introduced cavities, dislocations and sharp bends.

[0015] 3. The two different boundary states of the optical beam splitter described in the present invention can realize splitting at multiple frequencies. The splitting ratio of the beam splitter fluctuates in the range of 0.2-4.9 as the frequency of the incident light changes, and equal power beam splitting can be achieved at multiple frequencies. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to make the contents of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings.

[0017] Figure 1 It is a schematic diagram of the valley photonic crystal structure of the present invention.

[0018] Figure 2 Figure 2 shows the photon band structure diagram of the primitive cell, unit cell VPC1 and unit cell VPC2.

[0019] Figure 3 are different boundaries of the waveguide interface; among them, Figure 3 (a) indicates that the optical waveguide has a "Bridge" type boundary Figure 3 (b) shows that the optical waveguide has a "zig-zag" type boundary.

[0020] Figure 4 The band structures under two different boundary conditions are given; among them, Figure 4 (a) The edge mode represents the valley pseudo-spin state with a “Bridge” type boundary optical waveguide; Figure 4 The edge modes shown in (b) represent the valley pseudo-spin states with a “zig-zag” type boundary optical waveguide.

[0021] Figure 5 is the Ez electric field intensity distribution when the excitation source frequency is 102.7THz; where, Figure 5 (a) "Bridge" type boundary optical waveguide and its Ez electric field intensity distribution; Figure 5 (b) “zig-zag” type boundary optical waveguide and its Ez electric field intensity distribution.

[0022] Figure 6 is an optical waveguide structure with defects introduced; wherein, Figure 6(a) is the constructed Z-type waveguide structure with a 120° sharp bend; Figure 6 (b) is the constructed Z-type waveguide structure with a 60° sharp bend; Figure 6 (c) is a “Bridge” type boundary optical waveguide with a cavity introduced; Figure 6 (d) is a “zig-zag” type boundary optical waveguide with a cavity introduced; Figure 6 (e) is a “Bridge” type boundary optical waveguide with disorder introduced; Figure 6 (f) is a “zig-zag” type boundary optical waveguide with disorder introduced.

[0023] Figure 7 The transmission rate comparison of optical waveguides without defects and containing cavities, disorder and sharp bends is given; among them, Figure 7 (a) is the transmission rate comparison of the “Bridge” type boundary optical waveguide. Figure 7 (b) Comparison of transmission rates of “zig-zag” type boundary optical waveguides.

[0024] Figure 8 Schematic diagram of the optical beam splitter of the present invention.

[0025] Fig. 9 is the electric field distribution of the optical beam splitter at a frequency of 96.97 (THZ).

[0026] Fig.10 This is the test result of electromagnetic wave output power of optical beam splitter channel; Fig.10 (a) is the electromagnetic wave power value detected in each output channel of the optical beam splitter within the operating frequency range of 92-108 (THZ), Fig.10 (b) is the spectral ratio curve of the optical beam splitter system.

[0027] Explanation of the reference numerals in the specification: 10, primitive cell; 11, unit cell VPC1; 12, unit cell VPC2; 20, original silicon dielectric column; 21, first type of silicon dielectric column; 22, second type of silicon dielectric column; 3, waveguide interface; 41, first region; 42, second region; 43, third region; 44, fourth region; 51, optical input waveguide; 52, first optical output waveguide; 53, second optical output waveguide; 6, excitation source. DETAILED DESCRIPTION

[0028] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0029] like Figure 1As shown, the present invention provides a valley photonic crystal structure, which is formed by a plurality of regular hexagonal unit cells that diverge from the center of their cross sections in six directions around them and are arranged in an array manner, and the distance between the centers of adjacent regular hexagonal unit cells is a lattice constant a; a single regular hexagonal unit cell is formed by first-type silicon dielectric columns 21 and second-type silicon dielectric columns 22 that are evenly placed in the air, and the first-type silicon dielectric columns 21 and the second-type silicon dielectric columns 22 are respectively arranged in a triangular lattice and staggered to form a regular hexagonal basic unit, that is, the first-type silicon dielectric columns 21 and the second-type silicon dielectric columns 22 are alternately arranged at the six vertices of the regular hexagon; wherein the cross section of the first-type silicon dielectric column 21 is an equilateral triangle; and the cross section of the second-type silicon dielectric column 22 is an equilateral triangle ring with equilateral triangle holes.

[0030] In a specific embodiment, the distance from the center to the top of the first type silicon dielectric column 21 is , the distance from the center to the vertex of the second type silicon dielectric column 22 is r2=2r1, and the size of the equilateral triangle hole is the same as the size of the first type silicon dielectric column 21.

[0031] The energy valley photonic crystal structure of the present application is obtained from the original energy valley photonic crystal structure. Figure 1 As shown in (a), the original valley photonic crystal structure is composed of original silicon dielectric columns 20 uniformly placed in the air with a as a period, a is the lattice constant, and the cross section of the original silicon dielectric column 20 is an equilateral triangle. The regular hexagonal area represents the valley photonic crystal unit cell 10, each unit cell 10 contains six identical dielectric columns, with C6 rotational symmetry, and the original silicon dielectric column 20 in a single unit cell 10 is a shield-shaped shield divided by equilateral triangles, with a short side length of , that is, the distance from the center to the top corner of the original silicon dielectric column 20 , where the lattice constant a=346nm. In order to make the energy band of the valley photonic crystal have different properties in different valleys, it is necessary to break the spatial inversion symmetry of the structure while keeping the time reversal symmetry unchanged. Figure 1 As shown in (b), the short side of the dielectric column in one triangular lattice of the primitive cell is shortened to 1 / 2 of the original length to obtain the first type of silicon dielectric column 21; the dielectric column in another triangular lattice of the primitive cell is removed with an area of ​​the same size as the first type of silicon dielectric column 21 and filled with air to obtain the second type of silicon dielectric column 22. The unit cell when the upper vertex of the regular hexagon is the first type of silicon dielectric column 21 is recorded as unit cell VPC111, and the unit cell when the upper vertex of the regular hexagon is the second type of silicon dielectric column 22 is recorded as unit cell VPC212.

[0032] Figure 2 The photon band structure diagram of the unit cell 10, the unit cell VPC111 and the unit cell VPC212 is shown in FIG. Figure 2 As shown in (a), the system is protected by both time reversal and space inversion symmetry. The first and second energy bands are doubly degenerate at point K (K′), resulting in a Dirac point with a normalized frequency of 0.287c / a. The linear dispersion relation formed around the Dirac point is called the Dirac cone. The illustration is a schematic diagram of the first Brillouin zone of the honeycomb lattice unit cell. b1 and b2 are the reciprocal lattice basis vectors of the unit cell in reciprocal space, and Γ, K, K′, and M are high symmetry points in the first Brillouin zone. The C6 rotational symmetry of the honeycomb lattice ensures that there is always a Dirac cone with a linear dispersion relation near point K (K′) in the first Brillouin zone, which provides a platform for realizing topological valley transport with pseudospin locked to valley energy.

[0033] The photonic band structure of the unit cell VPC111 or the unit cell VPC212 is as follows Figure 2 As shown in (b), a photonic band gap with a normalized frequency of 0.336~0.397c / a is formed between the first and second energy bands. At this time, the rotational symmetry of the honeycomb lattice is reduced from the original C6 to C3, so the high symmetry point K (K′) of the first Brillouin zone is degenerate, and the Dirac cone is no longer protected by the spatial inversion symmetry, resulting in the disappearance of double degeneracy and the formation of two unequal energy valleys.

[0034] Figure 2 (c) and Figure 2 (d) shows the phase and amplitude distribution of the electron measurement field Ez of the unit cell VPC212 under K (K′). Figure 2 (c) The phase distribution diagram of the unit cell VPC212 structure shows that in the same band, K and K' have opposite vortex chirality, which is indicated by the clockwise and counterclockwise black arrows in the figure. They describe the right circular polarization (RCP) and left circular polarization (LCP) orbital angular momentum, respectively, and indicate the existence of band inversion, indicating that the valley photonic crystal has a topological phase transition. Figure 2 (d) shows the amplitude of the Ez component, where a phase reversal can be observed, indicating that a topological phase transition has occurred.

[0035] like Figure 3-7 As shown, the present invention also provides an optical waveguide, including a waveguide interface 3 and the valley photonic crystal structure as described above, wherein the valley photonic crystal structure includes a plurality of unit cells VPC111 and a plurality of unit cells VPC212; the plurality of unit cells VPC111 and the plurality of unit cells VPC212 are respectively distributed and arranged on both sides of the waveguide interface 3.

[0036] like Figure 3 As shown in (a), when the unit cell VPC111 and the unit cell VPC212 are symmetrically distributed with the waveguide interface 3 as the symmetry axis, the optical waveguide has a "Bridge" type boundary. Figure 3As shown in (b), when the second type of silicon dielectric pillars 22 of the unit cell VPC111 and the second type of silicon dielectric pillars 22 of the unit cell VPC212 are staggeredly distributed on the waveguide interface 3, the optical waveguide has a "zig-zag" type boundary.

[0037] Figure 4 The band structures under two different boundary conditions are given, where the gray solid line represents the topological state. It can be seen that the edge state formed on the boundary between the states runs through the entire photonic band gap. Figure 4 (a) The edge mode represents the valley pseudo-spin state with a “Bridge” type boundary optical waveguide; Figure 4 The edge mode shown in (b) represents the valley pseudo-spin state with a "zig-zag" type boundary optical waveguide. The slope of the curve indicates the propagation direction of the topological boundary state. These two topological boundary states have opposite slopes at the K valley and K' valley, that is, the frequency of the pseudo-spin down state is higher than the frequency of the pseudo-spin up state at the K valley, and the frequency of the pseudo-spin up state is higher than the frequency of the pseudo-spin down state at the K' valley. As a result, valley polarization selectivity appears, that is, the K valley and K' valley are selective for left-handed circularly polarized (LCP) light and right-handed circularly polarized (RCP) light, and then the pseudo-spin and valley-locked valley polarized light selective coupling and transmission can be achieved.

[0038] In order to more intuitively reflect the unidirectional transmission of the structure, Figure 5 As shown, four point light sources are placed at the waveguide interface. These four point light sources are evenly distributed on a circle with a radius of 0.03a. By controlling their phase difference, LCP or RCP light can be formed respectively as the excitation source 6 of the electric field, and the Ez electric field intensity distribution is tested when the frequency of the excitation source 6 is 102.7THz (hereinafter referred to as 102.7THz). Figure 5 (a) "Bridge" type boundary optical waveguide and its Ez electric field intensity distribution; Figure 5 (b) is a “zig-zag” type boundary optical waveguide and its Ez electric field intensity distribution. Figure 5 It can be seen that when the excitation source 6 (LCP and RCP) is located at the center of the boundary, the LCP light is transmitted to the left along the boundary, and the RCP light is transmitted to the right along the boundary, without obvious backscattering, and the electromagnetic waves are transmitted near the boundary, thus showing good unidirectionality and optical localization.

[0039] When light is transmitted in traditional micro-nano photonic devices, it is inevitable to encounter interference effects such as large-angle turns and defects, which will reduce the transmission efficiency of light in photonic devices. However, topological photonic devices are more robust to the above problems. In order to verify the robustness of the structure, the following is introduced: Figure 6 The defects shown, Figure 6 (a) is the constructed Z-type waveguide structure with a 120° sharp bend; Figure 6(b) is the constructed Z-type waveguide structure with a 60° sharp bend; Figure 6 (c) is a “Bridge” type boundary optical waveguide with a cavity introduced; Figure 6 (d) is a “zig-zag” type boundary optical waveguide with a cavity introduced; Figure 6 (e) is a “Bridge” type boundary optical waveguide with disorder introduced; Figure 6 (f) is a “zig-zag” type boundary waveguide with disorder introduced. Fig. 9 It can be seen that the light is not affected by the cavity defects and disorder and continues to propagate forward, demonstrating the anti-interference ability of the structure in the face of large-angle bending situations.

[0040] Figure 7 The transmission rate comparison of optical waveguides without defects and containing cavities, disorder and sharp bends is given, where Figure 7 (a) is the transmission rate comparison of the “Bridge” type boundary optical waveguide. Figure 7 (b) is a comparison of the transmission rates of the "zig-zag" type boundary optical waveguide. As can be seen from the figure, after the introduction of defects, the transmission efficiency of the "Bridge" type boundary optical waveguide and the "zig-zag" type boundary optical waveguide can reach more than 95% in the frequency range of 97-108THz, which has a strong transmission capacity.

[0041] like Figure 8-10 As shown, the present invention further provides an optical beam splitter 4, which is divided into a first region 41, a second region 42, a third region 43 and a fourth region 44, the first region 41 and the fourth region 44 are composed of a plurality of unit cells VPC111, the second region 42 and the third region 43 are composed of a plurality of unit cells VPC212, and the interface between the first region 41 and the second region 42, the interface between the first region 41 and the third region 43, and the interface between the second region 42 and the fourth region 44 respectively form optical waveguides as described above; wherein, the optical waveguide formed at the interface between the first region 41 and the second region 42 is an optical input waveguide 51, the optical waveguide formed at the interface between the first region 41 and the third region 43 is a first optical output waveguide 52, and the optical waveguide formed at the interface between the second region 42 and the fourth region 44 is a second optical output waveguide 53.

[0042] like Figure 8 As shown, in an optional embodiment, the light input waveguide 51 and the first light output waveguide 52 adopt a “Bridge” type boundary, and the second light output waveguide 53 adopts a “zig-zag” type boundary.

[0043] In order to analyze the transmission performance of the optical beam splitter as described above, the present application simulates the electric field distribution at a frequency of 96.97 THZ, as shown in FIG. Fig. 9As shown, a harmonic source 6 is placed at the port of the optical input waveguide 51 as the input end, and probes are placed at the input end, the first optical output end of the first optical output waveguide 52, and the second optical output end of the second optical output waveguide 53 to detect the transmission spectrum and calculate the electromagnetic wave output power in each output channel. Fig.10 It can be observed in (a) that the two transmission curves have multiple intersections, indicating that equal power beam splitting is achieved at the frequencies corresponding to these intersections. In order to intuitively compare the energy of the two output ports, a split ratio curve defined as Pout2 / Pout1 is drawn. Fig.10 As shown in (b), when the operating frequency is in the range of 92-108THZ, the fluctuation of the curve is in the range of 0.2-4.9, so the energy ratio of the two output waves can be adjusted.

[0044] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A valley photonic crystal structure, characterized in that: A plurality of regular hexagonal unit cells are arranged in an array manner, with the center of their cross section diverging in six directions around them, and the distance between the centers of adjacent regular hexagonal unit cells is a lattice constant a; a single regular hexagonal unit cell is composed of a first type of silicon dielectric column and a second type of silicon dielectric column uniformly placed in the air, and the first type of silicon dielectric column and the second type of silicon dielectric column are respectively arranged in a triangular lattice staggered to form a regular hexagonal basic unit, that is, the first type of silicon dielectric column and the second type of silicon dielectric column are alternately arranged at the six vertices of the regular hexagon; The cross section of the first type of silicon dielectric column is an equilateral triangle; the cross section of the second type of silicon dielectric column is an equilateral triangle ring with an equilateral triangle hole, and the size of the equilateral triangle hole is the same as that of the first type of silicon dielectric column.

2. The valley photonic crystal structure according to claim 1, characterized in that: The unit cell when the upper vertex of the regular hexagon is a first type silicon dielectric column is recorded as VPC1, and the unit cell when the upper vertex of the regular hexagon is a second type silicon dielectric column is recorded as VPC2.

3. The valley photonic crystal structure according to any one of claim 2, characterized in that: The distance from the center to the top of the first type of silicon dielectric column , the distance from the center to the vertex of the second type of silicon dielectric column is r2=2r1.

4. The valley photonic crystal structure according to claim 3, characterized in that: Lattice constant a=346nm.

5. An optical waveguide, characterized in that: It comprises a waveguide interface and a valley photonic crystal structure as claimed in any one of claims 2 to 4, wherein the valley photonic crystal structure comprises a plurality of unit cells VPC1 and a plurality of unit cells VPC2; the plurality of unit cells VPC1 and the plurality of unit cells VPC2 are respectively distributed and arranged on both sides of the waveguide interface.

6. The optical waveguide according to claim 5, characterized in that When the unit cell VPC1 and the unit cell VPC2 are symmetrically distributed with the waveguide interface as the symmetry axis, the optical waveguide has a “Bridge” type boundary.

7. The optical waveguide according to claim 5, characterized in that When the second type of silicon dielectric pillars of the unit cell VPC1 and the second type of silicon dielectric pillars of the unit cell VPC2 are staggeredly distributed on the waveguide interface, the optical waveguide has a "zig-zag" type boundary.

8. An optical beam splitter, characterized in that: The optical beam splitter is divided into a first region, a second region, a third region and a fourth region, the first region and the fourth region are composed of a plurality of unit cells VPC1, the second region and the third region are composed of a plurality of unit cells VPC2; the interface between the first region and the second region, the interface between the first region and the third region, and the interface between the second region and the fourth region respectively form the optical waveguide according to claim 5; Among them, the optical waveguide formed at the interface between the first area and the second area is a light input waveguide, the optical waveguide formed at the interface between the first area and the third area is a first light output waveguide, and the optical waveguide formed at the interface between the second area and the fourth area is a second light output waveguide; the light input waveguide and the first light output waveguide adopt a "Bridge" type boundary; the second light output waveguide adopts a "zig-zag" type boundary.

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