Topology energy valley photonic crystal, broadband topology slow light waveguide and topology beam splitter

Abstract

The invention relates to the technical field of topological photonic crystals, in particular to a topological energy valley photonic crystal, a broadband topological slow light waveguide and a topological beam splitter. The crystal comprises a substrate and a nanorod array arranged on the substrate, the nanopillar array is formed by arranging a plurality of unit cells with honeycomb structures, and the two-dimensional cross section of each unit cell is a regular hexagon; each unit cell is formed by arranging two kinds of nano columns with different section diameters at intervals; two adjacent unit cells share one edge; the two nano columns with different section diameters are respectively a first nano column and a second nano column, and the diameter of the first nano column is twice that of the second nano column; and the value range of the lattice constant a is 480-520 nm. The method has the advantages that the problem of broadband slow light regulation and control in the topological optical waveguide is solved; brillouin region winding of an energy valley boundary state mode is realized, and the group velocity of a light mode is reduced; the light transmission robustness is improved; the back scattering loss caused by manufacturing defects and disorder is reduced; and the applicable frequency range of the device is widened.

Description

Technical Field

[0001] The present invention relates to the technical field of topological photonic crystals, and in particular to a topological valley photonic crystal, a broadband topological slow light waveguide and a topological beam splitter. Background Art

[0002] Slow light technology enhances the interaction between light and matter by reducing the transmission speed of light. In recent years, on-chip devices based on slow light waveguides have been widely used in biochemical sensing, optical modulation, optical storage, optical communication, nonlinear optics and other fields. Periodic optical resonant cavity arrays and photonic crystal waveguides are the two most commonly used integrated slow light waveguide structures, which have the advantages of flexible design, high group refractive index, and compatibility with silicon-based integration processes. However, due to the limitations of micro-nano optical preparation processes, when the group refractive index of the slow light device exceeds 30, the transmission loss is as high as 100 dB / cm, which makes the slow light device impossible to be used in practice.

[0003] Slow-light devices increase the interaction time between light and matter by reducing the group velocity of light, thereby significantly enhancing the intensity of nonlinear effects and other optical phenomena. This is particularly critical for applications such as optical amplifiers, sensors, and quantum optics. However, traditional slow-light technology still has the following disadvantages: (1) Sensitivity to manufacturing defects and disorder. Traditional slow-light devices are prone to undesired backscattering effects and even Anderson localization due to the inevitable defects in the manufacturing process. This phenomenon greatly reduces the propagation efficiency of light and device performance. (2) The trade-off between bandwidth and group velocity. Traditional slow-light devices usually achieve low group velocity while significantly reducing bandwidth, which limits their broadband performance in practical applications. (3) Limitations in materials and frequency range: Some broadband topological slow-light schemes rely on strategies that break time reversal symmetry, but this requires the use of magneto-optical materials, which have extremely poor performance in the visible and near-infrared bands and are difficult to meet the application requirements of integrated photonics. In summary, traditional slow-light devices still have the problems of sensitivity to manufacturing defects and limited bandwidth.

[0004] With the development of on-chip micro-nano fabrication technology, photonic devices based on photonic crystals are getting smaller and smaller. Uncontrollable factors such as dislocation and defects in the fabrication process will cause large scattering losses, which greatly reduce the propagation efficiency of light and device performance. Topological photonics is a new method to solve the above-mentioned scattering loss problem, which can achieve robust transmission of light. Unlike traditional slow light devices, topological slow light devices use topological non-trivial boundary states to make light highly robust to structural defects, scattering and fabrication errors when propagating in waveguides. Therefore, topological photonic crystal waveguides can break through the bottleneck of the transmission length of slow light integrated devices. Topological photonic crystal waveguides are mainly realized by breaking the time and space inversion symmetry. Magneto-optical photonic crystals break the time reversal symmetry and realize the slow light properties with the help of unidirectional transmission boundary states in non-trivial topological band gaps. However, the lack of magnetic materials in the optical band restricts the application of this structure in the field of broadband slow light. To overcome these problems, researchers proposed to realize broadband topological slow light under the condition of destroying spatial inversion symmetry and realize topologically protected unidirectional light transmission. At the same time, through the Brillouin zone winding mechanism of the band structure, effective bandwidth expansion and slow light transmission can be achieved based on topological photonic crystal waveguides. In addition, the above-mentioned topological slow light mechanism also has extremely small group velocity dispersion properties, which can effectively suppress pulse broadening. In summary, the dispersion engineering of topological photonic crystals combined with broken spatial inversion symmetry and Brillouin zone winding is currently the most promising broadband slow light solution.

[0005] Although the current Brillouin zone winding strategy based on topological valley photonic crystals can reduce the group velocity, the breaking of the bulk symmetry at the topological boundary leads to the discontinuity of the mode dispersion curve and the generation of a small band gap, making it impossible to achieve truly broadband continuous slow light transmission. Therefore, by introducing topological photonics into slow light applications and breaking the spatial inversion symmetry, the dispersion engineering of Brillouin zone winding can not only achieve robust transmission of light, but also achieve the purpose of reducing the group velocity and expanding the bandwidth. Summary of the invention

[0006] In order to solve the above problems, the present invention provides a topological valley photonic crystal, a broadband topological slow light waveguide and a topological beam splitter.

[0007] The first object of the present invention is to provide a topological valley photonic crystal, comprising: a substrate, and a nanocolumn array arranged on the substrate; the nanocolumn array is composed of a plurality of honeycomb structured unit cells, each unit cell having a two-dimensional cross-section of a regular hexagon; the unit cell is composed of two types of nanocolumns with different cross-sectional diameters arranged in an interval manner; two adjacent unit cells share a side; The two nanorods with different cross-sectional diameters are respectively a first nanorod and a second nanorod, and the diameter of the first nanorod is twice the diameter of the second nanorod; Lattice constant aThe value range is 480~520nm.

[0008] Preferably, the lattice constant a =500nm.

[0009] Preferably, the material of the nanorod is dielectric silicon, with a relative dielectric constant of 12; the diameter of the first nanorod is 0.4 a , the diameter of the second nanorod is 0.2 a .

[0010] The second object of the present invention is to provide a broadband topological slow light waveguide, comprising: an original waveguide region, and topological valley photonic crystals arranged on both sides of the original waveguide region; The original waveguide region includes two rows of silicon column groups with the same structure. Each silicon column group is composed of five silicon columns with gradually decreasing cross-sectional diameters. The silicon column with the largest cross-sectional diameter is the same as the diameter of the first nanocolumn.

[0011] Preferably, the diameters of the five silicon pillars with decreasing cross-sectional diameters are d1=0.4 a d2=0.39 a d3=0.38 a d4=0.37 a d5=0.36 a .

[0012] Preferably, the broadband topological slow light waveguide is a straight waveguide, a defect waveguide or a Z-bend waveguide.

[0013] The third object of the present invention is to provide a topological beam splitter, comprising: N groups of periodically arranged broadband topological slow light waveguides and M groups of periodically arranged complementary structure waveguides, wherein the last complementary structure waveguide is connected to the first broadband topological slow light waveguide; The complementary structure waveguide includes a complementary waveguide region and topological valley photonic crystals arranged on both sides of the complementary waveguide region; Both M and N are natural numbers greater than or equal to 6.

[0014] Compared with the prior art, the present invention can achieve the following beneficial effects: The present invention provides a two-dimensional topological valley photonic crystal, a broadband topological slow light waveguide structure based on the topological valley photonic crystal, and a topological beam splitter, which solves the problem of regulating broadband slow light in topological optical waveguides. By optimizing the unit structure size at the domain wall position, the Brillouin zone winding of the valley boundary state mode is achieved, thereby greatly reducing the group velocity of the light mode. In order to solve the problems of limited frequency range of magneto-optical topological photonic crystals, discontinuous frequency bands in topological valley photonic crystals, and the restriction of slow light modes by small band gaps in the prior art, the present invention proposes a novel topological broadband slow light waveguide structure based on two frequency complementary topological valley photonic crystal waveguides. Furthermore, by combining two frequency complementary waveguide structures, the restriction of small band gaps on continuous slow light transmission is eliminated, forming a continuous broadband slow light waveguide structure. In addition, the present invention also designs a frequency selective beam splitter based on valley contrast unidirectional slow light propagation. By adjusting the phase difference of the electric dipole excitation source, the controllable selective output of optical signals at different ports is achieved, thereby realizing highly directional frequency selective beam splitting. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a cross-sectional schematic diagram of a topological valley photonic crystal provided according to an embodiment of the present invention.

[0016] Figure 2 Schematic diagram of a topological valley photonic crystal structure and a boundary mode band structure provided according to an embodiment of the present invention; (a) is a schematic diagram of a topological valley photonic crystal structure design; (b) is a band structure of a topological valley photonic crystal; (c) is a schematic diagram of a zigzag boundary structure of a topological valley photonic crystal; (d) is a band structure of a boundary mode of a topological valley photonic crystal; and (e) is a group refractive index distribution of a boundary mode of a topological valley photonic crystal.

[0017] Figure 3 It is a cross-sectional schematic diagram of a broadband topological slow light waveguide provided according to an embodiment of the present invention.

[0018] Figure 4 The projected band structure, group refractive index distribution and The electric field distribution diagram of the eigenmode when ; (a) is the projected band structure; (b) is the group refractive index distribution diagram; (c) is The electric field distribution of the eigenmode when .

[0019] Figure 5The robustness analysis results of the broadband topological slow light waveguide provided according to the embodiment of the present invention are shown in Figure (a). The transmission spectra of three different types of waveguides (straight, defective and Z-bend). (b) is the electric field distribution of the straight waveguide transmission mode; (c) is the structural schematic diagram of the defective waveguide and the electric field distribution of the transmission mode; (d) is the electric field distribution of the Z-bend waveguide transmission mode.

[0020] Figure 6 It is a schematic cross-sectional diagram of the overall structure of a topological beam splitter provided according to an embodiment of the present invention.

[0021] Figure 7 The topological beam splitter structure, band structure, directional distribution under point dipole excitation source, and electric field distribution diagram provided according to an embodiment of the present invention; (a) is a schematic diagram of the topological beam splitter structure design; (b) is the band structure of the broadband topological slow light waveguide and the complementary structure waveguide in the topological beam splitter; (c) is the directional distribution diagram under magnetic dipole excitation sources with different phase differences; (d) is the electric field distribution diagram of the waveguide mode with the switching point dipole phase difference.

[0022] Reference numerals: 1. Topological valley photonic crystals; 11. The first nanopillar; 12. The second nanopillar; 2. Original waveguide area; 3. Complementary waveguide region. DETAILED DESCRIPTION

[0023] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

[0024] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0025] The present invention provides a topological valley photonic crystal, comprising: a substrate, and a nanocolumn array arranged on the substrate; the nanocolumn array is composed of a plurality of honeycomb structured unit cells, each unit cell having a two-dimensional cross-section of a regular hexagon, and two types of nanocolumns with different cross-sectional diameters are arranged in an interval manner; two adjacent unit cells share a side; Lattice constant a The value range of is 480~520nm; in a specific embodiment, the lattice constant a =500nm; The nanorods are made of dielectric silicon with a relative dielectric constant of 12; the substrate is made of silicon; The silicon nanorod array is formed by deposition or etching.

[0026] See also Figure 1 The two nanorods with different cross-sectional diameters are the first nanorod 11 and the second nanorod 12. The diameter of the first nanorod 11 is twice the diameter of the second nanorod 12. In a specific embodiment, the diameter of the first nanorod 11 is 0.4 a , the diameter of the second nanorod 12 is 0.2 a ; Breaking the spatial inversion symmetry by two silicon nanopillars with different diameters opens a topological band gap.

[0027] See also Figure 2 , Figure 2 (a) is a schematic diagram of the structural design of a topological valley photonic crystal, where type A and type B structures are photonic crystal structures with two different valley Chern numbers.

[0028] The band structure of the transverse magnetic mode (TM, i.e., the magnetic field is distributed in the plane and the electric field is distributed out of the plane) of the topological valley photonic crystal is shown in the attached figure. Figure 2 As shown in (b), the shaded area is the topological band gap, which is located between the first and second energy bands, and the band gap topological Chern number is 0. The solid line and the dashed line are the situations before and after breaking the spatial inversion symmetry, respectively. The phase distribution diagram shows that there are singular points of phase vortex in the K and K' valleys, with a topological charge of ±1. The topological photonic crystal consists of two ∆d=0.2 a and ∆d=-0.2 a The energy valley photonic crystals are spliced ​​to form zigzag boundaries, and the topological boundary modes can be obtained by projecting the band structure. The wavelength of the energy valley boundary mode ranges from 1690.88 nm to 1823.56 nm, and the bandwidth relative to the band gap is 21.75%. In order to illustrate the slow light characteristics of the energy valley boundary mode, the group refractive index n is calculated. g . When k=0, π / a , 2π / a The maximum group refractive index of 3000 can be achieved at these positions. Figure 2 As shown in (d).

[0029] Principle description: Topological valley photonic crystals are composed of silicon nanopillars with periodic honeycomb structures. By breaking the spatial inversion symmetry, valley photonic crystals with complete topological band gaps are realized. According to the body-edge correspondence in topological theory, the boundary formed by two photonic crystals with opposite topological valley Chern numbers supports topological boundary states, which can turn at large angles and maintain anti-scattering transmission.

[0030] Based on the above-mentioned topological valley photonic crystal, the present invention provides a broadband topological slow light waveguide, comprising: an original waveguide region, and topological valley photonic crystals arranged on both sides of the original waveguide region; The original waveguide region includes two rows of silicon pillar groups with the same structure, each silicon pillar group is composed of five silicon pillars with decreasing cross-sectional diameters, and the silicon pillar with the largest cross-sectional diameter is the same as the diameter of the first nano-pillar; In a specific embodiment, the diameters of the five silicon pillars with decreasing cross-sectional diameters are d1=0.4 a d2=0.39 a d3=0.38 a d4=0.37 a d5=0.36 a .

[0031] See also Figure 3 The broadband topological slow light waveguide of the present invention comprises: an original waveguide region 2, and topological valley photonic crystals 1 arranged on both sides of the original waveguide region 2; the expansion of the slow light bandwidth of the mode is achieved by periodically and continuously changing the diameter of the silicon column at the domain wall, and the diameters of the silicon column at the boundary (decreasing in sequence) are d1, d2, d3, d4, and d5, respectively, and the sizes are d1=0.4 a d2=0.39 a d3=0.38 a d4=0.37 a d5=0.36 a , five groups are repeated periodically, and the new Brillouin zone formed is 1 / 5 of the original one, that is, 2π / 5 a The diameter change at the boundary is adjusted so that the bandwidth of the topological boundary mode matches the topological band gap, and the topological boundary mode is wound in the folded Brillouin zone. The number of windings is equal to the number of silicon pillars with different diameters. The group refractive index of the slow light mode is increased by 2 to 3 orders of magnitude, and the bandwidth is expanded to 31.7% relative to the band gap. Figure 4 As shown in (a) to (b). However, when k=0,π / Λ, 2π / Λ, four tiny band gaps will be generated due to the coupling between modes. It should be pointed out that the number of resonant cavities within the period can continue to increase so that the slow light bandwidth covers the entire topological band gap. Although the discontinuous band structure will limit some applications of slow light, the introduction of nanopillars with gradient sizes can further reduce the group velocity. In order to understand the relationship between the size of the nanopillar and the frequency of the near-flat band, by analyzing the distribution of the corresponding eigenmodes of each band, it is observed that the enhanced electric field in the low-frequency case occurs at the location with a large nanodiameter, and the enhanced electric field in the high-frequency case occurs at the location with a small nanodiameter, as shown in the attached figure. Figure 4 Middle (c).

[0032] Three different types of waveguides (straight, defective, and Z-bend) are constructed, and their transmission spectra are simulated. The numerical simulation results are shown in the figure below. Figure 5 The results show that the broadband topological slow light waveguide has anti-scattering transmission properties and is robust to straight waveguides, defect waveguides, and Z-shaped curved waveguides.

[0033] Brief description of the principle: A broadband topological slow-light waveguide mode is realized through dispersion engineering of Brillouin zone winding, that is, an array of silicon nanopillars with uniformly varying diameters of topological domain walls is used to effectively reduce the group velocity of the waveguide mode. The extended frequency range expands with the increase in the number of silicon pillar arrays with different diameters at the topological boundary, thereby realizing a broadband topological slow-light waveguide. However, this broadband expansion method will open a tiny band gap at the boundary of the Brillouin zone. In order to solve the problem of tiny band gaps, the lattice constant is kept unchanged, and the diameter of the silicon nanopillars at the topological boundary is optimized, so that the band structure of the complementary structure matches the tiny band gap, thereby realizing a continuous broadband slow-light waveguide.

[0034] The present invention also provides a frequency-selective topological beam splitter, comprising: N groups of periodically arranged broadband topological slow light waveguides and M groups of periodically arranged complementary structure waveguides, the last complementary structure waveguide is connected to the first broadband topological slow light waveguide; M and N are both natural numbers greater than or equal to 6; the complementary structure waveguide comprises a complementary waveguide region and topological energy valley photonic crystals arranged on both sides of the complementary waveguide region; See also Figure 6 The complementary waveguide region 3 includes two rows of silicon pillar groups with the same structure. Each silicon pillar group consists of four silicon pillars with perturbed cross-sectional diameters. The diameters of the four silicon pillars with perturbed cross-sectional diameters are d1=0.373 a d2=0.374 a d3=0.38 a d4=0.383 a .

[0035] In order to solve the problem of tiny band gap, a complementary structure waveguide is introduced, whose band structure matches the tiny band gap, thus realizing continuous broadband topological slow light. Figure 6 As shown in the figure, the dotted line divides the structure into two parts, 3 groups of broadband topological slow light waveguides and 3 groups of complementary structure waveguides. The design idea of ​​the complementary structure waveguide is to fix the lattice constant of the topological photonic lattice. The number of size-perturbed silicon pillars in the complementary waveguide region is the same as the number of band gaps in the original waveguide region in the broadband topological slow light waveguide. The frequency complementarity of the band structure is achieved only by adjusting the diameter of the nanopillars at the domain wall. The diameters of the silicon pillars in the complementary structure are 0.373 a , 0.374 a , 0.38 a and 0.383 a .

[0036] In order to realize the function of frequency selective topological beam splitter, a pair of a The point dipole excitation source can realize the directional output of the left and right ports by switching its phase difference to π / 2 or -π / 2. =-1, the light with the frequency corresponding to the tiny band gap of the initial structure is reflected into the complementary structure waveguide; in the above formula, P L represents the total energy output from port 2, P R represents the total energy output from port 1. For the frequency range where the two structures overlap, selective output of the left and right ports can be achieved by adjusting the phase difference of the point dipole excitation source, as shown in the attached figure. Figure 7 As shown in (a) to (d). Figure 7 The solid line in (b) is the band structure of the broadband topological slow light waveguide, and the dotted line is the band structure of the complementary structure waveguide; Figure 7 (c) shows the directional distribution under point dipole excitation sources with different phase differences; Figure 7 (d) is the dipole phase difference at the switching point ( )’s electric field distribution of the waveguide mode.

[0037] Principle: To realize the frequency selective beam splitter, two broadband topological slow light waveguides and complementary structure waveguides with complementary energy band structures are spliced ​​to realize continuous broadband slow light. a At the position, two point dipoles with phase differences of -π / 2 and π / 2 are placed respectively. By switching the phase difference of the point dipoles, the output of the frequency selective port is achieved, thereby realizing the function of a frequency selective beam splitter.

[0038] In view of the shortcomings of the prior art, the present invention provides a two-dimensional topological valley photonic crystal, a broadband topological slow light waveguide structure based on the topological valley photonic crystal, a topological beam splitter, and a method for using the same. First, broadband continuous topological slow light transmission is realized; a broadband topological slow light waveguide mode is realized by the dispersion engineering of Brillouin zone winding, that is, an array of silicon nanocolumns with uniformly varying diameters at the topological boundary is used to effectively reduce the group velocity of the waveguide mode. The extended frequency range expands with the increase in the number of silicon column arrays with different diameters at the topological boundary, thereby realizing a broadband topological slow light waveguide. However, due to the breaking of symmetry at the topological boundary, discontinuity of mode dispersion and the generation of small band gaps are caused. In order to solve the problem of tiny band gaps, a frequency-complementary topological waveguide structure is proposed at the same time, and the band frequency matches the tiny band gap frequency to realize continuous broadband slow light mode transmission in a true sense. Secondly, the robustness of optical transmission is improved. By utilizing the valley degree of freedom and topological protection characteristics, the backscattering loss caused by manufacturing defects and disorder is effectively reduced, ensuring the efficient optical transmission performance of slow light devices in complex environments. The present invention realizes efficient broadband slow light transmission through topological design without relying on magneto-optical materials, broadens the applicable frequency range of the device, especially for on-chip applications in the visible and near-infrared bands. Finally, in order to enhance the functionality of the application, the function of the frequency selective beam splitter is further realized, combined with the broadband slow light characteristics, to provide an efficient solution for optical communication integrated photonics systems.

[0039] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0040] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A topological valley photonic crystal, characterized in that: It includes a substrate, and a nanocolumn array arranged on the substrate; the nanocolumn array is composed of a plurality of honeycomb structured unit cells, each unit cell having a two-dimensional cross-section of a regular hexagon; the unit cell is composed of two nanocolumns with different cross-sectional diameters arranged in an interval manner; two adjacent unit cells share a side; The two nanorods with different cross-sectional diameters are respectively a first nanorod and a second nanorod, and the diameter of the first nanorod is twice the diameter of the second nanorod; Lattice constant a The value range is 480~520 nm.

2. A topological valley photonic crystal according to claim 1, characterized in that: The lattice constant a =500nm.

3. A topological valley photonic crystal according to claim 1, characterized in that: The material of the nanorod is dielectric silicon, with a relative dielectric constant of 12; the diameter of the first nanorod is 0.4 a , the diameter of the second nanorod is 0.2 a .

4. A broadband topological slow light waveguide, characterized in that: It comprises an original waveguide region, and a topological valley photonic crystal according to any one of claims 1 to 3 arranged on both sides of the original waveguide region; The original waveguide region includes two rows of silicon column groups with the same structure. Each silicon column group is composed of five silicon columns with gradually decreasing cross-sectional diameters. The silicon column with the largest cross-sectional diameter is the same as the diameter of the first nanocolumn.

5. The broadband topological slow light waveguide according to claim 4, characterized in that: The diameters of the five silicon pillars with decreasing cross-sectional diameters are d1=0.4 a d2=0.39 a d3=0.38 a d4=0.37 a d5=0.36 a .

6. The broadband topological slow light waveguide according to claim 4, characterized in that: The broadband topological slow light waveguide is a straight waveguide, a defect waveguide or a Z-shaped bending waveguide.

7. A topological beam splitter, characterized in that: Comprising N groups of periodically arranged broadband topological slow light waveguides as claimed in claim 4 and M groups of periodically arranged complementary structure waveguides, wherein the last complementary structure waveguide is connected to the first broadband topological slow light waveguide; The complementary structure waveguide comprises a complementary waveguide region and the topological valley photonic crystal according to claim 1 arranged on both sides of the complementary waveguide region; Both M and N are natural numbers greater than or equal to 6.

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