Compact second-order topological insulator quantum cascade laser and processing method thereof

Abstract

The invention provides a compact second-order topological insulator quantum cascade laser and a processing method thereof, and belongs to the technical field of topological photonics, the compact second-order topological insulator quantum cascade laser comprises a bottom metal electrode, a topological optical resonant cavity, a silicon dioxide insulating layer, a top metal electrode and a wire which are arranged from bottom to top, the topological optical resonant cavity is a QCL layer, the QCL layer is arranged above the bottom metal electrode, the silicon dioxide insulating layer wraps the periphery of the top of the QCL layer, the top metal electrode is deposited above the silicon dioxide insulating layer and above the interior of the QCL layer, and the wire is connected with the top metal electrode and used for introducing current to carry out electric pumping excitation. The problems that a traditional semiconductor laser is low in photoelectric conversion efficiency, unstable in output and difficult to integrate, and a second-order topological insulator quantum cascade laser is complex in design, difficult to manufacture, large in size and not beneficial to integration are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of topological photonics, and in particular to a compact second-order topological insulator quantum cascade laser and a processing method thereof. Background Art

[0002] Due to the limitations of structural design, traditional semiconductor lasers are difficult to achieve efficient photoelectric conversion and stable laser output, and are also difficult to achieve high integration with optoelectronic devices. Based on the topological resonant cavity proposed by topological photonics theory, by introducing the concept of second-order topological insulators, topologically protected zero-dimensional corner states can be realized in the cavity. These corner states significantly enhance the performance of the laser and have excellent anti-scattering and anti-defect capabilities. However, second-order topological insulator quantum cascade lasers still face many challenges in practical application and development.

[0003] In terms of structural design, its topological structure is extremely complex. The design process requires precise control of multiple parameters, involving profound mathematical models and complex physical principles, which requires extremely high professional qualities of designers, increasing the difficulty and cost of design. At the same time, accurate material selection is also a major problem. The optical, electrical and thermal properties of different materials vary greatly. Finding a material combination that meets specific topological structure requirements and has good performance matching is difficult in actual operation.

[0004] In terms of manufacturing process, complex topological structures and precise material requirements pose huge challenges to device manufacturing. Even a small process deviation may lead to a significant decline in device performance, which places almost stringent requirements on manufacturing equipment and process control accuracy, thereby increasing production costs and production cycles.

[0005] In addition, existing second-order topological insulator quantum cascade lasers generally have the problem of being large in size. As optoelectronic systems are currently developing towards high integration and miniaturization, larger device volumes not only take up too much space, increasing the overall size and weight of the system, but also seriously hinder integration with other optoelectronic devices, making it difficult to meet the needs of compact layout and multifunctional integration in fields such as on-chip optical systems. Summary of the invention

[0006] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a compact second-order topological insulator quantum cascade laser and a processing method thereof, utilizing the topological optical mode of the second-order topological insulator, constructing a resonant cavity with the help of a two-dimensional SSH model, adjusting the coupling strength by changing the position of the QCL dielectric column in the unit structure to obtain a topological band gap, utilizing the two-dimensional vein connection to play a medium role and conduct electricity, constructing a topological boundary to form a topologically protected localized mode; and by changing the periodic number of topological mediocre and topological non-trivial structures, the size of the topological cavity can be adjusted, thereby greatly improving the monolithic integration of the topological semiconductor laser.

[0007] To achieve the above object, the present invention provides the following solutions: A compact second-order topological insulator quantum cascade laser comprises a bottom metal electrode, a topological optical resonant cavity, a silicon dioxide insulating layer, a top metal electrode and a wire arranged from bottom to top, wherein the topological optical resonant cavity is a QCL layer, the QCL layer is arranged above the bottom metal electrode, the silicon dioxide insulating layer is coated on the top periphery of the QCL layer, the top metal electrode is deposited above the silicon dioxide insulating layer and above the inside of the QCL layer, and the wire is connected to the top metal electrode for introducing current for electrical pumping excitation.

[0008] Preferably, the QCL layer is made of isotropic GaAs material with a thickness of 10 μm, and the QCL layer has a topological interface composed of periodically distributed columnar structures running through the QCL layer, which is used to realize the injection of pump current into the columnar structure of the QCL layer and the formation of corner mode.

[0009] Preferably, the material of the QCL layer is one of triple-well resonant-phonon gallium arsenide or aluminum-containing gallium arsenide with a refractive index of 3.6.

[0010] Preferably, the column structure of the QCL layer is constructed of a mediocre domain column structure and a non-mediocre domain column structure, the cross-section of the mediocre domain column structure is circular or rhombus-shaped, and the cross-section of the non-mediocre domain column structure is four quarter circles or four quarter rhombuses.

[0011] Preferably, the cross-sectional shape of the columnar structure cell of the QCL layer is one of a circular “+” shape, a circular “×” shape or a diamond shape, specifically: When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "+" shape, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a circular "+" shape vein, the cross-section of the mediocre domain columnar structure is circular, and the cross-section of the non-mediocre domain columnar structure is a quarter circle; wherein, the characteristic size of the QCL layer columnar structure unit is 20 μm, the circular radius size is 7.07 μm, the width of the "+" shape vein is 2 μm, and the length is 20 μm, the centers of the two veins of the unit cell of the mediocre domain columnar structure are aligned with the center of the unit cell, the centers of the four veins of the non-mediocre domain columnar structure are respectively aligned with the centers of the four sides of the unit cell, and the overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 180 μm; When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "×" shape, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a circular "×"-shaped vein, the cross-section of the mediocre domain columnar structure is circular, and the cross-section of the non-mediocre domain columnar structure is a quarter circle; wherein the characteristic size of the QCL layer columnar structure unit is 20 μm, the circular radius size is 6.6 μm, the width of the "×"-shaped vein is 2 μm, and the length is 28.3 μm, the centers of the two veins of the mediocre domain columnar structure unit and the non-mediocre domain columnar structure unit are aligned with the center of the unit cell, and the overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 160 μm; When the cross-sectional shape of the columnar structure cell of the QCL layer is a rhombus, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a rhombus-shaped "+" vein, the cross-section of the mediocre domain columnar structure is a rhombus, and the cross-section of the non-mediocre domain columnar structure is a quarter of a rhombus; wherein, the characteristic size of the QCL layer columnar structure unit is 20 μm, the side length of the rhombus is 14.1 μm, and each QCL layer columnar structure unit has no "+" vein connection inside, but each columnar structure unit is connected by a "+" vein running through the entire cavity, and the overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 200 μm.

[0012] Preferably, the cross-section of the cell in the topological photonic crystal resonant cavity in the QCL layer is square, wherein in the mediocre domain, the center of the cross-section of the column structure is filled with gallium arsenide material, and the rest is filled with air; in the non-trivial domain, the cross-section of the column structure is filled with gallium arsenide material all around, and the central area is filled with air, and the cells are arranged in a two-dimensional array.

[0013] Preferably, the bottom metal electrode and the top metal electrode are both made of metal materials.

[0014] Preferably, the material of the bottom metal electrode and the material of the top metal electrode are both gold materials, the thickness of the bottom metal electrode is 300 nm, and the thickness of the top metal electrode is 250 nm.

[0015] Preferably, the thickness of the silicon dioxide insulating layer is 300 nm.

[0016] The present invention also provides a method for processing the above-mentioned compact second-order topological insulator quantum cascade laser, comprising the following steps: S1. Preparation + Doped GaAs substrate and THz QCL wafer layer, depositing a metal layer on the GaAs substrate and THz QCL wafer layer, and then forming a bottom contact gold layer by a gold / gold hot compression wafer bonding process; S2. Based on the structure of forming a bottom contact gold layer and carrying a terahertz QCL wafer layer, the terahertz QCL wafer layer is thinned using a polisher, and then wet-etched with a hydrofluoric acid solution to remove the gallium arsenide substrate remaining in the terahertz QCL wafer layer, exposing the QCL active area; S3. Depositing a silicon dioxide insulating layer on the structure where the QCL active area is exposed, and demarcating the pumping area on the structure where the silicon dioxide insulating layer is deposited by optical lithography, and then removing the silicon dioxide insulating layer above the pumping area by reactive ion etching, leaving only the silicon dioxide insulating layer on the top periphery of the terahertz QCL wafer layer; S4, based on step S3, optical lithography technology is used again to determine the mediocre domain and non-mediocre domain of the photonic crystal in the terahertz QCL wafer layer, and to determine the pore area in the photonic crystal structure, and then a contact layer is deposited in the pore area, and the contact layer in the remaining area is used as a hard mask; then the photoresist is removed, and the remaining contact layer forms a top contact point for current injection, and finally the QCL active area is dry-etched to obtain the QCL layer, i.e., the topological photonic crystal resonant cavity; S5. After completing S2~S4, the main substrate in the overall structure, i.e. n + The doped gallium arsenide substrate is thinned, and metal layers are deposited on the bottom and top to obtain a bottom metal electrode and a top metal electrode respectively. Finally, a wire is connected to the top metal electrode to complete the processing of a compact second-order topological insulator quantum cascade laser.

[0017] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: (1) The present invention constructs a topological photonic crystal resonator based on a two-dimensional SSH model, adopts a direct surface emitting laser scheme and an electric pump excitation method, which greatly simplifies the laser excitation process and reduces the dependence on complex optical components in the manufacturing process. This significantly reduces the difficulty of physical manufacturing of the device and effectively controls the cost, laying a solid foundation for large-scale production and application.

[0018] (2) By utilizing the special properties of the topological corner states of photonic crystals, the present invention has the advantages of low loss, high quality factor and Purcell factor due to its extremely small mode volume and wavelength characteristics in the topological band gap. This not only improves the energy conversion efficiency of the laser and makes laser emission more efficient, but also enhances the interaction between light and matter, ensuring the stability and high quality of the laser output. At the same time, this corner state has the largest state density in the same dimension, which is highly consistent with the integration requirements of compact devices and provides the possibility for the realization of highly integrated optoelectronic devices.

[0019] (3) Since the topological trivial and non-trivial unit structures in the topological photonic crystal resonator are isotropic, the Bloch modes in the photonic crystal are symmetrical. Under periodic boundary conditions in the positive and negative x directions and the positive and negative y directions, the band structures of the corresponding topological trivial and non-trivial photonic crystals have the same structure along the same path of the first Brillouin zone boundary.

[0020] (4) When the positive and negative x-direction boundaries are periodic and the positive and negative y-directions are truncated by the scattering boundary, the finite length trivial domain structure and the non-trivial domain structure form a boundary state with an energy gap. Compared with the conventional interface, the trivial structure unit at the boundary of the present invention adds a quarter circle or diamond filling, and combines it with the non-trivial structure to form a topological interface, which is convenient for injecting pump current into the corner state column.

[0021] (5) The present invention uses the near-field mode distribution of the topological photonic crystal cavity to accurately reveal the angular state position, and can flexibly adjust the cavity size by changing the number of periodic structural units. Under the premise of ensuring that the laser output quality factor is not less than 150, the minimum size design of the laser is achieved, taking into account the dual goals of miniaturization and high performance. In addition, the far-field distribution measurement provides an effective means for determining the working mode of the laser, which helps to accurately optimize and control the laser performance.

[0022] (6) The present invention adopts an electric pumping excitation method that relies only on the single adjustable degree of freedom of pump intensity, successfully breaking through many limitations of traditional optically pumped lasers. It has made major breakthroughs in achieving high performance and specificity such as large-area single-mode output, high-power emission, low divergence angle beam, room temperature operation, and flexible beam control, significantly broadening the application field of lasers and meeting the use needs of more complex scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0024] Figure 1 A schematic diagram of a circular “+” structure of a compact second-order topological insulator quantum cascade laser of the present invention; Figure 2 This is a flow chart of a processing method for a compact second-order topological insulator quantum cascade laser of the present invention; Figure 3An xy plane schematic diagram of a finite-size simulation structure of a circular “+”-shaped topological photonic crystal resonant cavity provided in Example 1 of the present invention; Figure 4 The circular "+" structure topologically mediocre and non-trivial unit structures and their corresponding topologically mediocre and non-trivial photonic crystal band structures provided in Example 1 of the present invention; wherein, Figure 4 (a) is a schematic diagram of the topologically trivial unit structure. Figure 4 (b) is a schematic diagram of a non-trivial unit structure; Figure 4 (c) is the band structure diagram of topological trivial photonic crystal. Figure 4 (d) is the band structure diagram of non-trivial photonic crystal; Figure 5 The diagram of the edge state unit composed of the topological mediocre and non-trivial unit structures of the circular "+" structure provided in Example 1 of the present invention and the corresponding photonic crystal edge state energy band structure; Figure 6 An angular near-field xy view of the simulation results of the circular “+”-shaped topological photonic crystal resonator provided in Example 1 of the present invention; Figure 7 The angular far-field observation position and xy view of the simulation result of the circular “+”-shaped topological photonic crystal resonator provided in Example 1 of the present invention; wherein, Figure 7 (a) is the angular state far-field observation position diagram of the simulation result of the circular “+” structure topological photonic crystal resonator. Figure 7 (b) is the xy view corresponding to the angular far-field observation position of the simulation result of the circular “+”-shaped topological photonic crystal resonator; Figure 8 A schematic diagram of a circular “×”-shaped structure of a compact second-order topological insulator quantum cascade laser provided in Example 2 of the present invention; Fig. 9 An xy plane schematic diagram of a finite-size simulation structure of a circular “×”-shaped topological photonic crystal resonant cavity provided in Example 2 of the present invention; Fig.10 The circular "X"-shaped structure topologically mediocre and non-trivial unit structures and their corresponding topologically mediocre and non-trivial photonic crystal band structures provided in Example 2 of the present invention; wherein, Fig.10 (a) is a schematic diagram of the topologically trivial unit structure. Fig.10 (b) is a schematic diagram of a non-trivial unit structure; Fig.10 (c) is the band structure diagram of topological trivial photonic crystal. Fig.10 (d) is the band structure diagram of non-trivial photonic crystal; Fig.11The diagram of the edge state unit composed of the topological mediocre and non-trivial unit structures of the circular "X"-shaped structure provided in Example 2 of the present invention and its corresponding photonic crystal edge state energy band structure; Fig.12 An xy plane view of the angular near-field of the circular “×”-shaped topological photonic crystal simulation result provided in Example 2 of the present invention; Fig.13 The angular far-field observation position and xy view of the simulation results of the circular “×”-shaped topological photonic crystal provided in Example 2 of the present invention; wherein, Fig.13 (a) is the angular state far-field observation position diagram of the simulation results of the circular "×"-shaped topological photonic crystal. Fig.13 (b) is the xy view corresponding to the angular far-field observation position of the simulation result of the circular “×”-shaped topological photonic crystal; Fig.14 A schematic diagram of a diamond structure of a compact second-order topological insulator quantum cascade laser provided in Example 3 of the present invention; Fig.15 An xy plane schematic diagram of a finite-size simulation structure of a rhombus-structured topological photonic crystal resonant cavity provided in Example 3 of the present invention; Fig.16 The diamond structure topologically mediocre and non-trivial unit structures and their corresponding topologically mediocre and non-trivial photonic crystal band diagrams provided in Example 3 of the present invention; wherein, Fig.16 (a) is a schematic diagram of the topologically trivial unit structure. Fig.16 (b) is a schematic diagram of a non-trivial unit structure; Fig.16 (c) is the band structure diagram of topological trivial photonic crystal. Fig.16 (d) is the band structure diagram of non-trivial photonic crystal; Fig.17 The diagram of the edge state unit composed of the topologically trivial and non-trivial unit structures of the diamond structure provided in Example 3 of the present invention and the corresponding photonic crystal edge state energy band structure; Fig.18 An xy view of the angular near-field of the simulation result of the rhombus-structured topological photonic crystal provided in Example 3 of the present invention; Fig.19 The angular far-field observation position and xy view of the simulation result of the rhombus structure topological photonic crystal provided in Example 3 of the present invention; wherein, Fig.19 (a) is the angular state far-field observation position diagram of the simulation results of the rhombus structure topological photonic crystal. Fig.19 (b) is the xy view corresponding to the angular far-field observation position of the simulation result of the rhombus-structured topological photonic crystal; Fig. 20This is a scatter plot comparison of the quality factors of the topological photonic crystal resonant cavity at different frequency points in Examples 1 to 3 of the present invention; wherein the quality factors at the angular state frequencies of each example are marked with numbers; Fig.21 It is a scatter comparison diagram of the effective mode volume corresponding to different frequency points of the topological photonic crystal resonant cavity in Examples 1 to 3 of the present invention; wherein the effective mode volume at the angular state frequency of each example is marked with numbers; Fig. 22 It is a scatter comparison diagram of the corresponding loss factors of the topological photonic crystal resonant cavity at different frequency points in Examples 1 to 3 of the present invention; wherein the loss factors at the angular state frequencies of each example are marked with numbers; Fig.23 It is a scatter comparison diagram of the Purcell factors corresponding to the topological photonic crystal resonant cavity at different frequency points in Examples 1 to 3 of the present invention; wherein the Purcell factors at the angular state frequencies of each example are marked with numbers.

[0025] Description of reference numerals: 1. Bottom metal electrode; 2. QCL layer; 3. Silicon dioxide insulating layer; 4. Top metal electrode; 5. Wire. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] The present invention provides a compact second-order topological insulator quantum cascade laser, comprising a bottom metal electrode, a topological optical resonant cavity, a silicon dioxide insulating layer, a top metal electrode and a wire arranged from bottom to top, wherein the topological optical resonant cavity is a QCL layer, the QCL layer is arranged above the bottom metal electrode, the silicon dioxide insulating layer is coated on the top periphery of the QCL layer, the top metal electrode is deposited above the silicon dioxide insulating layer and above the inside of the QCL layer, and the wire is connected to the top metal electrode for introducing current for electric pumping excitation.

[0029] In the above content, the QCL layer is composed of isotropic GaAs material with a thickness of 10μm, and the QCL layer has a topological interface composed of a periodically distributed columnar structure running through the QCL layer, which is used to realize the injection of pump current into the columnar structure of the QCL layer and the formation of angular modes. The material of the QCL layer is one of triple-well resonant-phonon gallium arsenide or aluminum-containing gallium arsenide with a refractive index of 3.6. The columnar structure of the QCL layer is constructed of a mediocre domain columnar structure and a non-mediocre domain columnar structure. The cross-section of the mediocre domain columnar structure is circular or rhombus-shaped, and the cross-section of the non-mediocre domain columnar structure is four quarter circles or four quarter rhombuses. The cross-sectional shape of the columnar structure cell of the QCL layer is one of a circular "+" shape, a circular "×" shape or a rhombus, specifically: When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "+" shape, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a circular "+" shape vein, the cross-section of the mediocre domain columnar structure is circular, and the cross-section of the non-mediocre domain columnar structure is a quarter circle; wherein, the characteristic size of the QCL layer columnar structure unit is 20μm, the circular radius size is 7.07μm, the width of the "+" shape vein is 2μm, and the length is 20μm. The centers of the two veins of the unit cell of the mediocre domain columnar structure are aligned with the center of the unit cell, and the centers of the four veins of the non-mediocre domain columnar structure are aligned with the centers of the four sides of the unit cell, respectively. The overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 180μm. When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "×" shape, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a circular "×"-shaped vein, the cross-section of the mediocre domain columnar structure is circular, and the cross-section of the non-mediocre domain columnar structure is a quarter circle; wherein, the characteristic size of the columnar structure unit of the QCL layer is 20μm, the circular radius size is 6.6μm, the width of the "×"-shaped vein is 2μm, and the length is 28.3μm. The centers of the two veins of the mediocre domain columnar structure unit and the non-mediocre domain columnar structure unit are aligned with the center of the unit cell, and the overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 160μm; When the cross-sectional shape of the columnar structure cell of the QCL layer is a rhombus, the columnar structure of the mediocre domain and the columnar structure of the non-mediocre domain are connected by a rhombus-shaped "+" vein. The cross-section of the columnar structure of the mediocre domain is a rhombus, and the cross-section of the columnar structure of the non-mediocre domain is a quarter of a rhombus. Among them, the characteristic size of the columnar structure unit of the QCL layer is 20μm, the side length of the rhombus is 14.1μm, and there is no "+" vein connection inside each columnar structure unit of the QCL layer. Instead, each columnar structure unit is connected by a "+" vein running through the entire cavity. The overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 200μm.

[0030] In addition, the cross-section of the cell in the topological photonic crystal resonant cavity in the QCL layer is square. In the mediocre domain, the center of the cross-section of the column structure is filled with gallium arsenide material, and the rest is filled with air; in the non-trivial domain, the cross-section of the column structure is filled with gallium arsenide material all around, and the central area is filled with air, and the cells are arranged in a two-dimensional array.

[0031] In addition to the above, the bottom metal electrode and the top metal electrode are both made of metal materials. Specifically, the material of the bottom metal electrode and the material of the top metal electrode are both gold materials, the thickness of the bottom metal electrode is 300nm, and the thickness of the top metal electrode is 250nm. The thickness of the silicon dioxide insulating layer is 300nm.

[0032] Reference Figure 2 The present invention provides a method for processing the above-mentioned compact second-order topological insulator quantum cascade laser, comprising the following steps: S1. Preparation + Doped GaAs substrate and THz QCL wafer layer, depositing a metal layer on the GaAs substrate and THz QCL wafer layer, and then forming a bottom contact gold layer by a gold / gold hot compression wafer bonding process; S2. Based on the structure of forming a bottom contact gold layer and carrying a terahertz QCL wafer layer, the terahertz QCL wafer layer is thinned using a polisher, and then wet-etched with a hydrofluoric acid solution to remove the gallium arsenide substrate remaining in the terahertz QCL wafer layer, exposing the QCL active area; S3. Depositing a silicon dioxide insulating layer on the structure where the QCL active area is exposed, and demarcating the pumping area on the structure where the silicon dioxide insulating layer is deposited by optical lithography, and then removing the silicon dioxide insulating layer above the pumping area by reactive ion etching, leaving only the silicon dioxide insulating layer on the top periphery of the terahertz QCL wafer layer; S4, based on step S3, optical lithography technology is used again to determine the mediocre domain and non-mediocre domain of the photonic crystal in the terahertz QCL wafer layer, and to determine the pore area in the photonic crystal structure, and then a contact layer is deposited in the pore area, and the contact layer in the remaining area is used as a hard mask; then the photoresist is removed, and the remaining contact layer forms a top contact point for current injection, and finally the QCL active area is dry-etched to obtain the QCL layer, i.e., the topological photonic crystal resonant cavity; S5. After completing S2~S4, the main substrate in the overall structure, i.e. n + The doped gallium arsenide substrate is thinned, and metal layers are deposited on the bottom and top to obtain a bottom metal electrode and a top metal electrode respectively. Finally, a wire is connected to the top metal electrode to complete the processing of a compact second-order topological insulator quantum cascade laser.

[0033] Example 1 like Figure 1 As shown, in this embodiment, the bottom metal electrode 1 is gold, with a thickness of 300nm, a length and width of 428μm; the QCL layer 2 uses triple-well resonant-phonon gallium arsenide with a refractive index of 3.6, a thickness of 10μm, a length and width of 428μm. The QCL column geometry is a column structure with a circle radius of 7.07μm, a "+" vein width of 2μm, a vein length of 20μm, a period of 20μm, and a characteristic size of 180μm. The lower left area is a topological non-trivial structure, and the outside of the area is a topological trivial structure. The two are arranged in a square, and the defect geometry is an arc octagon, that is, the air gap between the cells. The silicon dioxide insulating layer 3 is coated on the top periphery of the QCL layer 2, with a thickness of 300nm, a length and width of 428μm; the top metal electrode 4 is gold, deposited on the outer silicon dioxide insulating layer 3 and the inner QCL layer 2, and the thickness is 250nm. The wire 5 is gold, which is used to introduce current for pump excitation.

[0034] The processing method of the above-mentioned compact second-order topological insulator quantum cascade laser is: (1) To make the bottom contact gold layer, first, use an electron beam evaporator to dope n + A titanium (15nm) / gold (700nm) metal layer is deposited on the GaAs substrate and the terahertz QCL wafer, and then the bottom contact gold layer is formed by gold / gold hot compression wafer bonding.

[0035] (2) Use a polisher to thin the QCL wafer substrate to less than 80 μm. Then, use NH3·H2O / H2O2 / H2O (3 / 55 / 32 mL) and hydrofluoric acid (49%) solution for wet etching to remove the remaining GaAs substrate and Al 50 Ga 50 As etches the stop layer to expose the QCL active area, where the length and width of the active area are both 428 μm.

[0036] (3) The processing of the topological photonic crystal resonator begins with the deposition of a silicon dioxide (200 nm) insulating layer. The pumping region is determined by optical lithography and reactive ion etching, and the silicon dioxide layer above the central 9×9 period region is removed. The photonic structure is then determined on the QCL wafer by optical lithography. The topological mediocre region is a cylindrical structure with a circular “+” cross section, a circle radius of 7.07 μm, a “+” vein width of 2 μm, a vein length of 20 μm, 9 periods in the x and y directions, and a total size of 180 μm. The topological non-trivial region is a cylindrical structure with a cross section of four quarter-circular “+” shapes, a circle radius of 7.07 μm, a “+” vein width of 2 μm, a vein length of 20 μm, 5 periods in the x and y directions, and a total size of 100 μm.

[0037] (4) Deposit and peel off the contact layer (Ti / Au / Ti / SiO2, 15 / 450 / 15 / 450nm) on the top of the pore area. The contact layer in the remaining area is used as a hard mask. The photonic structure is obtained by reactive ion dry etching of the active area. The mixed gas used is Cl2 / BCl3 / CH4=5 / 100 / 20 standard cubic centimeters / minute. In this process, the silicon dioxide is etched away, and the remaining titanium / gold (≈250nm) will become the top contact point for current injection. Next, the main substrate is thinned to about 200μm with a polisher, and a titanium / gold layer (15 / 200nm) is deposited as the bottom electrode. Finally, the wire is connected to the top contact point.

[0038] Reference Figure 3 , showing the circular "+" cell structure (gray), arc octagonal defect structure (white) and peripheral silicon dioxide and gallium arsenide combination structure (diagonal shadow) of the resonant cavity. The cells are arranged in a two-dimensional array. In the simulation, the gray area represents the gallium arsenide material with a refractive index of 3.6, the white area represents the air with a refractive index of 1, and the diagonal shadow area is filled with gallium arsenide material with a real part of the refractive index of 3.6 and an imaginary part of 0.006, replacing the silicon dioxide and gallium arsenide combination structure. The internal cavity feature size is 180μm, and the total feature size is 428μm.

[0039] In addition, if Figure 4 As shown in (a), (b), (c) and (d) in the figure, the band structure is calculated along the first Brillouin zone boundary Γ-XM-Γ. The topologically trivial and non-trivial structural units are both square, with characteristic dimensions of 20μm in length and width and 10μm in height, forming the QCL column unit structure. In the mediocre unit, the interior of the column is filled with gallium arsenide material (gray) with a refractive index of 3.6, and the rest is filled with air (white). The top of the column is filled with an air column of the same size as the column (length, width and height are all 20μm) to simulate the top air environment of the physical device. The positive and negative x and positive and negative y directions of the unit structure are set as periodic boundary conditions. The entire plane below the QCL column unit structure, including the air, is set as an ideal conductor, and the gallium arsenide plane part above the column is set as an ideal conductor, simulating the bottom and top gold electrodes respectively. The plane on the top air column is set as a scattering boundary condition. Finally, the finite element analysis method is used in COMSOL Multiphysics to perform a three-dimensional simulation and solve the band diagram. Since the positive and negative x and positive and negative y directions are both infinite periods, the overall structure of the photonic crystals formed by the topological trivial and non-trivial structural units is the same, so it can be seen that the widths of the trivial band gap and the non-trivial band gap are the same.

[0040] Reference Figure 5 , the upper part of which consists of 10 Figure 4The topological non-trivial units in the QCL column are arranged vertically, and the lower part is composed of 10 topological trivial units arranged vertically. The length and width of the QCL column unit are both 20μm, the height is 10μm, and the height of the air column is 20μm. The positive and negative x directions are periodic boundary conditions, and the positive and negative y directions are scattering boundary conditions. The lower surface of the entire QCL column structure, including the air, is set as an ideal conductor, and only the gallium arsenide part of the upper surface is set as an ideal conductor. The upper plane of the top air column is also set as a scattering boundary condition. Finally, in COMSOL Multiphysics, a three-dimensional simulation is performed along a specific path to solve the boundary state energy band diagram. The simulation results show that although the topological trivial structure is directly in contact with the topological non-trivial structure by adding structural perturbations to facilitate current injection, the boundary state (black solid line) is still located within the band gap, verifying the topological robustness of the structure. Figure 6 The near-field observation position shown is located at the middle height of the photonic crystal resonator, 5 μm. The structure consists of topological non-trivial units with 4 periods in the x and y directions in the lower left region and topological trivial units with 9 periods in the x and y directions in the periphery, forming a two-dimensional array.

[0041] like Figure 7 As shown in (a) and (b), in order to facilitate the observation of the far-field distribution of the three-dimensional topological cavity, a hemispherical air envelope is laid on the top of the QCL device along the positive z-axis. The observation position is 6λ> from the top of the QCL device. R =2 L 2 / λ( L =180μm, λ=106μm), the results show that the brightest part of the far-field mode is located near the corner state in the middle of the near field.

[0042] Example 2 like Figure 8 As shown, in this embodiment, the bottom metal electrode 1 is gold, with a thickness of 300nm, a length and width of 428μm; the QCL layer 2 uses triple-well resonant-phonon gallium arsenide with a refractive index of 3.6, a thickness of 10μm, a length and width of 428μm. The QCL column geometry is a column structure with a circle radius of 7.07μm, an "×"-shaped vein width of 2μm, a vein length of 28.28μm, a period of 20μm, and a characteristic size of 160μm. The lower left area is a topological non-trivial structure, and the outside of the area is a topological trivial structure. The two are arranged in a square, and the defect geometry is an arc hexagon, that is, the air gap between the cells. The silicon dioxide insulating layer 3 is coated on the top periphery of the QCL layer 2, with a thickness of 300nm, a length and width of 428μm; the top metal electrode 4 is gold, deposited on the outer silicon dioxide insulating layer 3 and the inner QCL layer 2, and the thickness is 250nm. The wire 5 is gold, which is used to introduce current for pump excitation.

[0043] The processing method of the above-mentioned compact second-order topological insulator quantum cascade laser is different from the method of Example 1 in that, in step (3) of this embodiment, the remaining steps are the same as those of Example 1. Specifically, the processing of the topological photonic crystal resonant cavity starts with the deposition of a silicon dioxide (200nm) insulating layer. The pumping region is determined by optical lithography and reactive ion etching, and the silicon dioxide layer above the central 8×8 period is removed. Next, the photonic structure is determined on the QCL wafer by optical lithography, wherein the topological mediocre region is composed of a cylindrical structure with a circular "×" cross section, a circle radius of 7.07μm, a "×"-shaped vein width of 2μm, a vein length of 28.28μm, 8 periods in each of the x and y directions, and a total size of 160μm. The topological non-trivial region is composed of a cylindrical structure with a cross section of four quarter-circular "×" shapes, the circle radius is also 7.07μm, the "×"-shaped vein width is 2μm, the vein length is 28.28μm, 4 periods in each of the x and y directions, and a total size of 80μm.

[0044] Reference Fig. 9 , we can see the circular "×"-shaped cell structure (gray), the arc hexagonal defect structure (white), and the peripheral structure composed of silicon dioxide and gallium arsenide (diagonal shadow), and the cells are arranged in a two-dimensional array. In the simulation, the gray area is filled with gallium arsenide material with a refractive index of 3.6, the white part is filled with air with a refractive index of 1, and the diagonal shadow part is filled with gallium arsenide material with a real part of the refractive index of 3.6 and an imaginary part of 0.006, replacing the silicon dioxide and gallium arsenide combination structure. The characteristic size of the internal cavity is 160μm, and the total characteristic size is 428μm.

[0045] Reference Fig.10(a), (b), (c) and (d) in the figure, the band structure is calculated along the first Brillouin zone boundary Γ-XM-Γ. The topologically trivial and non-trivial structural units are square, with characteristic dimensions of 20μm in length and width and 10μm in height, forming a QCL column unit structure. In the mediocre unit, the interior of the column is filled with gallium arsenide material (gray) with a refractive index of 3.6, and the rest is filled with air (white). The top of the column is also filled with an air column with a length, width and height of 20μm to simulate the air environment at the top of the physical device. The positive and negative x and positive and negative y directions of the unit structure are set as periodic boundary conditions. The entire plane below the QCL column unit structure, including the air part, is set as an ideal conductor, and the gallium arsenide plane part above the column is set as an ideal conductor, simulating the bottom and top gold electrodes respectively. The upper plane of the top air column is set as a scattering boundary condition. Finally, the finite element analysis method is used in COMSOL Multiphysics to perform a three-dimensional simulation and solve the band diagram. Since the positive and negative x and positive and negative y directions are infinitely periodic, the overall structure of the photonic crystal formed by the topologically trivial and non-trivial structural units is the same. Therefore, the width of the trivial band gap and the non-trivial band gap is the same. In addition, Fig.11 As shown, the upper part consists of 10 Fig.10 The topological non-trivial units in the QCL are arranged vertically, and the lower part is composed of 10 topological trivial units arranged vertically. The length and width of the QCL unit are both 20μm, the height is 10μm, the height of the air column is 20μm, the positive and negative x-direction boundaries of the structure are set as periodic boundary conditions, and the positive and negative y-direction boundaries are set as scattering boundary conditions. The lower surface of the entire QCL column structure, including the air part, is set as an ideal conductor, and only the gallium arsenide part of the upper surface is set as an ideal conductor, and the upper plane of the top air column is set as a scattering boundary condition. Finally, COMSOL Multiphysics is used to perform a three-dimensional simulation along a specific path to solve the boundary state energy band diagram. It can be seen that the topological trivial structure directly contacts the topological non-trivial structure by increasing the structural perturbation, which helps current injection, but the boundary state (black solid line) is still located in the band gap, which proves the topological robustness of the structure.

[0046] like Fig.12 As shown, the near-field observation position is located at a height of 5 μm in the middle of the photonic crystal resonator. The structure consists of a topological non-trivial unit array with 3 periods in the x and y directions in the lower left region and a topological trivial unit array with 8 periods in the x and y directions in the periphery. Fig.13 As shown in (a) and (b), in order to facilitate the observation of the far-field distribution of the three-dimensional topological cavity, a hemispherical air envelope is laid on the top of the QCL device along the positive z-axis. The observation position is 5λ away from the top of the QCL device. R =2 L 2 / λ( L=160μm, λ=104μm), the results show that the brightest part of the far-field mode is located near the corner state in the middle of the near field.

[0047] Example 3 like Fig.14 As shown, in this embodiment, the bottom metal electrode 1 is gold, with a thickness of 300nm, a length and width of 428μm; the QCL layer 2 uses triple-well resonant-phonon gallium arsenide with a refractive index of 3.6, a thickness of 10μm, a length and width of 428μm. The QCL column geometry has a side length of 14.14μm, a period of 20μm, and a characteristic size of 200μm. The topological non-trivial structure in the lower left area and the topological mediocre structure in the periphery are arranged in a square shape, and the defect geometry is a diamond, that is, the air gap between the cells. The silicon dioxide insulating layer 3 is coated on the top periphery of the QCL layer, with a thickness of 300nm, a length and width of 428μm; the top metal electrode 4 is gold, deposited on the outer silicon dioxide insulating layer 3 and the inner QCL layer 2, and the thickness is 250nm. The wire 5 is gold, which is used to introduce current for pump excitation.

[0048] The processing method of the above-mentioned compact second-order topological insulator quantum cascade laser is different from the method of Example 1 in that, in step (3) of this embodiment, the remaining steps are the same as those of Example 1. Specifically, the processing of the topological photonic crystal resonator begins with the deposition of a silicon dioxide (200nm) insulating layer. The pumping region is determined by optical lithography and reactive ion etching, and the silicon dioxide layer above the central 10×10 period is removed. Next, the photonic structure is determined on the QCL wafer using optical lithography. The topologically mediocre region is a columnar structure with a diamond cross section, a side length of 14.14μm, 10 periods in each of the x and y directions, and a total size of 200μm. The topologically non-trivial region is a columnar structure with a cross section of four quarter diamonds, a side length of 10μm, 5 periods in each of the x and y directions, and a total size of 100μm. The "+" vein width of the overall structure is 2μm, and the vein length is 200μm.

[0049] from Fig.15 It can be seen that the resonant cavity has a diamond cell structure (gray), a diamond defect structure (white), and a peripheral silicon dioxide and gallium arsenide combination structure (diagonal shadow). The cells are arranged in a two-dimensional array. In the simulation, the gray area is filled with gallium arsenide material with a refractive index of 3.6, the white part is filled with air with a refractive index of 1, and the diagonal shadow is filled with gallium arsenide material with a real refractive index of 3.6 and an imaginary part of 0.006, replacing the silicon dioxide and gallium arsenide combination structure. The internal cavity characteristic size is 200μm, and the total characteristic size is 428μm. Reference Fig.16(a), (b), (c) and (d) in the figure, the band structure is calculated along the first Brillouin zone boundary Γ-XM-Γ. The topologically trivial and non-trivial structural units are square, with characteristic dimensions of 20μm in length and width and 10μm in height, forming a QCL column unit structure. In the mediocre unit, the interior of the column is filled with gallium arsenide material (gray) with a refractive index of 3.6, and the rest is filled with air (white). The top of the column is also filled with an air column with a length, width and height of 20μm to simulate the air environment at the top of the physical device. The boundaries in the positive and negative x directions and the positive and negative y directions are set as periodic boundary conditions. The entire plane below the QCL column unit structure (including the air part) is set as an ideal conductor, and only the gallium arsenide plane part above is set as an ideal conductor to simulate the gold electrodes at the bottom and top. The upper plane of the top air column is set as a scattering boundary condition. Finally, the finite element analysis method is used in COMSOL Multiphysics for three-dimensional simulation to solve the band diagram. Since the positive and negative x directions and the positive and negative y directions are both infinite periods, the overall structure of the photonic crystals formed by the topological trivial and non-trivial structural units is the same, so the widths of the trivial band gap and the non-trivial band gap are the same.

[0050] In addition, if Fig.17 As shown, the upper part consists of 10 Fig.16 The topological non-trivial units in the QCL column are arranged vertically, and the lower part is composed of 10 topological trivial units arranged vertically. The length and width of the QCL unit are 20μm, the height is 10μm, and the height of the air column is 20μm. The positive and negative x-direction boundaries are set as periodic boundary conditions, and the positive and negative y-direction boundaries are scattering boundary conditions. The lower surface of the entire QCL column structure, including the air part, is set as an ideal conductor, and only the gallium arsenide part of the upper surface is set as an ideal conductor. The upper plane of the top air column is also set as a scattering boundary condition. Finally, a three-dimensional simulation is performed in COMSOL Multiphysics along a specific path to solve the boundary state energy band diagram. It can be seen that the topological trivial structure is in direct contact with the topological non-trivial structure by adding structural perturbations, which facilitates current injection, but the boundary state (black solid line) is still located in the band gap, which confirms the topological robustness of the structure.

[0051] Reference Fig.18 The near-field observation position is located in the middle of the photonic crystal resonator, 5 μm from the bottom. The structure consists of topological non-trivial units with 4 periods in the x and y directions in the lower left region and topological trivial units with 9 periods in the x and y directions in the periphery, forming a two-dimensional array.

[0052] like Fig.19 As shown in (a) and (b), in order to facilitate the observation of the far-field distribution of the three-dimensional topological cavity, a hemispherical air envelope is laid on the top of the QCL device along the positive z-axis. The observation position is 8λ> from the top of the QCL device. R =2L 2 / λ( L =200μm, λ=104μm), the results show that the brightest part of the far-field mode is located near the corner state in the middle of the near field.

[0053] Based on the above-mentioned embodiments 1 to 3, comparison diagrams in three embodiments are also provided. Fig. 20 , Fig.21 , Fig. 22 and Fig.23 . Specifically include the following: first, Fig. 20 The results shown are calculated using the COMSOL Multiphysics electromagnetic wave frequency domain formula ewfd.Qfactor. It can be seen that the angular state quality factors of the three structures are higher than other modes. The quality factors of the circular "+", circular "×", and diamond angular states are 243, 222, and 172, respectively, which are all greater than the minimum laser quality factor requirement of 150.

[0054] Secondly, Fig.21 The results shown are calculated according to the following formula: ; Among them, ε ( r ) represents the dielectric constant of each position of the QCL device (calculated by the refractive index of the filling material), E ( r ) represents the electric field intensity (obtained by ewfd.normE in COMSOL). The effective mode volume is calculated by the ratio of the integral to the maximum value. It can be seen that the corner mode volumes of the three structures are significantly smaller than other modes. The effective mode volumes of the circular “+”, circular “×”, and diamond corner states are 5.14×10 3 μm 3 ,3.55×10 3 μm 3 , 6.97×10 3 μm 3 , demonstrating the compactness and high localization of the pattern.

[0055] again, Fig. 22 The results shown are calculated according to the following formula: ; in, , represents the effective refractive index, represents the imaginary part of frequency, cIt can be seen from the figure that the loss factors of the corner modes of the three structures are significantly lower than those of other modes. The loss factors of the circular “+” shape, circular “×” shape, and diamond corner modes are 6.8 cm -1 、8cm -1 、11cm -1 , proving that this mode can serve as an excellent excitation light source, helping to improve the life and performance of the laser.

[0056] at last, Fig.23 The results shown are calculated according to the following formula: ; exist Fig.23 It can be seen that the Purcell factors of the corner modes of the three structures are significantly higher than those of other modes. The Purcell factors of the circular "+", circular "×", and diamond corner modes are 197, 213, and 69, respectively. The Purcell factor is closely related to the quality factor and effective mode volume. It is precisely because the corner mode has a high quality factor, compact mode volume, and miniaturized device size that its Purcell factor is the highest.

[0057] Therefore, the above-mentioned compact second-order topological insulator quantum cascade laser and its processing method are adopted, the topological optical mode of the second-order topological insulator is utilized, and a resonant cavity is constructed with the help of a two-dimensional SSH model. The coupling strength is adjusted by changing the position of the QCL dielectric column in the unit structure to obtain a topological band gap, and the two-dimensional vein connection is utilized to play a medium role and conduct electricity, and a topological boundary is constructed to form a topologically protected localized mode; and by changing the periodic number of topologically trivial and topologically non-trivial structures, the size of the topological cavity can be adjusted, thereby greatly improving the monolithic integration of the topological semiconductor laser.

[0058] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A compact second-order topological insulator quantum cascade laser, characterized in that: It includes a bottom metal electrode, a topological optical resonant cavity, a silicon dioxide insulating layer, a top metal electrode and a wire arranged from bottom to top, wherein the topological optical resonant cavity is a QCL layer, the QCL layer is arranged above the bottom metal electrode, the silicon dioxide insulating layer is coated on the top periphery of the QCL layer, the top metal electrode is deposited above the silicon dioxide insulating layer and above the inside of the QCL layer, and the wire is connected to the top metal electrode for introducing current for electrical pumping excitation.

2. A compact second-order topological insulator quantum cascade laser according to claim 1, characterized in that: The QCL layer is made of isotropic GaAs material with a thickness of 10 μm, and has a topological interface composed of a periodically distributed columnar structure that runs through the QCL layer, which is used to realize the injection of pump current into the columnar structure of the QCL layer and the formation of an angular mode.

3. A compact second-order topological insulator quantum cascade laser according to claim 2, characterized in that: The material of the QCL layer is one of triple-well resonance-phonon gallium arsenide or aluminum-containing gallium arsenide with a refractive index of 3.

6.

4. A compact second-order topological insulator quantum cascade laser according to claim 3, characterized in that: The column structure of the QCL layer is constructed of a mediocre domain column structure and a non-mediocre domain column structure. The cross section of the mediocre domain column structure is circular or rhombus-shaped, and the cross section of the non-mediocre domain column structure is four quarter circles or four quarter rhombuses.

5. A compact second-order topological insulator quantum cascade laser according to claim 4, characterized in that: The cross-sectional shape of the columnar structure cell of the QCL layer is one of a circular "+" shape, a circular "×" shape or a diamond shape, specifically: When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "+" shape, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a circular "+" shape vein, the cross-section of the mediocre domain columnar structure is circular, and the cross-section of the non-mediocre domain columnar structure is a quarter circle; wherein, the characteristic size of the QCL layer columnar structure unit is 20 μm, the circular radius size is 7.07 μm, the width of the "+" shape vein is 2 μm, and the length is 20 μm, the centers of the two veins of the unit cell of the mediocre domain columnar structure are aligned with the center of the unit cell, the centers of the four veins of the non-mediocre domain columnar structure are respectively aligned with the centers of the four sides of the unit cell, and the overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 180 μm; When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "×" shape, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a circular "×"-shaped vein, the cross-section of the mediocre domain columnar structure is circular, and the cross-section of the non-mediocre domain columnar structure is a quarter circle; wherein, the characteristic size of the QCL layer columnar structure unit is 20 μm, the circular radius size is 6.6 μm, the width of the "×"-shaped vein is 2 μm, and the length is 28.3 μm, the centers of the two veins of the unit cell of the mediocre domain columnar structure and the unit cell of the non-mediocre domain columnar structure are aligned with the center of the unit cell, and the overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 160 μm; When the cross-sectional shape of the columnar structure cell of the QCL layer is a rhombus, the mediocre domain columnar structure and the non-mediocre domain columnar structure are connected by a rhombus "+"-shaped vein, the cross-section of the mediocre domain columnar structure is a rhombus, and the cross-section of the non-mediocre domain columnar structure is a quarter of a rhombus; wherein, the characteristic size of the QCL layer columnar structure unit is 20μm, the side length of the rhombus is 14.1μm, and each QCL layer columnar structure unit has no "+"-shaped vein connection inside, but each columnar structure unit is connected by a "+"-shaped vein running through the entire cavity, and the overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 200μm.

6. A compact second-order topological insulator quantum cascade laser according to claim 5, characterized in that: The cross-section of the cell in the topological photonic crystal resonant cavity in the QCL layer is square, wherein in the mediocre domain, the center of the cross-section of the column structure is filled with gallium arsenide material, and the rest is filled with air; in the non-trivial domain, the cross-section of the column structure is filled with gallium arsenide material all around, and the central area is filled with air, and the arrangement of the cells is a two-dimensional array arrangement.

7. A compact second-order topological insulator quantum cascade laser according to claim 1, characterized in that: The bottom metal electrode and the top metal electrode are both made of metal materials.

8. A compact second-order topological insulator quantum cascade laser according to claim 7, characterized in that: The material of the bottom metal electrode and the material of the top metal electrode are both gold materials. The thickness of the bottom metal electrode is 300 nm, and the thickness of the top metal electrode is 250 nm.

9. A compact second-order topological insulator quantum cascade laser according to claim 1, characterized in that: The thickness of the silicon dioxide insulating layer is 300 nm.

10. A method for processing a compact second-order topological insulator quantum cascade laser according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1. Preparation + Doped GaAs substrate and THz QCL wafer layer, depositing a metal layer on the GaAs substrate and THz QCL wafer layer, and then forming a bottom contact gold layer by a gold / gold hot compression wafer bonding process; S2. Based on the structure of forming a bottom contact gold layer and carrying a terahertz QCL wafer layer, the terahertz QCL wafer layer is thinned using a polisher, and then wet-etched with a hydrofluoric acid solution to remove the gallium arsenide substrate remaining in the terahertz QCL wafer layer, exposing the QCL active area; S3. Depositing a silicon dioxide insulating layer on the structure where the QCL active area is exposed, and demarcating the pumping area on the structure where the silicon dioxide insulating layer is deposited by optical lithography, and then removing the silicon dioxide insulating layer above the pumping area by reactive ion etching, leaving only the silicon dioxide insulating layer on the top periphery of the terahertz QCL wafer layer; S4, based on step S3, using optical lithography technology again, determining the mediocre domain and the non-mediocre domain of the photonic crystal in the terahertz QCL wafer layer, and determining the pore area in the photonic crystal structure, and then depositing a contact layer in the pore area, and using the contact layer in the remaining area as a hard mask; The photoresist is then removed, and the remaining contact layer forms the top contact point for current injection. Finally, the QCL active area is dry-etched to obtain the QCL layer, i.e., the topological photonic crystal resonant cavity; S5. After completing S2~S4, the main substrate in the overall structure, i.e. n + The doped gallium arsenide substrate is thinned, and metal layers are deposited on the bottom and top to obtain a bottom metal electrode and a top metal electrode respectively. Finally, a wire is connected to the top metal electrode to complete the processing of a compact second-order topological insulator quantum cascade laser.

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