A Compact Second-Order Topological Insulator Quantum Cascade Laser and Its Fabrication Method

Abstract

The present invention provides a compact second-order topological insulator quantum cascade laser and a processing method thereof, belonging to the technical field of topological photonics. The compact second-order topological insulator quantum cascade laser includes a bottom metal electrode, a topological optical resonator, a silica insulating layer, a top metal electrode and a wire arranged from bottom to top. The topological optical resonator is a QCL layer, the QCL layer is arranged above the bottom metal electrode, the silica insulating layer covers the top periphery of the QCL layer, the top metal electrode is deposited above the silica 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. The present invention solves the problems of low photoelectric conversion efficiency, unstable output, difficulty in integration of traditional semiconductor lasers, and complex design, difficult manufacturing and large volume not conducive to integration of second-order topological insulator quantum cascade lasers.

Description

Technical Field

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

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

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

[0004] In terms of manufacturing technology, the complex topological structure and precise material requirements pose huge challenges to the device manufacturing process. Tiny process deviations may lead to a significant decline in device performance, imposing almost demanding 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 large volume. In the current trend of the development of optoelectronic systems towards high integration and miniaturization, the large device volume not only occupies too much space, increases the overall size and weight of the system, but also seriously hinders the integration with other optoelectronic devices, making it difficult to meet the requirements for compact layout and multi-functional integration in fields such as on-chip optical systems. Summary of the Invention

[0006] In order to overcome the deficiencies of the prior art, the object of the present invention is to provide a compact second-order topological insulator quantum cascade laser and a processing method thereof. By utilizing the topological optical mode of the second-order topological insulator, a resonator is constructed with the help of the 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 bandgap. The two-dimensional vein connection plays a mediating role and conducts electricity, and a topological boundary is constructed to form a topologically protected local mode. And by changing the periodicity number of the topologically trivial and topologically non-trivial structures, the size of the topological cavity can be adjusted, thereby greatly improving the monolithic integration degree of the topological semiconductor laser.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] A compact second-order topological insulator quantum cascade laser, comprising a bottom metal electrode, a topological optical resonator, a silicon dioxide insulating layer, a top metal electrode and a wire arranged from bottom to top. The topological optical resonator is a QCL layer. The QCL layer is arranged above the bottom metal electrode. The silicon dioxide insulating layer covers the top periphery of the QCL layer. The top metal electrode is deposited above the silicon dioxide insulating layer and above the interior of the QCL layer. The wire is connected to the top metal electrode and is used to introduce current for electrical pumping excitation.

[0009] Preferably, the QCL layer is made of isotropic GaAs material with a thickness of 10 μm, and the QCL layer has a topological interface formed by a periodically distributed columnar structure penetrating 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 state modes.

[0010] Preferably, the material of the QCL layer is one of triple-well resonant-phonon gallium arsenide with a refractive index of 3.6 or aluminum-containing gallium arsenide.

[0011] Preferably, the columnar structure of the QCL layer is constructed by a trivial domain columnar structure and a non-trivial domain columnar structure. The cross-section of the trivial domain columnar structure is circular or rhombic, and the cross-section of the non-trivial domain columnar structure is four quarter-circles or four quarter-rhombuses.

[0012] Preferably, the cross-sectional shape of the columnar structure unit cell of the QCL layer is one of a circular "+", a circular "×", or a rhombus. Specifically:

[0013] When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "+", the trivial domain columnar structure and the non-trivial domain columnar structure are connected by a circular "+" vein. The cross-section of the trivial domain columnar structure is circular, and the cross-section of the non-trivial domain columnar structure is a quarter-circle. Among them, the characteristic size of the columnar structure unit of the QCL layer is 20 μm, the circular radius size is 7.07 μm, the width of the "+" vein is 2 μm, and the length is 20 μm. The centers of the two veins of the trivial domain columnar structure unit cell are both aligned with the center of the unit cell. The centers of the four veins of the non-trivial domain columnar structure are respectively aligned with the centers of the four sides of the unit cell. The overall characteristic size of the QCL layer columnar structure composed of the QCL layer columnar structure units is 180 μm.

[0014] When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "×" shape, a circular "×" shaped vein is used to connect the trivial domain columnar structure and the non-trivial domain columnar structure. The cross-section of the trivial domain columnar structure is circular, and the cross-section of the non-trivial domain columnar structure is a quarter circle. Among them, 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 trivial domain columnar structure and the unit cell of the non-trivial domain columnar structure are both aligned with the cell center. The overall characteristic size of the QCL layer columnar structure composed of QCL layer columnar structure units is 160 μm;

[0015] When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a rhombus, a rhombus "+" shaped vein is used to connect the trivial domain columnar structure and the non-trivial domain columnar structure. The cross-section of the trivial domain columnar structure is a rhombus, and the cross-section of the non-trivial domain columnar structure is a quarter rhombus. Among them, the characteristic size of the QCL layer columnar structure unit is 20 μm, the side length of the rhombus is 14.1 μm, and there is no "+" shaped vein connection inside each QCL layer columnar structure unit. Instead, the "+" shaped vein running through the entire cavity is used to connect each columnar structure unit. The overall characteristic size of the QCL layer columnar structure composed of QCL layer columnar structure units is 200 μm.

[0016] Preferably, the cross-section of the unit cell in the topological photonic crystal resonant cavity in the QCL layer is a square. Among them, in the trivial domain, the cross-section center of the columnar structure is filled with gallium arsenide material, and the rest is filled with air; in the non-trivial domain, the periphery of the cross-section of the columnar structure is filled with gallium arsenide material, and the central region is filled with air, and the arrangement of the unit cells is a two-dimensional array arrangement.

[0017] Preferably, both the bottom metal electrode and the top metal electrode are made of metal materials.

[0018] 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.

[0019] Preferably, the thickness of the silicon dioxide insulating layer is 300 nm.

[0020] The present invention also provides a processing method for the compact second-order topological insulator quantum cascade laser according to the above, including the following steps:

[0021] S1. Prepare n +A doped gallium arsenide substrate and a terahertz QCL wafer layer. A metal layer is deposited on the gallium arsenide substrate and the terahertz QCL wafer layer, and then a bottom contact gold layer is formed through a gold / gold thermocompression wafer bonding process;

[0022] S2. Based on the structure that forms the bottom contact gold layer and bears the terahertz QCL wafer layer, the terahertz QCL wafer layer is thinned using a polishing instrument, and then wet etched with a hydrofluoric acid solution to remove the residual gallium arsenide substrate in the terahertz QCL wafer layer, exposing the QCL active region;

[0023] S3. On the structure where the QCL active region is exposed, a silicon dioxide insulating layer is deposited, and the position of the pumping region is delineated on the structure deposited with the silicon dioxide insulating layer through optical lithography technology. Subsequently, through a reactive ion etching process, the silicon dioxide insulating layer above the pumping region is removed, and only the silicon dioxide insulating layer at the top periphery of the terahertz QCL wafer layer is retained;

[0024] S4. Based on step S3, again using optical lithography technology, the photonic crystal trivial domain and non-trivial domain are determined in the terahertz QCL wafer layer, and the pore region in the photonic crystal structure is determined. Subsequently, a contact layer is deposited in the pore region, and the contact layer in the remaining region is used as a hard mask; then the photoresist is removed, and the remaining contact layer forms the top contact point for current injection. Finally, the QCL active region is dry etched to obtain the QCL layer, that is, the topological photonic crystal resonator;

[0025] S5. After completing S2 - S4, the main substrate in the overall structure, that is, the n + doped gallium arsenide substrate is thinned, metal layers are deposited on its bottom and top to obtain the bottom metal electrode and the top metal electrode respectively, and finally a wire is connected to the top metal electrode to complete the processing of the compact second-order topological insulator quantum cascade laser.

[0026] According to the specific embodiments provided by the present invention, the following technical effects are disclosed by the present invention:

[0027] (1) The topological photonic crystal resonator constructed based on the two-dimensional SSH model in the present invention adopts a direct surface emission laser scheme and an electrical pumping excitation method, which greatly simplifies the excitation process of the laser, reduces the dependence on complex optical components during the manufacturing process, significantly reduces the physical manufacturing difficulty of the device, effectively controls the cost, and lays a solid foundation for large-scale production and application.

[0028] (2) By utilizing the special properties of the topological corner states of photonic crystals, the extremely small mode volume and wavelength characteristics in the topological bandgap of the present invention bring advantages of low loss, high quality factor, and Purcell factor. This not only improves the energy conversion efficiency of the laser, making the 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 maximum density of states under the same dimension, highly meeting the integration requirements of compact devices and providing the possibility for realizing highly integrated optoelectronic devices.

[0029] (3) Since the topological trivial and non-trivial unit structures in the topological photonic crystal cavity are isotropic, the Bloch modes in this photonic crystal exhibit symmetry. Under the 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 on the boundary of the first Brillouin zone.

[0030] (4) Under the condition that the boundaries in the positive and negative x directions are periodic and the boundaries in the positive and negative y directions are truncated by scattering boundaries, the finite-length trivial domain structure and non-trivial domain structure form boundary states with an energy gap. Compared with the conventional interface, the present invention adds a quarter circle or diamond filling to the trivial structure unit at the boundary and combines it with the non-trivial structure as a whole to form a topological interface, which is convenient for injecting the pump current into the corner state cylinder.

[0031] (5) The present invention accurately reveals the corner state position by means of the near-field mode distribution of the topological photonic crystal cavity. By changing the number of periodic structure units, the cavity size can be flexibly adjusted. On the premise of ensuring that the quality factor of the laser output is not less than 150, the minimum size design of the laser is realized, taking into account the dual goals of miniaturization and high performance. In addition, the far-field distribution measurement provides an effective means for judging the working mode of the laser, which is helpful for accurately optimizing and controlling the performance of the laser.

[0032] (6) The present invention adopts an electrical pumping excitation method that only depends on the single adjustable degree of freedom of the pump intensity, successfully breaking through many limitations of traditional optical pumping lasers. Great breakthroughs have been made in achieving high-performance and specific features 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 fields of lasers and meeting the usage requirements of more complex scenarios. Description of the Drawings

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required in the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0034] Figure 1 Schematic diagram of the circular "+" structure of a compact second-order topological insulator quantum cascade laser of the present invention;

[0035] Figure 2 Flowchart of the processing method of a compact second-order topological insulator quantum cascade laser of the present invention;

[0036] Figure 3 Schematic diagram of the x-y plane of the finite-size simulation structure of the circular "+" structure topological photonic crystal resonator provided in Embodiment 1 of the present invention;

[0037] Figure 4 Schematic diagrams of the circular "+" structure topological trivial and non-trivial unit structures and their corresponding topological trivial and non-trivial photonic crystal energy band diagrams provided in Embodiment 1 of the present invention; wherein, Figure 4 (a) in is the schematic diagram of the topological trivial unit structure, Figure 4 and (b) in is the schematic diagram of the non-trivial unit structure; Figure 4 (c) in is the topological trivial photonic crystal energy band diagram, Figure 4 and (d) in is the non-trivial photonic crystal energy band diagram;

[0038] Figure 5 Boundary state unit composed of the circular "+" structure topological trivial and non-trivial unit structures provided in Embodiment 1 of the present invention and its corresponding photonic crystal boundary state energy band diagram;

[0039] Figure 6 Angular state near-field x-y view of the simulation results of the circular "+" structure topological photonic crystal resonator provided in Embodiment 1 of the present invention;

[0040] Figure 7 Angular state far-field observation position and its x-y view of the simulation results of the circular "+" structure topological photonic crystal resonator provided in Embodiment 1 of the present invention; wherein, Figure 7 (a) in is the angular state far-field observation position diagram of the simulation results of the circular "+" structure topological photonic crystal resonator, Figure 7 and (b) in is the x-y view corresponding to the angular state far-field observation position of the simulation results of the circular "+" structure topological photonic crystal resonator;

[0041] Figure 8Schematic diagram of the circular "×" structure of the compact second-order topological insulator quantum cascade laser provided in Embodiment 2 of the present invention;

[0042] Figure 9 Schematic diagram of the x-y plane of the finite-size simulation structure of the topological photonic crystal resonator with a circular "×" structure provided in Embodiment 2 of the present invention;

[0043] Figure 10 Topologically trivial and non-trivial unit structures of the circular "×" structure provided in Embodiment 2 of the present invention and their corresponding topologically trivial and non-trivial photonic crystal energy band diagrams; among them, Figure 10 in (a) is a schematic diagram of the topologically trivial unit structure, Figure 10 in (b) is a schematic diagram of the non-trivial unit structure; Figure 10 in (c) is a topologically trivial photonic crystal energy band diagram, Figure 10 in (d) is a non-trivial photonic crystal energy band diagram;

[0044] Figure 11 Boundary state unit composed of topologically trivial and non-trivial unit structures of the circular "×" structure provided in Embodiment 2 of the present invention and its corresponding photonic crystal boundary state energy band diagram;

[0045] Figure 12 Angular state near-field x-y plane view of the simulation results of the topological photonic crystal with a circular "×" structure provided in Embodiment 2 of the present invention;

[0046] Figure 13 Angular state far-field observation position and its x-y view of the simulation results of the topological photonic crystal with a circular "×" structure provided in Embodiment 2 of the present invention; among them, Figure 13 in (a) is a diagram of the angular state far-field observation position of the simulation results of the topological photonic crystal with a circular "×" structure, Figure 13 in (b) is the corresponding x-y view of the angular state far-field observation position of the simulation results of the topological photonic crystal with a circular "×" structure;

[0047] Figure 14 Schematic diagram of the rhombus structure of the compact second-order topological insulator quantum cascade laser provided in Embodiment 3 of the present invention;

[0048] Figure 15 Schematic diagram of the x-y plane of the finite-size simulation structure of the topological photonic crystal resonator with a rhombus structure provided in Embodiment 3 of the present invention;

[0049] Figure 16 Topologically trivial and non-trivial unit structures of the rhombus structure provided in Embodiment 3 of the present invention and their corresponding topologically trivial and non-trivial photonic crystal energy band diagrams; among them, Figure 16 in (a) is a schematic diagram of the topologically trivial unit structure,Figure 16 In (b) is a schematic diagram of a non-trivial unit structure; Figure 16 In (c) is a band structure diagram of a topologically trivial photonic crystal, Figure 16 In (d) is a band structure diagram of a non-trivial photonic crystal;

[0050] Figure 17 This is the boundary state unit composed of topologically trivial and non-trivial unit structures of the rhombus structure provided in Embodiment 3 of the present invention and its corresponding photonic crystal boundary state band structure diagram;

[0051] Figure 18 This is the x-y near-field view of the corner state of the simulation result of the rhombus structure topological photonic crystal provided in Embodiment 3 of the present invention;

[0052] Figure 19 This is the far-field observation position and its x-y view of the corner state of the simulation result of the rhombus structure topological photonic crystal provided in Embodiment 3 of the present invention; among them, Figure 19 In (a) is the far-field observation position diagram of the corner state of the simulation result of the rhombus structure topological photonic crystal, Figure 19 In (b) is the corresponding x-y view of the far-field observation position of the corner state of the simulation result of the rhombus structure topological photonic crystal;

[0053] Figure 20 This is a scatter plot comparison diagram of the quality factors corresponding to different frequency points of the topological photonic crystal resonator in Embodiments 1 to 3 of the present invention; among them, the quality factors at the corner state frequencies of each instance are marked by numbers;

[0054] Figure 21 This is a scatter plot comparison diagram of the effective mode volumes corresponding to different frequency points of the topological photonic crystal resonator in Embodiments 1 to 3 of the present invention; among them, the effective mode volumes at the corner state frequencies of each instance are marked by numbers;

[0055] Figure 22 This is a scatter plot comparison diagram of the loss factors corresponding to different frequency points of the topological photonic crystal resonator in Embodiments 1 to 3 of the present invention; among them, the loss factors at the corner state frequencies of each instance are marked by numbers;

[0056] Figure 23 This is a scatter plot comparison diagram of the Purcell factors corresponding to different frequency points of the topological photonic crystal resonator in Embodiments 1 to 3 of the present invention; among them, the Purcell factors at the corner state frequencies of each instance are marked by numbers.

[0057] Explanation of reference numerals:

[0058] 1. Bottom metal electrode; 2. QCL layer; 3. Silicon dioxide insulating layer; 4. Top metal electrode; 5. Wire. Detailed implementation manners

[0059] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0060] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0061] The present invention provides a compact second-order topological insulator quantum cascade laser, which includes a bottom metal electrode, a topological optical resonator, a silicon dioxide insulating layer, a top metal electrode, and a wire arranged from bottom to top. The topological optical resonator is a QCL layer. The QCL layer is arranged above the bottom metal electrode. The silicon dioxide insulating layer covers 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. The wire is connected to the top metal electrode and is used to introduce current for electrical pumping excitation.

[0062] In the above content, the QCL layer is composed of isotropic GaAs material with a thickness of 10 μm, and the QCL layer has a topological interface composed of a periodically distributed columnar structure penetrating the QCL layer, which is used to realize the injection of the pumping current into the columnar structure of the QCL layer and the formation of the angular state mode. 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 by an ordinary domain columnar structure and a non-trivial domain columnar structure. The cross-section of the ordinary domain columnar structure is circular or rhombic, and the cross-section of the non-trivial domain columnar structure is four quarter circles or four quarter rhombuses. The cross-sectional shape of the columnar structure unit cell of the QCL layer is one of a circular "+", a circular "×", or a rhombus. Specifically:

[0063] When the cross-sectional shape of the columnar structure unit cell of the QCL layer is a circular "+", the ordinary domain columnar structure and the non-trivial domain columnar structure are connected by a circular "+" vein. The cross-section of the ordinary domain columnar structure is circular, and the cross-section of the non-trivial domain columnar structure is a quarter circle. Among them, the characteristic dimension of the columnar structure unit of the QCL layer is 20 μm, the circular radius dimension is 7.07 μm, the width of the "+" vein is 2 μm, and the length is 20 μm. The centers of the two veins of the ordinary domain columnar structure unit cell are aligned with the center of the unit cell, and the centers of the four veins of the non-trivial domain columnar structure are respectively aligned with the centers of the four sides of the unit cell. The overall characteristic dimension of the columnar structure of the QCL layer composed of the columnar structure units of the QCL layer is 180 μm.

[0064] When the cross-sectional shape of the columnar structural unit cell of the QCL layer is a circular "×" shape, the columnar structures in the trivial domain and the non-trivial domain are connected by a circular "×" shaped vein. The cross-section of the columnar structure in the trivial domain is circular, and the cross-section of the columnar structure in the non-trivial domain is a quarter circle. Among them, 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 columnar structure in the trivial domain and the unit cell of the columnar structure in the non-trivial domain are both aligned with the center of the unit cell. The overall characteristic size of the QCL layer columnar structure composed of QCL layer columnar structure units is 160 μm;

[0065] When the cross-sectional shape of the columnar structural unit cell of the QCL layer is a rhombus, the columnar structures in the trivial domain and the non-trivial domain are connected by a rhombus "+" shaped vein. The cross-section of the columnar structure in the trivial domain is a rhombus, and the cross-section of the columnar structure in the non-trivial domain is a quarter rhombus. Among them, the characteristic size of the QCL layer columnar structure unit is 20 μm, the side length of the rhombus is 14.1 μm, and there is no "+" shaped vein connection inside each QCL layer columnar structure unit. Instead, the "+" shaped vein that penetrates the entire cavity connects each columnar structure unit. The overall characteristic size of the QCL layer columnar structure composed of QCL layer columnar structure units is 200 μm.

[0066] In addition, the cross-section of the unit cell in the topological photonic crystal resonator cavity in the QCL layer is a square. Among them, in the trivial domain, the center of the cross-section of the columnar structure is filled with gallium arsenide material, and the rest is filled with air; in the non-trivial domain, the periphery of the cross-section of the columnar structure is filled with gallium arsenide material, and the central area is filled with air, and the arrangement of the unit cells is a two-dimensional array arrangement.

[0067] In addition to the above content, it also includes that both the bottom metal electrode and the top metal electrode are made of metal materials. Specifically, the materials of both the bottom metal electrode and the top metal electrode are gold materials. The thickness of the bottom metal electrode is 300 nm, and the thickness of the top metal electrode is 250 nm. The thickness of the silicon dioxide insulating layer is 300 nm.

[0068] Refer to Figure 2 , the present invention provides the above-mentioned processing method of a compact second-order topological insulator quantum cascade laser, including the following steps:

[0069] S1. Prepare an n + -doped gallium arsenide substrate and a terahertz QCL wafer layer. Deposit a metal layer on the gallium arsenide substrate and the terahertz QCL wafer layer, and then form a bottom contact gold layer through a gold / gold thermocompression wafer bonding process;

[0070] S2. Based on the structure that forms the bottom contact gold layer and bears the terahertz QCL wafer layer, use a polishing instrument to thin the terahertz QCL wafer layer, and then perform wet etching with hydrofluoric acid solution to remove the residual gallium arsenide substrate in the terahertz QCL wafer layer, exposing the QCL active region;

[0071] S3. On the structure where the QCL active region is exposed, deposit a silicon dioxide insulating layer, and use optical lithography technology to demarcate the position of the pump region on the structure deposited with the silicon dioxide insulating layer. Subsequently, through the reactive ion etching process, remove the silicon dioxide insulating layer above the pump region, only retaining the silicon dioxide insulating layer on the top periphery of the terahertz QCL wafer layer;

[0072] S4. Based on step S3, use optical lithography technology again to determine the trivial domain and non-trivial domain of the photonic crystal in the terahertz QCL wafer layer, and determine the air hole region in the photonic crystal structure. Subsequently, deposit a contact layer in the air hole region, and use the contact layer in the remaining region as a hard mask; then remove the photoresist, and the remaining contact layer forms the top contact point for current injection. Finally, perform dry etching on the QCL active region to obtain the QCL layer, namely the topological photonic crystal resonator;

[0073] S5. After completing S2 - S4, thin the main substrate in the overall structure, namely the n + -doped gallium arsenide substrate, deposit metal layers on its bottom and top to obtain the bottom metal electrode and the top metal electrode respectively. Finally, connect a wire to the top metal electrode to complete the processing of the compact second-order topological insulator quantum cascade laser.

[0074] Example 1

[0075] As Figure 1 shown, in this example, the bottom metal electrode 1 is gold, with a thickness of 300 nm, a length of 428 μm, and a width of 428 μm; the QCL layer 2 uses triple-well resonant-phonon gallium arsenide with a refractive index of 3.6, a thickness of 10 μm, a length of 428 μm, and a width of 428 μm. The QCL column geometry is a columnar structure with a circular radius of 7.07 μm, a “+” cross vein width of 2 μm, a vein length of 20 μm, a period of 20 μm, a characteristic size of 180 μm. The lower left region is a topological non-trivial structure, and the region outside is a topological trivial structure. The two are arranged in a square pattern, and the defect geometry is an arc octagon, that is, the air gap between the unit cells. The silicon dioxide insulating layer 3 is coated on the top periphery of the QCL layer 2, with a thickness of 300 nm, a length of 428 μm, and a width of 428 μm; the top metal electrode 4 is gold, deposited on the outer silicon dioxide insulating layer 3 and the internal QCL layer 2, with a thickness of 250 nm for both. The wire 5 is gold, used to introduce current for pumping excitation.

[0076] The processing method of the above-mentioned compact second-order topological insulator quantum cascade laser is as follows:

[0077] (1) Fabricate the bottom contact gold layer. First, deposit a titanium (15 nm) / gold (700 nm) metal layer on the doped n + -type gallium arsenide substrate and the terahertz QCL wafer using an electron beam evaporator, and then form the bottom contact gold layer through gold / gold thermocompression wafer bonding.

[0078] (2) Thin the QCL wafer substrate to less than 80 μm using a polisher. Then, perform wet etching using NH3·H2O / H2O2 / H2O (3 / 55 / 32 mL) and hydrofluoric acid (49%) solution to remove the residual GaAs substrate and the Al 50 Ga 50 As etching stop layer, exposing the QCL active region. The length and width of the active region are both 428 μm.

[0079] (3) The processing of the topological photonic crystal resonator starts with the deposition of a silica (200 nm) insulating layer. Determine the pumping area through optical lithography and reactive ion etching, and remove the silica layer above the central 9×9 periodic region. Then, determine the photonic structure on the QCL wafer through optical lithography. The topologically trivial region is a cylindrical structure with a cross-section of a circular “+” shape, the radius of the circle is 7.07 μm, the width of the “+” veins is 2 μm, the vein length is 20 μm, there are 9 periods in both the x and y directions, and the total size is 180 μm. The topologically non-trivial region is a cylindrical structure with a cross-section of four quarter-circular “+” shapes, the radius of the circle is 7.07 μm, the width of the “+” veins is 2 μm, the vein length is 20 μm, there are 5 periods in both the x and y directions, and the total size is 100 μm.

[0080] (4) Deposit and strip the contact layer (Ti / Au / Ti / SiO2, 15 / 450 / 15 / 450 nm) on the top of the pore region. The contact layer in the remaining region serves as a hard mask. The photonic structure is obtained through reactive ion dry etching of the active region, and the mixed gas used is Cl2 / BCl3 / CH4 = 5 / 100 / 20 standard cubic centimeters per minute. During this process, the silica is etched away, and the residual titanium / gold (≈250 nm) will become the top contact point for current injection. Then, thin the main substrate to about 200 μm using a polisher, deposit a titanium / gold layer (15 / 200 nm) as the bottom electrode, and finally connect a wire to the top contact point.

[0081] Refer to Figure 3, which shows the circular "+" cell structure (gray) of the resonant cavity, the arc octagon defect structure (white), and the combined structure of peripheral silicon dioxide and gallium arsenide (diagonal shading). The cells are arranged in a two-dimensional array. In the simulation, the gray area represents gallium arsenide material with a refractive index of 3.6, the white area represents air with a refractive index of 1, and the diagonal shading 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 combined structure of silicon dioxide and gallium arsenide. The internal cavity characteristic dimension is 180μm, and the total characteristic dimension is 428μm.

[0082] In addition, as shown in (a), (b), (c), and (d) in Figure 4 , the band structure is calculated along the boundary Γ-X-M-Γ of the first Brillouin zone. Both the topologically trivial and non-trivial structural units are squares with a length and width characteristic dimension of 20μm and a height of 10μm, forming the QCL column unit structure. In the trivial unit, the column is filled with gallium arsenide material with a refractive index of 3.6 (gray), and the rest is filled with air (white). An air column with the same size as the column (20μm in length, width, and height) is filled above the column to simulate the top air environment of the physical device. Periodic boundary conditions are set in the positive and negative x and positive and negative y directions of the unit structure. The entire plane including air below the QCL column unit structure is set as an ideal electric conductor, and the gallium arsenide plane part above the column is set as an ideal electric conductor, respectively simulating the bottom and top gold electrodes. The upper plane of the top air column is set as a scattering boundary condition. Finally, three-dimensional simulation is carried out using the finite element analysis method in COMSOL Multiphysics to solve for the band diagram. Since the positive and negative x and positive and negative y directions are both infinitely periodic, the overall photonic crystal structures formed by the topologically trivial and non-trivial structural units are the same, so the widths of the trivial band gap and the non-trivial band gap can be seen to be the same.

[0083] Referring to Figure 5 , the upper half is composed of 10 longitudinally arranged topologically non-trivial units in Figure 4 , and the lower half is composed of 10 longitudinally arranged topologically trivial units. The length and width dimensions of the QCL column unit are both 20μm, and the height is 10μm. The height of the air column is 20μm. Periodic boundary conditions are set in the positive and negative x directions, and scattering boundary conditions are set in the positive and negative y directions. The entire lower surface of the QCL column structure including air is set as an ideal electric conductor, and only the gallium arsenide part of the upper surface is set as an ideal electric conductor. The upper plane of the top air column is also set as a scattering boundary condition. Finally, in COMSOL Multiphysics, three-dimensional simulation is carried out according to a specific path to solve for the boundary state band diagram. The simulation results show that although the topologically trivial structure is made to be in direct contact with the topologically non-trivial structure by increasing the structural perturbation to facilitate current injection, the boundary state (black solid line) is still within the band gap, verifying the topological robustness of the structure. As shown in Figure 6The near-field observation position shown is at the middle height of 5 μm of the photonic crystal resonator. This structure consists of topologically non-trivial units with 4 periods in both the x and y directions in the lower left region and topologically trivial units with 9 periods in both the x and y directions in the periphery, forming a two-dimensional array.

[0084] As Figure 7 shown in (a) and (b) of [], in order to facilitate the observation of the far-field distribution of the three-dimensional topological cavity, a hemispherical air cladding is laid along the positive z-axis direction on the top of the QCL device, and the observation position is 6λ> R =2 L 2 / λ( L =180 μm, λ = 106 μm) away from the top of the QCL device. The results show that the brightest part of the far-field mode field is near the corner state in the middle of the near field.

[0085] Example 2

[0086] As Figure 8 shown, in this example, the bottom metal electrode 1 is gold with a thickness of 300 nm, a length of 428 μm, and a width of 428 μm; the QCL layer 2 uses triple-well resonant-phonon gallium arsenide with a refractive index of 3.6, a thickness of 10 μm, a length of 428 μm, and a width of 428 μm. The QCL column has a cylindrical structure with a circular radius of 7.07 μm, a "×"-shaped vein width of 2 μm, and a vein length of 28.28 μm, a period of 20 μm, a characteristic size of 160 μm. The lower left region is a topologically non-trivial structure, and the region outside is a topologically trivial structure. The two are arranged in a square, and the defect geometry is an arc hexagon, that is, the air gap between the unit cells. The silica insulating layer 3 is coated on the top periphery of the QCL layer 2 with a thickness of 300 nm, a length of 428 μm, and a width of 428 μm; the top metal electrode 4 is gold, deposited on the peripheral silica insulating layer 3 and the internal QCL layer 2, both with a thickness of 250 nm. The wire 5 is gold and is used to introduce current for pumping excitation.

[0087] The processing method of the above-mentioned compact second-order topological insulator quantum cascade laser is different from that of Example 1 in that step (3) of this embodiment is different, and the remaining steps are the same as those of Example 1. Specifically: The processing of the topological photonic crystal resonator starts from the deposition of a silicon dioxide (200 nm) insulating layer. The pump region is determined by optical lithography and reactive ion etching, and the silicon dioxide layer above the central 8×8 period is removed. Then, the photonic structure is determined on the QCL wafer by optical lithography. The topologically trivial region consists of a columnar structure with a circular "×" cross-section, with a circular radius of 7.07 μm, a "×"-shaped vein width of 2 μm, a vein length of 28.28 μm, 8 periods in both the x and y directions, and a total size of 160 μm. The topologically non-trivial region consists of a columnar structure with a cross-section of four quarter-circular "×" shapes, with the same circular radius of 7.07 μm, a "×"-shaped vein width of 2 μm, a vein length of 28.28 μm, 4 periods in both the x and y directions, and a total size of 80 μm.

[0088] Referring to Figure 9 , the circular "×"-shaped unit cell structure (gray), the arc hexagonal defect structure (white), and the structure composed of a combination of silicon dioxide and gallium arsenide in the periphery (diagonal shading) of the resonator can be seen, and the unit cell arrangement is a two-dimensional array arrangement. 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 shading 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 combined structure. The characteristic size of the internal cavity is 160 μm, and the total characteristic size is 428 μm.

[0089] Referring to Figure 10Among (a), (b), (c), and (d), the band structure is calculated along the boundary Γ-XM-Γ of the first Brillouin zone. The topological trivial and non-trivial structural units are squares with a characteristic length and width of 20 μm and a height of 10 μm, forming a QCL column unit structure. In the trivial 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). An air column with the same length, width, and height of 20 μm is also filled above the column to simulate the air environment at the top of the physical device. Periodic boundary conditions are set in the positive and negative x and y directions of the unit structure. The entire plane including the air part below the QCL column unit structure is set as a perfect electric conductor, and the gallium arsenide plane part above the column is set as a perfect electric conductor to simulate the bottom and top gold electrodes respectively. The upper plane of the top air column is set as a scattering boundary condition. Finally, three-dimensional simulations are performed using the finite element method in COMSOL Multiphysics to solve for the band diagram. Since the positive and negative x and y directions are both infinitely periodic, the overall photonic crystal structures formed by the topological trivial and non-trivial structural units respectively are the same. Therefore, the widths of the trivial bandgap and the non-trivial bandgap are the same. Additionally, as Figure 11 shown, the upper part is composed of 10 Figure 10 topologically non-trivial units arranged longitudinally, and the lower part is composed of 10 topologically trivial units arranged longitudinally. The length and width dimensions of the QCL unit are both 20 μm, the height is 10 μm, and the height of the air column is 20 μm. Periodic boundary conditions are set for the positive and negative x direction boundaries of the structure, and scattering boundary conditions are set for the positive and negative y direction boundaries. The lower surface of the entire QCL column structure including the air part is set as a perfect electric conductor, and only the gallium arsenide part of the upper surface is set as a perfect electric conductor. The upper plane of the top air column is set as a scattering boundary condition. Finally, three-dimensional simulations are performed using COMSOL Multiphysics along a specific path to solve for the boundary state band diagram. It can be seen that the topologically trivial structure helps with current injection by increasing the structural perturbation and directly contacting the topologically non-trivial structure, but the boundary state (black solid line) is still within the bandgap, demonstrating the topological robustness of the structure.

[0090] As Figure 12 shown, the near-field observation position is at a height of 5 μm in the middle of the photonic crystal resonator. The structure consists of an array of 3 periods of topologically non-trivial units in the x and y directions in the lower left region and an array of 8 periods of topologically trivial units in the x and y directions in the periphery. Referring to Figure 13 as shown in (a) and (b) of R =2 L 2 / λ( L= 160 μm, λ = 104 μm), and the results show that the brightest part of the far-field mode field is near the corner state in the middle of the near field.

[0091] Example 3

[0092] As Figure 14 shown, in this embodiment, the bottom metal electrode 1 is gold, with a thickness of 300 nm, a length and width of 428 μm; the QCL layer 2 uses triple-well resonant-phonon gallium arsenide with a refractive index of 3.6, a thickness of 10 μm, and a length and width of 428 μm. The QCL column geometry has a side length of 14.14 μm, a period of 20 μm, a characteristic size of 200 μm, and the topologically non-trivial structure in the lower left region and the topologically trivial structure in the periphery are arranged in a square pattern. The defect geometry is a rhombus, that is, the air gap between the unit cells. The silicon dioxide insulating layer 3 is coated around the top of the QCL layer, with a thickness of 300 nm, a length and width of 428 μm; the top metal electrode 4 is gold, deposited on the peripheral silicon dioxide insulating layer 3 and the internal QCL layer 2, both with a thickness of 250 nm. The wire 5 is gold and is used to introduce current for pumping excitation.

[0093] The processing method of the above compact second-order topological insulator quantum cascade laser is different from that of Example 1 in step (3) of this embodiment, and the remaining steps are the same as those of Example 1. Specifically: The processing of the topological photonic crystal cavity starts with the deposition of a silicon dioxide (200 nm) insulating layer. The pump region is determined by optical lithography and reactive ion etching, and the silicon dioxide layer above the central 10×10 period is removed. Then, the photonic structure is determined on the QCL wafer using optical lithography. The topologically trivial region is a columnar structure with a rhombus cross-section, a side length of 14.14 μm, 10 periods in both the x and y directions, and a total size of 200 μm. The topologically non-trivial region is a columnar structure with a cross-section of four quarter rhombuses, a side length of 10 μm, 5 periods in both the x and y directions, and a total size of 100 μm. The width of the "+" shaped vein of the overall structure is 2 μm, and the vein length is 200 μm.

[0094] From Figure 15 it can be seen the rhombic unit cell structure (gray), the rhombic defect structure (white), and the combined structure of silicon dioxide and gallium arsenide in the periphery (diagonal shading) of the cavity. The unit cell arrangement is a two-dimensional array arrangement. 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 shading is filled with gallium arsenide material with a real refractive index of 3.6 and an imaginary part of 0.006 to replace the combined structure of silicon dioxide and gallium arsenide. The internal cavity characteristic size is 200 μm, and the total characteristic size is 428 μm. Refer to Figure 16Among (a), (b), (c), and (d), the band structure is calculated along the boundary Γ-X-M-Γ of the first Brillouin zone. The topologically trivial and non-trivial structural units are squares with a characteristic length and width of 20 μm and a height of 10 μm, forming a QCL columnar unit structure. In the trivial unit, the column is filled with gallium arsenide material (gray) with a refractive index of 3.6, and the rest is filled with air (white). An air column with a length, width, and height of 20 μm is also filled above the column to simulate the air environment at the top of the physical device. Periodic boundary conditions are set for the positive and negative x-directions and the positive and negative y-directions, and the entire plane (including the air part) below the QCL columnar unit structure is set as a perfect electric conductor, and only the gallium arsenide plane part above is set as a perfect electric 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, three-dimensional simulation is performed using the finite element method in COMSOL Multiphysics to solve for the band diagram. Since the positive and negative x-directions and the positive and negative y-directions are both infinitely periodic, the overall photonic crystal structures formed by the topologically trivial and non-trivial structural units are the same, so the widths of the trivial bandgap and the non-trivial bandgap are the same.

[0095] In addition, as Figure 17 shown, the upper part is composed of 10 Figure 16 topologically non-trivial units in the

[0096] longitudinal arrangement, and the lower part is composed of 10 topologically trivial units in the longitudinal arrangement. The length and width of the QCL unit are 20 μm, and the height is 10 μm. The height of the air column is 20 μm. Periodic boundary conditions are set for the positive and negative x-directions, and scattering boundary conditions are set for the positive and negative y-directions. The entire lower surface of the QCL columnar structure, including the air part, is set as a perfect electric conductor, and only the gallium arsenide part on the upper surface is set as a perfect electric conductor. The upper plane of the top air column is also set as a scattering boundary condition. Finally, three-dimensional simulation is performed in COMSOL Multiphysics according to a specific path to solve for the boundary state band diagram. It can be seen that the topologically trivial structure is in direct contact with the topologically non-trivial structure by increasing the structural perturbation, which is convenient for current injection, but the boundary state (black solid line) is still located within the bandgap, confirming the topological robustness of this structure. Refer to Figure 18 , the near-field observation position is located in the middle of the photonic crystal cavity, 5 μm away from the bottom. This structure is composed of 4 periods of topologically non-trivial units in the x and y directions in the lower left region and 9 periods of topologically trivial units in the x and y directions in the periphery, forming a two-dimensional array.

[0097] As Figure 19 shown in (a) and (b) of R =2L 2 / λ( L = 200 μm, λ = 104 μm), the results show that the brightest part of the far-field mode field is located near the corner state in the middle of the near field.

[0098] Based on the above-provided Examples 1 to 3, control diagrams for the three examples are also provided. Refer to Figure 20 、 Figure 21 、 Figure 22 and Figure 23 . Specifically, it includes the following content:

[0099] First, Figure 20 The results shown are calculated through the COMSOL Multiphysics electromagnetic wave frequency domain formula ewfd.Qfactor. It can be seen that the corner state quality factors of the three structures are higher than those of other modes. The quality factors of the circular "+", circular "×", and diamond corner states are 243, 222, and 172 respectively, all greater than the requirement of the minimum quality factor of the laser, which is 150.

[0100] Second, Figure 21 The results shown are calculated according to the following formula:

[0101] ;

[0102] Among them, ε ( r ) represents the dielectric constant at each position of the QCL device (calculated through the refractive index of the filling material), E ( r ) represents the electric field strength (obtained through 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 state mode volumes of the three structures are significantly smaller than those of 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 , proving the compactness and high localization of this mode.

[0103] Third, Figure 22 The results shown are calculated according to the following formula:

[0104] ;

[0105] Among them, , represents the effective refractive index, represents the imaginary part of the frequency,c represents the speed of light. It can be seen from the figure that the angular mode loss factors of the three structures are significantly lower than those of other modes. The loss factors of the circular "+", circular "×", and rhombic angular states are 6.8 cm -1 , 8 cm -1 , 11 cm -1 respectively, which proves that this mode can be used as an excellent excitation light source, helping to improve the service life and performance of the laser.

[0106] Finally, Figure 23 the results shown are calculated according to the following formula:

[0107] ;

[0108] It can be seen in Figure 23 that the Purcell factors of the angular modes of the three structures are significantly higher than those of other modes. The Purcell factors of the circular "+", circular "×", and rhombic angular states are 197, 213, and 69 respectively. The Purcell factor is closely related to the quality factor and the effective mode volume. It is precisely because the angular mode has a high quality factor, a compact mode volume, and a small device size that its Purcell factor is the highest.

[0109] Therefore, by adopting the above-mentioned compact second-order topological insulator quantum cascade laser and its processing method, using the topological optical mode of the second-order topological insulator, constructing a resonant cavity with the help of the two-dimensional SSH model, adjusting the coupling strength by changing the position of the QCL dielectric column in the unit structure to obtain the topological bandgap, using the two-dimensional vein connection to play a mediating role and conduct electricity, constructing a topological boundary to form a topologically protected local mode; and by changing the periodicity number of the topologically trivial and topologically non-trivial structures, the size of the topological cavity can be adjusted, thereby greatly improving the monolithic integration degree of the topological semiconductor laser.

[0110] In this article, specific examples are used to elaborate on the principle and implementation mode of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be construed as a limitation to 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 electric pumping excitation; The column structure of the QCL layer is composed 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. The cross section of the column structure cell of the QCL layer is one of a circular "+" shape, a circular "×" shape or a rhombus 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 unit 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 "+" 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; 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.

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 1, characterized in that: The bottom metal electrode and the top metal electrode are both made of metal materials.

5. A compact second-order topological insulator quantum cascade laser according to claim 4, 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.

6. 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.

7. A method for processing a compact second-order topological insulator quantum cascade laser according to any one of claims 1 to 6, 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.

Images