Period-broken grating surface-emitting terahertz quantum cascade laser and optimization method

Abstract

The present invention discloses a period-broken grating surface-emitting terahertz quantum cascade laser and an optimization method. The present invention introduces a period-breaking structure in the electrode layer, and its potential periodicity ensures the formation of distributed feedback, which can realize the resonance and frequency selection inside the laser. At the same time, due to the non-periodic characteristics of the period-breaking structure locally, its resonant mode is between the symmetric mode and the antisymmetric mode, rather than the antisymmetric mode with low radiation efficiency, so it can effectively improve the radiation efficiency and power of the laser. At the same time, the present invention optimizes the grating parameters through a genetic algorithm, reducing the difficulty and time cost of forward design of the grating structure by analyzing the mode field, and has a high degree of design freedom and flexibility. The terahertz quantum cascade laser grating structure designed by the present invention has high radiation efficiency and can achieve high power output.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor lasers, and in particular relates to a period-broken grating surface-emitting terahertz quantum cascade laser and an optimization method thereof. Background Art

[0002] Terahertz (THz) quantum cascade lasers (QCLs) are compact, highly efficient, and easily integrated electrically pumped all-solid-state semiconductor lasers with important applications in chemical, biological, and medical imaging, astronomical remote sensing, and object detection. They typically have two different output modes: surface-emitting and edge-emitting. Compared to edge-emitting THz QCLs, surface-emitting structures offer significant advantages in external cavity coupling, optical alignment, packaging testing, and array integration. However, the output power of surface-emitting THz QCLs remains relatively low compared to near-infrared lasers, significantly limiting their application in various applications. Therefore, improving the output power of surface-emitting THz QCLs remains a key issue in THz QCL research.

[0003] Since terahertz quantum cascade lasers operate in a transverse magnetic field polarization mode, it is difficult to achieve vertical emission through a vertical cavity. However, surface emission can be achieved using a surface grating structure, a typical example of which is the second-order distributed feedback (DFB) structure. The second-order distributed feedback surface-emitting terahertz QCL has a single-period grating structure. The grating wave vector, the diffraction wave vector, and the incident guided wave vector in the cavity satisfy the Bragg diffraction relationship, which can achieve vertical terahertz radiation. The second-order distributed feedback surface-emitting terahertz QCL has two modes: symmetric mode and antisymmetric mode. The symmetric mode has higher radiation loss and higher threshold gain, making it impossible to achieve symmetric mode operation. The second-order DFB-QCL usually operates in the antisymmetric mode, which produces destructive interference in the far field, has low radiation efficiency and low output power.

[0004] Currently, in the research on grating structures, some scholars have adopted methods such as gradient photonic heterostructures, second-order and fourth-order hybrid grating structures, and quasi-periodic sequence grating structures to control the mode field, so as to achieve QCL lasing in a specified high radiation efficiency mode, thereby improving the QCL radiation efficiency and radiation power. However, the radiation efficiency of terahertz QCLs composed of these grating structures still has much room for improvement.

[0005] Patent application number CN 202310671204.7 proposes a segmented design of a first-order distributed feedback grating structure and a coupled output grating. The period length and number of the coupled grating are then adjusted to achieve stable single-mode terahertz output and adjust the quality factor of the laser resonator within a certain range. However, this segmented structure has low radiation efficiency and cannot independently adjust the output light angle and Q value.

[0006] Patent application number CN 202110100819.5 designs a non-uniform dipole antenna array consisting of a periodic narrow ridge structure. This allows the laser's light-emitting surface to be the entire dipole antenna array surface, achieving high output power and power slope efficiency. Furthermore, maximum power output can be achieved by properly designing the ridge width difference. However, the output power increase achieved by increasing the ridge width difference is relatively limited and still lags behind international advanced levels.

[0007] In summary, surface-emitting terahertz quantum cascade lasers using surface grating structures urgently need a new structure and design method with high radiation efficiency and high design flexibility. Summary of the Invention

[0008] At present, the surface-emitting terahertz quantum cascade lasers using grating structures are in urgent need of improvement in their output power and radiation efficiency. At the same time, the relatively complex mode control also brings challenges to the design of terahertz QCL waveguide structures. In order to solve the above problems, the present invention proposes a period-broken grating surface-emitting terahertz quantum cascade laser and an optimization method. The present invention introduces a period-broken grating structure. Since it is between a uniform periodic structure and a non-periodic structure in terms of grating arrangement, its mode field exhibits a combination of irregular symmetric modes and antisymmetric modes, and has the characteristics of high radiation efficiency, which can achieve high-power output of terahertz waves. Secondly, a method for optimizing the arrangement of period-broken gratings based on genetic algorithms is proposed. The optimization idea can simplify the complex mode field control and analysis process, and has high design flexibility.

[0009] The specific technical solutions adopted in the present invention are as follows:

[0010] In a first aspect, the present invention provides a period-broken grating surface-emitting terahertz quantum cascade laser, which adopts a bimetallic waveguide structure, including a substrate, a lower electrode layer, and a ridge waveguide structure;

[0011] The lower electrode layer is located on the upper surface of the substrate, and the ridge waveguide structure is located on the upper surface of the lower electrode layer;

[0012] The ridge waveguide structure is formed by stacking an upper electrode, an upper contact layer, an active area, and a lower contact layer in sequence from top to bottom; the upper electrode is located in the central area of ​​the upper contact layer and does not completely cover the upper contact layer, thereby forming an absorption boundary area at the edge of the upper contact layer surrounding the upper electrode; a periodic broken grating structure is opened in the central area of ​​the upper electrode area, and lead areas are retained on both sides of the central area; the periodic broken grating structure includes a number of parallel and equally spaced inherent slits, and the period between two adjacent inherent slits is one period. The goal of the periodic broken grating structure is to maximize the laser radiation efficiency, and a genetic algorithm is used to determine whether to open an additional broken slit in each period, as well as to determine the width of all slits in the periodic broken grating structure and the opening position of each broken slit.

[0013] As a preferred embodiment of the first aspect, in the period-broken grating structure, the period length is equal to the equivalent wavelength of the laser operating frequency in the laser waveguide structure.

[0014] As a preferred embodiment of the first aspect, the operating frequency of the laser is in the range of 1.2-5.4 THz.

[0015] As a preferred embodiment of the first aspect, the width of all slits in the period-broken grating structure is in the range of 0.05Λ to 0.5Λ, where Λ is the equivalent wavelength of the laser operating frequency in the laser waveguide structure.

[0016] In a second aspect, the present invention provides a genetic algorithm optimization method for a period-broken grating surface-emitting terahertz quantum cascade laser as described in any one of the solutions of the first aspect, comprising:

[0017] S1, according to the total number of periods n contained in the period-breaking grating structure, set the broken slit mark sequence b1b2b3...b n To record whether a broken slit is opened in n cycles, set the broken slit position sequence a1a2a3…a n To record the distance between the broken slit and the inherent slit on the specified side when a broken slit is opened in n cycles, set the slit width sequence w1w2w3…w 2n To record the widths of all inherent slits and the assumed widths of all broken slits in n cycles; mark the broken slits as the sequence b1b2b3…b n , the sequence of broken slit positions is a1a2a3…a i With the slit width sequence w1w2w3…w 2n Encode them into binary sequences as the chromosomes of each individual in the population, and generate a specified number of individuals as the initialized population through a random algorithm;

[0018] S2. Based on the initialized population and genetic algorithm, the population is optimized iteratively according to the process of selection, crossover and mutation; and during the iterative process, each individual needs to convert its own chromosome sequence into grating parameters and use them to model the finite element simulation model corresponding to the grating surface-emitting terahertz quantum cascade laser. The surface emission photon loss rate of the laser operating frequency is obtained through simulation and used as the individual's fitness to update the population;

[0019] S3. When the genetic algorithm reaches an iteration termination condition, the iteration is stopped, and the chromosome sequence of the individual with the highest fitness is converted into the design parameters of the period-breaking grating structure obtained by final optimization.

[0020] As a preferred embodiment of the second aspect, during the iterative process of the genetic algorithm, the population data is automatically read in by writing a script program and converted into grating parameters for modeling, and the finite element simulation software is used for simulation and the fitness of each individual is automatically solved.

[0021] As a preferred embodiment of the above-mentioned second aspect, the specific process of selection, crossover and mutation in the iterative process of the genetic algorithm is: first, a certain proportion of individuals in the population are selected as individuals of the new population according to the fitness of each individual; secondly, part of the chromosome fragments of adjacent individuals are exchanged according to the set exchange ratio to generate new individuals; finally, the genes at certain positions on the chromosomes of certain individuals in the group are mutated according to the set mutation probability, and the next generation of population is generated by simulating the evolutionary process of the biological population.

[0022] As a preferred embodiment of the second aspect, the iteration termination condition of the genetic algorithm is that the number of iterations reaches the set maximum number of iterations, or the iteration is stopped when the fitness of the individual with the highest fitness in the population reaches the set target fitness.

[0023] As a preferred embodiment of the second aspect, a method for obtaining the surface emission photon loss rate of the laser operating frequency through simulation is:

[0024] For the target individual, the chromosome sequence of the target individual is first converted into grating parameters, and a finite element simulation model corresponding to the grating surface-emitting terahertz quantum cascade laser is modeled according to the grating parameters; then, the characteristic frequency module in the finite element software simulation is used to simulate the electric field spatial distribution, the magnetic field spatial distribution and the characteristic frequency corresponding to different modes, and the mode loss calculation formula is used to calculate the mode loss at different characteristic frequencies, and then the mode loss spectrum between the characteristic frequency and the mode loss is obtained. The characteristic frequency corresponding to the lowest mode loss in the mode loss spectrum is used as the laser operating frequency, and then the surface emission photon loss rate at the laser operating frequency is calculated based on the results of the finite element simulation. The surface emission photon loss rate is obtained by dividing the time-averaged power flow integral in the vertical direction of the laser by the internal energy of the resonator.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention proposes a period-broken surface-emitting terahertz quantum cascade laser structure. The potential periodicity in the period-breaking structure ensures the formation of distributed feedback, which can realize resonance and frequency selection inside the laser. At the same time, due to the local non-periodic characteristics of the period-breaking structure, its resonant mode is between the symmetric mode and the antisymmetric mode, rather than the antisymmetric mode with low radiation efficiency, thus effectively improving the radiation efficiency and power of the laser.

[0027] The present invention also optimizes grating parameters through a genetic algorithm, reducing the difficulty and time cost of forward grating structure design via mode field analysis, thereby providing a high degree of design freedom and flexibility. After multiple optimization iterations, a period-breaking grating structure with high mode loss and high photon loss rate was obtained. The simulation results for mode loss and surface-emission photon loss rate are currently the highest for single-ridge waveguide terahertz QCLs, demonstrating that the designed terahertz quantum cascade laser grating structure has high radiation efficiency and can achieve high-power output. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic structural diagram of a surface-emitting terahertz quantum cascade laser according to the present invention;

[0029] Figure 2 is a schematic cross-sectional view of the structure of a surface-emitting terahertz quantum cascade laser according to the present invention;

[0030] Figure 3 Schematic diagram of the grating arrangement of the device's periodic broken grating structure (top view);

[0031] Figure 4 is a flow chart of a genetic algorithm according to an embodiment of the present invention;

[0032] Figure 5 This is the mode loss diagram of the device. In the figure, the frequency of the lowest loss mode is 3.34THz, and the lowest mode loss is 12.41cm -1 ;

[0033] Figure 6 This is the surface emission photon loss rate diagram of the device. The photon loss rate corresponding to the operating frequency in the figure is 96.25GHz;

[0034] The reference numerals in the figure are as follows: substrate 1, lower electrode layer 2, lower contact layer 3, active region 4, upper contact layer 5, absorption boundary region 6, lead region 7, periodic broken grating structure 8, and upper electrode region 9. DETAILED DESCRIPTION

[0035] The present invention will be further described and illustrated below with reference to the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention may be combined accordingly, provided that there is no conflict between them.

[0036] In order to make the above-mentioned objects, features and advantages of the present invention more clearly understood, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in the various embodiments of the present invention can be combined accordingly without conflicting with each other.

[0037] In the description of the present invention, it should be understood that when an element is considered to be "connected" to another element, it can be directly connected to the other element or indirectly connected, that is, there are intermediate elements. On the contrary, when an element is said to be "directly" connected to another element, there are no intermediate elements.

[0038] In the description of the present invention, it should be understood that the terms "first" and "second" are used solely for descriptive purposes and are not to be construed as indicating or implying relative importance or implicitly specifying the number of technical features being described. Therefore, features defined as "first" or "second" may explicitly or implicitly include at least one of such features.

[0039] See also Figure 1 and Figure 2 As shown, in a preferred embodiment of the present invention, a period-broken grating surface-emitting terahertz quantum cascade laser is provided. The device utilizes a bimetallic waveguide structure, specifically comprising a substrate 1, a lower electrode layer 2, and a ridge waveguide structure. The lower electrode layer 2 is located on the upper surface of the substrate 1, while the ridge waveguide structure is located on the upper surface of the lower electrode layer 2. The specific form and coordination relationship of the ridge waveguide structure are described below.

[0040] The ridge waveguide structure is formed by stacking the upper electrode 9, upper contact layer 5, active area 4, and lower contact layer 3 in order from top to bottom, forming a long, strip-shaped structure. The bottom surface of the lower contact layer 3 contacts the top surface of the lower electrode layer 2, the bottom surface of the active area 4 contacts the top surface of the lower contact layer 3, the bottom surface of the upper contact layer 5 contacts the top surface of the active area 4, and the bottom surface of the upper electrode 9 contacts the top surface of the upper contact layer 5.

[0041] The upper electrode 9 is located in the central area of ​​the upper contact layer 5 and does not completely cover the upper contact layer 5. There is an edge area in the upper contact layer 5 that is not covered by the upper electrode 9 and surrounds the upper electrode 9. This edge area is located in the horizontal and vertical directions of the ridge waveguide plane and can serve as the absorption boundary area 6. In addition, a periodic broken grating structure 8 is provided in the central area of ​​the upper electrode area 9. The parts on both sides of the central area located at the two ends of the ridge waveguide each retain a lead area 7 for connecting external wires. The periodic broken grating structure 8 includes a number of parallel and equally spaced inherent slits. The specific number of inherent slits can be reasonably adjusted according to the size and design parameters of the ridge waveguide. In the present invention, the period between two adjacent inherent slits is a period. However, it should be noted that since the inherent slits themselves have width, the center line of two adjacent inherent slits can be used as the boundary of the period in the embodiment. In each period, in addition to the original inherent slits, additional broken slits need to be selectively introduced. However, the introduction of broken slits is selective. Not every period needs to introduce broken slits, and the position of the introduced broken slits within the period also needs to be reasonably optimized. At the same time, the slit widths of all inherent slits and broken slits are also grating parameters that can be optimized.

[0042] Therefore, in the periodic broken grating structure 8 of the present invention, three types of grating parameters require optimization: the first is whether an additional broken slit is included in each period; the second is the width of all inherent slits in the broken grating structure 8 and their respective widths; and the third is the location of each broken slit in the broken grating structure 8. These three types of grating parameters are optimized using a genetic algorithm, with the optimization goal being to maximize the laser radiation efficiency. Theoretically, these three types of parameters can be optimized separately or jointly. In this embodiment, joint optimization is preferred.

[0043] For ease of description, combined Figure 3 As shown in the grating arrangement of the periodic broken grating structure, it is assumed that the laser periodic broken grating structure 8 contains n periods, the period length is L, and a broken slit is selectively introduced in the n periods of the grating; it is assumed that in i periods, the spacing between the broken slit and the fixed slit closest to its left is a i , that is, if there is a broken slit in the i-th period, its position is a i .

[0044] Therefore, in the above arrangement of grating slits, whether there is a broken slit in each period can be represented by a set of binary sequences b1b2b3…b n Indicates that b i = 0 means that there is no broken slit in the i-th period, b i =1 means there is a broken slit in the i-th cycle. At the same time, a set of binary sequences a1a2a3…a nTo record the distance between the broken slit and the inherent slit on the specified side when a broken slit is opened in n cycles, set a binary sequence w1w2w3…w 2n To record the widths of all inherent slits and the widths of all broken slits assumed to be open in n cycles.

[0045] In the above-mentioned period-breaking grating structure 8, the grating period length L is equal to the equivalent wavelength Λ of the laser operating frequency in the laser waveguide structure. The calculation formula of the equivalent wavelength Λ is: f is the designed laser frequency, n eff is the equivalent refractive index of the laser waveguide structure, and c is the speed of light in a vacuum. Generally speaking, the grating period L ranges from 15.4 μm to 69.4 μm, corresponding to an operating frequency range of 1.2-5.4 THz for the surface-emitting terahertz quantum cascade laser. Furthermore, the widths of all slits in the period-broken grating structure 8 should range from 0.05 Λ to 0.5 Λ.

[0046] Based on the above-mentioned ridge waveguide structure, terahertz waves are generated in the active region 4, resonate through the period-broken grating structure 8, and emerge from the slits. The light emerging from each slit undergoes constructive interference and is coupled into free space in the vertical direction. Due to the periodic characteristics of the period-broken grating structure, the terahertz waves generated in the active region 4 will undergo Bragg diffraction, which appears in the spectrum as a band gap and resonant modes located on both sides of the band gap. However, because the period-broken grating structure 8 exhibits local non-periodic characteristics, the modes located on both sides of the band gap will not exhibit the characteristics of symmetric and antisymmetric modes. Their mode field distribution is between symmetric and antisymmetric modes, rather than antisymmetric modes with low radiation efficiency. Each slit position can achieve far-field interference, realizing surface emission of terahertz waves.

[0047] In addition, in another embodiment of the present invention, a genetic algorithm optimization method for the above-mentioned period-broken grating surface-emitting terahertz quantum cascade laser is further provided, which is used to optimize the three types of grating parameters mentioned above. Figure 4 As shown in Figure 2, the process of the genetic algorithm optimization method includes:

[0048] S1, according to the total number of periods n contained in the period-breaking grating structure 8, set the broken slit mark sequence b1b2b3...b n To record whether a broken slit is opened in n cycles, set the broken slit position sequence a1a2a3…a n To record the distance between the broken slit and the inherent slit on the specified side when a broken slit is opened in n cycles, set the slit width sequence w1w2w3…w 2nTo record the widths of all inherent slits and the assumed widths of all broken slits in n cycles; mark the broken slits as the sequence b1b2b3…b n , the sequence of broken slit positions is a1a2a3…a i With the slit width sequence w1w2w3…w 2n They are respectively encoded as binary sequences as the chromosomes of each individual in the population, and a specified number of individuals are generated by a random algorithm as the initialized population.

[0049] Note that the slit position and width data may be non-integer numbers with decimals. Therefore, when encoding these two sets of positive decimal numbers into binary, the decimal width of each parameter must be limited to ensure that the chromosome length of each individual in the population is consistent.

[0050] In addition, the chromosomal binary data of each individual in the population is divided into three parts: whether there is a mark for the presence of a broken slit, the position of the broken slit within the cycle, and the width of the slit. By presetting the number of cycles, the position of the broken slit within the cycle, and the value of the slit width, the length of the binary array and the specific correspondence between the slit parameters and the binary array can be determined, and the effective reading of simulation data and the conversion between modeling data can be realized.

[0051] S2. Based on the initialized population and genetic algorithm, the population is optimized iteratively according to the process of selection, crossover and mutation; and in the iterative process, each individual needs to convert its own chromosome sequence into grating parameters and use them to model the finite element simulation model corresponding to the grating surface emitting terahertz quantum cascade laser. The surface emission photon loss rate of the laser operating frequency is obtained through simulation and used as the individual's fitness to update the population.

[0052] During the iterative process of the genetic algorithm, the population data can be automatically read in by writing a script program, and converted into grating parameters for modeling. The finite element simulation software can be used for simulation and the fitness of each individual can be automatically calculated.

[0053] In the iterative process of the above-mentioned genetic algorithm, the specific process of selection, crossover and mutation is as follows: first, a certain proportion of individuals in the population are selected as individuals of the new population according to the fitness of each individual; secondly, part of the chromosome fragments of adjacent individuals are exchanged according to the set exchange ratio to generate new individuals; finally, the genes at certain positions on the chromosomes of certain individuals in the group are mutated according to the set mutation probability, and the next generation of population is generated by simulating the evolutionary process of the biological population.

[0054] Therefore, the above genetic algorithm can simulate the evolution process of biological populations through the three operations of "selection", "crossover" and "mutation" to produce the next generation of populations and achieve individual update optimization. The specific process belongs to the existing technology and will not be repeated here.

[0055] The core of the genetic algorithm to update the population is to calculate the fitness of the individual. The optimization purpose of the present invention is to improve the laser radiation efficiency as much as possible. Since the photon loss rate is positively correlated with the laser radiation efficiency, that is: (η is the radiation efficiency, α w is the waveguide loss, n eff is the equivalent refractive index of the laser waveguide structure, and c is the speed of light in vacuum). Therefore, the fitness function based on the photon loss rate can measure the radiation efficiency of the laser. The optimization method based on the genetic algorithm can effectively improve the radiation efficiency and output power of the THz-QCL.

[0056] During the above genetic algorithm iteration process, the method for obtaining the surface emission photon loss rate of the laser operating frequency through simulation can be expressed as:

[0057] For the target individual, the chromosome sequence of the target individual is first converted into grating parameters, and a finite element simulation model corresponding to the grating surface-emitting terahertz quantum cascade laser is modeled according to the grating parameters; then, the characteristic frequency module in the finite element software simulation is used to simulate the electric field spatial distribution, the magnetic field spatial distribution and the characteristic frequency corresponding to different modes, and the mode loss calculation formula is used to calculate the mode loss at different characteristic frequencies, and then the mode loss spectrum between the characteristic frequency and the mode loss is obtained. The characteristic frequency corresponding to the lowest mode loss in the mode loss spectrum is used as the laser operating frequency, and then the surface emission photon loss rate at the laser operating frequency is calculated based on the results of the finite element simulation.

[0058] The mode loss calculation formula is as follows: where ω imag is the imaginary part of the characteristic frequency, n eff is the equivalent refractive index of the laser waveguide structure, and c is the speed of light in vacuum.

[0059] In addition, the surface emission photon loss rate is obtained by dividing the time-averaged power flow integral in the vertical direction of the laser by the internal energy of the resonator. Specifically, the surface emission photon loss rate is calculated as follows: The ratio of the total photon loss rate to the radiation efficiency is the radiation efficiency, where E and H represent the electric field and magnetic field of the laser operating mode, respectively. represents the normal vector of the integration surface, A represents the integration surface of the laser in the vertical direction, V represents the three-dimensional space of the laser active area, dS and dV represent the integration operators of area and volume respectively, ε represents the dielectric constant, μ represents the magnetic permeability; Φ is the time-averaged power flow integral in the vertical direction of the laser, Eres is the internal energy of the resonator, and both are solved by finite element simulation.

[0060] S3. When the genetic algorithm reaches an iteration termination condition, the iteration is stopped, and the chromosome sequence of the individual with the highest fitness is converted into the design parameters of the period-breaking grating structure obtained by final optimization.

[0061] In the above genetic algorithm, the iteration termination condition can be set to stop when the number of iterations reaches the set maximum number of iterations, or when the fitness of the individual with the highest fitness in the population reaches the set target fitness.

[0062] The following is a specific example to demonstrate the technical effects that can be achieved by the above-mentioned period-broken grating surface-emitting terahertz quantum cascade laser and optimization method.

[0063] Example

[0064] In this embodiment, according to Figure 1 and Figure 2 The device structure shown in FIG1 is a period-broken grating surface-emitting terahertz quantum laser device A, and the grating slit arrangement is determined by the genetic algorithm optimization shown in S1 to S3 above.

[0065] In this embodiment, the designed terahertz quantum cascade laser device A has a center frequency of 3.3 THz, and the equivalent refractive index of its active region material is approximately 3.6, corresponding to an active region wavelength of 25 μm. However, in actual situations, since a certain proportion of the evanescent field propagates outside the active region, resulting in a decrease in the propagation constant, the equivalent wavelength of the designed terahertz frequency in the waveguide structure is actually around 27 μm.

[0066] The material of substrate 1 is gallium arsenide with a thickness of 180 μm. The lower electrode 2 is a double-layer structure with a lower layer of titanium and an upper layer of gold. The thicknesses of the lower and upper layers are 20 nm and 600 nm respectively. The material of lower contact layer 3 is heavily doped gallium arsenide with a doping concentration of 1*10 18 cm 3 ~5*10 18 cm 3 The simulation example uses 3*10 18 cm 3 The thickness of the lower contact layer 3 is 50nm; the active region 4 adopts a quantum cascade structure formed by alternating gallium arsenide and aluminum gallium arsenide, with a thickness of 10μm; the upper contact layer 5 adopts heavily doped gallium arsenide with a doping concentration of 1*10 18 cm 3 ~5*10 18 cm 3 The simulation example uses 3*10 18 cm3 The thickness of the upper contact layer 5 is 300nm; the upper electrode region 9 adopts a double-layer structure, the lower layer is titanium, and the upper layer is superimposed with a layer of gold. The thicknesses of the lower and upper layers are 20nm and 200nm respectively.

[0067] In this embodiment, the grating parameters in the periodic broken grating structure 8 need to be optimized by genetic algorithm. The broken slit position is set to a fixed value a, which can be changed during optimization; the slit width is set to a fixed value w = 3um; the laser grating region length is designed to be 1.5mm, and the corresponding number of periods is 55. When the parameters are encoded according to the optimization method, since the broken slit position has only one value a, it can be directly encoded as a five-bit binary number A1...A5. Since there is only one fixed grating slit width parameter in this embodiment, there is no need to encode the width parameter. The binary sequence optimized by the genetic algorithm is in the form of b1...b i ...b 55 A1...A 5。

[0068] The grating slit arrangement of laser A obtained by performing multiple iterations of optimization using the optimization method in this embodiment is as follows: Figure 3 As shown;

[0069] The mode loss spectrum obtained by finite element software simulation in this embodiment is as follows Figure 5 As shown in the figure, the frequency of the lowest loss mode is 3.34THz, and the lowest mode loss is 12.41cm -1 ; Figure 6 This is the photon loss rate diagram of device A. The photon loss rate corresponding to its lowest loss mode in the diagram is 96.25 GHz, which shows that the periodic broken grating THz-QCL designed in this patent has high radiation efficiency characteristics.

[0070] In summary, the present invention proposes a periodic broken grating surface-emitting terahertz quantum cascade laser and design method based on a genetic algorithm. By introducing a periodic broken grating into the waveguide structure of the terahertz quantum cascade laser and selectively introducing broken slits through an optimization method based on a genetic algorithm, and designing parameters such as the broken slit position and slit width, a result with high radiation loss and high surface-emission photon loss rate is obtained, which can achieve an improvement in the radiation efficiency and radiation power of the terahertz quantum cascade laser. At the same time, the structure avoids the complex mode field analysis process and can use the optimization iterative process to design the laser according to specific needs. It has high flexibility and design freedom, so the present invention has high industrial value. The periodic broken grating structure and optimization method proposed in the present invention are applicable to quantum cascade lasers in both the terahertz band and the mid-infrared band, and can be used to improve the output radiation efficiency and output power of the quantum cascade laser.

[0071] The embodiments described above are merely some preferred implementations of the present invention and are not intended to limit the present invention. Persons skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent substitution or equivalent transformation falls within the scope of protection of the present invention.

Claims

1. A genetic algorithm optimization method for period-broken grating surface-emitting terahertz quantum cascade lasers, characterized in that: The device of the period-broken grating surface-emitting terahertz quantum cascade laser adopts a bimetallic waveguide structure, comprising a substrate (1), a lower electrode layer (2) and a ridge waveguide structure; The lower electrode layer (2) is located on the upper surface of the substrate (1), and the ridge waveguide structure is located on the upper surface of the lower electrode layer (2); The ridge waveguide structure is formed by stacking and compounding an upper electrode (9), an upper contact layer (5), an active region (4), and a lower contact layer (3) in sequence from top to bottom; the upper electrode (9) is located in the central region of the upper contact layer (5) and does not completely cover the upper contact layer (5), so that an absorption boundary region (6) is formed at the edge of the upper contact layer (5) surrounding the upper electrode (9); a periodic broken grating structure (8) is opened in the central region of the upper electrode (9), and lead regions (7) are retained on both sides of the central region; the periodic broken grating structure (8) includes a plurality of inherent slits arranged in parallel and at equal intervals, and a period is formed between two adjacent inherent slits. In the periodic broken grating structure (8), the laser radiation efficiency is maximized, and a genetic algorithm is used to determine whether an additional broken slit is opened in each period, and to determine the width of all slits in the periodic broken grating structure (8) and the opening position of each broken slit; The genetic algorithm optimization method comprises: S1. According to the total number of periods n contained in the period-breaking grating structure (8), a broken slit mark sequence b1b2b3...b n To record whether a broken slit is opened in n cycles, set the broken slit position sequence a1a2a3…a n To record the distance between the broken slit and the inherent slit on the specified side when a broken slit is opened in n cycles, set the slit width sequence w1w2w3…w 2n To record the widths of all inherent slits and the assumed widths of all broken slits in n cycles; mark the broken slits as the sequence b1b2b3…b n , the sequence of broken slit positions is a1a2a3…a i With the slit width sequence w1w2w3…w 2n Encode them into binary sequences as the chromosomes of each individual in the population, and generate a specified number of individuals as the initialized population through a random algorithm; S2. Based on the initialized population and genetic algorithm, the population is optimized iteratively according to the process of selection, crossover and mutation; and during the iterative process, each individual needs to convert its own chromosome sequence into grating parameters and use them to model the finite element simulation model corresponding to the grating surface-emitting terahertz quantum cascade laser. The surface emission photon loss rate of the laser operating frequency is obtained through simulation and used as the individual's fitness to update the population; S3. When the genetic algorithm reaches an iteration termination condition, the iteration is stopped, and the chromosome sequence of the individual with the highest fitness is converted into the design parameters of the period-breaking grating structure obtained by final optimization.

2. The genetic algorithm optimization method according to claim 1, wherein in the period-breaking grating structure (8), the period length is equal to the equivalent wavelength of the laser operating frequency in the laser waveguide structure.

3. The genetic algorithm optimization method according to claim 1, wherein: The operating frequency of the laser is in the range of 1.2-5.4 THz.

4. The genetic algorithm optimization method according to claim 1, wherein: The width of all slits in the periodic broken grating structure (8) ranges from 0.05Λ to 0.5Λ, wherein Λ is the equivalent wavelength of the laser operating frequency in the laser waveguide structure.

5. The genetic algorithm optimization method according to claim 1, wherein: During the iterative process of the genetic algorithm, the population data is automatically read in by writing a script program, and converted into grating parameters for modeling, and the finite element simulation software is used for simulation to automatically solve the fitness of each individual.

6. The genetic algorithm optimization method according to claim 1, wherein: During the iterative process of the genetic algorithm, the specific process of selection, crossover and mutation is as follows: first, a certain proportion of individuals in the population are selected as individuals of the new population according to the fitness of each individual; second, part of the chromosome fragments of adjacent individuals are exchanged according to the set exchange ratio to generate new individuals; finally, genes at certain positions on the chromosomes of certain individuals in the population are mutated according to the set mutation probability, and the next generation of population is generated by simulating the evolutionary process of the biological population.

7. The genetic algorithm optimization method according to claim 1, wherein: The iteration termination condition of the genetic algorithm is that the iteration number reaches the set maximum iteration number, or the iteration is stopped when the fitness of the individual with the highest fitness in the population reaches the set target fitness.

8. The genetic algorithm optimization method according to claim 1, wherein: The method for obtaining the surface emission photon loss rate of the laser operating frequency through simulation is: For the target individual, the chromosome sequence of the target individual is first converted into grating parameters, and a finite element simulation model corresponding to the grating surface-emitting terahertz quantum cascade laser is modeled according to the grating parameters; then, the characteristic frequency module in the finite element software simulation is used to simulate the electric field spatial distribution, the magnetic field spatial distribution and the characteristic frequency corresponding to different modes, and the mode loss calculation formula is used to calculate the mode loss at different characteristic frequencies, and then the mode loss spectrum between the characteristic frequency and the mode loss is obtained. The characteristic frequency corresponding to the lowest mode loss in the mode loss spectrum is used as the laser operating frequency, and then the surface emission photon loss rate at the laser operating frequency is calculated based on the results of the finite element simulation. The surface emission photon loss rate is obtained by dividing the time-averaged power flow integral in the vertical direction of the laser by the internal energy of the resonator.

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