Periodically broken grating surface emitting terahertz quantum cascade laser and optimization method

Abstract

The invention discloses a periodically broken grating surface emitting terahertz quantum cascade laser and an optimization method. The periodic broken structure is introduced into the electrode layer, the potential periodicity of the periodic broken structure ensures the formation of distributed feedback, resonance and frequency selection in the laser can be realized, and meanwhile, due to the non-periodic characteristic of the periodic broken structure locally, the resonance mode of the periodic broken structure is between a symmetric mode and an anti-symmetric mode, so that the frequency selection efficiency of the laser is improved. Therefore, the radiation efficiency and the power of the laser can be effectively improved. Meanwhile, grating parameters are optimized through the genetic algorithm, the difficulty and time cost of forward design of the grating structure by analyzing the mode field are reduced, and the method has high design freedom degree and flexibility. The terahertz quantum cascade laser grating structure designed by the invention has high radiation efficiency and can realize high-power output.

Description

Technical Field

[0001] The 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 laser (QCL) is a small-sized, high-efficiency, and easy-to-integrate electrically pumped all-solid-state semiconductor laser. It has important applications in the fields of chemistry, biology, and medical imaging, astronomical remote sensing, and object detection. It usually has two different output modes: surface emission and edge emission. Compared with edge-emitting THz QCL, the surface-emitting structure has significant advantages in external cavity coupling, optical alignment, packaging testing, and array integration. However, the output power of surface-emitting THz QCL is still lower than that of near-infrared lasers, and it has great limitations in various applications. Therefore, how to improve the output power of surface-emitting THz QCL is still an important issue in the current THz QCL research.

[0003] Since the terahertz quantum cascade laser operates in the transverse magnetic field polarization mode, it is difficult to achieve vertical emission through a vertical cavity. Surface emission can be achieved using a surface grating structure, a typical example of which is a second-order distributed feedback (DFB) structure. The second-order distributed feedback surface emitting terahertz QCL has a single-period grating structure, and 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, and cannot 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 study of 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 and realize 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] In the patent application number CN 202310671204.7, the first-order distributed feedback grating structure and the coupled output grating are designed in sections, and the period length and number of the coupled grating are further adjusted to achieve terahertz output in a stable single-mode state, and the quality factor of the laser resonant cavity is adjusted within a certain range. However, this segmented structure has low radiation efficiency and cannot achieve separate adjustment of the output light angle and Q value.

[0006] In the patent application number CN 202110100819.5, a non-uniform dipole antenna array composed of a periodic narrow ridge structure is designed, so that the light-emitting surface of the laser is the entire dipole antenna array surface, achieving higher output power and power slope efficiency, and the maximum power output can be achieved by reasonably designing the ridge width difference. However, the output power improvement brought by increasing the ridge width difference is relatively limited, and there is still a gap compared with the international advanced level.

[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-breaking grating surface-emitting terahertz quantum cascade laser and an optimization method. The present invention introduces a period-breaking 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, and can achieve high-power output of terahertz waves; secondly, a method for optimizing the arrangement of period-breaking gratings based on genetic algorithms is proposed. The complex mode field control and analysis process can be simplified through the optimization idea, and it has high design flexibility.

[0009] The specific technical solutions adopted by 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, the device 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, so that an absorption boundary area is formed 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 reserved on both sides of the central area; the periodic broken grating structure includes a plurality of inherent slits arranged in parallel and at equal intervals, and a period is formed between two adjacent inherent slits. The periodic broken grating structure aims to maximize the radiation efficiency of the laser, 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-breaking 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, wherein 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 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 needed in n cycles, set the slit width sequence w1w2w3…w 2n To record the widths of all inherent slits and all assumed broken slits in n cycles; label the broken slits as a sequence of 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 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, and the surface-emitting 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 the final optimization.

[0020] As a preferred embodiment of the above 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 are exchanged with adjacent individuals 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 a 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 and magnetic field spatial distribution and characteristic frequencies 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, and 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 result of the finite element simulation, and 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-broken structure ensures the formation of distributed feedback, and can realize resonance and frequency selection inside the laser. At the same time, due to the non-periodic characteristics of the period-broken structure locally, its resonant mode is between the symmetric mode and the antisymmetric mode, rather than the antisymmetric mode with low radiation efficiency, so the radiation efficiency and power of the laser can be effectively improved.

[0027] At the same time, the present invention optimizes the grating parameters through genetic algorithms, which reduces the difficulty and time cost of forward designing the grating structure by analyzing the mode field, and has 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 of its mode loss and surface-emission photon loss rate are the highest results of single ridge waveguide terahertz QCL at present, indicating 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 structural cross-sectional view of a surface-emitting terahertz quantum cascade laser according to the present invention;

[0030] Figure 3 Schematic diagram of the grating arrangement of the periodic broken grating structure of the device (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, upper electrode region 9. DETAILED DESCRIPTION

[0035] The present invention is further described and illustrated below in conjunction with the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0036] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation mode of the present invention is described in detail below in conjunction with 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 different from 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 each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0037] In the description of the present invention, it is to be understood that when an element is considered to be "connected" to another element, it may 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 only used for the purpose of distinguishing descriptions, and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features.

[0039] See also Figure 1 and Figure 2 As shown, in a preferred embodiment of the present invention, a period-breaking grating surface-emitting terahertz quantum cascade laser is provided, wherein the device adopts a bimetallic waveguide structure, and specifically comprises a substrate 1, a lower electrode layer 2, and a ridge waveguide structure. Among them, 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 specific form and matching relationship of the ridge waveguide structure are described below.

[0040] The ridge waveguide structure is formed by laminating and compounding the upper electrode 9, the upper contact layer 5, the active area 4, and the lower contact layer 3 in order from top to bottom, and the overall structure is a long strip. Among them, 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 a circle of edge areas in the upper contact layer 5 that are not covered by the upper electrode 9, which are located in the horizontal and vertical directions of the ridge waveguide plane and can be used as an absorption boundary area 6. In addition, a periodic broken grating structure 8 is provided in the central area of ​​the upper electrode area 9, and the parts located at both ends of the ridge waveguide on both sides of the central area retain lead areas 7 for connecting external wires. The periodic broken grating structure 8 includes a number of inherent slits arranged in parallel and at equal intervals, and 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, there is a period between two adjacent inherent slits, but it should be noted that since the inherent slit itself has a 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, and broken slits are not required to be introduced in every period, and the position of the introduced broken slits in 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, there are three types of grating parameters that need to be optimized. The first type is whether an additional broken slit is opened in each period, the second type is the width of all inherent slits and each broken slit in the broken grating structure 8, and the third type is the opening position of each broken slit in the broken grating structure 8. These three types of grating parameters are optimized by genetic algorithm, and the optimization goal is to maximize the radiation efficiency of the laser. In theory, the three types of parameters can be optimized separately or multi-objective jointly optimized. In this embodiment, joint optimization is preferably adopted.

[0043] For ease of description, combined Figure 3 The grating arrangement of the periodic broken grating structure shown in the figure assumes that the periodic broken grating structure 8 of the laser includes n periods, the period length is L, and a broken slit is selectively introduced in the n periods of the grating; assume 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 cycle, b i =1 indicates that there is a broken slit in the i-th period. 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 needed in n cycles, a set of binary sequences w1w2w3…w 2n To record the widths of all inherent slits in n cycles and the widths of all broken slits assumed to be opened.

[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 vacuum. Generally speaking, the value range of the grating period L is between 15.4um and 69.4um, corresponding to the operating frequency range of the surface-emitting terahertz quantum cascade laser between 1.2-5.4THz. In addition, the width range of all slits in the above-mentioned period-breaking grating structure 8 should be between 0.05Λ and 0.5Λ.

[0046] Based on the above-mentioned ridge waveguide structure, the terahertz wave is generated in the active area 4, resonates through the periodic broken grating structure 8 and emerges from the slits, and the light emerging from each slit undergoes constructive interference and is coupled to the free space in the vertical direction. Due to the periodic characteristics of the periodic broken grating structure, the terahertz wave generated in the active area 4 will undergo Bragg diffraction, which is manifested as a band gap and a resonant mode on both sides of the band gap in the spectrum. However, since the periodic broken grating structure 8 locally exhibits non-periodic characteristics, the modes on both sides of the band gap will not exhibit the characteristics of the symmetric mode and the antisymmetric mode, and the mode field distribution is between the symmetric mode and the antisymmetric mode, rather than the antisymmetric mode with low radiation efficiency. Each slit position can achieve far-field interference and realize the surface emission of the terahertz wave.

[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 above-mentioned three types of grating parameters. Figure 4 As shown, 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 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 needed in n cycles, set the slit width sequence w1w2w3…w 2nTo record the widths of all inherent slits and all assumed broken slits in n cycles; label the broken slits as a sequence of b1b2b3…b n , the sequence of broken slit positions is a1a2a3…a i With the slit width sequence w1w2w3…w 2n They are encoded into binary sequences as the chromosomes of each individual in the population, and a specified number of individuals are generated through a random algorithm as the initialized population.

[0049] It should be noted that the data of the position and width of the broken slit may be non-integer with decimals. Therefore, when encoding the two sets of positive numbers with decimals, the position and width of the broken slit data, into binary, the bit width of the decimal part 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 of a broken slit, the position of the broken slit in the cycle, and the width of the slit. By presetting the number of cycles, the position of the broken slit in 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 by iteratively following 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, and the surface-emitting 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 above-mentioned 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, and the finite element simulation software can be used for simulation and automatically solve the fitness of each individual.

[0053] In the above-mentioned 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; secondly, part of the chromosome fragments are exchanged with adjacent individuals 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 and 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, because 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, c is the speed of light in vacuum), so 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] In the above genetic algorithm iteration process, the method of 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 and magnetic field spatial distribution and characteristic frequencies 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, and 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 this to the total photon loss rate 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 the 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 FIG. 1 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 center frequency of the terahertz quantum cascade laser device A is 3.3 THz, and the equivalent refractive index of the active region material is about 3.6, corresponding to the active region wavelength of 25 um. However, in actual situations, due to a certain proportion of the evanescent field propagating outside the active region, the propagation constant is reduced, so the equivalent wavelength of the designed terahertz frequency in the waveguide structure is actually around 27 um.

[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, and the thicknesses of the lower layer and the upper layer 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. Set the broken slit position to a fixed value a, which can be changed during optimization; set the slit width to a fixed value w=3um; design the laser grating region length to be 1.5mm, and the corresponding number of periods is 55. When encoding the parameters 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. Finally, the binary sequence optimized by genetic algorithm is in the form of b1...b i ...b 55 A1...A 5。

[0068] In this embodiment, the grating slit arrangement of laser A obtained by multiple iterations of optimization using the optimization method 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 a 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 a broken slit through an optimization method based on a genetic algorithm, and designing parameters such as the position of the broken slit and the slit width, a result with high radiation loss and high surface-emission photon loss rate is obtained, which can achieve the improvement of 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 design the laser according to specific needs using the optimization iterative process, which 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 above-described embodiments are only some preferred implementations of the present invention, but are not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A period-broken grating surface-emitting terahertz quantum cascade laser, characterized in that: The device adopts a double metal waveguide structure, including 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 an upper electrode (9), an upper contact layer (5), an active region (4), and a lower contact layer (3) in order 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 region (9), and lead regions (7) are reserved 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 to open an additional broken slit 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.

2. The period-broken grating surface-emitting terahertz quantum cascade laser according to claim 1, characterized in that: 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 period-broken grating surface-emitting terahertz quantum cascade laser according to claim 1, characterized in that: The operating frequency of the laser is in the range of 1.2-5.4 THz.

4. The period-broken grating surface-emitting terahertz quantum cascade laser according to claim 1, characterized in that: The widths of all slits in the periodic broken grating structure (8) are in the range of 0.05Λ to 0.5Λ, wherein Λ is the equivalent wavelength of the laser operating frequency in the laser waveguide structure.

5. A genetic algorithm optimization method for a period-broken grating surface-emitting terahertz quantum cascade laser according to any one of claims 1 to 4, characterized in that: include: 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 needed in n cycles, set the slit width sequence w1w2w3…w 2n To record the widths of all inherent slits and all assumed broken slits in n cycles; label the broken slits as a sequence of 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 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, and the surface-emitting 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 the final optimization.

6. The genetic algorithm optimization method according to claim 5, characterized in that: 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 combined to simulate and automatically solve the fitness of each individual.

7. The genetic algorithm optimization method according to claim 5, characterized in that: In 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; secondly, new individuals are generated by exchanging some chromosome fragments with adjacent individuals according to the set exchange ratio; finally, 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 evolution process of the biological population.

8. The genetic algorithm optimization method according to claim 5, characterized in that: 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.

9. The genetic algorithm optimization method according to claim 5, characterized in that: 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 and magnetic field spatial distribution and characteristic frequencies 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, and 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 result of the finite element simulation, and 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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