In-plane heterogeneous photonic crystal quantum cascade laser

Abstract

The invention discloses an in-plane heterogeneous photonic crystal quantum cascade laser. The in-plane heterogeneous photonic crystal quantum cascade laser comprises a back metal electrode, a front metal electrode and an epitaxial wafer located between the back metal electrode and the front metal electrode. Wherein a photonic crystal layer is arranged between the epitaxial wafer and the front metal electrode; wherein the photonic crystal layer is composed of first-order photonic crystals, symmetrical bilattices and second-order photonic crystals which are periodically arranged and are sequentially symmetrical from outside to inside. The problems of low modal selectivity, low coupling efficiency and the like of the existing wide ridge emission QCL are solved, and a new thought is provided for researching the large-area QCL with high power and light beam quality.

Description

Technical Field

[0001] The present application relates to the technical field of semiconductor lasers, and in particular to an in-plane heterogeneous photonic crystal quantum cascade laser. Background Art

[0002] Wide-area (BA) semiconductor lasers, also known as large-spot lasers, are lasers that emit light in a wide emission area. Due to their high output power, they are used in fields such as fiber-optic communications, lidar, and bioimaging. In BA lasers, due to the large gain region, the current is unevenly distributed throughout the laser, causing overload in certain areas and resulting in local hot spots. In addition, as the output power increases, different modes compete with each other, resulting in uneven heat distribution and the formation of structures similar to "filaments" or "stripes" within the laser. This phenomenon is called the "filament effect" and is a common problem with high-power BA lasers. The filament effect is usually accompanied by a decrease in spectral and beam quality, causing the laser to be affected by multi-mode lasing and a large divergence angle. This limits the application of lasers, especially in fields such as laser processing and fiber-optic communications that have high requirements for beam accuracy and quality.

[0003] Quantum cascade lasers (QCLs) have rapidly developed over the past three decades, becoming the dominant laser source in the mid-infrared and terahertz wavelength ranges. This technology has benefited numerous applications, including environmental monitoring, free-space communications, chemical sensing, and other high-precision measurements. In most applications, high-output-power, single-mode operation is considered one of the most important performance criteria for QCLs, improving the accuracy, sensitivity, and detection range of sensing, imaging, and diagnostics. Currently, commonly used designs, such as master oscillator power amplifiers (MOPAs), coherent ridge waveguides, and stripe laser arrays, achieve high power by maintaining the fundamental mode with a narrow ridge-width waveguide and amplifying the signal with a power amplifier or coupled array. However, the power amplification portion of these designs cannot be amplified arbitrarily, resulting in limited output power. Designing QCLs for high quantum efficiency is an effective approach, but the power improvement achieved through active region design is limited. Increasing the area of ​​the QCL's gain region is the most direct way to achieve high power. Due to the relatively low linewidth enhancement factor, BAQCLs exhibit less pronounced filamentation than interband cascade lasers. This allows QCLs to achieve high power and high beam quality through BA designs. A straight FP cavity QCL with a ridge width of 400 μm and a lasing wavelength of 4.45 μm has achieved a peak power of 120 W. However, as the area increases, the number of modes in the laser increases, and the mirror losses of the FP cavity are low, causing the laser to operate in higher-order transverse modes.

[0004] To suppress higher-order modes, integrated photonic crystal DFB structures and slanted cavities have been proposed. For the monolithic integrated photonic crystal DFB approach, by setting different photonic crystal periods in the ridge width and cavity length directions, high-order transverse modes and longitudinal modes can be controlled separately. However, for edge-emitting QCLs, since the ridge width and cavity length usually differ by at least one order of magnitude, the coupling strength in the two directions is unbalanced. To suppress higher-order transverse modes, slanted cavities are integrated into photonic crystal DFBs. However, slanted cavity designs require a larger chip area and exhibit low modal selectivity under high pumping conditions. Summary of the Invention

[0005] To address the aforementioned deficiencies in the art, this application aims to provide an in-plane heterogeneous photonic crystal quantum cascade laser. By monolithically integrating a novel in-plane heterogeneous photonic crystal structure, single-mode, high-power operation of large-area mid-infrared and even terahertz quantum cascade lasers is achieved. This overcomes the low modal selectivity and coupling efficiency issues currently associated with wide-ridge edge-emitting QCLs, providing new insights into the development of high-power, high-beam-quality, large-area QCLs.

[0006] According to one aspect of the present application, an in-plane heterogeneous photonic crystal quantum cascade laser is provided, wherein the in-plane heterogeneous photonic crystal quantum cascade laser comprises a back metal electrode, a front metal electrode, and an epitaxial wafer located between the back metal electrode and the front metal electrode;

[0007] The surface of the epitaxial wafer in contact with the front metal electrode has a photonic crystal layer; the photonic crystal layer is an in-plane heterogeneous photonic crystal structure;

[0008] The photonic crystal layer is a first-order photonic crystal, a symmetrical double lattice and a second-order photonic crystal which are periodically arranged and symmetrical from the outside to the inside.

[0009] According to some embodiments of the present application, the symmetric double lattice is selected from one or more of ellipse, circle, and triangle.

[0010] According to some embodiments of the present application, the elliptical structure is rotated 45° and 135° respectively to form a symmetric dual lattice structure.

[0011] According to some embodiments of the present application, the first-order photonic crystal and the second-order photonic crystal are selected from circular, triangular, rectangular or polygonal shapes.

[0012] According to some embodiments of the present application, the first-order photonic crystal and the second-order photonic crystal are circular.

[0013] According to some embodiments of the present application, the epitaxial wafer includes, from bottom to top, a substrate, a lower waveguide layer, an active region, an upper confinement layer, an upper waveguide layer, a gradient-graded layer, and an ohmic contact layer;

[0014] According to some embodiments of the present application, the substrate is connected to the back metal electrode; the photonic crystal layer is located on the ohmic contact layer and is connected to the front metal electrode.

[0015] According to some embodiments of the present application, the substrate material is InP, and the doping concentration is 0.1 to 2×10 18 cm -3 ;

[0016] The lower waveguide layer is made of InP with a doping concentration of 1 to 10×10 16 cm -3 ;

[0017] The upper confinement layer is made of InGaAs with a doping concentration of 1 to 6×10 16 cm -3 ;

[0018] The upper waveguide layer is made of InP with a doping concentration of 2 to 10×10 16 cm -3 ;

[0019] The gradient layer material is InP, and the doping concentration is 1~5×10 17 cm -3 ;

[0020] The ohmic contact layer material is InP, and the doping concentration is 5~10×10 18 cm -3 ;

[0021] According to some embodiments of the present application, the active region is a superlattice structure in which InGaAs and InAlAs are alternately grown.

[0022] According to some embodiments of the present application, the substrate thickness is 100 to 200 μm; the lower waveguide layer thickness is 2 to 4 μm; the active region thickness is 1.5 to 4 μm; the upper confinement layer thickness is 0.1 to 0.5 μm; the upper waveguide layer thickness is 2 to 4 μm; the gradient layer thickness is 0.2 to 1 μm; and the ohmic contact layer thickness is 0.2 to 1 μm.

[0023] According to some embodiments of the present application, the front metal electrode material is Ti / Au with a thickness of 0.2 to 1.1 μm; the back metal electrode material is Ge / Au / Ni / Au with a thickness of 10 to 40 nm / 20 to 100 nm / 5 to 50 nm / 100 to 1000 nm.

[0024] According to some embodiments of the present application, an insulating layer is further provided between the front metal electrode and the photonic crystal layer;

[0025] According to some embodiments of the present application, the insulating layer is a SiO2 layer; the thickness of the insulating layer is 0.4-1 μm.

[0026] Compared with the prior art, this application has at least the following beneficial effects:

[0027] This application provides an in-plane heterogeneous photonic crystal quantum cascade laser (QCL). This device achieves single-mode, high-power operation of large-area mid-infrared and even terahertz QCLs by monolithically integrating a novel in-plane heterogeneous photonic crystal structure. This overcomes the low modal selectivity and coupling efficiency issues currently associated with broad-ridge edge-emitting QCLs, providing new insights into the development of high-power, high-beam-quality, large-area QCLs. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a 3D schematic diagram of the entire area of ​​the epitaxial wafer according to an embodiment of the present application.

[0029] Figure 2 3D schematic diagram of the entire area of ​​the in-plane heterogeneous photonic crystal structure prepared in the ohmic contact layer.

[0030] Figure 3 This is a 3D schematic diagram of the entire area where the luminous ridges are formed in an embodiment of the present application.

[0031] Figure 4 3D schematic diagram of the entire area for forming the front metal electrode injection window.

[0032] Figure 5 3D schematic diagram of the entire area of ​​the front metal electrode.

[0033] Figure 6 3D schematic diagram of the entire area of ​​the thickened front metal electrode.

[0034] Figure 7 3D schematic diagram of the entire area where the back metal electrode is formed.

[0035] Figure 8 3D schematic diagram of an in-plane heterogeneous photonic crystal quantum cascade laser.

[0036] Figure 9 The waveguide loss and mirror loss variation curves of the in-plane heterogeneous photonic crystal structure and FP cavity.

[0037] Figure 10 The band edge mode loss of the in-plane heterogeneous photonic crystal structure and the photonic band diagram of the subunit structure.

[0038] Figure 11 This is the SEM image of the prepared in-plane heterogeneous photonic crystal structure.

[0039] Figure 12 The power-voltage-current test curve of the in-plane heterogeneous photonic crystal QCL with a ridge width of 103μm at room temperature.

[0040] Figure 13 Spectral transformation diagram under different driving currents.

[0041] Figure 14 Far-field test and simulation curves of in-plane heterogeneous photonic crystal QCL and FP QCL with ridge width of 103μm.

[0042] Figure 15 3D schematic diagram of the dual-wavelength in-plane heterogeneous photonic crystal QCL.

[0043] Figure 16 Spectrum and far-field test curves of dual-wavelength in-plane heterogeneous photonic crystal QCL.

[0044] Description of reference numerals:

[0045] 01: InP substrate, 02: lower waveguide layer, 03: active area, 04: upper confinement layer, 05: upper waveguide layer; 06: gradient layer, 07, ohmic contact layer, 08: SiO2 layer, 09: front metal electrode layer, 10: back metal electrode layer. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions of this application in conjunction with the embodiments of this application. Obviously, the embodiments described are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of this application.

[0047] It is particularly important to note that similar substitutions and modifications made with respect to the present application are obvious to those skilled in the art and are considered to be included in the present application. Relevant persons can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of the present application to implement and apply the technology of the present application. Obviously, the embodiments described are only some of the embodiments of the present application, and not all of them.

[0048] If no specific conditions are specified in this application, the preparation shall be carried out in accordance with conventional conditions or the conditions recommended by the manufacturer. The raw materials or excipients used, as well as the reagents or instruments used, for which the manufacturers are not specified, are all conventional products that can be obtained commercially.

[0049] The following is a detailed description of this application.

[0050] At present, QCL has important application value in environmental monitoring, long-distance detection of harmful substances and explosives, biomedicine, infrared countermeasures and long-distance free-space optical communications. Since the sensitivity and range are proportional to the output power of the laser, further improving the output power of the device is crucial for its application. The most direct way to increase the power is to increase the area of ​​the gain region. However, due to the increase in the number of modes, wide-area FP cavity QCLs will inevitably have problems with poor spectral and beam quality. Even if it can be improved by integrating a tilted cavity or traditional photonic crystal, the propagation loss of the tilted cavity is large and the coupling emission efficiency is low, and the traditional photonic crystal structure cannot completely suppress the high-order modes when the ridge width is large. Therefore, it is necessary to propose a new design suitable for wide-ridge quantum cascade lasers to effectively control the mode of the laser in large areas, so as to improve the spectral and beam quality of the device and further promote its application in military and civilian fields.

[0051] To address these technical challenges, this application proposes a novel in-plane heterogeneous photonic crystal structure, which can be widely used in mid- and far-infrared wide-ridge QCLs. This structure consists of a first-order photonic crystal, a symmetrical double lattice, and a second-order photonic crystal structure. By rationally designing the in-plane heterogeneous photonic crystal based on the specific parameters of the epitaxial wafer, effective mode control can be achieved.

[0052] In some embodiments of the present application, the photonic crystal period Λ=3.003μm, and the etching depth is 300nm. In the ridge width and cavity length directions, the offset of the ellipse relative to the circle in the double lattice structure is set to 0.5Λ and 0.25Λ, respectively. The peak power of the prepared in-plane heterogeneous photonic crystal QCL with a ridge width of 103μm and a cavity length of 4mm reaches 15W. From the threshold to the peak current, the device maintains a stable single-mode characteristic, and the side mode suppression ratio is >30dB. And its far field presents a single spot with a divergence angle of 6.55°, which is close to the diffraction limit angle corresponding to the 103μm ridge width. In addition, in order to verify the feasibility of this design in difference-frequency terahertz QCL, a dual-wavelength in-plane heterogeneous photonic crystal QCL was prepared, whose spectrum exhibits dual wavelengths and the far field is still a single spot. Specifically, the experimental test results confirmed the feasibility of this application. The novel in-plane heterogeneous photonic crystal strategy provided by this application provides an effective method for mode control of wide-area QCL and promotes its further development in free-space optical communications, gas detection and other fields.

[0053] In some examples, the in-plane heterogeneous photonic crystal quantum cascade laser of the present application can be prepared by the following preparation method:

[0054] A lower waveguide layer, an active region, an upper confinement layer, an upper waveguide layer, a gradient layer and an ohmic contact layer with certain thickness and doping concentration are sequentially grown on the substrate;

[0055] Photolithography and etching are performed on the surface of the ohmic contact layer to form a novel in-plane heterogeneous photonic crystal structure;

[0056] forming a double groove on the surface of the ohmic contact layer and wet etching it to the InP lower waveguide layer to form a light-emitting ridge of the device;

[0057] Growing a SiO2 layer on the surface of the double trench and ohmic contact layer, and photolithography and etching to form an electric injection window;

[0058] preparing and growing a front metal electrode layer on the surface of the SiO2 layer and in the electrical injection window;

[0059] Thickening the front metal electrode layer by electroplating to form a thicker metal layer;

[0060] The substrate is thinned and polished, a back metal electrode layer is grown on the back of the substrate, and annealing and packaging steps are performed to obtain the novel in-plane heterogeneous photonic crystal quantum cascade laser.

[0061] The technical solution of the present application is further described below in conjunction with specific embodiments.

[0062] Example

[0063] (1) Using molecular beam epitaxy equipment or metal organic chemical vapor deposition equipment, the lower waveguide layer 02, active region 03, upper confinement layer 04, upper waveguide layer 05, gradient gradation 06, and ohmic contact layer 07 are grown in sequence on the InP substrate 01 to obtain an epitaxial wafer. Figure 1 The figure shows a 3D schematic diagram of the epitaxial wafer.

[0064] The material of substrate 01 is InP, and the doping concentration is 2×10 18 cm -3 The material of the lower waveguide layer 02 is InP, and the doping concentration is 3×10 16 cm -3 , thickness 3μm; the active region 03 is a superlattice structure of alternating InGaAs and InAlAs, with a thickness of 1.8μm; the upper confinement layer 04 is InGaAs with a doping concentration of 4×10 16 cm -3 , thickness 0.3μm; the upper waveguide layer 05 is InP, with a doping concentration of 3×10 16 cm -3 , thickness 3μm; the material of the gradient layer 06 is InP, and the doping concentration is 2×10 17 cm -3 , thickness 0.5μm; the material of the ohmic contact layer 07 is InP, and the doping concentration is 5×10 18 cm -3 , thickness 0.3μm.

[0065] (2) Using PECVD, a SiO2 layer with a thickness of 100 nm is grown on the surface of the epitaxial wafer ohmic contact layer 07. Photoresist Zep520a is coated on the surface of the SiO2 layer, and the pattern is defined by photolithography using EBL. Then, the SiO2 layer is etched using RIE, and the photoresist mask is removed. The patterned SiO2 layer is used as an etching hard mask, and the ohmic contact layer 07 is etched using ICP to form an in-plane heterogeneous photonic crystal pattern. Then, the resist is removed and the hard mask SiO2 layer is removed. Figure 2 As shown, it is a 3D schematic diagram of the entire area of ​​the in-plane heterogeneous photonic crystal prepared in the ohmic contact layer.

[0066] (3) S1805 photoresist is coated on the surface of the sample, and the window pattern is transferred to the sample by photolithography, exposure, and development. Then, an etching solution of HBr:HCl:H2O:H2O2=20:10:100:2 is used to form a light emitting ridge with a width of 103 μm. The etching depth reaches the lower waveguide layer O2, and the resist is removed. Figure 3 As shown, a 3D schematic diagram of the entire area of ​​the luminescent ridge is prepared to form a double groove.

[0067] (4) Use PECVD to cover the surface of the sample with a SiO2 layer 08, and transfer the window pattern to the sample through photolithography, exposure, and development. The photoresist used is AZ6130 with a thickness of 5μm. Then, use SiO2 etching solution HF: NH4F: H2O = 3:6:9 to remove the SiO2 layer at the front metal electrode window, and prepare the required front metal electrode window, followed by de-resisting. Figure 4 As shown, it is a 3D schematic diagram of the entire area for forming the front electrode injection window.

[0068] (5) Photolithography, exposure, and development are used to form the front metal electrode 09. The Ti / Au metal electrode pattern is transferred to the sample. The photoresist used is NR9 with a thickness of 5μm. The Ti / Au layer 09 is grown on the sample by electron beam evaporation with a thickness of 20nm / 300nm. The Ti / Au layer outside the pattern is removed by adhesive stripping technology. Figure 5 Shown is a 3D schematic diagram of the entire area for preparing the front metal electrode.

[0069] (6) Use photoresist AZ6130 with a thickness of 5μm for photolithography, exposure and development. Then use a plasma stripper to remove the glue for 15s, use a step meter to measure the height difference between the glue and the metal electrode layer, and perform constant current electroplating at a constant temperature of 30℃ and constant speed stirring at a current of 0.5~5mA. The electroplating time is determined by the thickness to be electroplated, generally 5μm. After the electroplating is completed, the height difference is measured with a step meter to determine the electroplating thickness. Electroplate the Au layer 10 to 5μm on the front Ti / Au metal layer evaporated by electron beam of the device. Then perform a stripping process with glue in acetone or a stripper to remove the excess electroplated Au layer, such as Figure 6 Shown is a 3D schematic diagram of the entire area of ​​the thickened front metal electrode.

[0070] (7) Thin the substrate of the sample and deposit the back metal electrode layer 11. Use mechanical thinning and physical and chemical polishing steps to thin the substrate to 200μm. Use electron beam evaporation to evaporate Ge / Au / Ni / Au: 10~40nm / 20~100nm / 5~50nm / 100~1000nm on the back of the sample as the back metal electrode. And perform thermal annealing treatment for 30-60s in N2 environment at 340-380℃. Figure 7 The figure shows a 3D schematic diagram of the entire area where the back metal electrode is formed.

[0071] (8) The entire epitaxial wafer is divided into independent tube cores along the reserved cavity surface and the lateral cleavage channel to obtain the in-plane heterogeneous photonic crystal QCL chip body. The conductive material In is deposited on the copper heat sink material. Then, at a temperature of 320°C, the single chip is sintered on the Cu heat sink material using a positive welding method, and the device is electrically interconnected by gold wire bonding. Figure 8 Figure 2 shows a 3D schematic diagram of the prepared in-plane heterogeneous photonic crystal QCL.

[0072] Experimental example

[0073] The finite element analysis method is used to analyze the waveguide and mirror losses of the in-plane heterogeneous photonic crystal structure and FP cavity of this application:

[0074] like Figure 9 Calculation results are shown in Figure 2. For a QCL with a wavelength of 9.5 μm, a ridge width of 103 μm, and a cavity length of 4 mm, it contains 18 TM modes. Due to the resonant coupling between the asymmetric modes in the photonic crystal structure and the surface plasmon, the waveguide loss of the in-plane heterogeneous photonic crystal structure is higher than that of the FP cavity.

[0075] Among them, the loss difference between the fundamental mode and the first high-order mode of the in-plane heterogeneous photonic crystal structure of the present application is 0.527 cm -1, this difference is sufficient to stabilize the fundamental mode and enable lasing preferentially. In addition, the waveguide loss differences between the fundamental mode of the novel structure of the present application and the 18th-order mode of the FP cavity are 1.18 and 0.64 cm respectively -1 , and the mirror loss difference of the FP cavity is 0.71 cm -1 , so the waveguide loss difference of the novel design of the present application > the waveguide loss difference and mirror loss difference of the FP cavity, and theoretically can fully suppress the eigenmode of the FP cavity.

[0076] The constituent units and photonic energy bands of the in-plane heterogeneous photonic crystal of the present application are as Figure 10 shown.

[0077] Among them, the symmetric double-lattice PC2 and PC4 have opposite diffraction directions, which can confine the fundamental mode in the center and compress the optical field. The PC3 structure located in the middle position can effectively improve the utilization rate of the active region and effectively expand the mode. The outermost PC1 structure increases the loss of the high-order modes near the edge, thereby further increasing the loss difference between the fundamental mode and the high-order modes, so as to stably achieve single-mode and single-spot operation of the device. The edge mode losses of PC1, PC2 and PC3 were obtained by the finite element method. The mode loss of mode A is the smallest and dominates in laser emission. Due to the small one-dimensional coupling coefficient of PC2, its photonic bandgap < PC1. The edge mode A of the PC2 structure is located within the photonic bandgap of PC1, so the wave in PC1 is reflected at the boundary of PC1, enhancing the in-plane confinement of the electric field. On the contrary, the mode A in the PC3 structure has the same band-edge frequency as that of PC2, so the wave can propagate on the interface between PC2 and PC3. In short, this composite in-plane heterogeneous structure photonic crystal design effectively enhances the in-plane localization effect, thereby strengthening the coupling strength in the ridge width direction and breaking the phenomenon of unbalanced coupling strength between the ridge width and the cavity length in previous studies.

[0078] The in-plane heterogeneous photonic crystal structure of the present application is prepared by the described EBL process, and its surface SEM pattern is as Figure 11 shown.

[0079] After the in-plane heterogeneous photonic crystal quantum cascade laser of the present application is fabricated and packaged, it is driven in pulse mode using a driving power supply at room temperature, and its power-current-voltage characteristics are characterized, as Figure 12 shown. The in-plane heterogeneous photonic crystal QCL of the present application is mounted and fixed on the temperature controller of a thermoelectric cooler, and the emitted optical power is collected and measured by a thermopile detector calibrated in front of the laser cavity surface. The pulse width and repetition frequency used in the test are set to ~90 ns and 10 kHz respectively. At room temperature, the threshold current of the device is 13.6 A, and the maximum output power is about 14.6 W.

[0080] The spectral characteristics of the in-plane heterogeneous photonic crystal QCL of this application under different driving currents are analyzed, and the test results are as follows: Figure 13 As shown. Because the in-plane heterostructure photonic crystal has a sufficiently strong coupling strength, its spectrum maintains stable single-mode characteristics from the threshold current to the peak current, and the wavelength peak basically does not shift with the increase of current, and is stable at around 9.5μm. The measured results are consistent with the theoretical results, proving the feasibility of the in-plane heterostructure photonic crystal design in wide-area edge-emitting QCLs. In addition, its maximum side mode suppression ratio is 30.25dB, which means that the side mode power is only 0.1% of the main mode power, proving its ability to emit high-quality monochromatic lasers.

[0081] Figure 14 The far-field distribution of this application is shown. The 103μm wide, 4mm long in-plane heterogeneous photonic crystal QCL exhibits a divergence angle of 6.55° near the threshold, while the theoretically calculated diffraction-limited angle is 6.44°. Therefore, the device fabricated using this application's solution exhibits near-diffraction-limited properties, based on the compression of the light field by the symmetrical double-lattice structure and the enhanced mode localization effect of the in-plane heterostructure.

[0082] In addition, in order to prove the feasibility of the in-plane heterogeneous photonic crystal design of this application in multi-wavelength QCL and further expand it to difference-frequency terahertz QCL, a dual-wavelength in-plane heterogeneous photonic crystal QCL was prepared and tested, as shown in the schematic diagram. Figure 15 As shown. It can be seen that the ridge width of the device of this application is still 103μm, the cavity lengths of the two wavelengths are set to 1.9mm respectively, and a 200nm trench is set in the middle of the device for electrical isolation. th Under the test, its spectral characteristics were tested and analyzed. The device exhibited lasing at 9.5 and 10.3 μm, peaks corresponding to the designed photonic crystal period. Furthermore, the dual-wavelength in-plane heterogeneous photonic crystal QCL exhibited a divergence angle of 10° near the threshold, while the diffraction-limited angles corresponding to the dual wavelengths were 6.45° and 7.0°, respectively. The tested divergence angle exceeded 7.0°. This is primarily due to the cavity length being halved compared to the single-mode device mentioned above, resulting in a corresponding decrease in coupling strength, which in turn weakens the compression and confinement of the mode.

[0083] The above embodiments are only intended to help understand the method and core concept of the present application. It should be noted that, for those skilled in the art, without departing from the principles of the present application, several improvements and modifications may be made to the present application, and such improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. An in-plane heterogeneous photonic crystal quantum cascade laser, characterized in that: The in-plane heterogeneous photonic crystal quantum cascade laser comprises a back metal electrode, a front metal electrode and an epitaxial wafer located between the back metal electrode and the front metal electrode; Wherein, the surface of the epitaxial wafer in contact with the front metal electrode has a photonic crystal layer; the photonic crystal layer is an in-plane heterogeneous photonic crystal structure; The photonic crystal layer is a first-order photonic crystal, a symmetrical double lattice and a second-order photonic crystal that are periodically arranged and symmetrical from the outside to the inside.

2. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 1, characterized in that The symmetrical double lattice is selected from a combination of an ellipse and a circle or a triangle.

3. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 2, characterized in that: The elliptical structures are rotated 45° and 135° respectively to form a symmetrical double lattice structure.

4. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 1, characterized in that The first-order photonic crystal and the second-order photonic crystal are selected from the group consisting of circular, triangular, rectangular or polygonal.

5. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 1, characterized in that The first-order photonic crystal and the second-order photonic crystal are circular.

6. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 1, characterized in that The epitaxial wafer includes, from bottom to top, a substrate, a lower waveguide layer, an active region, an upper confinement layer, an upper waveguide layer, a gradient layer and an ohmic contact layer; The substrate is connected to the back metal electrode; the photonic crystal layer is located on the ohmic contact layer and is connected to the front metal electrode.

7. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 6, characterized in that: The substrate material is InP, and the doping concentration is 0.1 to 2×10 18 cm -3 ; The lower waveguide layer is made of InP with a doping concentration of 1 to 10×10 16 cm -3 ; The upper confinement layer is made of InGaAs with a doping concentration of 1 to 6×10 16 cm -3 ; The upper waveguide layer is made of InP with a doping concentration of 2 to 10×10 16 cm -3 ; The gradient layer material is InP, and the doping concentration is 1 to 5×10 17 cm -3 ; The ohmic contact layer material is InP, and the doping concentration is 5 to 10×10 18 cm -3 ; The active region is a superlattice structure in which InGaAs and InAlAs grow alternately.

8. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 6, characterized in that: The thickness of the substrate is 100-200 μm; the thickness of the lower waveguide layer is 2-4 μm; the thickness of the active area is 1.5-4 μm; the thickness of the upper limiting layer is 0.1-0.5 μm; the thickness of the upper waveguide layer is 2-4 μm; the thickness of the gradient layer is 0.2-1 μm; and the thickness of the ohmic contact layer is 0.2-1 μm.

9. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 1, characterized in that: The front metal electrode material is Ti / Au with a thickness of 0.2 to 1.1 μm; the back metal electrode material is Ge / Au / Ni / Au with a thickness of 10 to 40 nm / 20 to 100 nm / 5 to 50 nm / 100 to 1000 nm.

10. The in-plane heterogeneous photonic crystal quantum cascade laser according to claim 1, characterized in that: An insulating layer is further provided between the front metal electrode and the photonic crystal layer; Optionally, the insulating layer is a SiO2 layer; the insulating layer has a thickness of 0.4 to 1 μm.

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