Semiconductor laser and terahertz light source for generating terahertz waves

Abstract

This application provides a semiconductor laser and a terahertz source for generating terahertz waves. The semiconductor laser includes: a substrate; a mesa located on the substrate; and a ridge waveguide and lateral coupling gratings located on the mesa, with the lateral coupling gratings located on both sides of the ridge waveguide. The lateral coupling gratings are configured such that the frequency difference between the fundamental transverse mode wavelength and the second-order transverse mode wavelength of the lasing light emitted by the semiconductor laser is 0.1 THz–10 THz. The semiconductor laser of this application, through the optimized configuration of the ridge waveguide and the lateral coupling gratings on both sides, precisely achieves simultaneous output of the fundamental and second-order transverse modes with a frequency difference on the order of terahertz. Combined with a polarization controller and a nonlinear crystal, it can stably output terahertz waves in the target frequency band. Compared with traditional solutions, the semiconductor laser of this application can simultaneously output the fundamental and second-order transverse modes with a frequency difference on the order of terahertz, offering advantages in miniaturization and low cost.

Description

Technical Field

[0001] This invention relates to the field of semiconductor optoelectronic devices, and more specifically to a semiconductor laser and a terahertz light source for use as a terahertz light source. Background Technology

[0002] Terahertz waves have shown great potential in fields such as spectral detection, radar technology, biological imaging, and environmental monitoring. However, the widespread application of terahertz waves is largely limited by the lack of efficient, compact terahertz light sources with a wide operating temperature range.

[0003] Currently, semiconductor quantum cascade lasers are an important terahertz source, but they typically require cryogenic operation, which significantly increases cost and energy consumption, limiting their widespread application. On the other hand, radiation sources based on the principle of optical difference frequency generation generate terahertz waves through the difference frequency of two laser beams in a nonlinear crystal, providing a feasible path to achieve room-temperature terahertz systems. However, traditional difference frequency systems usually use bulky and expensive CO2 lasers as pump sources, for example, generating 10μm wavelength lasers before difference frequency generation, which is also detrimental to system miniaturization and cost control. Summary of the Invention

[0004] Therefore, the object of the present invention is to overcome the defects of the prior art and provide a semiconductor laser for generating terahertz waves, the semiconductor laser comprising: a substrate; a mesa located on the substrate; and a ridge waveguide and a lateral coupling grating located on the mesa, the lateral coupling grating being located on both sides of the ridge waveguide; wherein the lateral coupling grating is configured such that the frequency difference between the fundamental transverse mode wavelength and the second transverse mode wavelength of the lasing light of the semiconductor laser is 0.1THz-10THz.

[0005] The semiconductor laser according to the present invention preferably further includes a first electrode located on the ridge waveguide and a second electrode located on the substrate.

[0006] In the semiconductor laser according to the present invention, preferably, the width of the ridge waveguide is 1.5-4 μm.

[0007] According to the semiconductor laser of the present invention, preferably, the grating period of the lateral coupling grating is 180-210nm, the height is 100-400nm, and the etching depth is 50-300nm.

[0008] According to the semiconductor laser of the present invention, preferably, the ridge waveguide is configured such that the third-order and higher transverse modes of the lasing light of the semiconductor laser are suppressed.

[0009] According to the semiconductor laser of the present invention, preferably, the lateral coupling grating includes a first portion and a second portion arranged on both sides of the ridge waveguide, wherein the first portion includes a first region and a second region, and the second portion includes a third region and a fourth region.

[0010] According to the semiconductor laser of the present invention, preferably, the first region and the second region are spaced apart by half a grating period, and the third region and the fourth region are spaced apart by half a grating period.

[0011] According to the semiconductor laser of the present invention, preferably, the width of the ridge waveguide is 1.8-2.5 μm; the grating period of the lateral coupling grating is 200 nm, the height is 100 nm, and the etching depth is 100 nm.

[0012] On the other hand, the present invention also provides a terahertz light source, comprising: a semiconductor laser according to the present invention; a polarization controller configured to control the polarization states of the fundamental transverse mode and the second-order transverse mode output by the semiconductor laser to satisfy a phase-matching condition; and a nonlinear crystal configured to cause a difference frequency between the two laser beams output by the polarization controller.

[0013] According to the terahertz light source of the present invention, preferably, the nonlinear crystal is a GaSe crystal or an orientation-patterned GaAs crystal (hereinafter referred to as OP-GaAS crystal).

[0014] The semiconductor laser of this application achieves a precise terahertz-level frequency difference between the fundamental transverse mode and the second-order transverse mode through an optimized configuration of a ridge waveguide and lateral coupling gratings on both sides. Combined with a polarization controller and a nonlinear crystal, it can stably output terahertz waves in the target frequency band. Compared to traditional solutions, the semiconductor laser of this application can simultaneously output the fundamental and second-order transverse modes with a terahertz-level frequency difference, offering advantages in miniaturization and cost, and promoting the expansion of the application range of terahertz waves. Attached Figure Description

[0015] The embodiments of the present invention will be further described below with reference to the accompanying drawings, wherein:

[0016] Figure 1 This is a three-dimensional structural diagram of a semiconductor laser according to an embodiment of the present invention;

[0017] Figure 2 This is a schematic cross-sectional view of a semiconductor laser according to an embodiment of the present invention;

[0018] Figures 3A-3D These are schematic cross-sectional views of the semiconductor laser fabrication process according to embodiments of the present invention.

[0019] Figure 4A and 4BSimulation results of the fundamental transverse mode optical field distribution and the second-order transverse mode optical field distribution of the lasing light from a semiconductor laser according to an embodiment of the present invention at the emission end face;

[0020] Figure 5A and 5B These are the test results of the lasing beam distribution of a semiconductor laser according to two preferred embodiments of the present invention;

[0021] Figure 6 This is the spectrum of lasing light from a semiconductor laser according to a preferred embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description, in conjunction with the accompanying drawings and specific embodiments, further illustrates this application. It should be understood that the described embodiments are only a part of, and not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application. It should be emphasized that the drawings are schematic and not drawn to scale. Specifically, for ease of explanation, the thickness of each layer in the drawings does not correspond to its actual thickness. Furthermore, the thickness of each layer is not proportional to its lateral dimension.

[0023] Embodiments of the present invention provide a semiconductor laser for a terahertz light source. See also Figure 1 and Figure 2 The schematic diagrams of the three-dimensional structure and cross-sectional structure of the semiconductor laser of this embodiment are shown. The laser includes: a substrate 101, a mesa 102, a lateral coupling grating 103, a ridge waveguide 104, a positive electrode 105, and a negative electrode 106. The mesa 102 is disposed on the substrate 101, the ridge waveguide 104 is disposed on the mesa 102, the lateral coupling grating 103 is disposed on the mesa 102 and on both sides of the ridge waveguide 104, the positive electrode 105 is disposed on the ridge waveguide 104, and the negative electrode 106 is disposed on the substrate 101 and on one side of the mesa 102.

[0024] In one embodiment of the invention, preferably, the substrate 101 is made of N-type doped GaAs material. In another embodiment, the substrate 101 is made of other N-type doped group III-V materials. This layer serves as the substrate of the laser, and other material layers on it need to be epitaxially grown by attaching to this layer, while also providing a pathway for the negative electrode 106.

[0025] In one embodiment of the present invention, such as Figure 2As shown, the mesa 102 includes an N-type doped layer 1021 and a quantum dot active region layer 1022. The lattice constant and bandgap of the N-type doped layer 1021 are matched with those of the quantum dot active region 1022 to eliminate interface lattice defects and reduce carrier recombination losses. Preferably, the N-type doped layer 1021 is made of N-type doped AlGaAs material, and the quantum dot active region 1022 is made of InAs material.

[0026] In one embodiment of the present invention, such as Figure 1 As shown, the lateral coupling grating 103 includes a first portion disposed on one side of the ridge waveguide 104 and a second portion disposed on the other side of the ridge waveguide 104. The first portion includes a first region 103.a and a second region 103.b, and the second portion includes a third region 103.c and a fourth region 103.d. Preferably, as... Figure 1 As shown, the first region 103.a and the second region 103.b are discontinuous, preferably separated by half a grating period; the third region 103.c and the fourth region 103.d are also discontinuous, preferably separated by half a grating period. This design of separating the first region 103.a and the second region 103.b, and separating the third region 103.c and the fourth region 103.d, can suppress unwanted higher-order transverse modes. In one embodiment of the invention, the lateral coupling grating 103 is made of p-type doped AlGaAs material. Preferably, the grating period is 180-210 nm. Preferably, the height of the grating 103 is 100-400 nm, and the etching depth is 50-300 nm. More preferably, the height of the grating 103 is 100 nm, and the etching depth is 100 nm. The lateral coupling grating 103 generates strong reflection of laser light within a specific wavelength range, thereby filtering out the target dual-wavelength laser light and forming a stable resonant cavity.

[0027] In an embodiment of the present invention, the lateral coupling grating 103 is configured such that the frequency difference between the fundamental transverse mode wavelength and the second-order transverse mode wavelength of the lasing light from the semiconductor laser is 0.1 THz-10 THz. Preferably, the structural parameters of the lateral coupling grating 103 are specifically configured such that the frequency difference between the fundamental transverse mode wavelength and the second-order transverse mode wavelength of the lasing light from the semiconductor laser is 0.1 THz-10 THz, wherein the structural parameters of the lateral coupling grating 103 include grating period, grating height, and grating etching depth.

[0028] In one embodiment of the present invention, such as Figure 2As shown, the ridge waveguide 104 includes a P-type doped layer 1031 and a P-type contact layer 1041 located on top of the P-type doped layer 1031. The P-type doped layer 1031 confines the light field within the ridge waveguide in the horizontal direction through the difference in refractive index with the outside environment; the P-type contact layer 1041 assists in carrier transport and contacts the positive electrode located thereon. Preferably, the P-type doped layer 1031 is a P-type doped AlGaAs material layer, and the P-type contact layer 1041 is a P-type doped GaAs material layer. The inventors have discovered that the intensity distribution of the fundamental transverse mode and the second-order transverse mode in the lasing light of a semiconductor laser is modulated by the width of the ridge waveguide. As the width of the ridge waveguide increases, the intensity of the second-order transverse mode increases while the intensity of the fundamental transverse mode decreases. To achieve terahertz waves, preferably, the width of the ridge waveguide is selected such that the fundamental transverse mode and the second-order transverse mode in the lasing light of the semiconductor laser can be output simultaneously, and more preferably, it is selected such that the intensities of the fundamental transverse mode and the second-order transverse mode in the lasing light of the semiconductor laser are comparable. In embodiments of the present invention, preferably, the width of the ridge waveguide 104 is 1.5-4 μm. When the width of the ridge waveguide 104 is less than 1.5 μm, the intensity of the second-order transverse mode is too weak to generate a difference frequency with the fundamental transverse mode. When the width of the ridge waveguide 104 is greater than 4 μm, the spacing between the lateral coupling gratings is too large, making it difficult to achieve wavelength filtering. More preferably, the width of the ridge waveguide 104 is 1.5-3 μm, preferably 1.8-2.5 μm, for example 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm. The laser beam from the semiconductor laser is confined and propagates longitudinally within the ridge waveguide 104, and exits near the boundary between the ridge waveguide 104 and the quantum dot active region 1022 on one end face of the laser. For example, Figure 1 107 is one of the end faces, from which the laser is emitted; alternatively, the laser is emitted from the other end face opposite to end face 107.

[0029] In a preferred embodiment, the width of the ridge waveguide 104 is configured such that the third and higher transverse modes of the lasing light from the semiconductor laser are suppressed, and only the fundamental transverse mode and the second transverse mode are output.

[0030] In another embodiment, an antireflection coating, preferably a 10% antireflection coating, is applied to the output facet. The antireflection coating can improve the laser output efficiency of the semiconductor laser, optimize beam quality, protect the device facet, and extend device lifespan.

[0031] In one embodiment of the present invention, the positive electrode 105 is made of P-type metal. The P-type metal positive electrode 105 can form a good ohmic contact with the P-type contact layer 1041, achieving low contact resistance and linear current-voltage characteristics. Preferably, the positive electrode 105 has a structure of Ti, Pt, Ti, and Au layered from top to bottom (Ti / Pt / Ti / Au structure) to further optimize contact resistance and thermal resistance.

[0032] In one embodiment of the present invention, the negative electrode 106 is made of N-type metal. The N-type metal negative electrode 106 can form a good ohmic contact with the N-type doped substrate 101, achieving low contact resistance and linear current-voltage characteristics. Preferably, the negative electrode 106 has a structure of Ni, Ge, and Au layered from top to bottom (Ni / Ge / Au structure) to further optimize contact resistance and thermal resistance.

[0033] When a voltage is applied between the positive electrode 105 and the negative electrode 106 of the semiconductor laser, holes in the P-type materials of the positive electrode 105, ridge waveguide 104, etc., are injected into the quantum dot active region 1022 under the action of the electric field, and electrons in the N-type materials of the N-type doped layer 1021, substrate 101, negative electrode 106, etc., are injected into the quantum dot active region 1022 under the action of the electric field. As the concentration of injected carriers in the quantum dot active region 1022 gradually increases, population inversion is achieved, and initial photons are generated through spontaneous emission, and then photon amplification is achieved through stimulated emission. The photons generated in the quantum dot active region 1022 are laterally confined by the ridge waveguide 104 and propagate longitudinally, and are amplified by the lateral coupling grating 103. Among them, the fundamental transverse mode and the second-order transverse mode that satisfy the Bragg condition can oscillate stably and finally be emitted from the end face 107.

[0034] In an embodiment of the present invention, the grating period of the lateral coupling grating 103 satisfies the following formula:

[0035] , formula (1)

[0036] Where Λ is the grating period, λ is the lasing wavelength, and n eff This is the effective refractive index corresponding to the lasing wavelength. A semiconductor laser may contain multiple transverse modes, each with a corresponding lasing wavelength λ and effective refractive index n. eff In an embodiment of the present invention, the fundamental transverse mode wavelength λ1 and the second-order transverse mode wavelength λ2 respectively satisfy Λ=λ1 / (2n eff1 ) and Λ=λ2 / (2n eff2 ).

[0037] In a preferred embodiment of the present invention, the ridge waveguide 104 of the semiconductor laser has a width of 3 μm, the grating 103 structure has a height of 100 nm relative to the quantum dot active region, the grating etching depth is 100 nm, and the quantum dot active region 1022 can provide gain for lasers with wavelengths ranging from approximately 1260 nm to 1370 nm. In this embodiment, the effective refractive index n of the fundamental transverse mode... eff1 The effective refractive index n of the second transverse mode is approximately 3.351. eff2The value is approximately 3.222. Based on the aforementioned formula (1) and combined with the following formula (2), since the lasing wavelength of the fundamental transverse mode needs to be within the gain range, the appropriate grating period is calculated to be 188.0 nm - 204.4 nm. In this preferred embodiment, according to the above conditions, the grating period is 200 nm. Based on formula (1) and combined with formula (2), the frequency difference between the fundamental transverse mode and the second-order transverse mode is calculated to be approximately 9 THz. Based on this, by performing frequency difference analysis on the fundamental transverse mode and the second-order transverse mode output by the semiconductor laser, a terahertz wave can be obtained.

[0038] , formula (2)

[0039] Embodiments of the present invention also provide a method for fabricating a semiconductor laser, see [link to documentation]. Figures 3A-3D The diagram shown is a cross-sectional view of the semiconductor laser fabrication process in this embodiment. The fabrication method of the semiconductor laser is as follows:

[0040] Step 1: An epitaxial wafer is formed by sequentially growing an N-type doped layer 1021, a quantum dot active region 1022, a P-type doped layer 1031, and a P-type contact layer 1041 on a substrate 101. Figure 3A As shown;

[0041] Step 2: Etch the P-type contact layer 1041 and the P-type doped layer 1031 to form a ridge waveguide structure in the central region. Dry etching is preferably used, wherein the etching depth of the P-type doped layer 1031 is less than its thickness. Figure 3B As shown.

[0042] Step 3: Etch the remaining portion of the p-type doped layer 1031 to form the lateral coupling grating 103, as shown below. Figure 3C As shown.

[0043] Step 4: Etch the quantum dot active region 1022 and the N-type doped layer 1021 to form mesa 102, preferably using wet or dry etching. The substrate 101 on both sides of the mesa 102 is exposed. Figure 3D As shown

[0044] Step 5: On the upper surface of the ridge waveguide 104 structure, a positive electrode 105 is fabricated using a metal deposition process; on the upper surface of the substrate 101, on one side of the mesa 102, a negative electrode 106 is fabricated using a metal deposition process, ultimately forming... Figure 2 The semiconductor laser structure shown.

[0045] To facilitate understanding, the present invention provides simulation results of the fundamental transverse mode optical field distribution and the second-order transverse mode optical field distribution of the lasing light from a semiconductor laser at the output end face 107 according to a preferred embodiment, as shown below. Figure 4A and 4BAs shown in the figure. The horizontal axis represents the horizontal length, and the vertical axis represents the vertical length. The black rectangle 401 in the figure represents the ridge waveguide 104, and the black line 402 below the ridge waveguide and the part below it represents the mesa 102. It can be seen that the fundamental transverse mode and the second-order transverse mode are emitted near the junction of the ridge waveguide 104 and the mesa 102.

[0046] To demonstrate the effectiveness of this invention, the present invention provides test results of the lasing beam distribution of semiconductor lasers from two preferred embodiments, such as... Figure 5A and 5B As shown, the fundamental transverse mode and the second-order transverse mode are emitted simultaneously, and their beams overlap. Figure 5A In the illustrated embodiment, the ridge waveguide of the semiconductor laser has a width of 1.5 μm, and the lateral coupling grating has a grating period of 200 nm, a height of 100 nm, and an etching depth of 100 nm. In this embodiment, the intensity of the second-order transverse mode in the lasing light of the semiconductor laser is relatively weak, while the intensity of the fundamental transverse mode is relatively strong. Figure 5B In the embodiment shown, the parameter values ​​of the lateral coupling grating are the same as those of the lateral coupling grating. Figure 5A The embodiment is the same, except that the width of the ridge waveguide is 3 μm. In this embodiment, the intensity of the second-order transverse mode is slightly weaker, and the intensity of the fundamental transverse mode is slightly stronger.

[0047] The present invention also provides test results for a semiconductor laser according to a preferred embodiment, wherein the width of the ridge waveguide of the semiconductor laser is 3 μm, and the grating period of the lateral coupling grating is 200 nm, the height is 100 nm, and the etching depth is 100 nm. See also Figure 6 The spectrum of the lasing light from the semiconductor laser in this embodiment is shown, with wavelength on the horizontal axis and intensity on the vertical axis. Figure 6 The two intensity peaks correspond to the wavelengths of the fundamental transverse mode (λ1) of 1319.62 nm and the second-order transverse mode (λ2) of 1312.61 nm, respectively. Substituting the actual values ​​of λ1 and λ2 as wavelengths into formula (2), the actual frequencies of the fundamental and second-order transverse modes can be calculated. The calculated actual frequency difference is approximately 1.18 Thz, which is within the terahertz range.

[0048] In the foregoing embodiments of the present invention, the upper part of the quantum dot active region 1022 is a P-type material and the lower part is an N-type material. In another embodiment, the upper part of the quantum dot active region 1022 is an N-type material and the lower part is a P-type material.

[0049] Embodiments of the present invention also provide a terahertz light source, comprising the aforementioned semiconductor laser, polarization controller, and nonlinear crystal according to the present invention. Preferably, the nonlinear crystal is a GaSe or OP-GaAs crystal. These devices are coupled sequentially in the order of semiconductor laser, polarization controller, and nonlinear crystal. The fundamental transverse mode and second-order transverse mode output from the semiconductor laser are passed through the polarization controller, which controls their polarization states to satisfy phase matching conditions. The collinear laser beams, polarization-controlled, are coupled into the nonlinear crystal. Utilizing the second-order nonlinear optical effect of the crystal, the two laser beams generate a difference frequency, thereby producing a terahertz wave with a frequency equal to the difference between the frequencies of the two pump lights. Preferably, a collection device, such as an optical fiber or optical lens, is provided after the nonlinear crystal of the terahertz light source to collect the generated terahertz wave. More preferably, a detector is provided after the collection device of the terahertz light source to detect whether the terahertz wave has been correctly generated and collected. The terahertz light source described in this embodiment is merely an example and not a limitation. Various modifications to the setup of the terahertz light source can be made without departing from the scope of the present invention.

[0050] The semiconductor laser of this application achieves a precise terahertz-level frequency difference between the fundamental transverse mode and the second-order transverse mode through the optimized configuration of the ridge waveguide and the lateral coupling gratings on both sides. When combined with a polarization controller and a nonlinear crystal, it can stably output terahertz waves in the target frequency band. Compared with traditional solutions, the semiconductor laser of this application can simultaneously output the fundamental transverse mode and the second-order transverse mode with a terahertz-level frequency difference, and has the advantages of miniaturization and low cost, which promotes the expansion of the application range of terahertz waves.

[0051] While the present invention has been described through preferred embodiments, it is not limited to the embodiments described herein, and various changes and modifications are made without departing from the scope of the invention.

Claims

1. A semiconductor laser for generating terahertz waves, the semiconductor laser comprising: Substrate; Mesa located on the substrate; and A ridge waveguide and a lateral coupling grating are located on the platform, with the lateral coupling grating located on both sides of the ridge waveguide; The lateral coupling grating is configured such that the frequency difference between the fundamental transverse mode wavelength and the second-order transverse mode wavelength of the lasing light from the semiconductor laser is 0.1 THz-10 THz.

2. The semiconductor laser according to claim 1, further comprising a first electrode located on the ridge waveguide and a second electrode located on the substrate.

3. The semiconductor laser according to claim 1 or 2, wherein the width of the ridge waveguide is 1.5-4 μm.

4. The semiconductor laser according to claim 1 or 2, wherein the lateral coupling grating has a grating period of 180-210 nm, a height of 100-400 nm, and an etching depth of 50-300 nm.

5. The semiconductor laser of claim 1, wherein the ridge waveguide is configured such that the third and higher transverse modes of the lasing light from the semiconductor laser are suppressed.

6. The semiconductor laser of claim 5, wherein the lateral coupling grating comprises a first portion and a second portion disposed on both sides of the ridge waveguide, wherein the first portion comprises a first region and a second region, and the second portion comprises a third region and a fourth region.

7. The semiconductor laser according to claim 6, wherein the first region and the second region are spaced apart by half a grating period, and the third region and the fourth region are spaced apart by half a grating period.

8. The semiconductor laser according to claim 1, wherein the width of the ridge waveguide is 1.8-2.5 μm; the grating period of the lateral coupling grating is 200 nm, the height is 100 nm, and the etching depth is 100 nm.

9. A terahertz light source, comprising: The semiconductor laser according to any one of claims 1-8; A polarization controller is configured to control the polarization states of the fundamental transverse mode and the second transverse mode output by the semiconductor laser to satisfy phase matching conditions. A nonlinear crystal is configured to cause a difference frequency in the two laser beams output by the polarization controller.

10. The terahertz light source according to claim 9, wherein the nonlinear crystal is a GaSe crystal or an OP-GaAs crystal.

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