A broad spectrum terahertz quantum cascade laser heterostructure active region

Abstract

The present application relates to a kind of wide spectrum terahertz quantum cascade laser heteroactive region, the heteroactive region is arranged between the lower electrode contact layer and the upper electrode contact layer of terahertz quantum cascade laser epitaxial structure;The heteroactive region is sequentially stacked by four kinds of sub-active regions with different center emission frequencies along the epitaxial growth direction;The four kinds of sub-active regions all adopt GaAs / AlGaAs two-trap fast phonon single period structure, and different center emission frequencies are formed by adjusting the thickness of two GaAs quantum well layers in each sub-active region.The present application uses four kinds of sub-active regions with different center emission frequencies to form heteroactive region, and utilizes the characteristics of the small thickness of two-trap active region single period, integrates more cascade cycles under the condition of limited total active region thickness, so as to obtain the total gain spectrum output wider than single active region.

Description

Technical Field

[0001] This invention belongs to the field of terahertz laser technology, and specifically relates to a heterogeneous active region for a broadband terahertz quantum cascade laser. Background Technology

[0002] Terahertz quantum cascade lasers (THz QCLs) are electrically injected coherent terahertz light sources that achieve lasing through interband transitions in semiconductor multi-quantum-well structures. They offer advantages such as high output power, compact structure, and designable emission frequencies. With the development of applications in terahertz spectroscopy, imaging, and communication, the demand for broadband terahertz emission is constantly increasing. However, under conventional single active region and cavity structure conditions, limited by cavity mode selection, mode competition, and electrical and thermal operating point constraints, the actual output of THz QCLs typically exhibits single-frequency or finite-narrowband emission, making it difficult to fully realize their wide-gain potential. To overcome the bandwidth limitation of single active regions, researchers have proposed heterogeneous THz QCL active region designs based on the scalable nature of THz QCL structures. By integrating multiple sub-active regions with different center frequencies in the same device, broadband output can be achieved. However, heterogeneous THz QCLs still face challenges such as current matching difficulties, increased structural and fabrication complexity, and exacerbated heat accumulation. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a heterogeneous active region for a broadband terahertz quantum cascade laser. The heterogeneous active region is composed of four sub-active regions with different center emission frequencies. By taking advantage of the small single-cycle thickness of the two-well active region, more cascaded cycles are integrated under the condition of finite total active region thickness, thereby obtaining a total gain spectrum output that is wider than that of a single active region.

[0004] This invention provides a heterogeneous active region for a broadband terahertz quantum cascade laser. The heterogeneous active region is disposed between the lower electrode contact layer and the upper electrode contact layer of the epitaxial structure of the terahertz quantum cascade laser. The heterogeneous active region is composed of four sub-active regions with different central emission frequencies stacked sequentially along the epitaxial growth direction. All four sub-active regions adopt a GaAs / AlGaAs two-well fast phonon single-period structure, and each sub-active region forms a different central emission frequency by adjusting the thickness of the two GaAs quantum well layers.

[0005] Preferably, the center emission frequencies of the four sub-active regions are 1.79 THz, 2.30 THz, 2.90 THz and 3.54 THz, respectively.

[0006] Preferably, the single-period sequence of the four sub-active regions are as follows:

[0007] The sub-active region with a central emission frequency of 1.79 THz has a single-cycle sequence of 4.5 / 8.9 / 3.8 / 17.3 nm.

[0008] The sub-active region with a central emission frequency of 2.30 THz has a single-cycle sequence of 4.5 / 8.7 / 3.8 / 17.5 nm.

[0009] The sub-active region with a central emission frequency of 2.90 THz has a single-cycle sequence of 4.5 / 8.5 / 3.8 / 17.7 nm.

[0010] The sub-active region with a central emission frequency of 3.54 THz has a single-cycle sequence of 4.5 / 8.3 / 3.8 / 17.9 nm.

[0011] Preferably, the number of periods corresponding to the four sub-active regions are 118, 79, 66 and 118, respectively.

[0012] Preferably, the barrier material of the two-well fast phonon single-period structure is AlGaAs, with an Al composition of 15%, and the thicknesses of the two barrier layers are 4.5 nm and 3.8 nm, respectively.

[0013] Preferably, a Si-doped region with a doping concentration of 3.00 × 10⁻⁶ nm is disposed in the central 5.00 nm region of the large well of each sub-active region. 16 cm -3 The doping type is N-type.

[0014] The present invention also provides an epitaxial structure comprising the heterogeneous active region of the broadband terahertz quantum cascade laser, wherein the epitaxial structure comprises, from bottom to top, a substrate, a buffer layer, an etch barrier layer, a lower electrode contact layer, the heterogeneous active region, an upper electrode contact layer, and a low-temperature growth protective layer.

[0015] The present invention also provides a terahertz quantum cascade laser, including the aforementioned epitaxial structure, and fabricating a metal-metal waveguide structure, electrodes, and a chip structure on the basis of the epitaxial structure.

[0016] In this invention, the gain spectrum of each sub-active region is calculated based on a rate equation model. First, an electric field scan is performed on each sub-active region to establish the correspondence between current density and electric field. Then, under the same target current density condition, the operating electric field corresponding to each sub-active region is determined, and the gain spectrum is calculated at the corresponding operating point. Finally, the gain spectra are weighted and superimposed according to the period ratio of each sub-active region to obtain the total gain spectrum output. The target current density is 0.2 kA / cm². 2 Under these conditions, the calculated full width at half maximum (FWHM) of the total gain spectrum is 2.04 THz.

[0017] Beneficial effects

[0018] This invention employs four sub-active regions with different center emission frequencies to form a heterogeneous active region. Taking advantage of the small single-cycle thickness of the two-well active region, more cascaded cycles are integrated under the condition of finite total active region thickness, thereby obtaining a total gain spectrum output wider than that of a single active region. The invention has a clear structural design, which facilitates the realization of broadband terahertz laser output and can be applied to multi-frequency terahertz radiation sources and terahertz frequency comb devices. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the THz QCL epitaxial structure containing the heterogeneous active region of the broadband terahertz quantum cascade laser of the present invention; wherein, 1 is the substrate, 2 is the buffer layer, 3 is the etching barrier layer, 4 is the lower electrode contact layer, 5 is the heterogeneous active region, 51 is the active region A, 52 is the active region B, 53 is the active region C, 54 is the active region D, 6 is the upper electrode contact layer, and 7 is the low-temperature growth protective layer.

[0020] Figure 2 The diagram shows the single-cycle structure and corresponding band structure of the four sub-active regions in this invention; where (a) is the sub-active region with a center emission frequency of 3.54 THz, (b) is the sub-active region with a center emission frequency of 2.90 THz, (c) is the sub-active region with a center emission frequency of 2.30 THz, and (d) is the sub-active region with a center emission frequency of 1.79 THz.

[0021] Figure 3 This is a schematic diagram showing the selection of the working electric field of four sub-active regions under the same target current density condition in this invention.

[0022] Figure 4 This is the gain contribution spectrum of the four sub-active regions in this invention after being weighted by the percentage of the number of periods, and the broadband total gain spectrum formed by their superposition. Detailed Implementation

[0023] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0024] Example 1

[0025] This embodiment provides a heterogeneous active region for a broadband terahertz quantum cascade laser, and the technical solution adopted is as follows:

[0026] (1) such as Figure 1As shown, the THz QCL epitaxial wafer containing a broadband terahertz quantum cascade laser heterogeneous active region comprises, from bottom to top, a substrate 1, a buffer layer 2, an etching barrier layer 3, a lower electrode contact layer 4, a broadband heterogeneous active region 5, an upper electrode contact layer 6, and a low-temperature growth protective layer 7. The broadband heterogeneous active region 5 is disposed between the lower electrode contact layer 4 and the upper electrode contact layer 6, and is composed of four sub-active regions with different center emission frequencies sequentially along the epitaxial growth direction: active region A 51, active region B 52, active region C 53, and active region D 54.

[0027] Furthermore, the broadband THz QCL heterogeneous active region of the present invention can be grown sequentially on a semi-insulating GaAs substrate 1 using molecular beam epitaxy (MBE) technology, consisting of a buffer layer 2, an etch barrier layer 3, a lower electrode contact layer 4, a broadband heterogeneous active region 5, an upper electrode contact layer 6, and a low-temperature growth protective layer 7. After epitaxial growth is completed, conventional semiconductor device processes can be used to fabricate metal-metal waveguide structures, fabricate electrodes, perform chip cleaving, packaging, and testing, thereby obtaining a broadband THz QCL device.

[0028] In this embodiment, the epitaxial structure of the broadband THz QCL and the parameters of each sub-active region are shown in Table 1.

[0029] The substrate 1 is a semi-insulating GaAs substrate with a thickness of 625±25 μm; the buffer layer 2 is GaAs with a thickness of 200 nm; the etching barrier layer 3 is AlGaAs with an Al composition of 55% and a thickness of 200 nm; the lower electrode contact layer 4 is GaAs with a thickness of 400 nm and a Si doping concentration of 5.00×10⁻⁶. 18 cm -3 The doping type is N + The upper electrode contact layer 6 is made of GaAs with a thickness of 50 nm and a Si doping concentration of 5.00 × 10⁶. 18 cm -3 The doping type is N + The low-temperature growth protective layer 7 is LTG GaAs with a thickness of 3.5 nm.

[0030] The broadband heterogeneous active region 5 is composed of four sub-active regions with different center emission frequencies, namely active region A 51, active region B 52, active region C 53, and active region D 54, with center emission frequencies of 3.54 THz, 2.90 THz, 2.30 THz, and 1.79 THz, respectively. Each sub-active region adopts a two-well fast phonon single-period structure, with a 5.00 nm region in the center of the large well being a Si-doped region with a doping concentration of 3.00 × 10⁻⁶. 16 cm -3 .

[0031] The active region A 51 has a center emission frequency of 3.54 THz, a single-period sequence of 4.5 / 8.3 / 3.8 / 17.9 nm, a corresponding period number of 118, and an alignment electric field of 14.05 kV / cm; the active region B 52 has a center emission frequency of 2.90 THz, a single-period sequence of 4.5 / 8.5 / 3.8 / 17.7 nm, a corresponding period number of 66, and an alignment electric field of 13.38 kV / cm; the active region C 53 has a center emission frequency of 2.30 THz, a single-period sequence of 4.5 / 8.7 / 3.8 / 17.5 nm, a corresponding period number of 79, and an alignment electric field of 12.74 kV / cm; and the active region D 54 has a center emission frequency of 1.79 THz, a single-period sequence of 4.5 / 8.9 / 3.8 / 17.3 nm, a corresponding period number of 118, and an alignment electric field of 12.19 kV / cm. kV / cm. This invention achieves the construction of four different central emission frequency sub-active regions by keeping the barrier layer thickness constant and adjusting the thickness of the two GaAs quantum well layers.

[0032] Table 1. Broadband THz QCL epitaxial structure and parameters of each sub-active region in this embodiment.

[0033]

[0034] like Figure 2 As shown, Figure 2 The energy band structure and associated wave function distribution of each sub-active region under the corresponding alignment electric field are shown to illustrate the design basis of the target radiation transition and the center emission frequency of each sub-active region. For each sub-active region, the transition between the upper laser level 3 and the lower laser level 2 in the nth module is taken as the target radiation transition. In this embodiment, the alignment electric field is defined as the electric field corresponding to the alignment between the injection state 1′ of the (n-1)th module and the upper laser level 3 of the nth module.

[0035] In this embodiment, the electrical transport and gain spectra of each sub-active region are calculated using the rate equation method at a lattice temperature of 10 K. Since the sub-active regions in the heterogeneous active region do not operate independently under the same applied electric field in actual devices, but rather operate together under the same injection current, this embodiment uses the same target current density to determine the operating electric field corresponding to each sub-active region, and obtains the overall gain spectrum accordingly. Specifically, the steps include:

[0036] S1. Perform electric field scanning on the four types of active regions respectively to obtain their respective current density-electric field relationship curves;

[0037] S2. Under the same target current density condition, determine the operating electric field corresponding to the four sub-active regions respectively; among them, such as Figure 3 As shown, at a target current density of 0.2 kA / cm2 Under these conditions, the four types of active regions correspond to different operating electric fields;

[0038] S3. Calculate the gain spectrum of each active region under the corresponding working electric field;

[0039] In this embodiment, the gain spectrum of each sub-active region under the corresponding working electric field can be calculated by the rate equation model, and its small-signal material gain can be expressed as:

[0040] ;

[0041] in, For elementary charge, The speed of light in a vacuum. The vacuum permittivity, For the refractive index of the material, The single-cycle length of the active region. Photon energy, and The first The and the first Individuals with energy levels Energy for inter-band transitions. For the dipole matrix elements corresponding to the transition, and These are the steady-state electron surface densities obtained from the rate equation. To use jump energy Centered on, with the following expansion parameters: The linear function. In this formula, the gain is determined not only by the transition energy and the dipole matrix element, but also by the steady-state electron population calculated from the rate equation. The determination is therefore able to reflect the effects of carrier injection, scattering relaxation, and population inversion on the gain spectrum.

[0042] For the heterogeneous active region described in this invention, since each sub-active region operates under the same injection current density in the actual device, rather than operating independently under the same applied electric field, the current density-electric field relationship of each sub-active region is first calculated separately, and then calculated under the same target current density. The working electric field corresponding to each sub-active region is determined below. Calculate the first under this working electric field. Gain spectrum of the active region Then, the total gain spectrum of the heterogeneous active region is obtained by weighting and superimposing the values ​​according to the proportion of the period number of each sub-active region:

[0043]

[0044] in, For the first The number of cycles in each sub-active region. In this embodiment, the number of cycles in the four sub-active regions are 118, 66, 79, and 118, respectively. The total gain spectrum obtained by the above method can simultaneously reflect the actual operating electric field, steady-state electron population, and cycle number contribution of each sub-active region, which is more consistent with the actual physical process of heterogeneous THz QCLs operating under the same injection current conditions.

[0045] S4. Based on the obtained gain spectra of each sub-active region, evaluate the total gain coverage of the heterogeneous active region.

[0046] like Figure 4 As shown, under the metal-metal waveguide condition, the contribution of each sub-active region to the total optical gain can be approximately regarded as being proportional to its repetition period. Therefore, the gain spectra of each sub-active region under the same target current density condition are weighted and superimposed according to the period ratio to obtain a broadband total gain spectrum. This period ratio takes into account both broadband coverage and spectrum continuity.

[0047] Figure 4 In the diagram, the dashed lines represent the weighted gain contribution spectra of each sub-active region, and the solid lines represent the total gain spectrum obtained by superposition. Figure 4 As can be seen, under the conditions of this embodiment, the obtained total gain spectrum has a full width at half maximum (FWHM) of 2.04 THz, corresponding to a FWHM frequency range of 2.44-4.48 THz. This indicates that the heterogeneous active region structure proposed in this invention can achieve broadband terahertz gain output. It should be noted that the center transmission frequency corresponds to the transition position under the aligning electric field, while the total gain spectrum is obtained by weighted superposition of the gain spectra of each transition under the operating electric field. Therefore, its FWHM range may be offset relative to the center transmission frequency range.

[0048] Without changing the basic idea of ​​this invention, the number of periods, doping concentration, width of doped region and composition order of each sub-active region can be adjusted to improve the gain contribution and spectral continuity of different frequency bands.

[0049] The features of this invention are as follows:

[0050] 1. This invention is aimed at low-frequency broadband THz output, and specifically proposes a heterogeneous active region composed of four types of two-well fast phonon sub-active regions with different center frequencies, which is a further concretization design of a specific low-frequency broadband active region structure.

[0051] 2. This invention covers a lower frequency terahertz band and has a wider gain bandwidth:

[0052] Four sub-active regions—1.79 THz, 2.30 THz, 2.90 THz, and 3.54 THz—are employed to target the lower-frequency THz band; with a target current density of 0.2 kA / cm². 2Under these conditions, the calculated full width at half maximum (FWHM) of the total gain spectrum is 2.04 THz, corresponding to a FWHM frequency range of 2.44–4.48 THz. Therefore, this invention has a more significant advantage in low-frequency THz broadband gain coverage.

[0053] 3. This invention employs a two-well fast phonon structure, suitable for multi-band heterogeneous broadband integration:

[0054] This invention employs a GaAs / Al0.15Ga0.85As two-well fast phonon structure with only 4 layers per cycle, resulting in a smaller single-cycle thickness, simpler structure, high lower-level extraction efficiency, and low threshold gain requirements. Under the constraint of limited total thickness in the active region, more cascaded cycles can be integrated (up to 118 cycles in a single sub-region), ensuring sufficient gain contribution across all frequency bands. This makes it more suitable for low-frequency multi-band heterogeneous broadband designs and also more advantageous for high-temperature operation.

[0055] 4. This invention obtains four center frequencies by adjusting the layer thickness and controls the gain contribution of each frequency band by allocating the number of cycles:

[0056] This invention explicitly fixes the thicknesses of the two AlGaAs barrier layers (4.5 nm and 3.8 nm), and precisely controls the four central emission frequencies simply by adjusting the thicknesses of the two GaAs quantum wells. The design method is clear and controllable. This invention sets the number of periods in the four sub-active regions to 118, 79, 66, and 118. Through differentiated period number allocation, the weight of each frequency band in the total gain spectrum can be flexibly adjusted, improving spectral width and continuity.

[0057] 5. This invention introduces electronic transport and matching of the same current operating point:

[0058] This invention employs a rate equation model, comprehensively considering transport factors such as carrier injection, scattering relaxation, and energy level population. More importantly, this invention explicitly proposes that the active regions of a heterogeneous THz QCL actually operate in series under the same injection current condition. Therefore, the operating electric field of each active region is first determined under the same target current density, and then the corresponding gain spectrum is calculated and weighted according to the period ratio. This method better reflects the actual operating mechanism of a heterogeneous THz QCL and more reasonably reflects the true operating point of each active region and its contribution to the overall gain spectrum.

[0059] The above description is merely a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. For those skilled in the art, any equivalent substitutions, simple modifications, or alterations made without departing from the technical concept of the present invention should be included within the scope of protection of the present invention.

Claims

1. A heterogeneous active region for a broadband terahertz quantum cascade laser, characterized in that, The heterogeneous active region is disposed between the lower electrode contact layer and the upper electrode contact layer of the epitaxial structure of the terahertz quantum cascade laser; the heterogeneous active region is composed of four sub-active regions with different central emission frequencies stacked sequentially along the epitaxial growth direction; the four sub-active regions all adopt a GaAs / AlGaAs two-well fast phonon single-period structure, and each sub-active region forms a different central emission frequency by adjusting the thickness of the two GaAs quantum well layers.

2. The heterogeneous active region of a broadband terahertz quantum cascade laser according to claim 1, characterized in that, The center emission frequencies of the four sub-active regions are 1.79 THz, 2.30 THz, 2.90 THz and 3.54 THz, respectively.

3. The heterogeneous active region of a broadband terahertz quantum cascade laser according to claim 2, characterized in that, The single-period sequence of the four sub-active regions are as follows: The sub-active region with a central emission frequency of 1.79 THz has a single-cycle sequence of 4.5 / 8.9 / 3.8 / 17.3 nm. The sub-active region with a central emission frequency of 2.30 THz has a single-cycle sequence of 4.5 / 8.7 / 3.8 / 17.5 nm. The sub-active region with a central emission frequency of 2.90 THz has a single-cycle sequence of 4.5 / 8.5 / 3.8 / 17.7 nm. The sub-active region with a central emission frequency of 3.54 THz has a single-cycle sequence of 4.5 / 8.3 / 3.8 / 17.9 nm.

4. The heterogeneous active region of a broadband terahertz quantum cascade laser according to claim 1, characterized in that, The four types of sub-active regions correspond to period numbers of 118, 79, 66 and 118, respectively.

5. The heterogeneous active region of a broadband terahertz quantum cascade laser according to claim 1, characterized in that, The barrier material of the two-well fast phonon single-period structure is AlGaAs, with an Al composition of 15%, and the thicknesses of the two barrier layers are 4.5 nm and 3.8 nm, respectively.

6. The heterogeneous active region of a broadband terahertz quantum cascade laser according to claim 1, characterized in that, A Si-doped region with a doping concentration of 3.00 × 10⁻⁶ nm is set in the central 5.00 nm region of the large well of each sub-active region. 16 cm -3 The doping type is N-type.

7. An epitaxial structure comprising a heterogeneous active region of a broadband terahertz quantum cascade laser as described in any one of claims 1 to 6, characterized in that, The epitaxial structure, from bottom to top, includes a substrate, a buffer layer, an etch barrier layer, a lower electrode contact layer, the heterogeneous active region, an upper electrode contact layer, and a low-temperature growth protective layer.

8. A terahertz quantum cascade laser, characterized in that, It includes the epitaxial structure as described in claim 7, and on the basis of the epitaxial structure, a metal-metal waveguide structure, an electrode and a chip structure are formed.

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