太赫兹量子级联激光器的单片集成研究

Terahertz (THz) laser sources with ultra-narrow linewidth or broad frequency range are very demanding for high precision terahertz spectroscopy. In this project, based on the optimization of terahertz quantum cascade laser (THz-QCL) material design, growth and processing, we will focus on two research areas. Firstly, we will use the monolithic integration of single-mode THz-QCL and power-amplifier to develop “master-oscillator power-amplifier THz-QCL”. We will clarify the mechanism of THz-wave propagation, amplification and feedback in the device. The equivalent facet of the amplifier formed by the diffraction grating and absorbing boundary is used to realize ultra-low reflectivity in order to suppress self-lasing. The laser gain will be measured precisely and maximized by controlling the doping concentration. By these means, high-power, narrow-linewidth, tunable THz laser will be demonstrated. Secondly, we will use the monolithic integration of THz-QCL and the double-chirped mirror dispersion compensation element to demonstrate the frequency comb operation of THz-QCL with high power and broad frequency range. We will precisely measure the group velocity dispersion of the laser and understand the effects of its three main components: the dispersion of the material, the modal dispersion of the waveguide and the gain dispersion. By growing equal-frequency spaced multiple active-regions to achieve octave-spanning and flat gain spectrum. This will suppress the gain dispersion and increase the comb operation frequency range. By integrating the double-chirped mirror to compensate the overall dispersion for the broad-band THz-QCL, we will realize THz-QCL frequency comb with high power and broad frequency range. The spectral resolution of terahertz spectroscopy is likely to reach hundreds kHz scale by the achievement of this project, which will greatly promote the development of related spectral techniques.

本项目围绕太赫兹(THz)精密光谱对极窄线宽的窄带及宽谱THz激光的需求，在解决太赫兹量子级联激光器(THz-QCL)材料与器件共性关键技术的基础上，重点研究两方面内容:(1)单模激光器与放大器的单片集成，研制主控振荡—功率放大THz-QCL。阐明器件中THz波传播、放大与反馈的机制；利用衍射光栅和吸收边界构建放大器的等效腔面，实现极低的反射率以抑制其自激振荡；精确测量增益并调控掺杂浓度实现激光器增益最大化；获得大功率窄带、可调谐太赫兹激光器。(2)研究THz-QCL与双啁啾色散补偿构件的单片集成；精确测量激光器群速度色散并阐明材料、波导与增益对色散的影响；通过生长等频率间隔的多个有源区实现倍频程且平坦化的增益谱，抑制增益色散并提高光频梳的频谱范围；单片集成双啁啾反射镜，实现宽谱的色散补偿；研制出宽谱高功率THz光频梳。本项目有望将THz光谱分辨率提高到百kHz量级，推动相关光谱技术发展。

本项目围绕太赫兹(THz)精密光谱对极窄线宽的窄带及宽谱THz激光的需求，在解决太赫兹量子级联激光器(THz-QCL)材料外延生长与器件制备关键技术的基础上，取得了以下成果：（1）利用分布反馈（DFB）激光器、锥型放大器、光栅耦合器以及吸收边界的单片集成，实现了国际上首个太赫兹“主控振荡-功率放大”量子级联激光器（THz-MOPA-QCL），建立了定量模型指导其结构设计。研制出频率约2.6 THz的单模、高光束质量THz-MOPA-QCL，温度20 K时，峰值输出功率228 mW，边模抑制比23 dB，光束发散角约为6度*16度；首次提出并实现了一种激光频率和偏振态精确可控的单模THz-MOPA-QCL，其结构由DFB激光器、预放大区和二维天线阵列组成。激光频率由DFB激光器决定，二维天线阵列的周期和天线指向以及天线间的相位关系决定偏振态。所研制的不同THz-MOPA-QCL的线偏振度与圆偏振度分别达到97.5%与99.3%。（2）基于束缚态到连续态跃迁和共振声子抽取相结合的有源区设计思想，优化设计了多个中心频率的THz-QCL有源区结构，通过提高GaAs/AlGaAs材料体系中势垒层的Al组分和优化注入垒的厚度，有效降低了载流子泄漏和器件的阈值电流密度。研制出不同频率的低阈值、连续波工作THz-QCL。研制的频率3.8-4.0 THz的高功率器件，在温度15 K连续波工作时，输出功率达到256 mW；在国际上首次实现了频率超过5 THz QCL的连续波工作，在温度15 K时，阈值电流密度仅约为130 A/cm2，输出功率79 mW，最高工作频率5.26 THz；研制出中心频率分别为3.3 THz和4.6 THz的THz-QCL光频梳，频率覆盖范围528 GHz。（3）提出了啁啾采样光栅技术，提升了单腔面单模输出功率。研制出国际上首个频率超过5 THz、连续波工作的单模THz-QCL，在温度15 K时，单模频率5.13 THz，输出功率48 mW；提出了利用含增益的光子晶体构建反射带宽极窄的布拉格反射镜，并在激光器的另一端构建光栅耦合器，来独立控制激光器模式选择与输出功率。研制出频率约3.3 THz的单模THz-QCL，温度20 K时，峰值功率达到366 mW，性能达到国际先进水平。本项目有力推动了单模大功率THz-QCL、以及THz-QCL光频梳等方向的研究进展。