太赫兹量子级联激光器与量子阱探测器研究进展（特邀）创刊五十周年特邀
Generally, the terahertz (THz) radiation spectrum is defined as the electromagnetic spectrum between 100 GHz to 30 THz (wavelength of 3 mm‒10 μm). THz waves have a wide bandwidth and low photon energy, their energy spectra cover the vibrational-rotational characteristics of numerous molecules, and THz waves can penetrate many non-polar media. These properties make them attractive for astronomical observations, public safety, biomedicine, and wireless communication. According to the different generation and detection methods of THz waves, THz technology can be divided into two categories: one is based on electronic technology-mainly related to microwaves in the low-frequency THz band, and the other is based on optical and photonics technology-mainly related to infrared light at higher frequencies. Further technical developments will bridge the gap between electronics and optics, enabling new THz spectroscopy and imaging methods for scientific exploration in physics, chemistry, biology, materials, devices, engineering, and other interdisciplinary fields.
Terahertz quantum cascade lasers (THz QCLs) and terahertz quantum-well photodetectors (THz QWPs) are semiconductor devices based on electron transitions within subbands. These devices are advantageous due to their small size, adjustable frequency, and fast response time. Over the past few years, their performance has improved and many techniques related to high-resolution spectroscopy, terahertz imaging, and wireless broadband communication have received considerable attention. Here, we review recent advances and discuss future research directions.
THz QCLs are the only practical and compact laser systems at this frequency and have significant impacts on the THz field. The first THz QCL was developed in 2002. After more than 20 years of development, device performance has made significant progress, including the emission frequency range from 1.2 THz to 5.6 THz, the maximum output power of 2.4 W, a single mode continuously tuning range of 650 GHz, a broad bandwidth of 2.6 THz, the maximum operating temperature of 261 K, and a great improvement in far-field beam quality (Figs. 3 and 4). The development of phase-locking technology has significantly improved the frequency stability of lasers, with the linewidth reaching a quantum limit of 100 Hz. This technology has been employed in high-resolution spectroscopy (Figs. 8 and 9). THz QCLs exhibit strong optical nonlinearity and short inter-subband transition lifetime, resulting in various applications. Room-temperature THz laser radiation is demonstrated through intra-cavity difference frequency generation in mid infrared lasers. Moreover, scientists have achieved active mode-locking, optical frequency combs, high-order wave mixing, inter-subband plasmons, and fast modulation and detection. These technologies have also been successfully used in metrology and THz imaging (Figs. 10 and 11).
THz QWPs are an extension of quantum-well infrared detectors to the THz band, with advantages such as fast response speed and small size. In 2004, the first THz QWP was successfully developed with a central detection frequency of 7.1 THz. Subsequently, by adjusting the energy of inter-subband transitions, the spectral response of THz QWP gradually covered the range of 1.5‒7.5 THz and 8.8‒15.0 THz. However, it is well documented that THz QWPs require low-temperature conditions, usually below 10 K, because the thermally activated current can rapidly exceed the photocurrent at high temperatures. Recently, the combination of device structure and metamaterials has effectively increased the operating temperature of the device to 60 K (Fig.7). In terms of applications, high-speed imaging and information transmission are demonstrated in the THz QCL and THz QWP systems (Fig.12), and THz QWPs are also used in broadband spectral measurements and nonlinear two-photon detection.
THz QCLs and THz QWPs have a significant impact on the THz field; however, their large-scale applications have not been realized. Currently, improving the high-temperature performance of devices is one of the main challenges because thermally excited carriers can obtain sufficient kinetic energy for transport between subbands at high temperatures. THz QCLs demolish the population inversion, whereas THz QWPs lead to an increase in the dark current. Additionally, a further increase in the frequency range poses challenges. Expanding to the low-frequency side becomes increasingly difficult owing to further reduction in photon energy, whereas expanding to the high-frequency side is limited by the reststrahlen band. The investigation of new active region designs, low-loss waveguide materials, metasurface structures, and photonic crystals is expected to improve the device performance. Moreover, wide-bandgap materials such as GeSi and quantum dots can be chosen for high-temperature devices. Additionally, high-power and low-noise optical frequency combs and ultrashort THz pulse technology have garnered considerable attention. Notably, understanding the phase relationship between the comb teeth and the mechanisms of pulsed operation can be helpful. Saturation absorbers based on inter-subband polaritons and harmonic combs have also been presented. THz QCLs with topological structures have opened up new directions. Topological structures result in unidirectional propagation of light, immunity to defects, and high-order light field regulation. Topological protection can facilitate the development of robust laser arrays, and topological chirality can modify far-field beam patterns and polarization for novel photonic devices.
The application and commercialization of THz QCLs and THz QWPs will become a driving force for the development of new methods for high-resolution spectroscopy, hyperspectral imaging, and terahertz communication. THz QCLs and THz QWPs combined with near-field microscopy can be employed for nanometer scale detection and used in fields such as material science and biomedicine.
1 引言
太赫兹（THz）辐射波长（3 mm~10 μm）介于微波和红外波长之间，THz辐射通常定义为频率在100 GHz~30 THz区间的电磁波。THz波的带宽宽、光子能量低，其能量谱线涵盖了众多分子的振动-转动特征，并且THz波可以穿透很多非极性介质，这些特性使THz波在光谱、成像、通信等领域中具有广阔的应用前景［1-3］。特别是近年来THz科学与技术的迅速发展，使得THz波在许多有价值的领域中显示出实用性，例如机场安全检查、天文探测、大气成分监测、药品及航天涂层检验、半导体集成电路故障诊断等［4-8］。当前，无线通信技术正在从5G时代进入6G时代，为了满足日益增长的带宽需求，通信频段也正在向高频方向拓展。随着无线通信技术的发展，电信技术也有望从微波频段过渡到太赫兹频段［9］。可以预见，THz技术的实用化和商业化，将是未来推动THz领域发展的核心动力之一。按照THz波的产生和检测方法的不同，THz技术可以分为两类：一类基于电子学技术，主要与THz波段较低频率端的微波有关；另一类基于光学和光子学技术，主要与较高频率端的红外光相关。太赫兹波段位于电子学和光学领域的交汇区域，其所涉及的物理、化学、生物、材料、器件、工程及交叉学科等众多领域，也将在光电融合方面迎来全新的科学发现和技术进步。
太赫兹量子级联激光器（THz QCL）是一种半导体激光器，基于量子结构中的子带间的电子跃迁，属于光子型器件，是目前该频率下唯一实用且紧凑的激光系统，对THz领域的影响巨大。第一个THz QCL于2002年研制成功［10］，经过二十多年的发展，器件性能获得了巨大的提高，其频率已经覆盖了从1.2 THz［11］到5.6 THz的频率范围［12］，最高输出功率达到2.4 W［13］，单模连续可调的范围达到650 GHz［14］，单管辐射带宽达到2.6 THz［15］，最高工作温度达到261 K［16］，远场光斑特性也得到显著改善［17］。在应用方面，锁相技术的发展使得激光的频率稳定性也大大提高［18-23］，可观测的半峰全宽已经达到量子极限，为100 Hz［24］，该技术已经应用于高分辨光谱的测试［25］。子带间跃迁寿命短、光学非线性效应强，利用电子的超快输运特性，可实现THz QCL的主动锁模［26］、快速调制［27］以及超快检测［28］；利用非线性效应，通过四波混频可实现THz光频梳［29］；通过中红外激光器腔内差频，可实现室温THz激光辐射［30］以及高阶波混合［31］、子带间等离子体激元等［32］。这些技术也已成功应用于计量学［33］与THz成像［34］等。
太赫兹量子阱探测器（THz QWP）也是基于子带间跃迁的半导体光电探测器，是量子阱红外探测器向THz波段的延伸，具有响应速度快、易集成等优点。2004年，第一个THz QWP研制成功，中心探测频率为7.1 THz［35］。随后，通过调节子带间跃迁的能量，THz QWP的光谱响应逐渐覆盖了1.5~7.5 THz 和8.8~15.0 THz范围［36］。THz QWP需要工作在低温（通常10 K以下）条件下，随着温度的升高，热激发形成的电流迅速超越光电流，造成器件性能下降或失效，如何抑制热激发产生的电流和如何增加光电流是THz QWP设计过程中时刻面临的问题。最近，研究者通过将器件结构与超材料相结合，有效地提高了器件的工作温度，达到60 K［37］。在应用方面，研究者将THz QWP与THz QCL相结合，进行了高速成像和信息传输的演示［38］，THz QWP也实现了高速调制［39］和非线性双光子探测［40］。
本文聚焦THz QCL和THz QWP的原理和最新进展，主要从以下三个方面进行介绍：1）THz QCL的原理、有源区结构与波导结构以及器件性能的进展；2）THz QWP的原理、器件结构以及器件性能的进展；3）与THz QCL和THz QWP相关的应用的进展。最后讨论了THz QCL和THz QWP研究面临的问题并展望了其未来发展前景。
2 太赫兹量子级联激光器
THz QCL是基于子带间跃迁的半导体激光器，与传统的半导体激光器相比，主要有以下三个特点：一是子带的结构与量子限制的尺寸相关，可以通过改变量子阱的尺寸来调节子带间的能量差，从而突破半导体材料的禁带宽度限制，在难以到达的THz频率范围内实现激光辐射。频率调节的灵活性也可以使THz QCL覆盖更宽的频谱范围。二是级联结构，即THz QCL的有源区是由多个周期结构相互连接构成的，一个电子可以在多个周期内跃迁产生多个光子，从而实现超高的量子效率和大功率输出。三是子带的色散关系相近，避免了带间激光器由于色散不同而出现线宽展宽，THz QCL也因此具有更窄的线宽。THz QCL的有源区和波导是决定器件性能的基本要素，本小节接下来将主要介绍THz QCL的有源区和波导结构以及器件性能的研究进展。
2.1 有源区结构
THz QCL的有源区为器件提供增益，实现光放大。依据电子注入和抽运的特点，有源区结构主要分为四种类型。在第一种类型中，电子注入和抽运主要依靠共振隧穿或者微带中的电子-电子散射等过程，其特点是电子输运控制精确，寄生电流小。如
图 1. 有源区能带结构图。（a）束缚态向连续态跃迁结构［41］；（b）共振声子结构［16］；（c）复合结构［42］；（d）抽运控制型结构［43］
Fig. 1. Band structures of active regions. (a) Bound-to-continuum structure[41]; (b) resonant phonon structure[16]; (c) hybrid structure[42]; (d) extraction-controlled structure[43]
THz QCL结构复杂，电子输运涉及多个子带间的多种散射机制［45-52］，如电子-电子散射、电子-声子散射以及电子-杂质散射等，同时THz QCL有源区的势垒较厚，退相干效应也影响电子的输运过程［53］，因此，THz QCL的有源区设计需要有效的电子输运模拟。目前，研究THz QCL载流子的方法主要有以下四种：第一种是率方程方法［54］，需要代入子带间的散射几率，最为简便；第二种为蒙特卡洛方法［55］，可以实时计算子带间的各种散射机制下的散射率，模拟精度更高；第三种是密度矩阵方法［56］，通过斜对角项，可以获得退相干效应的影响；第四种是格林函数方法［57］，基于量子理论，最为复杂也最为精确。尽管上述方法都能在一定程度上模拟THz QCL中的电子输运，但依然需要发展更为准确的算法和模型，特别是对电流-电压曲线的模拟还有待完善。
2.2 波导结构
波导的作用是将光场限制在增益区域实现光放大。从红外到THz波段，光的波长变长，自由载流子吸收变强，红外激光器常用的重掺杂介质波导不再适用。新型的THz波导结构也是THz QCL成功研制的关键，目前THz波导有两种类型：半绝缘衬底-表面等离子体（SISP）波导和金属-金属（MM）波导［8，10，58］，如
图 2. 波导结构、光场模式分布和光束轮廓图［8］。（a）半绝缘衬底-表面等离子体波导；（b）金属-金属波导
Fig. 2. Waveguide structures, light field mode distributions, and beam profiles[8]. (a) Semi-insulating substrate-surface plasma waveguide; (b) metal-metal waveguide
这两种波导结构各有优缺点，可以根据不同的需求选取合适的结构。SISP波导的优点是工艺简单、激光的出射效率高、远场光斑的质量较好，缺点是光场限制因子低，因此采用该波导的器件，激光辐射功率大，但工作温度较低。MM波导的优点是限制因子高，缺点是工艺复杂、激光辐射效率低、远场光斑质量差，采用该结构的器件一般工作温度较高。另外，波导金属材料也会影响波导损耗。一般情况下，采用金作为波导金属，其优点是稳定性好、工艺兼容性好；铜和银作为波导金属也已被报道［59-61］，并且显示出更低的波导损耗。
2.3 器件性能进展
近年来，THz QCL的器件性能取得了显著的进步，接下来将根据子带间跃迁的特点，从频率特性、激光辐射功率以及工作温度等方面介绍THz QCL器件性能的重要进展。
THz QCL的频率可以通过改变量子阱尺寸进行灵活调节，其频率特性主要表现在激光频率、宽谱性能以及单模调谐范围等方面。在激光频率方面，尽管THz QCL的频率可以在很宽的范围内灵活调节，但由于电子输运受到子带间各种散射的影响，在低频端和高频端实现激光辐射依然困难。在起始阶段，THz QCL的激射频率大多在2~5 THz区间［11，62］。在低频端，Walther等［11］通过优化电子注入和抽运的选择性以及抑制激光能级间的散射，实现了1.2 THz的激光辐射；随后Wade等［63］通过外加磁场，进一步削弱了激光能级间的散射，将激射频率降至0.68 THz；近期，多个小组在高频端取得突破，在大于5 THz的多个频点处实现了激射，最高脉冲激射频率达到5.6 THz［12］，单模连续激射最高频率为5.1 THz［64］，如
图 3. 激光光谱图。（a）高频单模器件［64］；（b）宽谱器件［15］；（c）单模可调器件［14］
Fig. 3. Laser spectra. (a) High-frequency single-mode device[64]; (b) wide-spectrum device[15]; (c) tunable single-mode device[14]
激光辐射功率是衡量器件性能的最重要的指标之一。最初，THz QCL的功率处于百毫瓦水平，Brandstetter等［72］通过将对称结构的器件相键合，率先突破了瓦级的功率输出；Li等［13］利用大尺寸器件，将多模激光器的最高功率进一步提升至2.4 W；随后，多个小组也都报道了超过1 W功率的THz QCL［73］。单模激光器的功率长期低于多模激光器，2018年，Curwen等［74］在垂直腔面发射激光器中观察到单模输出，峰值功率达到1.35 W；2020年，Jin等［75］采用锁相阵列，如
图 4. 不同温度条件下的强度-电流-电压关系图。（a）单模大功率器件［75］；（b）高温器件［16］
Fig. 4. Intensity-current-voltage relationship under different temperature conditions. (a) High-power single-mode device[75]; (b) high-temperature device[16]
THz QCL的工作温度较低，这也是限制其商业化的重要原因之一。由于THz QCL的光子能量（10 meV）小，高温乃至室温器件的研制极具挑战。在最初的十多年间，器件只能工作在液氦或液氮制冷的环境下；2019年，Bosco等［76］将器件工作温度提高到210 K（-63 ℃），达到了热电制冷的条件；随后，Khalatpour等［77］又将器件的工作温度大幅提高至250 K（-23 ℃），激光器在230 K的热电制冷条件下，可提供数十毫瓦的峰值输出功率，这大大提高了THz QCL的实用性；2023年，如
3 太赫兹量子阱探测器
THz QWP是量子阱红外光电探测器（QWIP）向THz波段的扩展，也是一种基于子带间跃迁的低维半导体量子器件，利用光激发的电子从子带的基态跃迁至准连续态的过程实现了光探测，具有结构简单、响应快、性能稳定、易于集成等优点，特别适用于高速检测和高速成像［6］。
3.1 器件结构
THz QWP的探测波长可以通过改变量子结构的尺寸进行调节，与THz QCL相比，其有源区更为简单，为多量子阱结构，如
图 5. 量子阱探测器示意图。（a）能带结构图［36］；（b）表面光栅式器件结构图［79］；（c）斜角45°入射式器件结构图［79］
Fig. 5. Schematics of QWP. (a) Band structure[36]; (b) schematic of device with surface gratings[79]; (c) schematic of device with 45° incident facet[79]
THz QWP只能探测具有特定偏振方向的光，即光的偏振方向与量子阱的生长方向垂直，直接垂直入射的光并不能产生光电流。为了得到可探测的偏振分量，THz QWP一般采用两种结构，如
3.2 器件性能进展
在探测频率范围方面，2004年，第一个THz QWP实现了对7.1 THz波的探测［35］。2014年，Gu等［79］对比了45°斜边入射和垂直入射两种器件结构，得到的探测中心频率位于5.85 THz，同时由于非平衡的LO声子产生的局部增强效应，还在纵向光学声子频率（8.87 THz）处发现了一个尖锐的光电流峰。2015年，Franke等［80］展示了双光子THz QWP，并研究了该探测器的非线性性能，中心频率为5.5 THz。2017年，Wang等［81］设计并开发了一种双色THz QWP，中心频率为3.75 THz和5.55 THz，光谱半峰全宽达到1.5 THz；2018年又演示了一种宽带电压调节的THz QWP，峰值频率可以从4.5 THz调节到6.5 THz［82］。THz QWP在接近低频和剩余射线带区域也面临与THz QCL类似的困难。
在提高探测器响应度方面，2013年，Guo等［83］研究了表面等离子体增强光栅结构和金属微腔结构对探测器耦合效率的影响，理论结果表明，表面等离子体增强光栅结构可以将耦合效率提高30倍，金属微腔结构可以将耦合效率提高100倍。2014年，Zhang等［84-85］制作了具有不同光栅结构的THz QWP器件，并优化了光栅的尺寸，结果表明，表面光栅耦合垂直入射器件比斜入射器件有更高的峰值响应度，且二维光栅的效果优于一维光栅。如
图 6. 太赫兹量子阱探测器显微照片和峰值响应曲线［85］。（a）器件台面的照片；（b）表面二维光栅的照片；（c）峰值响应度与光栅尺寸的关系曲线
Fig. 6. Microscope pictures and peak responsivity curve of THz QWPs[85]. (a) Photo of device table; (b) picture of surface 2D grating; (c) peak responsivity versus grating size
在温度特性方面，近期也取得了重要的进展。Jeannin等［37］于2020年报道了一种基于三维超材料的太赫兹高温量子阱探测器，将探测器的工作温度提升至60 K。如
图 7. 高温太赫兹量子阱探测器［37］。（a）超材料单元结构示意图；（b）能带结构图；（c）超材料显微照片；（d）谐振器与耦合天线示意图；（e）不同温度条件下的光电流谱
Fig. 7. High-temperature THz QWP[37]. (a) Schematic of metamaterial unit cell; (b) band structure; (c) micrograph of metamaterial; (d) schematic of resonator and coupling antenna; (e) photocurrent spectra at different temperatures
在理论设计方面，Guo等［87］考虑了多体效应的影响，计算了三种不同的峰值频率，结果表明，多体效应对THz探测器峰值频率的影响远大于红外器件，该方法将峰值检测频率的设计误差从30%降低到5%，提高了THz QWP的设计精度。2015年，Jia等［88］优化了不同峰值频率下的掺杂浓度，计算结果表明，当引入功率密度为819 W/cm2的太赫兹光束时，器件的光子噪声限性能温度可以提高到77 K以上。
4 太赫兹子带间跃迁器件的应用进展
THz波作为电磁波的一部分，也同其他波段一样，在光谱、成像、信息传输等方面有广泛的应用。本小节将着重介绍与THz QCL和THz QWP相关的技术进展。
4.1 太赫兹高分辨光谱技术
THz QCL线宽窄、输出功率高，而且频率连续可调，这些特性使其在高分辨率光谱应用中占有优势。而气体分子或原子通常具有尖锐的吸收峰，更需要高分辨的光谱技术对其进行检测。这里将围绕气体分子光谱的检测，介绍THz高分辨光谱及锁相技术的研究进展。
目前，基于THz QCL的气体光谱技术研究仍然主要集中在光谱方法的开发上，主要包括直接吸收光谱、外差光谱和光声光谱等，如
图 8. 太赫兹高分辨光谱方法示意图和测试图［8］。（a）直接吸收光谱方法示意图；（b）差频探测方法示意图；（c）光声光谱方法示意图；（d）差频方法探测到的氧原子谱线（4.7 THz）
Fig. 8. Schematics of THz high-resolution spectroscopic methods and measured spectrum[8]. (a) Schematic of direct-absorption spectroscopic method; (b) schematic of difference frequency detection method; (c) schematic of photoacoustic spectroscopic method; (d) oxygen atomic line detected by difference frequency method (4.7 THz)
高分辨光谱的分辨率受到THz QCL频率稳定性的影响，一般可以达到1.6 MHz。为了进一步提高分辨率，多种锁相技术被用来提高THz QCL的频率稳定性。2005年，Betz等［18］将THz QCL与气体激光器锁定，使得THz QCL的半峰全宽减小至65 kHz；后来利用该方法又将半峰全宽进一步降低至4 kHz［19］。2009年，Rabanus等［20，97-98］利用微波倍频源作为参考信号锁定THz QCL，由于微波源频率稳定性高，该方法将THz QCL的稳定性提高到了100 Hz。2010年，以甲醇气体吸收线作为参考的锁相方法被用于THz QCL，锁定后的半峰全宽达到了51 kHz［21，99］。以光频梳作为参考信号进行锁相的方法也被应用于THz QCL，测得的拍频信号的半峰全宽可以达到1 Hz，但绝对的频率稳定性没有测量［22-23］。2014年，利用铯原子钟标准频率作为参考的光频梳被用于THz QCL的锁相和甲醇气体的测试，如
图 9. 基于频梳稳频的QCL的光谱方法示意图［25］
Fig. 9. Schematic of spectroscopic method with QCL stabilized to frequency comb[25]
4.2 太赫兹成像技术
THz QCL输出功率高，并且可以穿透非极性和非金属介质，再加上THz波段的特性，因此基于THz QCL的成像技术快速发展。基于THz QCL的成像技术按原理可分为非相干成像和相干成像，非相干成像只能获得强度信息，呈现二维图像，而相干成像还可获得相位信息，可以呈现三维图像。目前，非相干成像的研究较多，主要集中在分辨率、成像速度、成像距离等方面。
非相干成像的分辨率与THz QCL发光波长和光束质量有关，大多在亚毫米级别。例如，2004年，Darmo等［100］率先利用THz QCL对小鼠的脑部成像，分辨率为0.25 mm。2014年，Tan等［101］使用THz QCL作为光源，演示了金属图案和隐蔽金属叶片的反射成像，获得了0.33 mm的成像分辨率。在提高分辨率方面，2012年，de Cumis等［102］利用光阑改善了THz QCL的光束质量，如
图 10. 太赫兹成像图。（a）~（c）照片与THz成像的对比图［102，106，108］；（d）QWP-LED与Si CCD相机拍摄的THz QCL光斑像［109］
Fig. 10. Pictures by THz imaging. (a)‒(c) Comparison of photographs and THz images[102,106,108]; (d) THz QCL spot images taken by QWP-LED and Si CCD camera[109]
初始的THz成像大多采用单点扫描的方式，成像速度较慢，一般在1 pixel/s左右。2013年，Rothbart［105］提出扫描镜与快速Ge∶Ga探测器相结合的方法，实现了二维图像的快速扫描，成像速度达到了4140 pixel/s。2018年，Qiu等［106］构建了一种基于快速旋转平移结构的扫描成像系统，使用具有快速检测性能的THz QWP作为探测器，进行了快速扫描反射成像，如
在相干成像方面，2010年，Danylov等［111］采用气体激光作为本振参考信号，利用其与THz QCL发出的光的差频，实现了逆合成孔径雷达（ISAR）成像，如
图 11. 基于太赫兹量子级联激光器的逆合成孔径雷达成像［111］。（a）ISAR成像装置示意图；（b）比例模型坦克照片；（c）比例模型坦克的THz图像，像素分辨率为0.4 mm×0.6 mm
Fig. 11. THz QCL-based ISAR imaging[111]. (a) Schematic of ISAR imaging device; (b) photograph of scale model tank; (c) THz image of scale model tank with pixel resolution of 0.4 mm×0.6 mm
4.3 太赫兹通信技术
THz波段是未来宽带通信的拓展方向，因此THz通信技术得到了很多国家和研究机构的重视。2009年，Grant等［116］首次将THz QWP应用于3.8 THz无线传输系统，演示了实时音频信号的传输，实现了基于1 THz以上辐射的无线信号传输的突破。2011年，Chen等［117-118］采用4.1 THz连续波QCL作为发射器，采用频谱匹配的THz QWP作为接收器，实现了实时音频信号的无线传输。在距离为2 m的传输过程中使用幅度调制，器件电路限制的调制带宽约为580 kHz；2013年，通过进一步优化THz QCL的驱动电路和THz QWP的信号处理电路，实现了距离为2.4 m、传输速率为2.5 Mbit/s的3.9 THz无线链路，演示了实时视频信号的无线传输［38］，如
图 12. 基于太赫兹量子级联激光器和量子阱探测器的视频传输［38］。（a）信息传输装置示意图；（b）调制信号与传输后的信号；（c）视频传输结果
Fig. 12. Video transmission based on THz QCL and THz QWP[38]. (a) Schematic of information transmission device; (b) modulating signal and transmitted signal; (c) video transmission result
高速调制和解调是THz通信技术的关键环节。THz QCL和THz QWP的皮秒量级的载流子寿命，使得它们具有高速调制和解调的潜力。对于THz QCL而言，传统的电子调制方法在根本上受到了寄生阻抗的限制。2020年，Dunn等［119］报道了一种全新的声波调制方法，利用飞秒激光激发的声波脉冲扰动激光器的能带结构，从而实现了对激光辐射强度的高速调制，使调制时间降低至约800 ps，调制上升时间增加约800 ps，最大调制深度达到6%。对于THz QWP而言，2017年，Tan等［120］对调制THz光信号进行检测，最快响应频率为0.5 GHz。通过改进太赫兹QWP的封装，实现了对6.2 GHz调制光信号的直接检测［39］，器件的噪声等效功率（NEP）也达到了10-13 W·Hz-1/2。另外，Wang等［121-122］在2021年报道了利用THz QCL产生毫米波的新方法，为高频无线通信中的低噪声本地振荡器提供了可能的设计方案。
4.4 太赫兹光频梳技术
THz光频梳技术和超短脉冲生成可以应用于THz光谱、成像和通信领域。基于量子级联激光器的小型化太赫兹光频梳在2014年研制成功［29］，但其性能受到了色散的影响。最初的色散补偿采用啁啾波导结构，随后又尝试采用在激光器背面集成Gires-Tournois干涉仪和外腔调制等方法。2021年，di Gaspare等［123］以单层石墨烯光栅门控调制器作为无源色散补偿器，研制出具有98个等距光学模式的光频梳，将频梳光谱覆盖范围拓宽到太赫兹水平，达到1.2 THz。2019年，Lu等［124］报道了一种室温THz半导体光频梳的实现方法，该方法利用激光材料的非线性，使中红外激光与中红外光频梳在量子级联激光器中产生差频，并在2.2~3.3 THz区间产生5条间距为245 GHz的梳状线。
光频梳最重要的特征是光学模式之间有紧密的相位关系，对其进行研究需要精确的相位测量和调控方法。移波干涉傅里叶变换光谱（SWIFT）可用于测试光频梳梳齿之间的相位关系，但其实时性受到机械部件扫描速度的限制。Cappelli等［125］在2019年提出一项新技术，依靠多外差检测方案，能够实时跟踪光频梳发射模式的相位，并对光频梳的性能进行更准确的评估。同年，Consolino等［126］进一步实现了对光频梳模式间距和频率偏移的独立控制，使频梳的每个发射模式都表现出亚赫兹的相对频率稳定性和高度相干性。该技术为计量级用途奠定了基础。Li等［127］在2019年设计了基于多层石墨烯反射器的复合腔结构，如
图 13. 基于石墨烯反射器的锁模技术［127］。（a）石墨烯耦合THz QCL示意图；（b）探测信号强度随延迟时间的变化
Fig. 13. Mode locking technology based on graphene reflector[127]. (a) Schematic of graphene-coupled THz QCL; (b) probe signal intensity versus delay time
5 展望和总结
THz QCL和THz QWP对THz领域的影响巨大，但其大规模应用还有待实现。目前，提高器件温度特性是其面临的主要挑战之一，其原因是THz光子能量（10 meV）小，热激发的载流子可以获得足够的动能实现子带间的跃迁，这对于THz QCL来说，其粒子数反转变得困难，而对于THz QWP来说，则会出现暗电流的增加。另外，波长的进一步拓展也面临挑战，向低频段的拓展由于光子能量的进一步减小而愈加困难，而向高频段的拓展则受到剩余射线带的限制。新的有源区设计、低损耗波导材料、超表面结构和新型光子晶体的探索将为提高器件性能开辟新的途径。宽禁带半导体、GeSi、量子点等材料也可能成为高温器件选择［128］。高功率低噪声的光频梳和光频梳超短脉冲技术依然受到关注。研究梳齿间的相位关系及产生脉冲的新机制与新方法有助于解决高性能光频梳难以制备的难题，比如基于子带间极化激元的饱和吸收器以及谐波梳等。具有拓扑结构的THz QCL开创了新的方向［129］，利用拓扑光子技术可实现光的单向传播、对缺陷的免疫以及高维度的光场调控，实现传统光电子器件难以实现的性能［130-131］。如拓扑保护有助于研制稳健的激光器阵列和点发射激光源；利用拓扑结构的手征性进行波前调控，可实现对远场光斑和偏振状态的控制［132］，如涡旋光等；还可构建非厄米系统，用于新型拓扑光电子器件的研制和拓扑物理的研究。
国内的THz QCL和THz QWP的研究基本与国外同步，在器件性能和系统应用研究的很多方面都处于国际先进水平，如THz QCL的输出功率达到了瓦级，频率连续电调谐范围达到了80 GHz等。目前高性能器件的有源区设计多源自国外，自主设计的器件还有待进一步改进。特别是对于THz QCL来说，有源区结构是决定器件性能的重要因素，每次工作温度的提升基本都是采用了改进的有源区设计，但也不可否认除了有源区设计外，材料生长与器件工艺等也同样重要。
THz QCL和THz QWP的发展及其商用化的需求将进一步带动相关领域的研究，特别是高分辨光谱、高光谱成像以及太赫兹通信等。与近场显微镜相结合，还可将THz检测缩小至纳米尺度，并用于材料科学和生物医药等领域的基础研究中。
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Article Outline
曹俊诚, 韩英军. 太赫兹量子级联激光器与量子阱探测器研究进展（特邀）[J]. 中国激光, 2024, 51(1): 0114001. Juncheng Cao, Yingjun Han. Recent Progress in Terahertz Quantum Cascade Lasers and Quantum Well Detectors (Invited)[J]. Chinese Journal of Lasers, 2024, 51(1): 0114001.
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