Development of Room Temperature Terahertz Quantum Cascade Lasers

提出的研究旨在开发在室温及以上工作的太赫兹量子级联激光器（THz qcl）。这种发展将对太赫兹频率的科学和技术产生重大影响，其潜在应用前景广阔，包括探测化学和生物制剂、医疗和安全应用成像、天体物理学、等离子体诊断、遥感大气和监测以及高带宽自由空间通信。技术：自2012年以来，最高工作温度Tmax为200 K，直到2019年。在此期间，PI的小组调查了阻碍更高Tmax发展的可能机制。首先，他们开发了一种新的方法来提取太赫兹量子激光器输出功率热降解过程中的活化能值。利用这种方法，他们确定了隧道屏障上方的热泄漏通道，这是以前被忽视的。基于这一发现，采用更高的屏障来抑制这种泄漏通道。基于较高的势垒，首次在室温下在太赫兹QCL器件中观察到负差分电阻（NDR）。为了提高在高温下的高态寿命，该领域已经认识到应该使用对角过渡结构。在这些结构中，上下能级波函数的空间分离减少了两者之间的电子散射。PI的团队首先意识到需要使用更高的载流子浓度来补偿降低的振荡器强度。在对掺杂对Tmax影响的系统研究中，PI的团队发现，在低掺杂水平下可以忽略不计的充电效应，在高载流子浓度下变得严重，并对器件性能产生负面影响。为了减轻充电效应，PI的小组研究了一种新的直接声子方案来减少低激光能级的数量。基于这两个特点，高屏障和直接声子方案，Tmax = 200k的长期记录终于在今年被打破，首先是由一个欧洲小组打破了210k，然后由PI的团队打破了250k。该项目将利用这些最新的突破，并涉及相当大的设计工作，因为量子阱的数量将会很大，它们的组合将会很复杂。如果孤立量子阱可以看作是一维的“人工原子”，那么多量子阱（MQW）结构就是一个“人工分子”。这个项目无非是设计和制造这样的人工分子来执行太赫兹激光器所期望的功能。所提议的活动产生的更广泛的影响：随着最近的突破，首席研究员已被邀请在许多著名会议上进行邀请/全体会议/主题演讲，并且工作也在广泛社区的媒体上进行了报道。通过合作，PI小组开发的太赫兹激光器通过增加一个关键的使能组件，帮助增强了其他机构在太赫兹相关活动中的基础设施。首席研究员计划将研究项目中的元素纳入本科课程《信号与系统》。如果可以开发出室温太赫兹qcl， PI计划与最近成立的一家基于该技术的初创公司合作，将紧凑型太赫兹成像系统商业化。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The proposed research seeks to develop terahertz quantum-cascade lasers (THz QCLs) that operate at and above room temperature. Such a development will have a significant impact on the science and technologies in THz frequencies, where potential applications are promising in detection of chemical and biological agents, imaging for medical and security applications, astrophysics, plasma diagnostics, remote atmospheric sensing and monitoring, and high-bandwidth free-space communications.Technical: Since 2012, the highest operating temperature Tmax was 200 K, until 2019. During this period the PI’s group investigated the possible mechanisms that hindered the development of higher Tmax. First, they developed a novel method to extract the value of activation energy in the thermal degradation of output power of THz QCLs. Using this method, they identified a thermal leakage channel over the tunnel barriers that were previously overlooked. Based on this finding, taller barriers were used to suppress this leakage channel. Based on the taller barriers, for the first time negative differential resistance (NDR) was observed at room temperature in a THz QCL device. In order to increase the upper-state lifetime at elevated temperatures, it has been recognized in the field that diagonal transition structures are to be used. In those structures, the spatial separation of the upper- and lower-level wavefunctions reduces electron scattering between the two. The PI’s group first realized that a higher carrier concentration needs to be used to compensate for the reduced oscillator strength. In a systematic investigation of the doping effect on Tmax, the PI’s group discovered that charging effect, which was negligible at low doping levels, became severe at high carrier concentrations and it negatively impacted the device performance. To mitigate the charging effect, the PI’s group investigated a new direct-phonon scheme for the depopulation of the lower lasing level. Based on these two features, tall barriers and direct-phonon scheme, the long-held record of Tmax = 200 K was finally broken this year, first to 210 K by a European group and then to 250 K by the PI’s team. The project will leverage those recent breakthroughs and involve a considerable design effort, as the number of quantum wells will be large and their combination will be complicated. If an isolated quantum well can be viewed as a one-dimensional "artificial atom", then a multiple quantum-well (MQW) structure is an "artificial molecule". This project is nothing short of designing and making such artificial molecules which perform the desired function of THz lasers. Broader impacts resulting from the proposed activity: Following the recent breakthroughs, the principal investigator has been invited to give invited/plenary/keynote talks at many prestigious conferences and the work has also been reported in media for broad communities. Through collaborations, the THz lasers developed in the PI's group have helped to enhance the infrastructures at other institutions in THz-related activities by adding a crucial enabling component. The principal investigator plans to incorporate elements in the research project into a undergraduate course Signals and Systems. If room-temperature THz QCLs can be developed, the PI plans to work with a recently founded start-up company based on this technology, to commercialize compact THz imaging systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.