Development of Room Temperature Terahertz Quantum Cascade Lasers

拟议中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，在太赫兹频率的潜在应用中，化学和生物制剂的检测、医疗和安全应用的成像、天体物理学、等离子体诊断、远程大气传感和监测以及高带宽自由空间通信等方面都是有希望的。自2012年以来，最高工作温度Tmax为200 K，直到2019年。在此期间，PI小组研究了阻碍Tmax升高的可能机制。首先，他们开发了一种新的方法来提取THz QCL输出功率热降解中的活化能值。使用这种方法，他们发现了以前被忽视的隧道势垒上的热泄漏通道。基于这一发现，使用更高的屏障来抑制该泄漏通道。基于更高的势垒，首次在室温下在THz QCL器件中观察到负微分电阻（NDR）。为了增加高温下的上态寿命，在本领域中已经认识到要使用对角过渡结构。在这些结构中，上能级和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振荡器强度的降低。在对Tmax的掺杂效应进行系统研究时，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，并且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，用于减少较低的激光能级。基于这两个特征，高势垒和直接声子方案，Tmax = 200 K的长期记录终于在今年被打破，首先由欧洲小组打破到210 K，然后由PI的团队打破到250 K。该项目将利用这些最新的突破，并涉及相当大的设计工作，因为量子威尔斯的数量将是巨大的，他们的组合将是复杂的。如果孤立量子阱可以看作是一维的“人造原子”，那么多量子阱（MQW）结构就是“人造分子”。这个项目就是设计和制造这样的人造分子，它们可以实现太赫兹激光器的预期功能。拟议活动产生的更广泛影响：在最近的突破之后，主要研究员应邀在许多著名会议上发表特邀/全体/主旨演讲，媒体也为广大社区报道了这项工作。通过合作，PI小组开发的THz激光器通过增加一个关键的使能组件，帮助加强了其他机构在THz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。如果能够开发出室温THz QCL，PI计划与一家最近成立的基于该技术的初创公司合作，将紧凑型THz成像系统商业化。该奖项反映了NSF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.