Development of Room Temperature Terahertz Quantum Cascade Lasers

这项拟议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(THzQCL)。这一发展将对太赫兹频率的科学和技术产生重大影响，在化学和生物制剂的检测、医疗和安全应用的成像、天体物理、等离子体诊断、遥感大气传感和监测以及高带宽自由空间通信方面具有潜在的应用前景。技术：自2012年以来，最高工作温度Tmax为200K，直到2019年。在此期间，PI的研究小组研究了阻碍高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准分子激光器输出功率热退化过程中的激活能值。使用这种方法，他们发现了之前被忽视的隧道屏障上的热泄漏通道。基于这一发现，使用更高的势垒来抑制这种泄漏通道。基于较高的势垒，首次在THzQCL器件中观察到了室温下的负差分电阻。为了提高高温下的上态寿命，采用对角跃迁结构已在该领域得到公认。在这些结构中，高能级和低能级波函数的空间分离减少了两者之间的电子散射。PI的小组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，充电效应在低掺杂水平下可以忽略不计，但在高载流子浓度时会变得严重，并对器件性能产生负面影响。为了减轻电荷效应，Pi的小组研究了一种新的直接声子方案来减少较低的激光能级的布居。基于高势垒和直接声子方案这两个特点，Tmax=200K的长期纪录终于在今年被打破，首先是一个欧洲团体达到210K，然后是PI的团队达到250K。该项目将利用这些最近的突破，并涉及相当大的设计工作，因为量子井的数量将很大，它们的组合将是复杂的。如果孤立的量子阱可以看作是一维的“人造原子”，那么多量子阱(MQW)结构就是一个“人造分子”。这个项目完全是设计和制造这样的人造分子，以实现THz激光器的预期功能。拟议活动产生更广泛的影响：在最近取得突破之后，首席调查员应邀在许多著名会议上作特邀/全体会议/主旨演讲，广泛社区的媒体也报道了这项工作。通过合作，PI小组开发的太赫兹激光通过增加一个关键的启动部分，帮助加强了与太赫兹相关活动的其他机构的基础设施。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NSF的法定使命，并通过使用基金会的智力优势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.