Development of Room Temperature Terahertz Quantum Cascade Lasers

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

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.