Operation of terahertz quantum cascade lasers above 160 K covering a frequency range of 2-4 THz

Abstract

The development of terahertz quantum cascade lasers (QCLs) has progressed significantly in the past ten years. Widely different types of QCLs have been demonstrated covering a frequency range from 1:2 THz to 5 THz (when operating without the existence of an external magnetic field). Improvement of operating temperatures of terahertz QCLs is one of the primary goals to make such devices viable for important terahertz applications. Some of the best techniques to obtain high operating temperatures have relied on electron-phonon scattering assisted depopulation. This paper reviews terahertz QCLs operating in a frequency range of 1:4 THz to 4:7 THz with such design schemes. Operation above a temperature of 160 K has been obtained across a broad range of frequencies from 1:8 THz - 4:3 THz. While the temperature degradation mechanisms are still not completely understood, it is speculated that collisional broadening of subbands may result in degradation of resonant tunneling transport at higher temperatures, which is critical to establishing population inversion in the QCL structure. The recently developed scattering-assisted injection techniques may mitigate subband broadening effects at higher temperatures, which is supported by experimental results. Further advances in the active region design as well as choice of different materials for growth and design of superlattices may result in even higher operating temperatures for terahertz QCLs.