  Room temperature operation of Terahertz Quantum Cascade Lasers (THz QCLs) has
been a long-pursued goal to realize compact semiconductor THz sources. The
progress toward high-temperature operation in THz QCLs has been relatively slow
compared to infrared QCLs owing to more significant challenges at THz
frequencies. Recently, the maximum operating temperature of THz QCLs was
improved to 250 K, and the achievement revitalized hope in the THz community in
pursuit of higher temperature operations. In this paper, we report on further
improvement in operating temperature to ~261 K (-12 0C) by judiciously
optimizing key parameters and discuss the challenges ahead in achieving room
temperature operation.
