Quantum cascade lasers-chasing room temperature

Research is continuing in the quest to demonstrate room-temperature terahertz quantum cascade lasers (THz QCL). The main problem is that the gain of a THz QCL depends on the product between population inversion and oscillator strength, which are difficult to optimize simultaneously. Saeed Fathololoumi and co-workers from the National Research Council of Canada, the University of Waterloo, the Massachusetts Institute of Technology, Technische Universität München and Shanghai Jiao Tong University have now used a density matrix approach to maximize the gain of a THz QCL and achieve lasing at a temperature of 199.5 K. The THz QCL was based on GaAs/Al0.15Ga0.85As and had a three-well structure. The active region thickness, ridge width and length of the device were 10 ?m, 144 ?m and 1 mm, respectively. The researchers reduced the waveguide loss within the QCL by fabricating a Cu–Cu double-metal waveguide without a top n+ contact layer. The device produced single-mode emission at a wavelength of 3.22 THz when electrically driven with a pulse duration of 300 ns at a repetition rate of 300 Hz. The researchers measured a threshold current of around 1.92 kA cm?2.