Frequency tuning of THz quantum cascade lasers

This paper introduces the continuously tunable THz radiation through sideband generation of a free running and solid-nitrogen-cooled THz quantum cascade laser. The 2.324 THz QCL operating in a single longitudinal mode (SLM) in continuous-wave (cw) was mixed with a swept synthesized microwave signal by a THz Schottky-diode-balanced mixer. Through sideband generation, two frequency branches were observed at low and high frequency, characterized with a Fourier-transform spectrometer. At low frequency, the sideband generates frequencies from -50 GHz to +50 GHz. At high frequency, it generates sideband frequencies from 70 GHz to 115 GHz. The total +/- 100 GHz tuning range can be further expanded with higher frequency millimeter wave amplifier/multiplier source. The sideband generates total 1 mu W of output power at both upper and lower frequency with 200 mu W of driven power from the THz QCL, showing a power conversion efficiency of 5 x 10(-3). The demonstration of this SM, continuously tunable THz source enables its applications where SM, spatially coherent beam is required.