The terahertz frequency range (1-10 THz) has long remained undeveloped, mainly due to the lack of compact, coherent radiation sources. Transitions between subbands in semiconductor quantum wells were suggested as a method to generate long wavelength radiation at customizable frequencies. However, because of difficulties in achieving population inversion between narrowly separated subbands and mode confinement at long wavelengths, THz lasers based on intersubband transitions were developed only very recently. Taking a completely novel approach, we have developed THz quantum-cascade lasers based on resonant-phonon-assisted depopulation and using metal-metal waveguides for mode confinement. The schematics of both features are illustrated in the top-left of Figure 1. Based on the combination of these two unique features, we have developed many THz QCLs with record performance, including a maximum pulsed operating temperature at 186 K (top-right), a maximum cw operating temperature at 117 K (bottom-right), and a maximum power level of ~250 mW (bottom-left).

Figure 1: Top left: Band diagram and subband wave functions of a THz quantum-cascade laser based on resonant-phonon design. Top right: Power-current (L-I) curves of a THz laser device with a maximum operating temperature of ~186 K. Bottom right: Cw power-current (L-I) curves of a THz laser device with a maximum operating temperature of ~117 K. Bottom left: Power-current (L-I) curves of a THz laser device with a maximum power level of ~250 mW.