Two-well terahertz quantum-cascade laser with direct intrawell-phonon depopulation

We report the simplest quantum-cascade laser (QCL) to-date with only two quantum wells per QCL period, and at low temperatures, only three subband levels participate in electron transport. The lower laser level is directly depopulated via intrawell longitudinal-optical phonon scattering, which is in contrast with all the previously demonstrated terahertz QCLs, and maintains an ultrashort lower level lifetime under all operating bias and temperatures. Optical gain is due to a diagonal photon-assisted tunneling transition. Laser operation at 4.6 THz is obtained up to a heat-sink temperature of 121 K with a low-temperature threshold current density of 350 A/cm(2). Due to the simplicity of a two-well design, its electrical transport behavior could be analyzed in greater detail. A thermally activated carrier leakage due to higher-energy parasitic levels is speculated to be the most likely cause of a steep rise in the lasing threshold current density with temperature. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3243459]