Phase-locked arrays of surface-emitting terahertz distributed feedback quantum cascade lasers

We first describe a novel concept to excite the fundamental radiative mode in second-order distributed-feedback (DFB) surface-emitting THz quantum cascade (QC) lasers, which leads to high wall-plug power efficiencies. A graded photonic heterostructure (GPH) is implemented to spatially separate the field distributions of radiative and non-radiative modes, hence favoring the former one. The GPH lasers then realize single-mode emissions in a wide spectral range with very high power efficiencies, as well as low-divergence single-lobed emission patterns. The use of a low-threshold bound-to-continuum active region permits continuous-wave (CW) operation with output power of the order of ~ 4 mW (unpurged) at 12K.