Phase-locking of surface-emitting THz quantum cascade laser arrays

We first discuss and review a novel technique to obtain efficient surface-emission in terahertz quantum cascade lasers (THz-QCLs). We report high-power, single-mode surface-emission by using graded photonic heterostructures (GPH) as resonators. Peak output powers of ∼100 mW and differential efficiency of 230mW/A are achieved at frequency ∼ 3.4THz, the maximum operation temperature being 120K in pulsed mode. Combining with the active region based on the bound-to-continuum design, we also achieved continuous-wave (CW) operation of GPH THz QC lasers, with the maximum output power of 3.8 mW at 2.7THz. We then discuss the prospects to develop phase-locked arrays of these devices, in order to take advantage of their good wall-plug efficiency (<0.3%) and single-lobed directional beam pattern. As an initial result, a very robust approach to phase-lock second-order distributed feedback (DFB) THz QC laser arrays (pairs in this case) is demonstrated.