Low divergence, single-lobed, surface emission from THz photonic-crystal quantum cascade lasers

We demonstrate full control of the spectral and spatial emission properties of terahertz quantum cascade lasers using metallic photonic-crystals for lateral confinement. Single-mode, surface-emission is obtained at a frequency of 2.8-2.9 THz using a photonic-crystal design with a square lattice geometry. The resonator quality factor is then optimised, using a graded photonic-crystal architecture, and this leads to an increased maximum operating temperature of 151 K. Furthermore, the introduction of a carefully designed pi-shift yields single-lobed surface emission (~12deg times 8deg), with a peak output power of 5 mW at 78 K.