THz Quantum Cascade Lasers with Optimized Beam Divergence

Single-mode surface-emitting terahertz quantum cascade lasers based on distributed feedback mechanism with high symmetry is presented. By inserting a central $\pi$-phase shift or using a non-uniform grating period, an optimized 2-D far-field radiation pattern is obtained}. Single-mode emission is achieved with a side-mode suppression ration above 25 dB.