Ring cavity induced threshold reduction in single-mode surface emitting quantum cascade lasers

We present ring cavity surface emitting (RCSE) quantum cascade lasers operating at temperatures as high as 380 K and above. A reduction in threshold current density and enhanced radiation efficiency are observed as compared to Fabry-Peacuterot (FP) lasers. In continuous wave, the maximum operation temperature of RCSE lasers is 50 K higher than in FP emitters. The devices exhibit single-mode emission at a wavelength around 8 mu m with a side mode suppression ratio of 30 dB at room temperature. A lithographic tuning of the resonance is achieved by a variation in the grating period. The emitters exhibit a low divergent ring-shaped beam pattern with a lobe separation of similar to 1.5 degrees.