Low divergence single-mode surface-emitting concentric-circular-grating terahertz quantum cascade lasers

We report the design, fabrication and experimental characterization of surface-emitting terahertz (THz) frequency quantum cascade lasers (QCLs) with distributed feedback concentric-circulargratings. Single-mode operation is achieved at 3.73 THz with a side-mode suppression ratio as high as ̃30 dB. The device emits ̃5 times the power of a ridge laser of similar dimensions, with little degradation in the maximum operation temperature. Two lobes are observed in the far-field emission pattern, each of which has a divergence angle as narrow as ̃13.5°×7°. We demonstrate that deformation of the device boundary, caused by anisotropic wet chemical etching is the cause of this doublelobed profile, rather than the expected ring-shaped pattern.