"Single-Mode" Whispering-Gallery Terahertz Quantum-Cascade Lasers with Controlled Degeneracy

We present whispering-gallery mode terahertz quantum-cascade lasers with either "single-mode" or "double- mode" emission depending on the rotational symmetry of such resonators. Strong mode confinement in the growth and in-plane directions are provided by a double-plasmon waveguide and due to the strong impedance mismatch between the gain material and air. These ultra-compact devices exhibit increased temperature performance up to 95 K in continuous-wave mode operation and threshold currents as low as 13.5 mA. We have observed dynamical frequency pulling of the resonator mode on the GHz scale. Thus, we were able to estimate the peak gain of the material to 27 cm^-1. Finite-difference time-domain calculations were performed to obtain the emission spectra from such microdisk terahertz quantum-cascade lasers.