Dynamical frequency pulling of degenerated and nondegenerated modes in small mode volume whispering-gallery terahertz quantum-cascade lasers

We present terahertz quantum-cascade lasers based on sub-wavelength circular-shaped double-metal microcavities whose single-mode emission can be fine-tuned via dynamical frequency pulling. This allows to estimate the peak gain of the material to 27cm-1 and the shift of the cavity mode towards the gain maximum by 30 GHz. 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 threshold currents as low as 13.5 mA. We lifted the natural two-fold degeneracy of the whispering-gallery modes by lifting the rotational symmetry of such resonators.