Ultra-compact low threshold whispering-gallery terahertz quantum-cascade lasers

We present terahertz quantum-cascade lasers with small mode volumes based on whispering-gallery resonators. The strong mode confinement in the growth and in-plane directions are provided by a double-metal waveguide. This allows laser emission from devices with overall dimensions much smaller than the free-air emission wavelength. These ultra-small devices lase inherently single-mode and exhibit threshold currents as low as 4.8 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 simulations of these microcavity lasers validates that the experimentally observed lasing emission can be attributed to whispering gallery modes.