Silicon integrated terahertz quantum cascade ring laser frequency comb

Abstract

We demonstrate terahertz quantum cascade lasers realized in “ideal” ring resonators without discontinuities from, e.g., contacting pads. We realize this by mounting rings episide-down on a silicon substrate by a die-bonding technique. This technique allows one to realize ideal conditions for optical confinement as well as heat dissipation and provides the basis for future Si integrated THz devices. The lasers emit light around 3.8 THz and show much reduced threshold current densities. When operated in continuous-wave operation, frequency comb formation with a spectral bandwidth of 70 GHz is observed. Frequency comb operation is indicated by a narrow beat note signal at 8.55 GHz with a signal-to-noise ratio up to 40 dB. The experimentally measured spectral behavior of ring devices is described accurately by the results obtained from numerical simulations based on the Maxwell–Bloch formalism.