Up to now, terahertz quantum-cascade vertical-external-cavity surface-emitting lasers (QC-VECSELs) have tended to oscillate in only one or two lasing modes at a time. This is due to the fact that the interaction between all of the longitudinal external cavity modes and the QC gain material is mediated through a single metasurface resonance whose spatial overlap changes little with frequency; this suppresses spatial-hole-burning induced multi-mode operation. In this Letter, a VECSEL external cavity is demonstrated using an output coupler based upon a highresistivity silicon etalon which presents a periodic reflectance spectrum that is nearly matched with the external cavity mode spectrum. As the cavity length is varied, a systematic transition between a single/double-mode lasing regime and a multi-mode lasing regime is realized due to Vernier effect. Up to nine modes lasing simultaneously with a free-spectral-range of approximately 21 GHz is demonstrated. This result provides a path towards the multi-mode operation necessary for eventual frequency comb operation.
The manuscriptThe demand for high-resolution, broadband, and fast spectroscopy/sensing in THz frequency range has promoted the study of THz quantum-cascade (QC) laser based frequency combs. [1][2][3][4][5] The QC material itself exhibits a broadband spectral response and octave-spanning multi-mode lasing has been demonstrated in waveguide-based THz QC-lasers, typically with mode spacings in the range of 5-20 GHz as set by the free-spectral-range (FSR). 6 Such dense multi-mode