Multi-mode lasing in terahertz metasurface quantum-cascade VECSELs

Abstract

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

RF Injection Locking of THz Metasurface Quantum‐Cascade VECSEL

“…This is particularly notable, as multimode lasing in QC‐VECSELs has been extremely difficult to achieve due to the lack of spatial hole burning within the metasurface; before now at most 9 lasing modes had been observed. [ 22 ] There are several obvious avenues for improvement. First, RF attenuation and impedance mismatch severely limits the modulation efficiency, and strong RF reflections impede the detection of the electrical beat note signal using a spectrum analyzer.…”

“…[ 21 ] Even though the gain bandwidth of the metasurface used was at least 100 times larger than the free spectral range, only a single lasing mode was observed. This is mainly due to a lack of spatial hole burning within the QC‐VECSEL metasurface which suppresses multimode instabilities, [ 22 ] although it is perhaps compounded by the fact that no effort toward dispersion engineering has yet been attempted. Despite these challenges, there is strong interest in achieving active mode‐locked QCLs and frequency combs within the QC‐VECSEL architecture.…”

Continuous wave operation of terahertz metasurface quantum-cascade VECSEL with a long intra-cryostat cavity

“…This single mode operation is primarily due to the lack of spatial hole burning in QC-VECSELs, as all the longitudinal modes supported in the external cavity interact with the QCgain material through the same metasurface resonance with nearly the same spatial overlap. 5 In contrast, the FTIR spectra in pulsed mode show lasing bandwidth around 58 GHz. Though we are not able to spectrally resolve individual modes due to the limited FTIR resolution of 7.5 GHz, it indicates multi-mode lasing operation of ~10 modes.…”

“…However, due to increasing diffraction loss, the maximum plano-plano cavity lengths have been limited to ~10 mm, and this was only achieved by using a metasurface with a large central circular bias area of diameter 2.4 mm. 5 Such a metasurface consumes large electrical power, which prevents continuous wave (cw) operation. Nonetheless, there are many circumstances which would necessitate a longer cavity, for example, insertion of additional optical elements (e.g.…”