Comb operation in terahertz quantum cascade ring lasers

Abstract

Ring resonators are an interesting alternative cavity solution to the commonly used ridge-type waveguide for terahertz (THz) quantum cascade lasers. They either support a standing-wave pattern showing spatial hole burning if there are defects implemented or a traveling mode in a defect-free cavity. Here, we report on ring-shaped THz quantum cascade lasers emitting between 3.2 and 4.1 THz operating in four different emission regimes. The presence of defects in the cavities force the THz quantum cascade laser into a standing-wave pattern. The measurements show a complex behavior highlighting the effect of strong confinement and the optical nonlinearities leading to the generation of a harmonic state, as well as to a fundamental comb, exhibiting over 30 equidistant modes and covering a bandwidth of 622 GHz. The results are explained by numerical calculations based on the Maxwell–Bloch formalism, including the linewidth enhancement factor and reflection points. The compact geometry and high output power (4 mW detected) make these devices extremely appealing for on-chip frequency comb applications in the terahertz region.

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

“…This is the longest cavity length reported to date with the FSR easily accessible for beatnote measurement and injection locking using modest RF instrumentation. 17,18 It is notable that there is no evidence of spontaneous frequency comb operation in QC-VECSELs as that has been demonstrated in ridge waveguide QC-lasers 19,20 or ring cavity QC-lasers 21,22 this may be related the lack of spatial hole burning within the metasurface, or the fact that no effort towards dispersion compensation has been attempted yet. Nonetheless, we consider this cavity design as an excellent platform to study THz QC-VECSEL based frequency combs through RF modulation of the QC-gain.…”

Self-starting microring laser solitons from the periodic cubic complex Ginzburg-Landau equation

“…On the other hand, the laser operates much closer to the TO phonon resonance and is more sensitive to the cavity geometry, which can yield very large GVD values up to ±10 5 fs 2 /mm [37]. This high GVD could compensate and allow soliton operation also in THz ring QCLs [38].…”

RF Injection Locking of THz Metasurface Quantum‐Cascade VECSEL

“…[6][7][8] Its inherently high optical nonlinearity induces self-phase locking through four-wave mixing, which promotes the generation of spontaneous frequency combs; these DOI: 10.1002/lpor.202300007 have been demonstrated in waveguidebased Fabry-Pérot [9][10][11] and ring QClasers. [12,13] Based on that, THz dualcomb spectroscopy has been demonstrated, surpassing the precision and speed of traditional Fourier spectrometers by several orders of magnitude. [14][15][16][17] In separate experiments, THz quantum-cascade lasers have recently been implemented in the verticalexternal-cavity surface-emitting laser (VECSEL) architecture, which exhibits watt-level output power, near diffractionlimited beam quality, and ≈20% continuous fractional single-mode tuning.…”