Comb Formation in Ring Quantum Cascade Lasers Under Radio-Frequency Injection

On-chip frequency combs have drawn increasing attention due to their wide possible range of applications in spectroscopy and the intriguing mechanisms that govern their formation [1]. In the molecular fingerprint region of the mid-infrared, Quantum Cascade Lasers (QCLs) provide a flexible platform for developing such combs with high output powers, small device footprints and extensive spectral coverage [2]. However, in the established frequency-modulated (FM) combs, the bandwidth of such a QCL frequency comb is limited by dispersion, gain curvature, losses and carrier lifetimes [3], and is thus determined by both active region design and fabrication. Radio-frequency (RF) injection allows for some degree of additional broadening of these combs, thereby providing a powerful way to tune the bandwidth and dynamics of QCLs [4].