Mode management in Bottom-Up, Parity-Time-Symmetric Micro-Cavity Lasers

The intrinsic non-Hermiticity of photonic devices with tunable optical gain and loss makes them excellent platforms to explore and implement applications based on parity-time symmetry, and a notable example is mode management in micro-cavity lasers. Thus far, parity-time-symmetric lasers are fabricated via conventional top-down etching processes, which are known to cause cavity sidewall roughness that is potentially detrimental to laser performance. Bottom-up fabrication of parity-time-symmetric lasers, however, has seen limited success due to strict requirements on the uniformity of cavity morphology. Here, parity-time-symmetric lasing is demonstrated in coupled InP micro-ring cavities grown directly by selective area epitaxy. With a facet engineering technique, ring laser cavities with a highly deterministic morphology are realized, enabling parity-time-symmetric laser designs. Furthermore, benefiting from the versatility and controllability of this bottom-up process, lasing mode selectivity can be enhanced through coupling gap tuning and cavity shape engineering, leading to single-mode lasing with a peak side mode suppression ratio exceeding 17 dB and a threefold single-mode brightness enhancement compared to a single, uncoupled laser cavity. This work unlocks a lasing mode management strategy that is previously unavailable to bottom-up laser cavities, which is a major step toward the realization of on-chip, low-loss, and single-mode micro-cavity lasers.