Bandwidth-restricted microcombs in photonic crystal ring resonators (Invited Paper)

Soliton microcombs are established as reliable sources of low-noise, high-repetition-rate optical frequency combs, leveraging cascaded four-wave mixing in resonators with anomalous group-velocity dispersion (GVD). However, their intrinsic sech² spectral envelope limits their utility in applications like optical telecommunications, where a rectangular envelope with equal power per line is desired. Here, we introduce a novel method to control the spectral extent of microcombs using meta-dispersion in photonic crystal ring resonators. By patterning a complex corrugation along the resonator waveguide, we induce wavelength-dependent bidirectional coupling via Bragg reflection, hybridizing modes and precisely shifting resonance frequencies. This approach enables tailored control of the phase-matching condition, allowing us to shape the spectral envelope of the microcomb. We demonstrate this principle in both anomalous and normal GVD regimes: in the former, the soliton profile is strongly altered to enhance central lines; in the latter, we achieve comb generation in strongly normal dispersion waveguides, overcoming material dispersion by engineering near-zero meta-dispersion over a defined spectral range.