Symmetrically dispersion-engineered microcombs

Normal-dispersion microcombs have gained significant attention for their features, including high conversion efficiency, deterministic generation, and thermal management-free operation. However, most of the demonstrated microcombs in the normal-dispersion regime heavily rely on asymmetric local dispersion anomalies, which introduce odd-order dispersion components and originate asymmetric spectral characteristics. In this study, we present a scheme that employs two symmetrically positioned local dispersion alterations on either side of the pump mode. This configuration enables direct mode-locked microcombs, referred to as ‘dark pulses’ or ‘platicons’, while preserving spectral symmetry. The platicon microcombs exhibit efficient conversion, spectral symmetry, and can be generated with high repeatability. Furthermore, we demonstrate the deterministic generation of perfect platicon crystals with highly symmetric spectra by precisely controlling the position of the two symmetric dispersion alterations relative to the pump mode. Our proposed method offers a reliable approach for achieving power-efficient microcombs with highly symmetric spectra, and can be transferred to other integrated nonlinear platforms.