Anomalous-dispersion phase matching in a hypergap crystal

While two perfect phase-matching mechanisms exist in homogeneous single crystals, birefringent phase-matching (BPM) dominates for nonlinear optical applications. The anomalous-dispersion phase matching (ADPM), though demonstrated in gases and liquids, has not been established in crystals due to the parasitic material absorption that induces the anomalous dispersion in the first place. This absorption bottleneck can be overcome by a crystal possessing an additional spectral gap in its absorption spectrum, beyond the regular band gap, which we term the ``hypergap''. Here, we theoretically propose and experimentally demonstrate such a hypergap and the ADPM-enabled third harmonic generation in potassium chromate ${\mathrm{K}}_{\text{2}}{\mathrm{CrO}}_{\text{4}}$. Our results introduce a class of nonlinear optical materials that do not rely on lattice anisotropy or periodic poling.