Ultralow level all-optical self-switching empowered by merging bound states in the continuum

We report all-optical switching driven by ultralow intensities in silicon photonic crystal nanostructures empowered by merging bound states in the continuum (BICs). A merging BIC is realized by modulating the thickness of the silicon photonic crystal and is confirmed by the distribution of topological charges. Compared to the quasi-BIC near an isolated BIC, the quasi-BIC near a merging BIC exhibits significantly higher Q factors and stronger field localization at the same in-plane wavevector. Benefiting from the advantages of merging quasi-BICs in significantly reduced radiation loss and enhanced local field, all-optical switching via the Kerr effect of silicon can be realized at an ultralow level of 1 W/cm2, with the transmittance changing from 0% to 90%. The results indicate that the concept of merging BICs in nonlinear platforms holds significant promise for high-performance all-optical devices.