Low-Contrast BIC Metasurfaces with Quality Factors Exceeding 100,000.

Dielectric metasurfaces operating at quasi-bound states in the continuum (qBICs) can achieve exceptionally high radiative quality (Q) factors by introducing small asymmetries into their unit cells. However, fabrication imperfections often impose major limitations on the experimentally observed Q factors. In this study, we experimentally demonstrate BIC metasurfaces with a Q factor of 101,000 under normal excitation of light in the telecom wavelength range achieved by employing low-contrast silicon pairs. Our findings show that such free-space accessible ultrahigh-Q factors can be attained by leveraging both the high radiative Q factors of higher-order qBIC modes and reduced scattering losses in shallow-etched designs. Additionally, we demonstrate stable sub-picometer-level wavelength fluctuations in water, with a limit of detection of 10^-5 for environmental refractive index changes. The proposed approach can be extended to BIC metasurfaces with many other configurations and operating wavelengths for ultrahigh-Q applications in both fundamental physics and advanced devices.