Low-ContrastBIC Metasurfaces with Quality FactorsExceeding 100,000

Dielectric metasurfaces operating at quasi-bound states in the continuum (qBICs) can achieve exceptionally high radiative quality (<i>Q</i>) factors by introducing small asymmetries into their unit cells. However, fabrication imperfections often impose major limitations on the experimentally observed <i>Q</i> factors. In this study, we experimentally demonstrate BIC metasurfaces with a <i>Q</i> 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-<i>Q</i> factors can be attained by leveraging both the high radiative <i>Q</i> 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-<i>Q</i> applications in both fundamental physics and advanced devices.