Angle-insensitive dual bound states in the continuum on germanium metasurface

Metasurface-enabled bound states in the continuum (BICs) provide a novel solution for achieving exceptionally high quality factors (Q factors), which could overcome the limitations of traditional mid-infrared filters, sensors, lasers, and nonlinear sources. However, most BIC metasurfaces are restricted by their sensitivity to specific incident angles, limiting their practical applications. Here, we introduced a germanium-based metasurface that supports two BIC modes for different polarizations, exhibiting robust angle insensitivity. By leveraging geometric asymmetry, we effectively controlled BIC leakage and coupling. The device maintained infinite Q factors under oblique incidence with preserved symmetry, and exhibited stable quasi-BIC resonance wavelengths and linewidths even with broken symmetry, regardless of TE or TM polarization. This angular robustness has been validated both theoretically and experimentally, demonstrating its potential for broadening the applicability of high-performance mid-infrared optical devices.