Ultrathin BIC Metasurfaces Based on Ultra-Low-Loss Sb₂Se₃ Phase-Change Material

Ultrathin and low-loss phase-change materials (PCMs) are highly valued for their fast and effective phase transitions and applications in reconfigurable photonic chips, metasurfaces, optical modulators, sensors, photonic memories, and neuromorphic computing. However, conventional PCMs mostly suffer from high intrinsic losses in the near-infrared (NIR) region, limiting their potential for high quality factor (<i>Q</i>-factor) resonant metasurfaces. Here we present the design and fabrication of tunable bound states in the continuum (BIC) metasurfaces using the ultra-low-loss PCM Sb₂Se₃. Our BIC metasurfaces, only 25 nm thick, achieve high modulation depth and broad resonance tuning in the NIR with high <i>Q</i>-factors up to 130. Experimentally, we employ these BIC metasurfaces to modulate photoluminescence in rare earth-doped upconversion nanoparticles, reducing the excitation power for multiphoton photoluminescence and enabling emission polarization manipulation. This work offers a promising platform for developing active resonant metasurfaces, with broad applications including optical modulation, ultrafast switches, color filtering, and optical sensing.