Non-Volatile Reconfigurable Transmissive Notch Filter Using Wide Bandgap Phase Change Material Antimony Sulfide

Reconfigurable free-space metasurfaces with subwavelength-scale tunable nano-scatterers can manipulate light for many applications ranging from bio-medical imaging, light detection and ranging to optical computing. Several endeavors have been made to achieve tunable metasurfaces using thermo-optic, electro-optic effects, liquid crystals, and phase change materials (PCMs). PCMs stand out, particularly for low-tuning frequency and low-power consumption applications, thanks to their non-volatile nature and drastic index modulation, leading to zero-static power and a small footprint. Antimony sulfide (Sb₂S₃) is an emerging low-loss PCM with the widest bandgap reported so far, enabling operation at low wavelengths down to ∼600 nm in the visible spectrum. In addition, Sb₂S₃ has slow crystallization speed, which enables amorphization of large-volume Sb₂S₃ without unintentional recrystallization. This makes Sb₂S₃ suitable for application in reconfigurable metasurfaces, where the switching area (usually > hundreds of μm²) is significantly larger than photonic integrated circuits (tens of μm²). Herein, we experimentally demonstrate an electrically tunable notch filter at a wavelength of ∼1150 nm on a Sb₂S₃-cladded silicon-on-sapphire platform. The notch filter is enabled by a 2-dimensional symmetry-protected quasi-bound-state-in-the-continuum (quasi-BIC) metasurface. We experimentally observed a quality factor of up to ∼200 and demonstrated reversible tuning of a record large volume (4.5 μm³) of Sb₂S₃. Thanks to the large modulation provided by Sb₂S₃, we observed a resonance shift as high as ∼4 nm in situ using a doped silicon microheater. Our work paves the way for compact and low-power nonvolatile notch filters. Moreover, due to the low loss of Sb₂S₃ in the visible, this work also lays the foundation for phase-only modulation in the visible using PCMs.