Near-IR Ultra-High-Contrast Optical Switch Based on AIST Multilayer Fabry-Perot Stacks

Optical switches for quantum communication and photonic logic require exceptionally high on/off contrast, but platforms based on conventional phase-change materials are limited by intrinsic optical loss. Here we show a lithography-free thin-film Fabry-Perot cavity that exploits the non-volatile phase transition of Ag5In5Sb60Te30 (AIST) to deliver ultra-high-contrast modulation in the near-infrared. Through admittance engineering, the amorphous state is impedance-matched to free space, yielding near-zero reflectance at the design wavelength; crystallization perturbs both phase and loss, breaking destructive interference and restoring strong reflectance. Varying the AIST thickness deterministically sets the switching wavelength and maximizes contrast across the telecommunications band. The device attains a contrast ratio of 9156 at 1525 nm (defined as R_c/R_a) and is realized entirely by deposition and annealing, without nanopatterning. This materials-and-interference co-design offers a scalable route to high-contrast, reconfigurable photonic components and positions AIST-based thin-film cavities as a practical alternative to patterned metasurfaces for integrated optical switching.