Ultracompact Programmable Silicon Photonics Using Layers of Low-Loss Phase-Change Material Sb₂Se₃ of Increasing Thickness

High-performance programmable silicon photonic circuits are considered to be a critical part of next-generation architectures for optical processing, photonic quantum circuits, and neural networks. Low-loss optical phase-change materials (PCMs) offer a promising route toward nonvolatile free-form control of light. Here, we exploit the direct-write digital patterning of waveguides using layers of the PCM Sb₂Se₃ with thickness values from 20 to 100 nm, demonstrating the scaling of induced optical phase shift with thickness and the ability to strongly increase the effect per pixel for thicker layers. We exploit the excellent refractive index matching between Sb₂Se₃ and silicon to achieve a low-loss hybrid platform for programmable photonics. A 5-fold reduction in the modulation length of a Mach-Zehnder interferometer is achieved with increasing thickness compared to the 20 nm thin-film Sb₂Se₃ devices, which decreased to 5 μm in this work. Application of the thicker PCM layers in direct-write digital programming of a multimode interferometer shows a corresponding 3-fold reduction of the number of programmed pixels to below 10 pixels per device. The demonstrated scaling of performance with Sb₂Se₃ layer thickness is important for establishing the optimum working range for hybrid silicon-Sb₂Se₃ devices and holds promise for achieving ultracompact, programmable photonic circuits.