Ultra-compact and broadband nonvolatile 2 × 2 optical switch based on a waveguide crossing

Emerging silicon-photonic applications, particularly large-scale programmable photonic integrated circuits (PICs), demand optical switches with an ultra-compact footprint, low power consumption, and low optical loss. In this work, we present a simulation study of a 2 × 2 nonvolatile optical switch based on a waveguide crossing, with a low-loss phase-change material (PCM), Sb₂S₃, embedded along the crossing diagonal. In the crystalline state, the refractive index of Sb₂S₃ is well matched to that of silicon, enabling near-lossless transmission through the PCM region. In the amorphous state, the large refractive-index change (Δn ≈ 0.6) drives total internal reflection (TIR) at the PCM interface, thereby routing light efficiently between output ports. To further enhance the device performance, an adjoint-based inverse shape optimization approach is employed to refine the waveguide-crossing geometry. The optimized device, incorporating an integrated indium tin oxide (ITO) microheater, occupies a compact footprint of ∼ 6.4 × 9 µm², maintaining a worst-case insertion loss of 1.24 dB and crosstalk of -12.5 dB over an ultra-broad bandwidth exceeding 500 nm (1300-1800nm). Moreover, a C-band-optimized design achieves an insertion loss below 0.26 dB with crosstalk better than -19.1 dB over a 70 nm bandwidth centered at 1550 nm. These simulation results indicate the potential of the proposed structure as a compact nonvolatile switching element for programmable PIC platforms.