Reconfigurable mode converter based on a Sb₂Se₃ phase change material and inverse design

In this paper, we combine the inverse design with a silicon-Sb₂Se₃ hybrid platform to design an on-chip mode converter that converts basic modes to higher-order modes. Firstly, we present a 1 × 2 mode converter with dimensions of 4.8 × 2.7 µm² that enables TE₀ mode input, TE₀ or TE₁ output in the C-band (1530 nm to 1565 nm) with an insertion loss (IL) of less than 0.8 dB and a crosstalk (CT) of less than -13 dB. Secondly, the device is extended to a 1 × 3 switchable three-mode converter. Using two controllable phase change regions as drivers, it can flexibly control the switching from TE₀ mode input to three modes of TE₀, TE₁, or TE₂ outputs, which enables mode switching and signal routing. The device can be switched between three modes and has broad application potential in broadband optical signal processing for mode division multiplexing systems, as well as optical interconnections. Finally, the device is extended to a 1 × 2 controllable (mode and power) beam splitter, which can control the power ratio between output modes. By modulating the crystallinity of Sb₂Se₃, the simulation achieves a multilevel switching of 36 levels (> 5-bit). These devices pave the way for high integration densities in future photonic chips.