Six Bit Optical Phase States Realized in Nonvolatile\nPhase Shifter Based on N‑Doped Sb<sub>2</sub>Se<sub>3</sub>

Due to the nonvolatile and large\noptical contrast of phase-change\nmaterials (PCMs), PCM-based optical phase shifters have been considered\nas a powerful technology for a variety of emerging applications, such\nas optical switching, optical memory, and neuromorphic computing.\nHowever, due to the lack of a phase-change material with low-loss\nand high optical contrast, the phase modulation space of current PCM-based\nphase shifters still falls short of meeting the requirements for real-world\napplications. To address these issues, it is necessary to develop\na phase-change material with a high optical contrast to enhance the\nphase modulation capability of optical phase shifters. In this work,\nwe have designed a new optical PCM by doping Sb<sub>2</sub>Se<sub>3</sub> with the nitrogen element (N), which can improve the optical\nrefractive index of Sb<sub>2</sub>Se<sub>3</sub> from 0.67 to 1.10\nand keep its optical loss near 0 at communication bands. By integrated\nN-doped Sb<sub>2</sub>Se<sub>3</sub> with unbalanced Mach–Zehnder\ninterferometer (MZI), an optical phase shifter with a high phase modulation\nspace is successfully developed and achieves 84 distinct optical phase\nstates (>6 bit). Finally, an artificial neural network (ANN) is\nestablished\nusing the optical phase shifter as an optical synapse. The multiwavelength\nand multistate encoding enabled by this optical phase shifter can\nnot only reduce hardware costs by more than 75%, but also maintain\na high image recognition rate (96.67%). This work provides an effective\napproach for designing PCMs with high phase modulation space, which\nis significant for constructing high-performance programmable optical\nphase shifts.