Ultra-compact, low-loss silicon photonic phase shifter enabled by a ferroionic two-dimensional material.

Tunable optical materials are essential for enabling electro-optic functionality in photonic integrated circuits (PICs). Two-dimensional (2D) materials offer strong light-matter interaction and are promising candidates for compact tunable elements. However, achieving low-loss, efficient, and broadband phase modulation in the short-wave infrared (SWIR) remains challenging. Here, we demonstrate electro-refractive tuning in silicon photonic imbalanced Mach-Zehnder interferometers (UMZIs) via hybrid integration of multilayer CuCrP₂S₆ (CCPS), a ferroionic 2D material. The modulation is driven by reversible Cu ion migration at the metal-semiconductor interface, yielding a refractive index change of 1.5 × 10^-2 RIU. A consistent redshift of 7.0-7.4 nm at 8 V (0.8π phase shift) is observed across all measured devices. The two-terminal CCPS phase shifter achieves a half-wave voltage-length product (Vπ·L) of 0.033 V·cm with no measurable optical loss at 1.55 µm, outperforming prior results based on transition metal dichalcogenides (TMDs). These results highlight the potential of CCPS for high-efficiency, ultra-compact phase shifters in silicon photonics.