Record Index-Bandgap Trade-Off: CdPS₃ as a High-Index van der Waals Platform for Ultraviolet-Visible Nanophotonics

The development of nanophotonics is hindered by a fundamental trade-off between a material's refractive index (n) and its electronic bandgap (E _g ), which severely restricts the choice of materials for short-wavelength applications. This challenge is particularly acute in the visible and ultraviolet (UV) spectra, where high-performance devices require materials that are simultaneously highly refractive and transparent. Here, we report on the van der Waals (vdW) crystal cadmium phosphorus trisulfide (CdPS₃) as a solution to this long-standing problem. Through comprehensive optical and structural characterization, we show that CdPS₃ possesses an anomalously high in-plane refractive index across the visible spectrum approaching three in the near-UV, combined with a wide indirect bandgap. This combination of properties is validated by first-principles calculations and direct near-field imaging of highly confined waveguide modes. These findings establish CdPS₃ as a leading material for UV-visible photonics, opening a new pathway for the development of high-density integrated circuits and metasurfaces.