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

TL;DR

This paper introduces CdPS3, a van der Waals crystal with a high refractive index and wide bandgap, overcoming the traditional trade-off and enabling advanced UV-visible nanophotonics applications.

Contribution

The study demonstrates that CdPS3 uniquely combines high refractive index and wide bandgap, challenging Moss's law and providing a new material platform for UV-visible nanophotonics.

Findings

CdPS3 has an in-plane refractive index approaching 3 in the near-UV.

CdPS3 exhibits a wide indirect bandgap suitable for UV-visible applications.

First-principles calculations confirm the anomalous optical properties.

Abstract

The development of nanophotonics is hindered by a fundamental trade-off between a material's refractive index (n) and its electronic bandgap (Eg), 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 3 in the near-UV, combined with a wide indirect bandgap. This combination of properties, which circumvents the empirical Moss's law, 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.