Maksim R Povolotskiy, Aleksandr S. Slavich, Georgy A. Ermolaev, D. V. Grudinin, Nikolay V. Pak, Ilya A. Zavidovskiy, Konstantin Kravtsov, Anton Minnekhanov, Mikhail K. Tatmyshevskiy, Alexander V. Syuy, Dmitry I. Yakubovsky, Arslan Mazitov, Liudmila A. Klimova, Adilet N. Toksumakov, Alexander Melentev, Elena Zhukova, Davit Ghazaryan, Gleb Tselikov, Ivan Kruglov, Sergey M. Novikov, Andrey A. Vyshnevyy, Aleksey V. Arsenin, Kostya S. Novoselov, Valentyn S. Volkov
ABSTRACT 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 (CdPS 3 ) as a solution to this long‐standing problem. Through comprehensive optical and structural characterization, we show that CdPS 3 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 CdPS 3 as a leading material for UV–visible photonics, opening a new pathway for the development of high‐density integrated circuits and metasurfaces.