Anomalous refractive index modulation and giant birefringence in 2D ferrielectric CuInP$_2$S$_6$

TL;DR

This paper reports a groundbreaking discovery of large, thickness-dependent refractive index changes and giant birefringence in 2D ferrielectric CuInP$_2$S$_6$, with potential applications in photonics and electro-optics.

Contribution

It reveals unprecedented optical property modulation in 2D CuInP$_2$S$_6$, including anomalous refractive index changes and record-breaking birefringence at room temperature.

Findings

Refractive index change as large as 23.2% in 2D CuInP$_2$S$_6$

Giant birefringence of ~1.24 in the blue-ultraviolet regime

Optical constants linked to Cu(I) ferroelectric polarization

Abstract

2D ferroelectric (FE) materials have opened new opportunities in non-volatile memories, computation and non-linear optics due to their robust polarization in the ultra-thin limit and inherent flexibility in device integration. Recently, interest has grown in the use of 2D FEs in electro-optics, demanding the exploration of their electronic and optical properties. In this work, we report the discovery of an unprecedented anomalous thickness-dependent change in refractive index, as large as $\delta n$ $\sim$ 23.2$\%$, in the 2D ferrielectric CuInP$_2$S$_6$, far above the ultra-thin limit, and at room temperature. Furthermore, CuInP$_2$S$_6$ exhibits a giant birefringence in the blue-ultraviolet regime, with a maximum $\vert n_{OOP} - n_{IP}\vert$ $\sim$ 1.24 at $t \sim$ 22 nm and $\lambda$ = 339.5 nm, which is, to the best of our knowledge, the largest of any known material in this wavelength regime. We relate changes in CuInP$_2$S$_6$ optical constants to changes in the Cu(I) FE polarization contribution, influenced by its ionic mobility, opening the door to electronic control of its optical response for use in photonics and electro-optics.