Giant and tunable excitonic optical anisotropy in single-crystal CsPbX$_3$ halide perovskites

TL;DR

This paper demonstrates that by chemically altering halogen composition in single-crystal CsPbX$_3$ perovskites, one can achieve continuous and significant tuning of optical anisotropy, enabling advanced photonic applications.

Contribution

It introduces a method to tune optical anisotropy in halide perovskites via halogen ratio modification, revealing the largest anisotropy among non-van der Waals materials.

Findings

Optical anisotropy up to 0.6 in visible range.

Continuous anisotropy tuning by 0.14 through halogen ratio.

Anisotropy depends on perovskite shape and orientation.

Abstract

During the last years, giant optical anisotropy demonstrated its paramount importance for light manipulation which resulted in numerous applications ranging from subdiffraction light guiding to switchable nanolasers. In spite of recent advances in the field, achieving continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through chemical alteration of the ratio of halogen atoms (X = Br or Cl) in single-crystal CsPbX$_3$ halide perovskites. It turns out that the anisotropy originates from an excitonic resonance in the perovskite, which spectral position and strength are determined by the halogens composition. As a result, we manage to continually modify the optical anisotropy by 0.14. We also discover that the halide perovskite can demonstrate optical anisotropy up to 0.6 in the visible range -- the largest value among non-van der Waals materials. Moreover, our results reveal that this anisotropy could be in-plane and out-of-plane, depending on perovskite shape -- rectangular and square. Hence, it can serve as an additional degree of freedom for anisotropy manipulation. As a practical demonstration, we created perovskite anisotropic nanowaveguides and show a significant impact of anisotropy on high-order guiding modes. These findings pave the way for halide perovskites as a next-generation platform for tunable anisotropic photonics.