Optical activity of chiral excitons

TL;DR

This paper develops an analytical model to explain the optical activity of chiral excitons, linking spin-splitting phenomena to chiroptical effects in chiral semiconductors, with applications to 2D hybrid perovskites and nanocrystals.

Contribution

It introduces a new effective mass model that captures parity mixing and magnetic dipole transitions in chiral excitons, connecting structural asymmetry to optical activity.

Findings

Circular dichroism arises from spin-splitting and cross coupling of spin textures.

The model explains chiroptical properties in chiral 2D perovskites.

Chiroptical effects can occur without chiral lattice distortions in nanocrystals.

Abstract

Recent activity in the area of chiroptical phenomena has been focused on the connection between structural asymmetry, electron spin configuration and light matter interactions in chiral semiconductors. In these systems, spin-splitting phenomena emerge due to inversion symmetry breaking and the presence of extended electronic states, yet the connection to chiroptical phenomena is lacking. Here, we develop an analytical effective mass model of chiral excitons, parameterized by density functional theory. The model accounts for parity mixing of the band edge Bloch functions resulting from polar distortions, resulting in magnetic dipole allowed transitions. Through the study of a prototypical chiral 2D hybrid perovskite semiconductor, we show that circular dichroism of the chiral exciton and its interband continuum emerges from spin-splitting via cross coupling of Rashba-like and chiral/helical spin-texture components. As a counterpoint, we apply our model to describe chiroptical properties of excitons in perovskite nanocrystals that occur without chiral lattice distortions.