Optical Activity of Chiral Excitons.

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 allowed magnetic dipole 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. To demonstrate the generality of our approach, and as a counterpoint, we apply our model to describe chiroptical properties of three-dimensionally confined excitons in perovskite nanocrystals that occur without chiral lattice distortions.