# Twisted Molecular Excitons as Mediators for Changing the Angular Momentum of Light

## TL;DR

This paper demonstrates how molecules with specific symmetries can mediate the transfer and change of optical angular momentum through excitonic states, offering a new approach without relying on nonlinear optical effects.

## Contribution

It introduces a molecular mechanism for changing light's angular momentum using twisted excitons, verified through tight-binding and density functional theory models.

## Key findings

- Angular momentum is conserved in exciton interactions.
- Molecules can mediate angular momentum transfer to light.
- The mechanism works in many-body, multi-level systems.

## Abstract

Molecules with C_N or C_Nh symmetry can absorb quanta of optical angular momentum to generate twisted excitons with well-defined quasi-angular momenta of their own. Angular momentum is conserved in such interactions at the level of a paraxial approximation for the light beam. A sequence of absorption events can thus be used to create a range of excitonic angular momenta. Subsequent decay can produce radiation with a single angular momentum equal to that accumulated. Such molecules can thus be viewed as mediators for changing the angular momentum of light. This sidesteps the need to exploit nonlinear light-matter interactions based on higher-order susceptibilities. A tight-binding paradigm is used to verify angular momentum conservation and demonstrate how it can be exploited to change the angular momentum of light. The approach is then extended to a time-dependent density functional theory setting where the key results are shown to hold in a many-body, multi-level setting.