# Circular Dichroism in Rotating Particles

## TL;DR

This paper investigates how the internal geometry of rotating nanostructures influences their circular dichroism, revealing strong dependence on shape and rotation speed through a quantum-mechanical model.

## Contribution

It introduces a quantum-mechanical framework for analyzing the polarizability of rotating particles, highlighting the geometry-dependent circular dichroism and superradiance effects.

## Key findings

- Nanorings and nanocrosses show a splitting of $2\Omega$ in optical resonances.
- Compact particles exhibit weak dichroism at low $\Omega$ but strong at high $\Omega$.
- Optical frequency cutoff for superradiance deviates from rotation frequency $\Omega$.

## Abstract

Light interaction with rotating nanostructures gives rise to phenemona as varied as optical torques and quantum friction. Here we reveal that circular dichroism of rotating optically-isotropic particles has an unexpectedly strong dependence on their internal geometry. In particular, nanorings and nanocrosses exhibit a splitting of $2\Omega$ in the particle optical resonances, while compact particles display weak circular dichroism at low rotation frequency $\Omega$, but a strong circular dichroism at high $\Omega$. We base our findings on a quantum-mechanical description of the polarizability of rotating particles, which has not been rigorously addressed so far. Specifically, we use the random-phase approximation and populate the particle electronic states according to the principle that they are thermally equilibrated in the rotating frame. We further provide insight into the rotational superradience effect and the ensuing optical gain, originating in population inversion as regarded from the lab frame, in which the particle is out of equilibrium. Surprisingly, we find the optical frequency cutoff for superradiance to deviate from the rotation frequency $\Omega$. Our results unveil a rich, unexplored phenomenology of light interaction with rotating objects.