# Electromagnetic helicity in complex media

## TL;DR

This paper extends the concept of optical helicity density to dispersive media, aligning it with biorthogonal electromagnetism, energy density, and light's canonical momentum and spin, with applications to various complex media.

## Contribution

It introduces a consistent definition of optical helicity density in dispersive media, bridging several theoretical frameworks and exploring practical examples.

## Key findings

- Helicity density defined for dispersive media.
- Consistency with biorthogonal electromagnetism and energy density.
- Applications to dielectrics, negative-index materials, and chiral interactions.

## Abstract

Optical helicity density is usually discussed for monochromatic electromagnetic fields in free space. It plays an important role in the interaction with chiral molecules or nanoparticles. Here we introduce the optical helicity density in a dispersive isotropic medium. Our definition is consistent with biorthogonal Maxwell electromagnetism in optical media, the Brillouin energy density, as well as with the recently-introduced canonical momentum and spin of light in dispersive media. We consider a number of examples, including electromagnetic waves in dielectrics, negative-index materials, and metals, as well as interactions of light in a medium with chiral and magnetoelectric molecules.