# Giant optical activity and Kerr effect in type-I and type-II Weyl semimetals

## TL;DR

This paper demonstrates giant optical activity and Kerr effects in both thin films and bulk of type-I and type-II Weyl semimetals, driven by their unique topological properties and axion electrodynamics.

## Contribution

It reveals the mechanisms behind optical activity and Kerr effects in Weyl semimetals, highlighting the roles of Hall conductivity and axion electrodynamics in different regimes.

## Key findings

- Giant Kerr angle proportional to Q/ω in thin films.
- Optical activity in bulk WSM persists without optical transitions.
- Large polar Kerr effect and Voigt effect depend on surface Fermi arc states.

## Abstract

We explore optical activity in thin films and bulk of type-I and type II Weyl semimetals (WSM), and demonstrate the existence of a giant Kerr effect in both. In time-reversal symmetry broken WSM thin films, the polarization rotation is caused by the optical Hall conductivity including the anomalous Hall term. The Kerr angle is found to be $\propto Q/\omega$, with $Q$ and $\omega$ being the Weyl node separation and the optical frequency, respectively. In contrast, the optical activity in the bulk WSM is dominated by axion electrodynamics, which persists even in the Pauli blocked regime of no optical transitions. In bulk WSM, $Q$ acts analogous to the magnetization in magnetic materials, leading to large `polar Kerr effect' (linear in $Q$) when light is incident on WSM surface without Fermi arc states, and the `Voigt effect' (quadratic in $Q$), when light is incident on surface with Fermi arc states.