Optical responses and optical activity of ultrathin films of topological insulator

TL;DR

This study explores how ultrathin topological insulator films exhibit strong optical anisotropy and activity under in-plane magnetic fields, affecting polarization states and enabling polaroid-like behavior due to broken time reversal symmetry.

Contribution

It demonstrates the magnetic field-induced optical anisotropy and activity in ultrathin topological insulator films, revealing their potential as tunable optical devices.

Findings

Optical conductivity shows strong anisotropy due to magnetic field and surface state overlap.

Ultrathin films can induce elliptic polarization in reflected and transmitted light.

Films can act as polaroids, with near-linear polarization under certain conditions.

Abstract

We investigate the optical properties of an ultrathin film of a topological insulator in the presence of an in-plane magnetic field. We show that due to the combination of the overlap between the surface states of the two layers and the magnetic field, the optical conductivity can show strong anisotropy. This leads to the effective optical activity of the ultrathin film by influencing the circularly polarized incident light. Intriguingly, for a range of magnetic fields, the reflected and transmitted lights exhibit elliptic character. Even for certain values almost linear polarizations are obtained, indicating that the thin film can act as a polaroid in reflection. All these features are discussed in the context of the time reversal symmetry breaking as one of key ingredients for the optical activity.