Suppressed magnetic circular dichroism and valley-selective magneto-absorption due to the effective mass anisotropy in bismuth

TL;DR

This study investigates the magneto-optical properties of bismuth, revealing how effective mass anisotropy causes suppressed magnetic circular dichroism in electron transitions, with implications for valley polarization techniques.

Contribution

It provides the first quantitative explanation of reduced electron dichroism in bismuth due to effective mass anisotropy, linking optical measurements to electronic structure.

Findings

Hole transitions show 100% magnetic circular dichroism.

Electron dichroism is reduced to 13%, explained by mass anisotropy.

Effective mass anisotropy enables contactless measurement and valley polarization control.

Abstract

We have measured the far-infrared reflectivity and Kerr angle spectra on a high-quality crystal of pure semimetallic bismuth as a function of magnetic field, from which we extract the conductivity for left- and right handed circular polarisations. The high spectral resolution allows us to separate the intraband Landau level transitions for electrons and holes. The hole transition exhibits 100% magnetic circular dichroism, it appears only for one polarisation as expected for a circular cyclotron orbit. However the dichroism for electron transitions is reduced to only $13\pm 1$%, which is quantitatively explained by the large effective mass anisotropy of the electron pockets of the Fermi surface. This observation is a signature of the mismatch between the metric experienced by the photons and the electrons. It allows for a contactless measurement of the effective mass anisotropy and provides a direction towards valley polarised magneto-optical pumping with elliptically polarised light.