Giant Faraday rotation in atomically thin semiconductors

Abstract

Faraday rotation is a fundamental effect in the magneto-optical response of solids, liquids and gases. Materials with a large Verdet constant find applications in optical modulators, sensors and non-reciprocal devices, such as optical isolators. Here, we demonstrate that the plane of polarization of light exhibits a giant Faraday rotation of several degrees around the A exciton transition in hBN-encapsulated monolayers of WSe ₂ and MoSe ₂ under moderate magnetic fields. This results in the highest known Verdet constant of -1.9 × 10 ⁷ deg T ⁻¹ cm ⁻¹ for any material in the visible regime. Additionally, interlayer excitons in hBN-encapsulated bilayer MoS ₂ exhibit a large Verdet constant ( V _IL ≈ +2 × 10 ⁵ deg T ⁻¹ cm ⁻²) of opposite sign compared to A excitons in monolayers. The giant Faraday rotation is due to the giant oscillator strength and high g-factor of the excitons in atomically thin semiconducting transition metal dichalcogenides. We deduce the complete in-plane complex dielectric tensor of hBN-encapsulated WSe ₂ and MoSe ₂ monolayers, which is vital for the prediction of Kerr, Faraday and magneto-circular dichroism spectra of 2D heterostructures. Our results pose a crucial advance in the potential usage of two-dimensional materials in ultrathin optical polarization devices.

Abstract

Here, the authors perform Faraday rotation spectroscopy around the excitonic transitions in hBN-encapsulated WSe ₂ and MoSe ₂ monolayers, and interlayer excitons in MoS ₂ bilayers. They measure a large Verdet constant - 1.9 × 10 ⁷ deg T ⁻¹cm ⁻¹ for monolayers, and attribute it to the giant oscillator strength and high g-factor of the excitons.