Giant Faraday rotation in atomically thin semiconductors

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 WSe₂ and MoSe₂ under moderate magnetic fields. This results in the highest known Verdet constant of -1.9 × 10⁷ deg T^-1 cm^-1 for any material in the visible regime. Additionally, interlayer excitons in hBN-encapsulated bilayer MoS₂ exhibit a large Verdet constant (V_IL ≈ +2 × 10⁵ 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 WSe₂ and MoSe₂ 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.