Electrostatic Control of Magneto-Optic Excitonic Resonances in the van der Waals Ferromagnetic Semiconductor Cr$_2$Ge$_2$Te$_6$

TL;DR

This study demonstrates electrostatic control of magneto-optic properties in the 2D ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$, revealing tunable excitonic resonances and significant modulation of light polarization for potential magneto-photonic applications.

Contribution

It provides the first detailed characterization of the magneto-optic Kerr effect in Cr$_2$Ge$_2$Te$_6$ and shows how electrostatic gating can modulate its magneto-optic response.

Findings

Strong magneto-optic Kerr effect observed around 1.43 eV.

Electrostatic gating modulates magneto-optic strength by over 65%.

Magneto-optic efficiency, not saturation magnetization, is affected by gating.

Abstract

Two-dimensional magnetic materials exhibit strong magneto-optic effects and high tunability by electrostatic gating, making them very attractive for new magneto-photonic devices. Here, we characterize the magneto-optic Kerr effect (MOKE) spectrum of thin Cr$_2$Ge$_2$Te$_6$ from 1.13 to 2.67 eV, and demonstrate electrostatic control over of its magnetic and magneto-optic properties. The MOKE spectrum exhibits a strong feature around 1.43 eV which we attribute to a magnetic exchange-split excitonic state in Cr$_2$Ge$_2$Te$_6$, in agreement with \textit{ab-initio} calculations. The gate dependence of the MOKE signals shows that the magneto-optical efficiency - rather than the saturation magnetization - is affected by electrostatic gating. We demonstrate a modulation of the magneto-optical strength by over 1 mdeg, with some wavelengths showing a modulation of 65% of the total magneto-optical signals, opening the door for efficient electrical control over light polarization through two-dimensional magnets. Our findings bring forward the fundamental understanding of magneto-optic processes in two-dimensional magnets and are highly relevant for the engineering of devices which exploit excitonic resonances for electrically-tunable magneto-photonic devices.