Metallic Electro-Optic Effect in Twisted Double-Bilayer Graphene

TL;DR

This paper uncovers a new electro-optic mechanism in twisted double-bilayer graphene driven by Berry curvature and orbital magnetic moments, enabling giant, tunable terahertz dichroism for ultrafast optical control.

Contribution

It introduces an alternative electro-optic effect mechanism in metallic systems based on Berry curvature and magnetic moments, specific to moiré bands in TDBG, beyond the Berry curvature dipole paradigm.

Findings

Giant, gate-tunable linear and circular dichroism in terahertz regime.

Dominance of the magnetoelectric electro-optic response in $C_{3z}$-symmetric TDBG.

New mechanism for ultrafast electro-optic modulation in 2D materials.

Abstract

Recent theoretical advances have highlighted the role of Bloch state intrinsic properties in enabling unconventional electro-optic (EO) phenomena in bulk metals, offering novel strategies for dynamic optical control in quantum materials. Here, we identify an alternative EO mechanism in bulk metallic systems that arises from the interplay between Berry curvature and the orbital magnetic moment of Bloch electrons. Focusing on twisted double-bilayer graphene (TDBG), we show that the enhanced intrinsic properties of moir\'e Bloch bands give rise to a sizable linear magnetoelectric EO response, a first-order, electric-field-induced non-Hermitian correction to the gyrotropic magnetic susceptibility. This mechanism dominates in $C_{3z}$-symmetric TDBG, where EO contributions originating from the Berry curvature dipole (BCD) are symmetry-forbidden. Our calculations reveal giant, gate-tunable linear and circular dichroism in the terahertz regime, establishing a robust and tunable platform for ultrafast EO modulation in two-dimensional materials beyond the BCD paradigm.