Giant Faraday rotation induced by Berry phase in bilayer graphene under strong terahertz fields

TL;DR

This paper demonstrates that strong terahertz fields induce a giant Faraday rotation in bilayer graphene due to Berry phase effects, enabling potential ultrafast electro-optical applications.

Contribution

The study applies Berry phase theory to bilayer graphene under strong THz fields, revealing a significantly larger Faraday rotation compared to monolayer MoS2.

Findings

Giant Faraday rotation (~1 rad) observed in bilayer graphene under THz fields.

Bilayer graphene's larger Berry curvature enhances nonlinear optical effects.

Potential for ultrafast electro-optical device applications.

Abstract

High-order terahertz (THz) sideband generation (HSG) in semiconductors is a phenomenon with physics similar to high-order harmonic generation but in a much lower frequency regime. It was found that the electron-hole pairs excited by a weak optical laser can accumulate Berry phases along a cyclic path under the driving of a strong THz field. The Berry phases appear as the Faraday rotation angles of the emission signal under short-pulse excitation in monolayer MoS$_2$. In this paper, the theory of Berry phase in THz extreme nonlinear optics is applied to biased bilayer graphene with Bernal stacking, which has similar Bloch band features and optical properties to the monolayer MoS$_2$, such as time-reversal related valleys and valley contrasting optical selection rules. The bilayer graphene has much larger Berry curvature than monolayer MoS$_2$, which leads to a giant Faraday rotation of the optical emission ($\sim$ 1 rad for a THz field with frequency 1 THz and strength 8 kV/cm). This provides opportunities to use bilayer graphene and low-power THz lasers for ultrafast electro-optical devices.