Light-Induced Valleytronics in Pristine Graphene

TL;DR

This paper demonstrates that valley-selective excitation and high-harmonic generation in pristine graphene can be achieved using two counter-rotating circularly polarized laser fields, enabling all-optical control and measurement of valley polarization.

Contribution

It introduces a novel all-optical method for valley-selective excitation in pristine graphene using dual circularly polarized fields, overcoming previous symmetry limitations.

Findings

Valley-selective excitation achieved in pristine graphene.

Controlled valley polarization via phase tuning of laser fields.

Method for measuring valley polarization with a weak probe pulse.

Abstract

Electrons in two-dimensional hexagonal materials have valley degree of freedom, which can be used to encode and process quantum information. The valley-selective excitations, governed by the circularly polarised light resonant with the material's band-gap, continues to be the foundation of valleytronics. It is often assumed that achieving valley selective excitation in pristine graphene with all-optical means is not possible due to the inversion symmetry of the system. Here we demonstrate that both valley-selective excitation and valley-selective high-harmonic generation can be achieved in pristine graphene by using the combination of two counter-rotating circularly polarized fields, the fundamental and its second harmonic. Controlling the relative phase between the two colours allows us to select the valleys where the electron-hole pairs and higher-order harmonics are generated. We also describe an all-optical method for measuring valley polarization in graphene with a weak probe pulse. This work offers a robust recipe to write and read valley-selective electron excitations in materials with zero bandgap and zero Berry curvature.