Light-induced valleytronics in pristine graphene

M S Mrudul,Gopal Dixit,Álvaro Jiménez-Galán,Misha Ivanov

doi:10.1364/optica.418152

Abstract

Electrons in two-dimensional hexagonal materials have an extra degree of freedom, the valley pseudospin, that can be used to encode and process quantum information. Valley-selective excitations, governed by the circularly polarized light resonant with the material’s bandgap, are 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 a combination of two counter-rotating circularly polarized fields, the fundamental and its second harmonic. Controlling the relative phase between the two colors 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.

Highlights

Electrons in two-dimensional hexagonal materials have valley degree of freedom, which can be used to encode and process quantum information
With its zero bandgap, zero Berry curvature, and identical dispersion near the bottom of the valleys, pristine graphene appears unsuitable for valleytronics – a disappointing conclusion in view of its exceptional transport properties
Our proposal offers an all-optical route to valleytronics in pristine graphene, complementing approaches based on creating a gap by using a heterostructure of graphene with hexagonal boron nitride [16,17,18,19], or by adding strain and/or defect engineering [20,21,22,23,24]

Summary

Introduction

Electrons in two-dimensional hexagonal materials have valley degree of freedom, which can be used to encode and process quantum information. Valley selectivity is achieved by matching the helicity of a circularly polarized pump pulse, resonant to the bandgap, to the sign of the Berry curvature [5,6,7,8,9]. Single-cycle pulses with the controlled phase of carrier oscillations under the envelope offer a route to valleytronics in gapped graphene-type materials even when such pulses are linearly, not circularly, polarized [14].

Results

Conclusion