Optically Induced Avoided Crossing in Graphene

TL;DR

This paper demonstrates a non-equilibrium method to lift degeneracies in graphene by optically driving lattice vibrations, resulting in new vibronic states and altered vibrational properties, with potential implications for material phase control.

Contribution

It introduces a novel non-equilibrium approach to lift degeneracies in solids through coherent optical driving of lattice modes, specifically in graphene, revealing new vibronic states and mode coupling effects.

Findings

Laser-driven coupling of degenerate modes in graphene.

Observation of mode splitting and increased lifetimes via Raman.

Potential generalization to other materials systems.

Abstract

Degenerate states in condensed matter are frequently the cause of unwanted fluctuations, which prevent the formation of ordered phases and reduce their functionalities. Removing these degeneracies has been a common theme in materials design, pursued for example by strain engineering at interfaces. Here, we explore a non-equilibrium approach to lift degeneracies in solids. We show that coherent driving of the crystal lattice in bi- and multilayer graphene, boosts the coupling between two doubly-degenerate modes of E1u and E2g symmetry, which are virtually uncoupled at equilibrium. New vibronic states result from anharmonic driving of the E1u mode to large amplitdues, boosting its coupling to the E2g mode. The vibrational structure of the driven state is probed with time-resolved Raman scattering, which reveals laser-field dependent mode splitting and enhanced lifetimes. We expect this phenomenon to be generally observable in many materials systems, affecting the non-equilibrium emergent phases in matter.