Topological resonance and single-optical-cycle valley polarization in gapped graphene

TL;DR

This paper predicts that intense ultrashort circularly polarized pulses can induce significant valley polarization in gapped graphene through topological resonance, with polarization increasing with the bandgap.

Contribution

It introduces the concept of topological resonance in gapped graphene and demonstrates how ultrashort pulses induce valley polarization dependent on the bandgap.

Findings

Valley polarization increases with bandgap size.

Ultrashort pulses excite electrons across a broad energy range.

Topological resonance explains the polarization mechanism.

Abstract

For gapped graphene, we predict that an intense ultrashort (single-oscillation) circularly-polarized optical pulse can induce a large population of the conduction band and a large valley polarization. With an increase in the bandgap, the magnitude of the valley polarization gradually increases from zero (for the native gapless graphene) to a value on the order of unity. The energy bandwidth of the electrons excited into the conduction band can be very large ($\gtrsim 10$ eV for a reasonable pulse amplitude of $\sim 0.5$ $\mathrm{V/\AA}$). These phenomena are due to the effect of topological resonance: the matching of the topological (geometric) phase and the dynamic phase. Gapped graphene with tunable bandgap can be used as a convenient generic model of two-dimensional semiconductors with honeycomb generic lattice structures and broken inversion symmetry, such as transition metal dichalcogenides.