Coherent Control of Photocurrents in Graphene and Carbon Nanotubes

TL;DR

This paper investigates how coherent optical excitations induce asymmetric photocurrents in graphene and carbon nanotubes, revealing dependence on polarization, phase, and electronic properties, with implications for optoelectronic applications.

Contribution

It introduces a long wavelength theory for low energy electronic states, analyzing coherent excitations and their effects on photocurrent directionality in graphene and nanotubes, including higher order corrections.

Findings

Coherent superpositions produce momentum space asymmetry in graphene.

Photocurrent directionality depends on polarization, phase, and electronic gap.

Robust third order nonlinearity in semiconducting nanotubes, absent in conducting ones.

Abstract

Coherent one photon ($2 \omega$) and two photon ($ \omega$) electronic excitations are studied for graphene sheets and for carbon nanotubes using a long wavelength theory for the low energy electronic states. For graphene sheets we find that coherent superposition of these excitations produces a polar asymmetry in the momentum space distribution of the excited carriers with an angular dependence which depends on the relative polarization and phases of the incident fields. For semiconducting nanotubes we find a similar effect which depends on the square of the semiconducting gap, and we calculate its frequency dependence. We find that the third order nonlinearity which controls the direction of the photocurrent is robust for semiconducting t ubes and vanishes in the continuum theory for conducting tubes. We calculate corrections to these results arising from higher order crystal field effects on the band structure and briefly discuss some applications of the theory.