Valley control by linearly polarized laser pulses

TL;DR

This paper demonstrates that valley control in valleytronics can be achieved using pairs of linearly polarized femtosecond laser pulses, expanding the understanding beyond circular polarization effects.

Contribution

It introduces a new method for valley control using linearly polarized pulses and identifies the key parameters and phase structures involved.

Findings

Almost complete valley control with two linearly polarized pulses

Pulse timing and polarization as key control parameters

Robust effect demonstrated in a transition metal dichalcogenide

Abstract

Underpinning the field of "valleytronics" is the coupling of the helicity of circularly polarized light to the valley degree of freedom, and this remains the only known lightform to exhibit this remarkable effect. Here we show that on femtosecond time scales valley coupling is a much more general effect. We find that two time separated linearly polarized pulses allow almost complete control over valley excitation, with the pulse time difference and polarization vectors emerging as key parameters for valley control. In contrast to the Berry curvature that underpins the effect for circularly polarized light, we demonstrate that a different phase structure drives this effect, with excitations during each linear pulse acquiring a valley discriminating phase involving the polarisation angle of linear light. Unimportant in a single linear pulse, for pairs of pulses these can constructively and destructively interfere. Employing state-of-the-art time dependent density function theory, we show that the effect is robust to the complexities of charge dynamics in a real material with the example of a transitional metal dichalcogenide.