Enhancement of valley-selective excitation by a linearly polarized two-color laser pulse

Abstract

Here, we propose valley-selective excitations via a two-color $(\ensuremath{\omega}+2\ensuremath{\omega})$ laser field, made by superimposing two linearly polarized pulses at frequencies $\ensuremath{\omega}$ and $2\ensuremath{\omega}$. We have studied the intensity ratio between a few-cycle pulse of an $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ laser, and its enhancement factor by employing time-dependent first-principles calculations. The valley polarization depends on the carrier-envelope phases (CEPs) of the pulses and the intensity ratio ${I}_{\ensuremath{\omega}}/{I}_{2\ensuremath{\omega}}$. We found that a two-color field enhances valley polarization by as much as 1.2 times larger than a single-color pulse. Maximum valley asymmetry is achieved for an intensity ratio ${I}_{\ensuremath{\omega}}/{I}_{2\ensuremath{\omega}}$ of 36 with a relative CEP of $\ensuremath{\pi}$. In our previous work, we found that the asymmetric vector potential induces valley polarization [A. Hashmi et al., Phys. Rev. B 105, 115403 (2022)]. In this paper, we find that the asymmetry of the electric field modulates the valley polarization. Our two-color scheme offers another path toward the optical control of valley pseudospins.