Valley-Selective High Harmonic Generation and Polarization Induced by an Orthogonal Two-Color Laser Field

The valley pseudospin properties of electrons in two-dimensional hexagonal materials result in many fascinating physical phenomena, which opens up the new field of valleytronics. The valley-contrasting physics aims at distinguishing the valley degree of freedom based on valley-dependent effects. Here, we theoretically demonstrate that both of the valley-selective high harmonic generation and valley-selective electronic excitation can be achieved by using an orthogonal two-color (OTC) laser field in gapped graphene. It is shown that the asymmetry degrees of harmonic yields in the plateaus, cutoff energies of generated harmonics and electron populations from two different valleys can be precisely controlled by the relative phase of the OTC laser field. Thus, the selectivity of the dominant valley for the harmonic radiation and electronic polarization can be switched by adjusting the relative phase of the OTC laser field. Our work offers an all-optical route to produce the valley-resolved high harmonic emissions and manipulate the ultrafast valley polarization on a femtosecond timescale in condensed matter.