All-Optical Ultrafast Valley Switching in Two-Dimensional Materials

Abstract

Electrons in two-dimensional materials possess an additional quantum attribute, the valley pseudospin, labeled as $\mathbf{K}$ and ${\mathbf{K}}^{\mathrm{\ensuremath{'}}}$---analogous to the spin up and spin down. The majority of research to achieve valley-selective excitations in valleytronics depends on resonant circularly polarized light with a given helicity. Not only acquiring valley-selective electron excitation but also switching the excitation from one valley to another is quintessential for bringing valleytronics-based technologies in reality. Present work introduces a coherent control protocol to initiate valley-selective excitation, de-excitation, and switch the excitation from one valley to another on the fly within tens of femtoseconds --- a timescale faster than any valley decoherence time. Our protocol is equally applicable to both gapped and gapless two-dimensional materials. Monolayer graphene and molybdenum disulfide are used to test the universality. Moreover, the protocol is robust as it is insensitive to significant parameters of the protocol, such as dephasing times, wavelengths, and time delays of the laser pulses. Present work goes beyond the existing paradigm of valleytronics, and opens an alternative realm of valley switch at petahertz rate.