Floquet-Bloch Valleytronics

TL;DR

This paper demonstrates the creation and control of valley-polarized Floquet-Bloch states in monolayer WSe2 using chiral light pulses, linking Floquet engineering with valleytronics in 2D materials.

Contribution

It introduces the concept of Floquet-Bloch valleytronics and experimentally shows how to generate and manipulate valley-polarized states with ultrafast light in transition metal dichalcogenides.

Findings

Valley-polarized Floquet-Bloch states are formed in WSe2.

Quantum path interference depends on valley pseudospin and light polarization.

Circular dichroism reveals control over orbital character of Floquet states.

Abstract

Driving quantum materials out-of-equilibrium makes it possible to generate states of matter inaccessible through standard equilibrium tuning methods. Upon time-periodic coherent driving of electrons using electromagnetic fields, the emergence of Floquet-Bloch states enables the creation and control of exotic quantum phases. In transition metal dichalcogenides, broken inversion symmetry within each monolayer results in a non-zero Berry curvature at the K and K$^{\prime}$ valley extrema, giving rise to chiroptical selection rules that are fundamental to valleytronics. Here, we bridge the gap between these two concepts and introduce Floquet-Bloch valleytronics. Using time- and polarization-resolved extreme ultraviolet momentum microscopy combined with state-of-the-art ab initio theory, we demonstrate the formation of valley-polarized Floquet-Bloch states in 2H-WSe$_2$ upon below-bandgap coherent electron driving with chiral light pulses. We investigate quantum path interference between Floquet-Bloch and Volkov states, showing that this interferometric process depends on the valley pseudospin and light polarization-state. Conducting extreme ultraviolet photoemission circular dichroism in these nonequilibrium settings reveals the potential for controlling the orbital character of Floquet-engineered states. These findings link Floquet engineering and quantum geometric light-matter coupling in two-dimensional materials. They can serve as a guideline for reaching novel out-of-equilibrium phases of matter by dynamically breaking symmetries through coherent dressing of winding Bloch electrons with tailored light pulses.