Terahertz Quasiparticle Acceleration: From Electron-Hole Collisions To Lightwave Valleytronics

Intense lightwaves can accelerate quasiparticles inside solids. This strong-field light-matter interaction results in the emission of high-harmonic or high-order sideband radiation. While the former process relies on a complex coupling between simultaneously driven interband polarization and intraband currents, high-order sidebands originate from a ballistic acceleration of the quasiparticles within the bands. This mechanism allows for the implementation of a quasiparticle collider in order to study those entities in close analogy to conventional collision experiments. Accelerating electrons and holes in a monolayer of transition metal dichalcogenides extends this scheme to internal quantum degrees of freedom. Our experiments show a lightwave-indu-ced switching of the valley pseudospin, paving the way to ultimately fast valleytronics.