Speaker
Description
Ultrafast electron diffraction is now a relatively mature approach for directly probing structural transformations in molecules and materials at the atomic-level. More recently this pump-probe technique has been extended beyond crystallographic/Bragg features to include phonon-diffuse scattering, which interrogates non-equilibrium structural fluctuations (about the average structure) and provides a remarkable window on a variety of quantum phenomena that emerge from electron-phonon coupling. Here I will report on one such phenomenon; how valley-selective carrier excitation in a monolayer transition metal dichalcogenide using circularly polarized light directly influences electron–phonon coupling and the quantum kinetics of chiral phonon generation. We report ultrafast electron phonon-diffuse scattering measurement of valley-polarized phonon populations in 1L-MoS2 that are selectively excited at either the K or K′ valleys depending on the light helicity. This transient vibrational state is characterized by a distinctive chirality, which lifts time-reversal symmetry of the lattice. In the context of this workshop, one could consider this effect a material-mediated chiral interaction between free electrons and light.