Realizing and Imaging Ballistic Transport in Van der Waals Semiconductors

Achieving long-range ballistic (coherent) electron flow in materials at room temperature is a long-standing goal that could unlock lossless energy harvesting and wave-based information technologies. The key challenge is to overcome short-range scattering between electrons and lattice vibrations (phonons). I will describe several avenues to achieve ballistic transport by harnessing strong interactions between coherent and incoherent excitations in solid-state lattices. The first is to leverage polaritons, part-light part-matter quasiparticles resulting from hybridization between photons and semiconductor excitations. The second is to leverage strong electronic and electron-phonon interactions, yielding either highly delocalized excitons or acoustic polarons that are intrinsically shielded from phonon scattering. In all cases, we develop ultrafast optical imaging capabilities enabling us to track the propagation of these quasiparticles with femtosecond resolution and few-nanometer sensitivity over a frequency range spanning the visible to the mid-IR, providing a precise measurement of quasiparticle velocity, scattering pathways, and transition from coherent to incoherent transport.