Time-Resolved Pump-Probe and Kerr-Effect Spectroscopy of Collective Lattice Modes and Electronic Relaxation in Chiral Weyl Semimetals*

One of the special properties of chiral Weyl semimetals is that their Weyl points are separated in energy due to the lack of mirror plane symmetries of the crystalline lattice. Another special property of chiral crystal structures is that they can also host topological phonon Weyl and Dirac points with similar characteristics to the electronic quasiparticles that have attracted the bulk of the scientific community's interest. These phonon modes can be observed by time-resolved ultrafast spectroscopies in which a femtosecond timescale laser pulse impulsively excites collective excitations that are measured in the time-domain by a separate mechanically delayed pulse. Depending upon the symmetry of the mode, this probe pulse either experiences a time-dependent change in reflectance that is measured as pump-probe spectroscopy, or a change in polarization that is measured as a time-dependent Kerr rotation. In this talk, we describe our time-resolved pump-probe and Kerr effect measurements of these phonon modes in space group 198 compounds and discuss our observations in terms of the electronic Weyl band structure.