Development of Broadband Time-Resolved Spectroscopy and Investigation of the Dynamics of Photoexcited Carriers and Lattice Excitations in Chiral Weyl Semimetals

Abstract

This thesis presents both technological developments of broadband time-resolved ultrafast spectroscopy and investigation of the dynamics of photoexcited carriers and topological phonons in chiral Weyl semimetals. In the first portion of this thesis, we describe a novel, optical pump supercontinuum probe technique based upon a nonlinear photonic crystal fiber and a digital micro-mirror device enabling the use of single element detector and lock-in demodulation of the signal, along with rapid, repeated averaging over the spectrum. Using a bismuth test sample, we demonstrate that the apparatus is capable of measuring time-resolved changes in reflectivity ΔR of a sample over the 1.5 - 3.0 eV energy range with 25 fs temporal resolution, while also being sensitive to relative changes in reflectivity as low as ΔR/R ~ 10^(-4). A more conventional broadband pump-probe technique was applied to the chiral Weyl semimetal RhSi (S.G. 198) with the aim of investigating the dynamics of chiral single particle excitations and collective modes of topological, Weyl quasiparticles. In S.G. 198 materials, lack of crystallographic mirror symmetries allows for Weyl nodes to exist with a relative displacement of ~ 330 meV in energy, permitting optical investigation of the dynamics of a single Weyl node without interference from other bands or the opposite chirality Weyl node. The probe wavelength was independently scanned over the 0.4 -1.0 eV energy range to monitor the photoinduced changes in reflectivity fixing the pump wavelength at energies 0.57, 0.73 & 1.03 eV in order to excite one node, two nodes, and non-topological portions of the band structure, respectively. A single fast decay process (relaxation time τ ~ 300 - 500 fs) was observed over the entire energy regime studied, consistent with previous measurements of photoexcited charge dynamics in other Dirac and Weyl semimetals. Significantly, measurements of time-resolved Kerr effect spectroscopy yielded evidence of a T representation chiral lattice excitation whose observed frequency matched the calculated frequency using density functional theory.