Polarized Transient Reflectance of Type II Weyl Semimetals WTe₂ and NbIrTe₄

We characterize ultrafast optical anisotropy in type-II Weyl semimetals WTe₂ and NbIrTe₄, using polarized transient reflectance. We study single nanoflakes excited by an ultrafast (140 fs) 800 nm pulse, and monitor the dynamic reflectance of a tunable polarized NIR-MIR probe pulse. CW polarized reflectance measurements establish the biaxial symmetry of a flake's optical anisotropy. A half-wave plate sweeps the probe polarization between critical optical axes and records the dynamic response as a function of angle. The intensity and phase of the transient signal sensitively depends on the probe polarization in both materials, whereas characteristic lifetimes show no clear correlation. In WTe_2, we observe a rapid ~1.0 ps decay followed by a slow nanosecond-scale relaxation. Likewise in NbIrTe₄, we observe a fast ~0.6 ps initial decay followed by a slower nanosecond process. We interpret these results in the context of carrier thermalization and lattice cooling in topological semimetals.