Optical Signatures of Ultrafast Symmetry Switching in Weyl Semimetal TaIrTe₄

Td phase Weyl semimetals are of technological interest due to their crystal polarization and nontrivial topology being dependent on the relative orientation of their van der Waals stacked layers. Optical switching of crystal symmetry via layer sliding has been reported in Td phase Weyl semimetals WTe₂ and MoTe₂, but is unexplored in isostructural TaIrTe₄, which has recently demonstrated a more pronounced nonlinear Hall effect and greater environmental stability than WTe₂. We investigate the optical response of the TaIrTe₄ crystal symmetry following an ultrafast, NIR pump via transient reflectance and time-resolved phase-sensitive second harmonic generation (SHG). From the reflectance probe, we isolate a pump-induced excitation of an interlayer phonon mode (~15wn) and observe a threshold pump fluence above which the phonon oscillation inverts phase. Examining phase-sensitive SHG, a minority of polarization configurations undergo a near complete phase inversion of the SHG emission following an intense pump. This process largely occurs within 100 fs of the pump arrival, and decays over the following nanoseconds. Efforts are ongoing to assign the space group of this metastable state via the polarization anisotropy of the SHG phase and intensity shifts, in addition to structural modeling. This demonstration of optical control of lattice symmetry reveals another dimension to TaIrTe₄'s topological applications.