Using polarized mid-IR pump-probe spectroscopy to identify band-to-band transitions in Weyl semimetals

Because Weyl semimetals do not have a gap, it has been extremely difficult to use optical spectroscopy to identify band-to-band transitions. While optical transitions connect states below the Fermi energy to above, a 0.6 eV photon potentially connect up to a dozen possible bands. However, if a 1.5 eV pump pulse excites electrons and holes well away from the Fermi energy (which is near the Weyl crossing), upon relaxation a nonequilibrium hot distribution of electrons is seen at the Fermi level which thermalizes within picoseconds. If a delayed probe pulse detects the changed reflectivity caused by the presence of this non-thermal distribution, it is possible to identify those optical transitions which connect to initial or final states which are near the Fermi level. Since topological semimetals typically have very few bands which cross the Fermi level (the bands which form the Weyl crossings among them), such a modulated spectroscopy significantly reduces the number of potential bands which are involved. By using circular or linear polarization, it may make possible an unambiguous identification of bands which are connected to states near the Fermi level. In this talk we describe preliminary polarized transient reflectivity measurements from 0.3 to 1.0 eV on the Weyl semimetals WTe₂, NbIrTe₄ and MoTe₂ which exhibit derivative-like optical transitions which are distinctly different for these three different materials.