Revealing Optical Transitions and Carrier Dynamics within the Bulk Band Structure of Chiral Tellurium Nanosheets*

Tellurium is a chiral crystal consisting of helical chains arranged in a hexagonal array along the c-axis. The bulk band structure due to the spin-orbit interaction exhibits Weyl nodes with opposite chirality at the H and H’ points in the Brillouin zone. Here we use polarized mid-infrared pump-probe spectroscopy on nanosheets of tellurium to map the band-edge electronic structure and interrogate carrier relaxation processes over a wide energy range (0.3 to 1.2 eV). We observe a series of transitions between all three valence bands at H-point of the Brillouin zone (H₄, H₅ and H₆ symmetries) with the doubly degenerate conduction band with Weyl points (H₆ Symmetry) with dynamic response of polarization anisotropy. The main decay of photoexcited carriers occurs within the first 50 ps, followed by a weak long-lived decay of carriers. The carrier thermalization process varies with polarization state of the probe beam at different bands. Such knowledge of electronic structure and carrier dynamics provides a foundation for understanding the topological and anisotropic nature of the material.