Terahertz Emission from Bismuth Thin Films Induced by Excitation with Circularly Polarized Light

Bismuth is remarkable for its strong spin-orbit coupling and interband effects, and electrons that behave as Dirac particles. Consequently, it efficiently converts electric current to spin current via the spin Hall effect (SHE). This study shows that furthermore bismuth thin films can emit light in the key terahertz frequency range, when irradiated by circularly polarized femtosecond laser pulses. Optically generated spin current plus the inverse SHE combine to generate ultrafast electric current, and thus terahertz emission. These results demonstrate the potential of Dirac electrons in bismuth for a simple, controllable terahertz source for ultrafast spintronic applications and beyond.