Terahertz Emission From Bismuth Thin Films Induced By Excitation With Circularly Polarized Light

We demonstrate that Bi thin films emit terahertz radiation when irradiated by circularly polarized femtosecond laser pulses. Terahertz emission that is polarized in a direction perpendicular to the plane of incidence is significantly enhanced when the pump pulse is circularly polarized compared with when it is linearly polarized. The enhanced terahertz emission reverses its polarity with the reversal of pump helicity. This helicity dependent terahertz emission is absent in the horizontal direction along the plane of incidence, and is odd with respect to the incident angle. Combined with the layer thickness dependence measurement, the dominant origin of terahertz emission is attributed to the photoinduced inverse spin Hall effect (ISHE) in Bi. The temporal dynamics of the ultrafast photocurrents are extracted and the spin and charge dynamics are discussed. The observation of terahertz emission from the photoinduced ISHE in a thin film consisting of a single material indicates the unique character of Bi, i.e., the long spin diffusion length and the large spin Hall angle, demonstrating the potential ability of the material to be applied to terahertz emitters and terahertz spintronics.