Terahertz emission from circular photogalvanic effect in bismuth thin films*

When circularly polarized light is shined onto a sample, optical selection rules lead to spin-dependent excitations. This can cause helicity-dependent spin-polarized photocurrents within the sample, referred to as the circular photogalvanic effect (CPGE). Bismuth is a highly expected material to host spin-dependent photocurrents, due to its large spin Hall angle and Rashba-like surface states. Recently, helicity dependent photocurrents have been demonstrated in bismuth/copper heterostructures by dc transport measurements [1]. Correspondingly, ultrafast transient photocurrents are expected to occur under the illumination of femtosecond laser pulses, resulting in the emission of terahertz (THz) pulse radiation.
Here, we present the observation of THz emission from bismuth thin films under near-infrared (800 nm, 1.55 eV) pulse excitation. A polarization dependent THz emission is observed in both bismuth thin films and bismuth/metal heterostructures. The opposite polarities of the emitted THz pulse with the left/right-hand circular polarized excitation evidences the transient photocurrent with the opposite direction due to CPGE.

[1] Hirose et al., Appl. Phys. Lett. 113, 222404 (2018)