Different spin relaxation properties observed in linearly and circularly polarized laser induced terahertz emission from a Bi/Co bilayer

Recently, a helicity-dependent photocurrent was reported in Bi single-layer thin films. It is proposed that the origin of this photocurrent is the combination of photon-spin conversion and spin-charge conversion effects in Bi, and efficient spin conversion in Bi is expected. In this study, we measure two types of terahertz (THz) emissions from Bi/Co bilayer films induced by spin current generation using laser induced demagnetization of the Co layer and the photon-spin conversion effect in the Bi layer to investigate the spin current induced by the two mechanisms simultaneously. We clearly observe different Bi thickness dependences of peak intensity and bandwidth for THz spin current in two experiments, i.e., spin current induced by demagnetization of Co and by photon-spin conversion in Bi. The different Bi thickness dependences of spin current intensity and bandwidth in two experiments are caused by different spin relaxation properties of optically excited spin currents in Bi layers.

Automated summary

A recent study reported a helicity-dependent photocurrent in Bi single-layer thin films. This photocurrent is believed to originate from the combination of photon-spin conversion and spin-charge conversion effects in Bi, with a high efficiency of spin conversion expected in Bi. In this study, two types of terahertz (THz) emissions from Bi/Co bilayer films were measured to investigate spin current induced by laser induced demagnetization of the Co layer and photon-spin conversion effect in the Bi layer simultaneously. The observed different thickness dependences of peak intensity and bandwidth for THz spin current in the two experiments are attributed to the distinct spin relaxation properties of optically excited spin currents in Bi layers.