Prof. Ryo Shimano Profile

Prof. Ryo Shimano

at Univ of Tokyo

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Proceedings Article | 3 October 2022 Presentation + Paper

Helicity-dependent terahertz-wave emission from the Dirac electrons in bismuth

Y. Hirai, N. Yoshikawa, H. Hirose, M. Kawaguchi, M. Hayashi, R. Shimano

Proceedings Volume 12205, 1220502 (2022) https://doi.org/10.1117/12.2632879

KEYWORDS: Bismuth, Terahertz radiation, Polarization, Electrons, Thin films, Pulsed laser operation, Femtosecond phenomena, Crystals

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We demonstrated that thin films of bismuth emit helicity-dependent terahertz-waves when illuminated with circularly polarized near-infrared femtosecond laser pulses from an oblique incidence. The helicity-dependent terahertz-wave appears only in the polarization perpendicular to the incident plane and is the most dominant contribution to terahertz emission in this polarization. By increasing the thickness of the film, the helicity-dependent terahertz-wave emission enhances significantly, taking a maximum at around 70-nm-thickness, which is well beyond the penetration depth of the near-infrared laser. From this thickness dependence, we identify the photoinduced inverse spin Hall effect as the most plausible mechanism behind the helicity-dependent terahertz emission. By lowering the temperature, we find a significant enhancement of the high-frequency component of the helicity-dependent terahertz-waves for the 30-nm-thick sample. The current dynamics are extracted from the terahertz-waves, and we find that the enhancement comes from the increasing photocurrent’s relaxation rate when the temperature is lowered. By considering two different spin relaxation mechanisms, namely the Elliott-Yafet mechanism and the D’yakonov-Perel’ mechanism, we attribute the sharp increase of the relaxation rate seen for the 30-nm-thick film to the D’yakonov-Perel’ mechanism. Our findings highlight the unique characteristics of bismuth as a beneficial platform for terahertz spintronics, and the potential of terahertz emission spectroscopy as a useful probe for ultrafast spin/charge dynamics.

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