Dr. Naotaka Yoshikawa Profile

Dr. Naotaka Yoshikawa

at Univ of Tokyo

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Publications (2)

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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.

Proceedings Article | 7 March 2022 Presentation + Paper

Amplitude mode-driven ultrafast transition into a hidden state in a thin film of transition metal dichalcogenide

Ryo Shimano, Naotaka Yoshikawa

Proceedings Volume 11999, 119990B (2022) https://doi.org/10.1117/12.2611742

KEYWORDS: Terahertz radiation, Transition metals, Ultrafast phenomena, Spectroscopy, Picosecond phenomena

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Ultrafast non-thermal control of quantum materials has gained growing interest over decades. Contrary to the conventional knowledge that the photoexcitation causes heating of materials and destroys the low temperature ordered phases, recent developments of ultrafast light sources have shown the possibility of creating symmetry-broken ordered phases before the system reaches thermal equilibrium state. As a new route for such a light-induced phase transition, we have investigated the effect of strong excitation of amplitude mode in a charge density wave (CDW) phase in a layered transition-metal dichalcogenide compound, 3R-Ta_1+xSe₂¹. A soft phonon mode associated with the CDW phase transition, namely the amplitude mode, is identified at 2.3 THz at the lowest temperature through the optical pump and optical probe experiments. When this amplitude mode is coherently driven by an intense THz pulse through the two-photon excitation process, a dynamical suppression of the CDW order is manifested by the mode softening of the CDW amplitude mode with intense THz excitation. Furthermore, a gap opening is observed in the THz-frequency optical conductivity spectrum, indicating that an insulating-like metastable state is induced by the amplitude mode excitation. The formation dynamics of the gap synchronizes with the oscillation of CDW amplitude mode, which indicates the intimate interplay between the order parameters of the equilibrium CDW and the induced metastable hidden state. In this presentation, we overview the above results which have been recently published in Ref.1.

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