Keisuke Kaneshima

VO ₂ is a prototypical strongly correlated material that undergoes photoinduced phase transitions on the femtosecond scale. To explore its potential for ultrafast all-optical phase modulation, we constructed a femtosecond time-resolved interferometer to directly probe the optical phase response, revealing the change in the real part of the refractive index during the phase transition. The observed optical phase dynamics exhibit faster and slower components, reflecting different stages of the phase transition process. The overall magnitude of the phase shift is largely determined by the ultrafast response, demonstrating the feasibility of femtosecond all-optical phase modulation in VO ₂ thin films.

Tapered undulators are designed to generate broadband, chirped radiation for advanced synchrotron light sources. However, the complete electric-field waveform of the radiation has never been directly measured. Here, we report the first, to the best of our knowledge, direct characterization of a chirped wavepacket generated from a tapered undulator using spectral phase interferometry for direct electric-field reconstruction (SPIDER). The retrieved spectral phase reveals a distinct linear chirp, and its magnitude shows essential agreement with theoretical predictions. This work provides the first experimental validation of the chirped waveform generated from an undulator, enabling advanced temporal control of next-generation light sources.

This study introduces a compact, portable femtosecond fibre laser system designed for synchronization with SPring-8 synchrotron X-ray pulses in a uniform filling mode. Unlike traditional titanium–sapphire mode-locked lasers, which are fixed installations, our system utilizes fibre laser technology to provide a practical alternative for time-resolved spectroscopy, striking a balance between usability, portability and cost-efficiency. Comprehensive evaluations, including pulse characterization, timing jitter and frequency stability tests revealed a centre wavelength of 1600 nm, a pulse energy of 4.5 nJ, a pulse duration of 35 fs with a timing jitter of less than 9 ps, confirming the suitability of the system for time-resolved spectroscopic studies. This development enhances the feasibility of experiments that combine synchrotron X-rays and laser pulses, offering significant scientific contributions by enabling more flexible and diverse research applications.

Abstract Observing ultrafast pulse-to-pulse dynamics of highly photoexcited materials could foster a comprehensive understanding of the initial stage of irreversible photoinduced events, such as phase change, structural deformation, and laser ablation. In this study, using high-repetition-rate single-shot spectroscopy and a laser microscope, the pulse-to-pulse ultrafast dynamics of energy relaxation in Ge₂Sb₂Te₅ thin films are revealed under high-density photoexcitation that induces sequential events involving the crystalline-to-amorphous phase transition, melt and quench processes, and formation of laser-induced periodic surface structures (LIPSSs). Above the threshold excitation density for LIPSS formation, the first excitation pulse induces the transient transmittance change of the crystalline phase in a picosecond timescale, and subsequent pulses provoke the amorphous phase energy relaxation with a long decay time of hundreds of picoseconds. We observed that the subsequent pulses gradually reduce the amplitude and decay time of the transient transmittance, leading to efficient energy relaxation and LIPSS formation in the photoinduced amorphous phase.