Yoshihito Tanaka

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.