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.

Objective lenses are frequently employed in state-of-the-art ultrafast time-resolved microscopy techniques. While it enables tight spatial focusing, its dispersion causes a longer optical pulse duration. Since an objective lens is a combination of different lens materials, finding its correct dispersion can be challenging. In this paper, we propose a dispersion measurement method for an objective lens using white light interferometry. Our proposed method enables the experimental determination of the lens dispersion and improves the temporal resolution of ultrafast time-resolved microscopy techniques.

Abstract We have investigated the dependence of terahertz wave emissions on the internal electric field in undoped GaAs/n-type GaAs epitaxial structures irradiated by ultrashort laser pulses. The undoped layer has an electric field, the strength of which was controlled by the temperature in addition to the undoped layer thickness. We observed the electric field dependence of the terahertz waveform, and the results were explained by the calculation of the transient dynamics of electrons and phonons under electric fields. Furthermore, we indicated that the terahertz amplitude can be linearly controlled by the electric field strength in a wide electric field range.

We have realized element-specific pump-probe time-resolved hard X-ray photoelectron spectroscopy (TR-HAXPES) for the study of ultrafast electron dynamics in condensed matter. TR-HAXPES has been applied for the first time to determine the temporal evolution of space-charge effects on Ti 1s core-level photoelectron spectra of SrTiO₃ at the SACLA X-ray free electron laser facility. We found that the temporal evolution of Ti 1s kinetic energy shifts can be well explained by a mean-field model for the electron propagation in the vacuum. We also report preliminary results of a real-time laser-assisted valence-transition in YbInCu₄, observed by measuring Yb 3d_5/2 core-level HAXPES spectra at BL19LXU of SPring-8, under ON/OFF conditions of an optical pump-laser. We observed drastic changes in intensities of the Yb²⁺ and Yb³⁺ spectral components induced by pump-laser irradiation.