Keisuke Kaneshima

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.

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.