Investigation of void nucleation on photoinduced ultrafast melting process

Abstract

Melting is a universal phase transition that occurs in various materials. In a phase diagram, the melting transition occurs on a specific coexistence line, and each line point assumes an equilibrium state under each condition. In terms of time scale, the conventional understanding of melting is based on a long time scale that approaches at least a quasi-equilibrium state. Despite the universality of melting, there is a lack of understanding of its microscopic and dynamic knowledge. Furthermore, the emergence of photoinduced ultrafast melting with the development of ultrafast laser sources has raised questions about how well we understand the melting process. In photoinduced ultrafast melting, light energy is selectively absorbed by electrons, and a highly nonequilibrium state with high electron and cold-lattice temperatures is established. Microscopic and dynamic knowledge of the subsequent energy transfer of this nonequilibrium state is essential for understanding ultrafast photoinduced melting. Ultimately, this understanding can lead to the control of ultrafast melting through different laser parameters such as polarization and fluence. An experimental method is required to study photoinduced ultrafast melting and observe the spatiotemporally inhomogeneous system. Single-shot time- resolved coherent diffraction imaging (CDI) is an effective experimental technique that provides picosecond temporal resolution and nanometer spatial resolution. This thesis investigates the photoinduced ultrafast melting process of gold nanoparticles using this technique. The electron cloud of a metallic nanoparticle is sensitive to light polarization. Therefore, it is a good target sample for studying the effects of light polarization in photoinduced ultrafast melting. In addition to the polarization dependence, the effect of fluence was observed by comparing the experimental results with two different fluences. These experimental results were analyzed using two-temperature molecular dynamics simulations. The obtained observations and analysis reveal that the transient pressure generated by photoexcited hot electrons controls the nucleation of voids during ultrafast melting. To improve the experimental method, we developed a multiplexing experimental setup to simultaneously measure small- and wide-angle X-ray scattering and X-ray emission spectrum. The small-angle X-ray scattering signal can be used for CDI. Moreover, wide-angle X-ray scattering can be used to observe the average atomic-scale change during ultrafast melting. The X- ray emission spectrum provides information about the filled electrons. Each measurement signal covers a different observation area and complements the others. The performance of this multiplexing experimental setup was successfully verified through a commissioning experiment using an X-ray free- electron laser. The time-resolved experiment was tested along with the static measurement, revealing its versatility in measuring dynamic phenomena induced by ultrafast lasers. The multiplexing experimental setup is expected to provide a valuable platform for studying various aspects of ultrafast phase transitions.