Study of Ultrafast Electron Dynamics in Non-equilibrium Warm Dense Matter using Femtosecond Laser and X-ray Free Electron Laser

Abstract

The non-equilibrium warm dense matter generated by irradiation of ultrafast laser on a solid density medium is highly transient, unstable, and intermediate between condensed matter and plasma. This condition has a high temperature and is strongly coupled. The studies of this extreme condition are still questionable and challenging because of the extraordinary experimental conditions and barely known theories, unlike condensed matter and plasma. This thesis presents a series of experiments using an ultrafast optical laser and X-ray free electron laser to interpret the ultrafast electron dynamics and non-equilibrium states. Furthermore, a novel model to study ultrafast electron dynamics using non-thermal electron kinetics is proposed which provides crucial evidence of the abnormal electronic thermalization in the non-equilibrium states by comparing with experimental results. To monitoring ultrafast free-electron dynamics, experiments using optical lasers are performed. At first, the fluence and pulse duration-dependent self-reflection, transmission, and absorption of a femtosecond laser pulse were measured for warm dense Au. The optical properties were varied by increasing the laser fluence and pulse duration, which can be hardly described using the traditional model with an instantaneous thermalization approach. Instead, the experimental features can be reproduced with our non-thermal electron kinetic model. The time-dependent reflectance in warm dense Al was observed using a femtosecond laser pump and chirped pulse probe technique within around 10 ps. The transient reflectance was interpreted with the hydrodynamic simulation of FLASH. The visualization of the d-band unoccupied state dynamics in the highly excited Cu was investigated using the femtosecond-resolved X-ray absorption spectroscopy. Thus, a significant lowering of energy of the d-band top-edge with the d-electron depopulation was observed in a few picoseconds. Furthermore, the X-ray absorption simulation with the transient d-band density of states based on the finite-temperature density functional theory reproduced the measurements with non-thermal electron kinetics. Finally, advanced diagnostical techniques such as laser-induced breakdown spectroscopy and an XFEL cross-correlator system at the X-ray Scattering and Spectroscopy (XSS) beamline in PAL-XFEL were introduced.