Femtosecond Electron Pulse as an Ultrafast Probe

The thesis presents the recent development of the 3rd generation femtosecond electron diffractometer in Professor Jim Cao's group and its application to study ultrafast processes in real time. The research activities cover two main subjects: photoinduced structural phase transition (PIPT) in colossal magnetoresistive (CMR) materials and the dynamics of electron emission and the associated residual charge redistribution in targets during the early stage of laser ablation. In the study of PIPT in CMR materials, a direct and real time measurement of photoinduced structure phase transition in single crystal La0.84Sr0.16MnO3 and LaMnO3 was performed by using femtosecond electron diffraction. The melting of orthorhombic lattice ordering under femtosecond optical excitation is found involving two distinct processes with different time scales, an initial fast melting of orthorhombic phase in 3 ps and a subsequent much slower transformation in 50 ps and longer timescales. The fast process can be attributed to the initial melting of orthorhombic phase induced by the Mn-O bond change that is driven by the quenching of the Jahn-Teller distortion following the photo-excitation. The slow process is associated with the growing of newly formed structure domain from excited sites to the neighboring non-excited orthorhombic sites. In the second project, two new techniques, namely femtosecond electron shadow imaging and ultrafast electron deflectometry, were developed. These two complementary techniques provide both a global view and local prospect of the associated transient electric field and charge expansion dynamics. The results reveal that the charge cloud above the target surface is predominantly composed of thermally ejected electrons and the charge cloud expands with a fast front-layer speed exceeding 107 m/s. The average electric field strength of the charge cloud induced by a pump fluence of 2.2 J/cm2 is estimated to be on the order of ~2.4x105 V/m. For the temporal evolution of residual charges on the target, the results show that residual charges in metals can redistribute themselves almost instantly, abiding by the boundary conditions and Maxwell equations in the same way as they would at electrostatic equilibrium condition. However, residual charges in dielectrics are confined within the excited area for hundreds of picoseconds and beyond. These observations provide an experimental support to the alleged coulomb explosion phenomenon in previous studies, as well as a reference for modeling residual charge dynamics. In addition, a 1-D molecular dynamics simulation of coherent lattice motion in laser excited thin film is presented in the last section of this thesis. Using this simulation, both the displacement and expansion at each lattice site along the 1-D atomic chain can be traced as a function of delay time. In particular, the simulation shows that the electronic thermal stress is responsible for driving the lattice motion at the early stage, which matches very well with our FED experimental data obtained in the study of ultrafast heating of free-standing metal films.