Investigation Of Transient Surface Electric Field And Ultrafast Structural Dynamics Of Metals Induced By Femtosecond Laser Irradiation

Based on a home-made laser-pump electron-probe setup, which is capable of operating in three models: Ultrafast Electron Shadowgraph, Deflection and Diffraction, we investigated the transient surface electric field and structural dynamics induced by femtosecond laser irradiation of metallic samples. These studies consist of three parts:Part I: Investigation of transient surface electric fieldUnder an intense laser irradiation on the order of 1014 W/cm2, the transient electric field(TEF) is mainly contributed by the temporal evolution of laser-induced plasmas, whose evolution during the first few tens of picoseconds is of critical importance in inertial fusion and laser-plasma accelerator studies. However, due to the lack of chargedparticle probes, the initial evolutions of laser-plasmas are mainly studied by numerical simulation. Employing Ultrafast Electron Shadowgraph and Deflection, we investigated the picosecond evolution of global and local electric field distributions of plasmas induced by laser-sliver interaction, respectively. In addition to a demonstration of the symmetric distribution of such electric field, this study also illustrated a method that can investigate transient electric filed with a simultaneous picosecond temporal and micrometer spatial resolution.Under moderate laser intensities on the order of 1010 W/cm2, which is well below the damage threshold of metals such as aluminum, TEF is mainly due to the chargeseparation effects on the sample surface. Such TEF may distort the trajectory of probe electrons and reduce the detection resolution and accuracy of time-resolved electron scattering experiments(Such as Ultrafast Electron Diffraction and Time-resolved Angular-resolved Photoelectron Spectroscopy). Meanwhile, TEF effect is also a critical influencing factor for the optimization of photocathodes. Employing Ultrafast ElectronDeflection, we investigated the TEFs induced by moderate femtosecond laser irradiation of an aluminum nano-film. At pump intensities range from 2.9 to 7.1×1010 W/cm2, the TEFs last at least one nanosecond with a maximum field strength of 3.2~5.3 × 104 V/m at 120 μm above the aluminum surface. Such TEFs and the associated evolutions of photoelectrons were explained by a “three-layer” model. The potential influence of such fields on reflection ultrafast electron diffraction and time-resolved angle-resolved photoemission spectroscopy were evaluated.Part II: Simultaneous investigation of transient surface electric field and ultrafast structural dynamicsAccording to the investigations in Part I, TEFs generally exist in Ultrafast Electron Diffraction studies. Therefore, it is of critical importance to distinguish the effects of TEFs in electron diffraction data. In this study, the ultrafast structural dynamics and surface TEFs, which are concurrently induced by laser excited electrons of an aluminum nano-film, have been simultaneously investigated by the same transmission electron diffraction patterns. These two processes are found to be significantly different and distinguishable by tracing time dependent changes of electron diffraction and deflection angles, respectively. This study also provides a practical means to evaluate simultaneously the effect of TEF during the study of structural dynamics under low pump fluences by transmission ultrafast electron diffraction.Part III: Investigation of ultrafast structural dynamicsDetecting and understanding of ultrafast structural dynamics with an atomic spatialtemporal resolution is the key for the ultimate control of material properties. With the method introduced in Part II, it is possible to extract ultrafast structure dynamics with the presents of transient electric fields in a transmission Ultrafast Electron Diffraction configuration. Therefore, we further studied the structural dynamics of aluminum films and gold nanoparticles. For the aluminum experiments, we built an optical parametricamplifier to evaluate the parallel band transition effects by comparing the structural dynamic results obtained from 1.25 μm and 800 nm pump pulses. The preliminary results indicated that, the parallel band transition, which corresponded to the photon energy of 800 nm pump laser, contributes a non-thermal heating mechanism. For gold nanoparticle experiments, it was found that, at pump intensities ranging from 2.0~4.0 m J/cm2, the change of lattice equilibrium position was delayed by ~4 ps comparing to that of the lattice heating. This is due to the time-dependent strain-stress evolutions within gold nanoparticles. Meanwhile, the expansion of lattice plane distance, electron–phonon coupling time increased linearly with respect to pump laser energy, which indicates a pure thermal heating mechanism of gold nanoparticles under femtosecond laser irradiation.