Ultrafast electronic and structural dynamics in solids using femtosecond laser techniques.

摘要

This dissertation investigates the electronic and structural dynamics in solid materials driven by femtosecond laser pulses. Two series of experiments are presented. Firstly, the femtosecond laser induced periodic surface structures (LIPSSs) generated on different noble metal surfaces are systematically studied. For the first time, it is found that LIPSSs show distinctly different morphological appearance on different noble metals. The experimental observations suggest that the electron-phonon coupling strength plays the dominant role, which is supported by numerical analysis we performed subsequently on heat transfer in metals using the finite difference in time domain (FDTD) technique. Some special LIPSSs generated on metal surfaces using femtosecond lasers are also observed, and their generation mechanisms are investigated. Extraordinary uniform type-c LIPSSs that are parallel orientated to the incident light polarization can be formed on metal surfaces with p-polarized incident laser pulses at relatively large incident angles. A study of the angular dependence of the structures' periods supports the conjecture that surface plasmons play an important role during the formation of the type-c LIPSSs. Formation of mirror symmetrically orientated periodic structures by using circularly polarized light are also observed for the first time, which may be the only phenomena that can permanently record light helicity with an optically inactive material.;In the second set of experiments, coherent acoustic phonons in solids generated by femtosecond laser excitation are studied. A sensitive surface plasmon probing technique is applied to measure coherent acoustic phonon dynamics in metal thin films. The effect of hot electrons excited by femtosecond laser pulses is studied. It is shown that hot electron pressure contributes to the generation of coherent acoustic phonons in metal thin films. Using a metal thin film as a transducer, long-lived propagating acoustic phonons are generated in bulk dielectrics and are detected by using a two-color femtosecond laser pump-probe technique. Acoustic attenuation of the dielectric material for sound frequency from 20 GHz to 50 GHz is also directly measured through pump-probe experiments.