The role of thermal and electronic pressure in the picosecond acoustic response of femtosecond laser-excited solids

摘要

In this work we apply ultrafast time-resolved X-ray diffraction to study the dynamics of coherent acoustic phonons in laser-excited Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal driving forces. For Ge our measurements reveal that the relative strength of the electronic pressure decreases with increasing laser fluence. For larger laser fluences the thermal pressure exceeds the electronic one, and only at low excitation strength the electronic pressure becomes the dominant driving force, as predicted by theory [1]. For the case of Au the data are well described within the established theoretical framework using the known values for those material parameters which determine the laser-induced pressure, namely the energy relaxation time and the electronic and lattice Gruneisen parameters.