On signatures of sonic wavepackets in time-resolved X-ray diffractometry of metal single crystals absorbing pulse from ultrafast laser

TL;DR

This study investigates how ultrafast laser pulses induce sonic wavepackets in metal single crystals, revealing differences in acoustic phonon generation and wavepacket dynamics in copper and gold through time-resolved X-ray diffraction.

Contribution

It provides new insights into the generation and propagation of acoustic phonons in metals under ultrafast laser excitation, highlighting the role of electron diffusion and absorption layers.

Findings

Oscillations in rocking curves correlate with acoustic wave bouncing.

In gold, acoustic waves originate from the absorption layer.

In copper, phonon generation occurs throughout the bulk, faster than expected.

Abstract

Copper (Cu), gold (Au) (111) crystals were illuminated with 120 fs pulses and probed by 600 fs X-ray pulses. Rocking curves were measured versus 267 nm (UV) pump- 0.154 nm probe delay time. Curve width broadening, peak diffracted intensity and angular shift were recorded for the range of UV excitation intensities of 2-8 mJ/cm2 . Observed oscillations in time delay dependences of shift, width and of peak intensity of rocking curves are correlated, as described by acoustic pulse bouncing between crystal surfaces. In (111) Au, acoustic pulse originates from pump absorption layer, and peak intensity drops by a factor of two (at delay time at which sonic wave reaches Au-substrate boundary) of its initial value prior to absorption of laser pulse. In Cu, correlated rocking curves dynamics is also observed, however buildup of rocking curve broadening is faster than one can expect from sonic wavepacket originating in pump absorption layer. This effect is attributed to acoustic phonon generation outside of pump absorption layer, it is much stronger in Cu than in Au. Phonon generation in Cu is distributed throughout the whole bulk of crystal due to delocalization of mobile hot electrons diffusing from pump absorption layer.