Generation of a large compressive strain wave in graphite by ultrashort-pulse laser irradiation.

Xiaocui Wang,C.-M Tu,M Wulff,Å U J Bengtsson,H Enquist,M N Pedersen,J Larsson,J C Ekström,A Jarnac,F Dorchies,A Jurgilaitis

doi:10.1063/1.5089291

Abstract

We have studied strain wave generation in graphite induced by an intense ultrashort laser pulse. The study was performed in the intensity regime above the ablation threshold of graphite. The aim was to maximize the strain and, thus, also the internal pressure (stress). Laser pulses with a 1 ps temporal duration melt the surface of graphite resulting in a molten material which initially exists at the solid density. As the molten material expands, a compressive strain wave starts propagating into the crystal below the molten layer. The strain pulse was studied with time-resolved X-ray diffraction. At a temporal delay of 100 ps after laser excitation, we observed >10% compressive strain, which corresponds to a pressure of 7.2 GPa. This strain could be reproduced by hydrodynamic simulations, which also provided a temperature map as a function of time and depth.

Highlights

Strain in graphite has been extensively studied in order to understand the role of non-thermal processes in strain generation1–3 and phonon-phonon interaction4 and to study heat transport in this anisotropic material.5 Graphite is one of the carbon allotropes
Raman et al used time-resolved electron diffraction (TRED) to observe interlayer compression in graphite induced by a femtosecond laser pulse with a fluence of 77 mJ/cm2, and their results suggest that an increased population of 2pz orbitals may induce interlayer attraction in the graphite lattice
The laserperturbed pattern shifted towards larger values of the momentum transfer vector in reciprocal space, indicating that the graphite lattice was compressed in real space after laser excitation

Summary

Introduction

Strain in graphite has been extensively studied in order to understand the role of non-thermal processes in strain generation and phonon-phonon interaction and to study heat transport in this anisotropic material. Graphite is one of the carbon allotropes. Raman et al used time-resolved electron diffraction (TRED) to observe interlayer compression in graphite induced by a femtosecond laser pulse with a fluence of 77 mJ/cm, and their results suggest that an increased population of 2pz orbitals may induce interlayer attraction in the graphite lattice.. Ultrafast laser ablation has been studied experimentally and theoretically at fluences higher than 185 mJ/cm. Small quantities of nano-diamonds have been created following laser excitation in multi-pulse exposures.. A mechanism that may be involved in this phase transition is the compressive strain wave, which is launched by the ablation process and propagates into the bulk graphite. The application of these techniques to graphite has focused on strain measurements along the c-axis ([0 0 1]) at fluences below the ablation threshold. No direct time-resolved

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Conclusion