Heat Absorption, Transport and Phase Transformation in Noble Metals Excited by Femtosecond Laser Pulses

Abstract

With the rapid development of short pulse lasers, femtosecond (fs) lasers have been used in technologies such as materials modification (Vorobyev & Guo, 2008), micromachining (Liu et al., 1997; Gattass & Mazur, 2008), and surface patterning (Miyaji & Miyazaki, 2008). However, details on energy absorption, heat dissipation, and phase transformation during the initial excitation of these intense pulses are poorly understood, despite their importance in determining the final microstructure and morphology of the laser-processed materials. Scientifically, intense laser pulses drive materials into highly non-equilibrium states, which provide us new opportunities in studying the fundamental properties of these states. By carefully controlling the relaxation of the excited materials, it is also possible to create novel metastable structures. Before we can achieve these goals, we need a detailed knowledge on how materials respond to these intense pulses. This is a non-trivial task since the optical, thermodynamics and transport properties can change drastically upon intense laser irradiation. In this chapter, we summarize our recent works on Ag to illustrate some of the complexities involved in both heat transport and absorption at laser fluencies near the melting and ablation threshold. We observe significant heat confinement and absorption enhancement at high laser fluences. These differences derive mainly from the excitation of d-band electrons at high electron temperatures, which significantly changes the electronic properties, although many of the details remain poorly understood. From a more applied consideration, we show that by using fs-pulses, we are able to confine melting and ablation to the very top layers of the materials, which has important implications for pulsed laser deposition of ultra-thin layers and micromachining. Furthermore, we demonstrate that by utilizing the ultrafast quenching of the surface layer after laser excitations, we can create a pure metallic liquid at a temperature as low as 0.6 Tm, where Tm is the melting temperature of the metal. This degree of undercooling has not been achieved by other experimental techniques in pure metals and the transformation kinetics of this far-from-equilibrium state is not well-understood. Indeed, our results show that classical solidification theories cannot explain the experimental results and new atomistic mechanisms are needed in order to explain the measured kinetics.