The advent of femtosecond lasers has shed new light on non-equilibrium physics. The ultrafast energy absorption by electrons, and the finite rate of their energy transfer to the lattice, triggers a new class of non-thermal processes in solids of primordial importance in material science. The sub-ps dynamics of this electron–ion interplay is still poorly understood, as the experimental tools used up-to-now has provided an incomplete insight. The development of optical pump-probe experiments allows the study of these non-equilibrium processes on a femto-second time scale [Z. Chen et al, 2013]. For example, over such duration, ion temperature increases and electronic temperature decreases due to electron-phonon coupling. The description of the evolution of this electron-phonon coupling constant for this non-equilibrium superheated solid phase is crucial for the simulation of the time evolution of the system but it is still an open question. In order to describe ultrafast transformations in solid induced by ultra-short laser irradiation, we need to understand the fundamental processes on a microscopic level. The goal of this workshop is to advance our understanding of non equilibrium electronic structure.
When submitted to femtosecond laser heating, matter undergoes ultrafast phase transitions from solid to extreme states of matter. The study of this non-equilibrium physics up to Warm Dense Matter (WDM) conditions is fundamental to the optimization of processes (laser machining and ablation for industrial and medical applications), as well as for the fundamental understanding of dynamical properties of matter. Pump-probe experiments allowed the study of non-equilibrium superheated solid with Te>>Ti. This was based on heating of an ultrathin metal target by a femtosecond laser pump pulse followed by optical reflectivity and transmitivity measurements with a femtosecond laser probe pulse before ion motion causes significant hydrodynamic expansion (T. Ao et al, 2006, Y. Ping et al, 2008). In some materials (Al), the loss of crystalline structure is observed to occur on a ps time scale that is understood as a thermal phase transition [M. Kandyla et al, 2007]. The heated electrons transfer their energy to the ions, and solid-liquid transition occurs when the ionic temperature (Ti) exceeds the melting temperature. In some semiconductors (Si, InSb, ..), “non-thermal” mechanisms were invoked to reproduce the sub-ps time scales observed (M. Harb et al, 2008, E.S. Zijlstra et al, 2008). On the other hand, melting of gold is observed to last over a few ps, indicating increased lattice stability (“bond hardening”) in non-equilibrium conditions (and high electron temperature, Te) [Recoules et al, 2006]. Recently, new techniques of ultrafast electron and X-ray diffraction [Ernstorfer et al, 2009, B. Cho et al 2011 ] succeed to investigate the non-equilibrium superheated solid phase of metals and phonon hardening was proposed as an alternative mechanism for superheating . At the same time, the dynamics of melting through the vanishing of the lattice long-range order was studied using X-ray diffraction [Enrstorfer et al, 2009] or X-Ray absorption [Levy et al, 2012]. These observations demonstrate a strong correlation between electronic structure and lattice bonding, whose dynamics is still poorly understood.
From the theoretical side, in order to describe ultrafast transformation, such as change in optical properties, magnetic properties, or phase transition, we need to understand the fundamental processes on a microscopic level. The evolution of these two temperatures can be described by rate equations in what is known as Two Temperature Models (TTM) and is widely used to describe laser-heated materials (E. Karim et al 2012, Z. Lin et al, 2010, Mazevet et al, 2005). The heat transfer rate is accounted for by electron-phonon scattering (ref). Then, electron-phonon coupling constant is a crucial input for the TTM and at this time, there is still a controversy on the description of this coupling constant for two-temperature systems. The solid states physics approach consider that the electron kinetic is transferred to the degrees of freedom of bound ions, described as phonons (Z. Lin et al 2008, B. Arnaud et al 2011, Z. Chen et al 2013) and on the other side the plasmas physics approaches consider the scattering of electrons with plasmons (C. Dharma-Wardana 2012, J. Vorberger et al, 2010). This seriously challenged theoretical calculation of electronic structure of nonequilibrium solids.