Ultrafast dynamics of the condensed phase

Ultrafast dynamics of the condensed phase under the action of ultrashort laser pulses

We performed experimental time-resolved studies of the dynamics of electron and lattice subsystems, phase dynamics and surface ablation plasma dynamics on the surface and in the bulk of materials excited by ultrashort laser pulses to determine the fundamental mechanisms of the treatment of materials and their key physical parameters. The peak reverberations of the acoustic wave in a short-lived film of the melt of materials were first revealed, which enabled us to study for the first time not only the melting dynamics but also the melt ablation dynamics. Methods were proposed to study the super-intense electron and ion emission from the surface of materials under the action of ultrashort laser pulses using buffer gas media. We developed and used in experiments methods of exciting ultra-intense (up to several TPa) shock waves on the surfaces of hard materials and their quantitative registration.
In particular, in the course of the studies of PSS generation under the action of USP with weakly subthreshold energy densities we first found not regular sequences of nanorelief grooves, but  two-dimensional arrays of nanospikes; moreover, these spikes periodically emerge on lines of USP–SEW interference maxima as the result of the cavitation instability of the surface melt (earlier, within the framework of calculations using the method of molecular dynamics, a possibility of forming a nonperiodic nanorelief was predicted). The microscopic mechanism of the cavitation dynamics, preceding the formation of nanospikes as the result of a partial “frozen” split-off of the surface film of the melt, as well as of the other – fragmentation and ionization –  mechanisms of ablation of solid-state materials under the action of USP, was first studied by us using time-resolved optical microscopy. These studies have first shown the possibility of monitoring the movement of the melting front in a material by  observing acoustic reverberations in the surface film of the melt with the acoustic impedance distinct from the impedance of a solid-state material, as well as the movement of the cavitation region from the surface into the bulk of the melt due to the cooling of the surface at its adiabatic expansion followed by a splitoff-ejection from the surface of the target of the cooled surface layer at subnanosecond times, apparently due, mainly, not to stress (rarefaction) waves circulating in the melt, but to steam pressure in the subsurface region of  cavitation (nanofoam). At higher USP energy densities, we noted an irreversible fragmentation removal of a higher temperature surface layer of the melt.
The fragmentation mechanism of the surface ablation of solid-state materials under the action of USP, as well as a higher-energy mechanism of their ablation  via direct ionization were also studied using the method of contact-free broadband ultrasonic diagnostics, adapted for the registration of extra-power shock waves (SW) generated during the ablation using USP. It is known that at the travel of extra-power SW in materials as the result of a shock-wave loading, the latter undergo processes of plastic deformation (elastoplastic transition) and polymorphic transformations, gradually significantly transforming the profile of the propagating SW even in rather thin targets (of submicron and micron thickness), which proves especially essential at the generation of extra-power SW in the ablation of targets under the action of highly intense USP with peak intensities of 0.1–1 PW/cm2. For this reason, we proposed a new approach to the study of extra-power SW generated at the ablation of materials by highly intense USP, by observing the evolution of the shock wave in the adjacent medium of high shock-wave and optical strength, for instance, in the air atmosphere. We implemented an experimental scheme of registering extra-power SW using a method of contact-free broadband ultrasonic diagnostics by observing the propagation of a shock wave in the air from an ablated surface to a broadband piezoelectric receiver, free of the dimensional effect for the thickness of the target, enabling, among other things, in situ investigations with micron spatial resolution of absolutely different types of materials with various surface relief profiles. We performed tabletop studies of extra-power shock waves of sub- and multi-megabar level, excited on the surface of aluminium at its ablation in the air atmosphere by highly intense (£ 1 PW/cm2) linearly polarized USP. Assessments of initial pressure and shockwave (ablation plume) velocity are well consistent with the literature data obtained by various methods for shock waves propagating inside an ablated target. 

1. A.A. Ionin, S.I. Kudryashov, S.V. Makarov, L.V. Seleznev, D.V. Sinitsyn,Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses. JETP Lett., 94(1), 34–38 (2011).
2. A.A. Ionin, S.I. Kudryashov, A.E. Ligachev, S.V. Makarov, L.V. Seleznev, D.V. Sinitsyn, Nanoscale cavitation instability of the surface melt along the grooves of one-dimensional nanorelief gratings on an aluminum surface. JETP Lett., 94(4), 266–269 (2011).
3. A.A. Ionin, S.I. Kudryashov, L.V. Seleznev, D.V. Sinitsyn, Dynamics of the spallation ablation of a GaAs surface irradiated by femtosecond laser pulses. JETP Lett., 94, 753 (2011).
4. T. Apostolova, A.A. Ionin, S.I. Kudryashov, L.V. Seleznev, D.V. Sinitsyn, Self-limited ionization in band-gap renormalized GaAs at high femtosecond laser intensities. Opt. Eng., 51(12), 121808 (2012).
5.  A.A. Ionin, S.I. Kudryashov, S.V. Makarov, P.N. Saltuganov, L.V. Seleznev, D.V. Sinitsyn, A.R. Sharipov, Ultrafast electron dynamics on the silicon surface excited by an intense femtosecond laser pulse. JETP Lett., 96, 375–379 (2012).
6. A.A. Ionin, S.I. Kudryashov, L.V. Seleznev, D.V. Sinitsyn, A.F. Bunkin, V.N. Lednev, S.M. Pershin, Thermal melting and ablation of silicon surface by femtosecond laser pulses.  JETP 116(3), 347–362 (2013).
7. Vasily N. Lednev, Sergey M. Pershin, Andrey A. Ionin, Sergey I. Kudryashov, Sergey V. Makarov, Alexander E. Ligachev, Andrey A. Rudenko, Roman A. Chmelnitsky, Alexey F. Bunkin, Laser ablation of polished and nanostructured titanium surfaces by nanosecond laser pulses. Spectrochim. Acta Part B: Atomic Spectrosc., 88, 15–19 (2013).