Studies of the laser-induced ultrafast processes in thin films are of significant importance for the development of microelectronics. These processes include the heating of an electron subsystem, relaxation and transport of the absorbed energy, and generation and propagation of picosecond acoustic waves. In view of this circumstance, to study the dynamics of variation of the differential reflection coefficient ΔR(t)/R0 of a 73-nm-thick Ni film on a glass substrate, pump–probe measurements have been performed in this work with the synchronous detection of a ΔR(t)/R0 signal. High absorbed fluences up to 11 mJ/cm2 have been reached by increasing the pulse-repetition interval tcool of heating (pump) pulses. An increase in tcool makes it possible to better cool the film after heating. As a result, record temperatures Te ≈ 3 kK and Ti ≈ 1 kK and stresses up to 7 GPa have been reached for the first time to the best of our knowledge. These high values have allowed the observation of nonlinear effects for the first time in experiments with synchronous detection.

The evolution of the optical properties of a 25-nm-thick nickel film on a glass substrate excited by a subpicosecond terahertz pulse with a field strength of 11 MV/cm has been studied by femtosecond interference microscopy with a time resolution of 10–13 s. The measurements of the complex reflection coefficient and the permittivity in the visible range of the spectrum indicate the non-equilibrium heating of the electron subsystem of nickel to several thousands of degrees, which is accompanied by the induced increase in reflection at the initial time of irradiation and by the subsequent melting after 5–10 ps. Scanning electron and atomic force microscopy studies of the morphology of the modified surface have indicated the local melting of the nanofilm and its delamination from the substrate at this field strength.

Intense ultrashort laser pulses can drive the electrons and the lattice in a solid material far out of equilibrium creating a highly excited matter with extraordinary properties. Here, we report on an experimental approach to study the relaxation of electrons in a bulk gold using time-resolved pump-probe reflectivity measurements with phase-sensitive signal detection in a wide range of absorbed energy densities ranging more than two orders of magnitude. As a result, the process of electron relaxation in gold can be seamlessly studied with femtosecond time resolution in a wide range of initial electron temperatures ranging from several tens to several thousands of kelvins by implying appropriate models of transient thermal, mechanical and optical properties. As far as we know, the experimental validation of the theories existing at present time on behavior of such crucial values as electron-phonon coupling parameter, electron thermal conductivity and electron heat capacity is almost lacking in gold driven far out of equilibrium with elevated electrons temperatures ranging from 1 to 20 kK.

The behavior of molybdenum under the action of load pulses of picosecond duration is studied in an experiment. Using the method of spectral interferometry in the single-exposure mode in the picosecond range, changes in the phase and amplitude of the diagnostic pulse reflected from the free surface of the sample are recorded. In a film sample of molybdenum of submicron thickness, compressive stresses reaching 89 GPa are realized and are accompanied by a significant increase in the surface reflectance.

Non-destructive and non-contact all-optical methods for characterization of thermal and mechanical properties of layered materials with nanoscale spatial and femtosecond/picosecond temporal resolutions are in ever-increasing demand. Here, we report on an experimental study of energy relaxation in a strongly excited electron subsystem of a Nickel film on a substrate upon irradiation with an ultrashort laser pulse using time-resolved pump-probe reflectivity measurements with phase-sensitive signal detection. The temporal dynamics of changes in reflectivity at initial electron temperatures reaching several thousand kelvins contains essential information about the ultrafast electron relaxation, electron-phonon coupling parameter, electron thermal conductivity and heat capacitance of the metal film, as well as elastic and mechanical properties revealed by detection of propagating picosecond coherent acoustic phonons in the gigahertz frequency range both in the metal and substrate. We believe these experimental data to be useful for validation of the theories existing at present time and the further development of predictive modelling.

The movement of the rear free surface of submicron cobalt film samples exposed to picosecond laser pulses with different energy densities was studied experimentally. The displacement of the free rear surface of the sample determined in the picosecond range in single-pulse mode using spectral interferometry. Data were obtained on the spall strength cobalt in a condensed state for a strain rate of 109 s1.

Modification of titanium microstructure after propagation of a melting shock wave (SW) generated by a femtosecond laser pulse is investigated experimentally and analyzed using hydrodynamic and atomistic simulations. Scanning and transmission electron microscopy with analysis of microdiffraction is used to determine the microstructure of modified subsurface layers of titanium. We found that two layers are modified beneath the surface. A top surface polycrystalline layer of nanoscale grains is formed from shock-molten material via rapid crystallization. In a deeper subsurface layer, where the shock-induced melting changes into plastic deformation due to attenuation of SW, the grain structure of solid is considerably affected, which results in a grain size distribution differing from that in the intact titanium. Molecular dynamics simulation of single-crystal titanium reveals that the SW front continues to melt even after its temperature drops below the melting curve Tm(P). The enormous shear stress of ∼12 GPa generated in a narrow SW front leads to free slip of atomic planes, collapse of the crystal lattice, and formation of a supercooled metastable melt. Such melt crystallizes in an unloading tail of SW. The mechanical melting ceases after drop in the shear stress giving rise to the shock-induced plastic deformation. The last process triggers a long-term rearrangement of atomic structures in solid. The overall depth of modified layers is limited by SW attenuation to the Hugoniot elastic limit and can reach several micrometers. The obtained results reveal the basic physical mechanisms of surface hardening of metals by ultrashort laser pulses.

The method of interference microscopy is used to study the features of iron ablation under single exposure to laser pulses with a duration of 60 fs of moderate intensity 10 12 –10 13 W/cm 2 . The value of the reflection coefficient is measured and the value of the threshold of thermomechanical ablation is determined from the absorbed energy density. The evolution of the crater bottom morphology and the dependence of their depth on the energy density of laser pulses are studied.

Laser-driven shock wave phenomena in a sub-micrometer Mg–4Al–2Zn alloy film are studied using spectral interferometry with spatial and temporal (1 ps) resolution. Upon irradiating the film through a glass substrate by 500 fs laser pulses, the ultrashort elastic compression pulses with the peak stress up to 4.6 GPa at a propagation distance of 0.5 μm were generated. Depending on the laser fluence, either spall fracture near the rear surface in the solid state or cavitation near the metal–glass interface in the liquid state was observed. The spall strength of the solid Mg alloy and the upper limit of the cavitation threshold in the melt at the strain rate of ∼109 s−1 were extracted from the free surface velocity history. The depth of fracture initiation was retrieved from the instant of the spall pulse exit, and the thickness of the molten layer was estimated to be 100–160 nm depending on laser fluence. The investigation of the residual morphology by scanning electron and atomic force microscopies revealed the presence of melting and nucleation within the irradiated area. The experimental findings are of interest for predicting the behavior of magnesium alloys in the condensed state at extremely high strain rates, for studying the physics of metastable states and for simulating the interaction of ultrashort laser pulses with thin film materials.

Laser-assisted nanostructuring of silicon interfaces provides a unique procedure for leading-edge technologies. We report on a new embossing technique with tightly focused Gaussian-shaped ultrashort laser pulses near the ablation threshold in liquid. We take advantage of a primary microbubble for controllable spatial-modulation of light intensity of succeeding pulses. Such a bubble, generated in liquid near the molten silicon surface by the first pulse, serves as an obstacle for the second pulse following with a sub-millisecond time delay, to produce a diffraction ring pattern. Variation of laser intensity can be utilized to guide the diffraction patterns. Thus the various annular patterns deeply embossed up to hundreds nanometers on the surface are produced with high reproducibility. Morphology of modified surface layer is investigated in detail using atomic-force microscopy, as well as scanning and transmission electron microscopies. Full-wave EM modeling of laser beam diffraction by the bubbles with various radii and shapes shows that the peak intensity in silicon is up to 1.7 times higher than in bubble-free liquid. Atomistic simulation of ultrafast heating with such a diffraction peak reveals that an annular microdimple surrounded by rims is formed by high-gradient pressure in molten silicon to be frozen after several nanoseconds.

Ultrashort laser pulses with a duration from several to about a thousand optical cycles have significant importance in modern science and engineering. Such a pulse transfers a metal to an excited two-temperature state with hot electrons where the temperature of the electron subsystem T e is much higher than the temperature of the ion subsystem T i . The thermal conductivity in such systems differs from well-known reference values. The thermal conductivity κ and the energy exchange rate between the electron and ion subsystems α are the key parameters of the two-temperature model, which are still poorly studied, although studies of these parameters, particularly α, are numerous. New theoretical and experimental results that make it possible to determine the parameters κ and α for gold have been reported in this work.

The features of detection of propagating in silicon coherent acoustic phonons generated upon strong laser excitation of the top metal nanofilm via time-domain Brillouin scattering are investigated. The generation of electron-hole plasma in silicon along with Beer–Lambert law result in a shrinkage of the silicon layer thickness to be studied.

Ультракороткие лазерные воздействия с длительностью от нескольких до порядка 1000 световых осцилляций имеют огромное значение в современных науке и технике. Такой импульс переводит металл в возбужденное состояние с горячими электронами: двухтемпературное состояние Te ≫ Ti, где Te, Ti - это температуры электронной и ионной подсистем. При этом меняется коэффициент теплопроводностипо сравнению с хорошо известными справочными значениями. Теплопроводность κ и темп обмена энергией между электронной и ионной подсистемами α - это ключевые параметры 2T модели. До сих пор они остаются плохо изученными. Хотя работ, направленных на их исследование, особенно это касается α, много. В настоящей статье представлены новые теоретико-экспериментальные данные, позволяющие определить κ и α на примере с золотом.

В эксперименте исследовано поведение молибдена под действием импульсов нагрузки пикосекундной длительности. Методом спектральной интерферометрии в режиме однократного воздействия в пикосекундном диапазоне регистрировалось изменение фазы и амплитуды диагностического импульса, отраженного от свободной поверхности образца. В пленочном образце молибдена субмикронной толщины реализованы напряжения сжатия, достигающие 89 ГПа и сопровождающиеся существенным ростом коэффициента отражения поверхности.

Spectral interferometry is used to study picosecond acoustic pulse propagation in a submicron titanium film heated by femtosecond laser pulse through a glass substrate. The melting depth was estimated from a moment of spall pulse arrival, formed due to cavitation in melt during tensile stress relaxation.

The temporal dynamics of the change in the thermally-induced induced reflection coefficient of polycrystalline gold, aluminum, and molybdenum at a wavelength of 800 nm as a result of exposure to a femtosecond laser pulse (at a wavelength of 400 nm) with an intensity of up to 0.2 TW in the in the picosecond time range is studied using pump-probe optical scheme with phase-sensitive detection. New experimental data are obtained on the change in the reflection coefficient under strongly nonequilibrium conditions when the electron subsystem is heated to temperatures of 10 kK, which are relevant for the development of the experimental base for numerical simulation.

The article studies the motion of the rear free surface of a film sample of a magnesium alloy of submicron thickness under the action of subpicosecond laser pulses. Continuous diagnostics of the displacement of the free rear surface of the sample were carried out in the picosecond range in a single-pulse mode using spectral interferometry. The value of the spall strength at a strain rate of 109 s–1, the value of which is about 53

Проведено экспериментальное исследование движения тыльной свободной поверхности субмикронных пленочных образцов кобальта при воздействии лазерными импульсами пикосекундной длительности с различной плотностью энергии. Диагностика смещения свободной тыльной поверхности образца осуществлялась в пикосекундном диапазоне в одноимпульсном режиме методом спектральной интерферометрии. Получены данные об откольной прочности кобальта в конденсированном состоянии при скорости деформирования ~10 9 с –1 .