This study addresses the significant gap in understanding microsecond-to-millisecond phase evolution in silicon during laser ablation. While femtosecond-to-nanosecond dynamics are well-documented, the longer timescales governing phase transformation and quenching remain largely unexplored. We bridge this gap by combining time-resolved X-ray diffraction (TR-XRD) with molecular dynamics (MD) simulations to analyze the multi-stage crater formation process following nanosecond laser ablation (similar to 10(11) W/cm(2)). Our results reveal a three-stage mechanism tracked by TR-XRD: initial nanosecond-scale crater seeding and shock wave generation; microsecond-scale shock wave propagation (similar to tens of GPa) with diffraction profile broadening indicating phase transitions through metastable states (Si-II/Si-XI); and final sub-millisecond thermodiffusion and recrystallization. We demonstrate that the final polymorphic phases (Si-III/XII) observed at crater peripheries are metastable products quenched from a high-pressure liquid state, with their spatial distribution governed by extreme temperature gradients exceeding 7000 K at the epicenter. MD simulations establish the crucial link between the initial non-equilibrium shock wave state and the final quenched microstructure. Notably, we report evidence of residual Si-II phase, challenging previous assumptions about its stability at ambient pressure. This work establishes a comprehensive methodology connecting ultrafast initiation with long-timescale material modification, providing new insights into phase transformation dynamics under extreme conditions.

We propose that the anomalous (non-monotonic) behavior of physical properties of supercritical fluids (SCF) in the Widom delta is attributed to the formation of medium-sized clusters. This hypothesis is experimentally verified for carbon dioxide using both experimental methods and molecular dynamics simulations. From a microscopic point of view, the non-monotonic behavior of the nonlinear refractive index, speed of sound, and Raman scattering efficiency is caused by the formation of quasi- linear clusters of medium size (5-200 molecules per cluster). Within the clusters, the molecule concentration is close to that of the liquid phase, while outside the clusters, it resembles the gas phase, leading to experimentally observed high (similar to 15 %) density fluctuations. Isolated linear clusters exhibit high second-order hyperpolarizability, resulting in an increase in the molecular contribution to the nonlinear refractive index and the intensity of Raman scattering. The appearance of multiple Widom lines on the pressure-temperature (p-T) diagram, each associated with unique physical properties, arises from the combined effects of cluster-specific and density-related factors. This interplay results in the divergence of Widom lines and the formation of the characteristic feature known as the Widom delta.

The dynamics of femtosecond laser impact on water was experimentally studied and reconstructed using numerical modeling based on the classical molecular dynamics method in combination with the two-temperature model and dynamical rate equations. This process occurs in several stages. Initially, a femtosecond laser pulse interacts with the electron subsystem, generating plasma due to multiphoton, tunnel, and impact ionization. The energy transfer from plasma electrons to atoms, as shown using the two-temperature model, leads to ultrafast heating of the substance to a temperature of 10 000 K, and the pressures achieved in the irradiated area are 15 GPa, which leads to the generation of a shock wave. The temperatures and pressures exceeding the critical values, combined with high density fluctuations and clustering, indicate the transition of the substance to a supercritical state. The pressures and temperatures exceeding the critical values are achieved in a region slightly exceeding the cavitation zone, and this region experiences oscillations with a period close to the period of oscillations of the cavitation bubble. In the case of the femtosecond laser impact, the experimentally measured deposited energy density can be used as an initial condition under assumtion of unstantaneous heating of the medium, which significantly simplifies numerical modeling. Both the pressures achieved at the shock wave front and the dynamics of cavitation bubbles are successfully reconstructed within the framework of this approach.

The control of mechanical effects, such as shock waves, induced by ultrashort laser pulses in water is crucial for applications in biomedicine and material processing. However, optimizing these effects requires a detailed understanding of how laser parameters, particularly pulse duration, influence the underlying energy deposition mechanisms. This study systematically investigates the dependence of shock wave amplitude on fluence (up to 10 J/cm2) and pulse duration (200 fs to 10 ps) of near-infrared laser pulses under tight focusing conditions (Numerical aperture NA = 0.42), using a combined experimental and numerical approach based on the dynamical rate equation model. Our key finding is that the shock wave amplitude is governed by the total kinetic energy of the electrons in the laser-induced plasma, leading to a distinct maximum at approximately 5 ps (confidence interval: 4.5–5.5 ps) and saturation at fluences ~7 J/cm2. This optimum arises from a balance between the increasing effectiveness of avalanche ionization for longer pulses and the competing effects of electron recombination and reduced photoionization efficiency. Consequently, these results identify a practical parameter window—pulse durations of 4–6 ps at moderate fluences—for optimizing laser-induced mechanical effects in applications such as laser surgery in aqueous media.

The dynamics of submicron gold particle formation during laser ablation in supercritical and liquid CO2 are investigated using time-resolved Mie scattering absorption spectroscopy. It is found that a high density of CO2 of approximately 800 kg/m3, regardless of the phase state, leads to the formation of laser-induced medium fluctuations on time scales of 10 to 1000 μs, during which the defocusing of subsequent laser pulses may occur. Under conditions of the reduced fluid density in the vicinity of the Widom region (P = 8.5 МРa, T = 310 K), it is demonstrated that nanoparticles can leave the interaction zone unhindered as evidenced by the exponential increase in integral absorption. Data from the absorption spectra allow the observation of the nanoparticle formation dynamics with an average diameter of approximately 160 nm and a log-normal size distribution, the width of which is determined by the thermodynamic conditions of CO2.

An approach to simulating the Raman spectra of supercritical carbon dioxide was demonstrated. The approach is based on the Fourier analysis of the autocorrelation function of the dipole moment calculated by molecular dynamics. Based on these data, the Raman spectra of carbon dioxide were calculated along the isotherm at 310 K in the pressure range of 0.1–15 MPa. It was determined that, in the Widom delta, the spectral lines are broadened and the pressure dependences of the Raman shifts have extrema. The results obtained agree well with the known experimental data and indicate a significant effect of cluster formation on the macroscopic properties of carbon dioxide in the vicinity of the critical point.

We created a laser-plasma X-ray source based on the femtosecond fiber laser with high yield $\sim 2 \times 10^{9} \mathrm{phot} / \mathrm{s} / 2 \pi(3-12$ keV), and with a source size diameter of approximately 10 microns. The X-ray yield and the source size were optimized by using artificial intelligence, the He flow and nanosecond pre-pulse.

The synchronization of laser and X-ray sources is essential for time-resolved measurements in the study of ultrafast processes, including photo-induced piezo-effects, shock wave generation, and phase transitions. On the one hand, optical diagnostics (by synchronization of two laser sources) provides information about changes in vibration frequencies, shock wave dynamics, and linear and nonlinear refractive index behavior. On the other hand, optical pump–X-ray probe diagnostics provide an opportunity to directly reveal lattice dynamics. To integrate two approaches into a unified whole, one needs to create a robust method for the synchronization of two systems with different repetition rates up to the MHz range. In this paper, we propose a universal approach utilizing a field-programmable gate array (FPGA) to achieve precise synchronization between different MHz sources such as various lasers and synchrotron X-ray sources. This synchronization method offers numerous advantages, such as high flexibility, fast response, and low jitter. Experimental results demonstrate the successful synchronization of two different MHz systems with a temporal resolution of 250 ps. This enables ultrafast measurements with a sub-nanosecond resolution, facilitating the uncovering of complex dynamics in ultrafast processes.

The dynamics of the diffraction peak 0012 parameters of LiNbO3:Fe crystals with a time resolution of less than 1 ns were recorded by synchronizing nanosecond laser pulses with electron bunches of the KISI-Kurchatov synchrotron source. The influence of a laser pulse (λ = 532 nm, t = 4 ns, energy density 0.6 J/cm2) at different polarization directions of the laser radiation causes a change in the peak intensity, which depends on the angle between the polarization direction of the laser radiation and the crystallographic axes. The obtained results are supplemented with wavelet analysis of experimental data. The observed polarization dependence correlates with published data on the photovoltaic effect.

We studied the evolution of shock waves and cavitation bubbles generated by femto- and nanosecond laser pulses over a time scale ranging from femtoseconds to microseconds in various molecular environments, including supercritical states. Through time-resolved shadow photography techniques, we observed that when highly intense laser pulses are focused into a supercritical fluid like carbon dioxide, cavitation bubbles do not collapse as in liquid media. Instead, clusters ranging from 1 to 200 microns in size are formed, presenting a significant differnce from the typical dynamics of cavitation bubbles induced by laser in liquids which exhibit size fluctuations

Unusual time-delayed changes in the x-ray diffraction parameters of LiNbO3 and LiNbO3 : Fe crystals were observed under nanosecond laser impact. Subnanosecond time resolution in the registration of diffraction rocking curve (DRC) dynamics was achieved through the synchronization of a 4 ns laser pulse with the circulation phase of electron bunches within a synchrotron storage ring. The response of the crystals to optical impact resulted in a reversible center-of-mass shift and integral intensity decrease of the DRC recovering in similar to 35 ns. The dynamics of lattice deformation indicates the process of formation and subsequent decay of an electrical charged layer near the surface due to directed migration of photoelectrons as a result of the bulk photovoltaic effect. The drop in the integral intensity of the DRCs is apparently caused by a running wave generated by a sharp change in the deformation of the crystal lattice. In the case of the nominally undoped crystal, the time-delayed processes occur within the same time interval but with significantly smaller amplitudes.

Продемонстрирован подход к моделированию рамановских спектров сверхкритического диоксида углерода, основанный на Фурье-анализе автокорреляционной функции дипольного момента, рассчитанного методом молекулярной динамики. На его основе рассчитаны рамановские спектры диоксида углерода вдоль изотермы 310 К в диапазоне давлений 0,1—15 МПа. Установлено, что в дельте Видома наблюдается уширение спектральных линий, а также экстремумы в зависимостях рамановских сдвигов от давления. Полученные результаты хорошо согласуются с известными экспериментальными данными и свидетельствуют об существенном влиянии кластерообразования на макроскопические свойства диоксида углерода в окрестности критической точки An approach to modeling the Raman spectra of supercritical carbon dioxide based on Fourier analysis of the autocorrelation function of the dipole moment calculated by the molecular dynamics method is demonstrated. On its basis, Raman spectra of carbon dioxide were simulated along the 310 K isotherm in the pressure range 0.1–15 MPa. It has been established that in the Widom delta there is a broadening of spectral lines, as well as extrema in the dependences of Raman shifts on pressure. The results obtained are in good agreement with known experimental data and indicate a significant influence of cluster formation on the macroscopic properties of carbon dioxide in the vicinity of the critical point

Using time-resolved techniques we revealed the dynamics of Si lattice under impact of intense nanosecond and femtosecond laser pulse. The energy transfer from laser-induced plasma to the atomic subsystem occurs on a sub-ps timescale, generates a shock wave that leads to the cascade of phase transitions (Si-X => Si-VII => Si-VI => Si-XI => Si-XII=>Si-III).

The spectra of transmisson coefficients and absorption indices of single-domain and multidomain LaBGeO5 samples have been measured. It is shown that, to measure more exactly the optical rotation ρ, it is necessary to use the spectra of transmission coefficients not only for the cases of parallel and crossed polarizers but also at other angles between them. The obtained ρ values for both samples are described quite well by only dispersion using the Drude formula. This is in agreement with the fact that the ρ value should not change during transition to the single-domain state of the crystal at a given symmetry (P31 in the ferroelectric phase and P3121 in the paraelectric phase). It is shown that the Cherenkov-type second harmonic generation (SHG) is observed only in a polydomain sample, while the second-harmonic radiation is not polarized. The domain structure of the samples was observed by scanning electron microscopy and piezoelectric force microscopy. The presence of a labyrinth-like domain structure was shown for the multidomain sample, whereas for a single-domain sample no changes in contrast were observed within the scanned region.

Radiographic imaging using X-rays is a tool for basic research and applications in industry, materials science, and medical diagnostics. In this article, we present a novel approach for the generation of X-rays using a vacuum-free microplasma by femtosecond fiber laser. By tightly focusing a laser pulse onto a micrometer-sized solid density near-surface plasma from a rotating copper target, we demonstrate the generation of Cu K-photons (8-9 keV) with high yield ∼ 1.6 × 10 9 phot/s/2π, and with a source size diameter of approximately 10 microns. Femtosecond fiber laser allows working with a high repetition rate (∼2 MHz) and moderate energy levels (10-40 µJ), ensuring the effective quasi-continuous generation of X-ray photons. Furthermore, we introduce a hybrid scheme that combines the tightly focusing laser-plasma X-ray generator with an online control unit for microplasma size source based on the back-reflected second harmonic generated in the laser-induced microplasma. The compactness and high performance of this vacuum-free femtosecond fiber laser microplasma X-ray source makes it a promising solution for advanced radiographic applications. Our preliminary results on the creation of a microfocus X-ray source provide insights into the feasibility and potential of this innovative approach.

In this study, we reconstructed the dynamics of the impact of mid-IR-range (4.6 μm) femtosecond laser pulses on bulk silicon under tight focusing conditions (NA = 0.5). Our experimental results show that under this impact, the deposited energy density (DED) reaches approximately 4 kJ/cm3 (at an energy slightly above the plasma-formation threshold). Initially, the femtosecond pulse energy is absorbed by the laser-induced plasma, with a lifetime of approximately 160–320 fs (depending on the laser pulse energy). The energy transfer from the plasma to the atomic subsystem occurs on a sub-ps timescale, which generates a shock wave and excites coherent phonons on a sub-ps scale. The shift of atoms in the lattice at the front of the shock wave results in a cascade of phase transitions (Si-X => Si-VII => Si-VI => Si-XI => Si-II), leading to a change in the phonon spectra of silicon.

Progress in the field of machine learning has enhanced the development of self-adjusting optical systems capable of autonomously adapting to changing environmental conditions. This study demonstrates the concept of self-adjusting optical systems and presents a new approach based on reinforcement learning methods. We integrated reinforcement learning algorithms into the setup for tuning the laser radiation into the fiber, as well as into the complex for controlling the laser-plasma source. That reduced the dispersion of the generated X-ray signal by 2–3 times through automatic adjustment of the position of the rotating copper target and completely eliminated the linear trend arising from the ablation of the target surface. The adjustment of the system was performed based on feedback signals obtained from the spectrometer, and the movement of the target was achieved using a neural network-controlled stepper motor. As feedback, the second harmonic of femtosecond laser radiation was used, the intensity of which has a square root dependence on the X-ray yield. The developed machine learning methodology allows the considered systems to optimize their performance and adapt in real time, leading to increased efficiency, accuracy, and reliability.

The advent of free-electron lasers opens new routes for experimental high-pressure physics, which allows studying dynamics of condensed matter with femtosecond resolution. A rapid compression, that can be caused by laser-induced shock impact, leads to the cascade of high-pressure phase transitions. Despite many decades of study, a complete understanding of the lattice response to such a compression remains elusive. Moreover, in the dynamical case (in contrast to quasi-static loading) the thresholds of phase transitions can change significantly. Using the third harmonic pump–probe technique combined with molecular dynamics to simulate the terahertz (THz) spectrum, we revealed the dynamics of ultrafast laser-induced phase transitions in MgF 2 in all-optical experiment. Tight focusing of femtosecond laser pulse into the transparent medium leads to the generation of sub-TPa shock waves and THz coherent phonons. The laser-induced shock wave propagation drastically displaces atoms in the lattice, which leads to phase transitions. We registered a cascade of ultrafast laser-induced phase transitions (P42/mnm ⇒ Pa-3 ⇒ Pnam) in magnesium fluoride as a change in the spectrum of coherent phonons. The phase transition has the characteristic time of 5–10 ps, and the lifetime of each phase is on the order of 40–60 ps. In addition, phonon density of states, simulated by molecular dynamics, together with third-harmonic time-resolved spectra prove that laser-excited phonons in a bulk of dielectrics are generated by displacive excitation (DECP) mechanism in plasma mediated conditions.

A new approach is proposed to form a jet with submicron aggregates for femtosecond laser neutron generation under nonlinear interaction with relativistically intense laser pulse. Aggregates are formed through the rapid expansion into vacuum of the supercritical mixture of CO 2 + CD 3 OD (3:1). For the first time, fusion neutrons (2.45 MeV) with a peak output of 3 × 10 3 neutron/pulse/4 π and efficiency of 6 × 10 4 neutron J −1 were obtained under interaction of Ti:Sa laser pulse having 3 × 10 18 W cm −2 intensity with submicron aggregates produced from supercritical CO 2 + CD 3 OD mixture.

We studied the dynamics of laser-induced shock waves in supercritical CO2 (scCO2) for different pressures and temperatures under nanosecond optical breakdown. We estimated the shock wave pressure and energy, including their evolution during shock wave propagation. The maximal shock wave pressure ~0.5 GPa was obtained in liquid-like scCO2 (155 bar 55 °C), where the fluid density is greater. However, the maximal shock wave energy ~25 μJ was achieved in sub-critical conditions (67 bar, 55 °C) due to a more homogeneous microstructure of fluid in comparison with supercritical fluid. The minimal pressure and energy of the shock wave are observed in the Widom delta (a delta-like region in the vicinity of the critical point) due to the clusterization of scCO2, which strongly affects the energy transfer from the nanosecond laser pulse to the shock wave.