arXiv subjects

D. Ksenzov

Publications and source records attributed to D. Ksenzov.

5 recordsLinked to original sources

arXiv2026

Time-domain extreme ultraviolet diffuse scattering spectroscopy of nanoscale surface phonons

We report the observation of dynamic fringe patterns in the diffuse scattering of extreme ultraviolet light from surfaces, following femtosecond optical excitation. At each point on the detector, the diffuse scattering intensity exhibits oscillations at well-defined frequencies that correspond to surface phonons with wave vectors determined by the scattering geometry, indicating that the optical excitation generates coherent surface phonons propagating in all directions and spanning a wavelength range from 60 to 300 nm. This phenomenon is observed on a variety of samples, including single-layer and multilayer metal films, as well as bulk semiconductors. The measured surface phonon dispersions show good agreement with theoretical calculations. By comparing signal amplitudes from samples with different surface morphologies, we find that the excitation mechanism is linked to the natural surface roughness of the samples. However, the signal is still detectable on extremely smooth surfaces with sub-nanometer roughness. Our findings demonstrate a simple and effective method for optically exciting coherent surface phonons with nanoscale wavelengths on a wide range of solid samples and establish a foundation for surface phonon spectroscopy in a wave vector range well beyond the limit of conventional surface Brillouin scattering.

cond-mat.mtrl-sci↗

arXiv2025

Transient laser-induced periodic surface structures revealed by time-resolved EUV diffuse scattering

The formation of permanent laser-induced periodic surface structures (LIPSS) on solid surfaces under impulsive laser irradiation above the damage threshold has been subject of extensive research. We demonstrate the formation of transient surface displacement patterns under femtosecond laser irradiation at fluences well below this threshold. Time-resolved extreme ultraviolet scattering measurements reveal distinct reciprocal-space features similar to those observed for permanent LIPSS but dissipating on the hundreds-of-picoseconds time scale. We show that the transient surface displacement patterns responsible for these features are produced via thermal expansion by the spatial modulation of absorbed laser intensity caused by scattering of the laser radiation by surface roughness and present a model accounting for the experimental observations. We suggest that our experiment revealed a universal phenomenon that will be observed on any strongly absorbing material under ultrafast laser irradiation.

physics.optics↗

arXiv2020

Nanoscale transient magnetization gratings excited and probed by femtosecond extreme ultraviolet pulses

We utilize coherent femtosecond extreme ultraviolet (EUV) pulses derived from a free electron laser (FEL) to generate transient periodic magnetization patterns with periods as short as 44 nm. Combining spatially periodic excitation with resonant probing at the dichroic M-edge of cobalt allows us to create and probe transient gratings of electronic and magnetic excitations in a CoGd alloy. In a demagnetized sample, we observe an electronic excitation with 50 fs rise time close to the FEL pulse duration and ~0.5 ps decay time within the range for the electron-phonon relaxation in metals. When the experiment is performed on a sample magnetized to saturation in an external field, we observe a magnetization grating, which appears on a sub-picosecond time scale as the sample is demagnetized at the maxima of the EUV intensity and then decays on the time scale of tens of picoseconds via thermal diffusion. The described approach opens prospects for studying dynamics of ultrafast magnetic phenomena on nanometer length scales.

cond-mat.mes-hall↗

arXiv2015

Structural information extracted from the diffraction of XFEL fs-pulses in a crystal

We present a theoretical justification for a method of extracting of supplementary information for the phase retrieval procedure taken from diffraction of fs-pulses from X-ray Free Electron Laser facilities. The approach is based on numerical simulation of the dynamics of the electron density in the crystal composed of different atoms in the unit cell, namely a bi-atomic crystal containing heavy and light atoms. It is shown that evaluation of diffraction intensities measured by means of different values of XFEL pulse parameters enables to find absolute values of structure factors for both types of atoms and their relative phase. The accuracy of structural information is discussed in terms of fluctuations of the evaluated atomic scattering factors. Our approach could be important for improvement of phase retrieval methods with respect to a more efficient determination of atomic positions within the unit cell of macromolecules.

cond-mat.mtrl-sci↗

arXiv2013

Time dependence of X-ray diffraction intensity of a crystal induced by an intense femtosecond X-ray pulse

The time evolution of the electron density and the resulting time dependence of X-ray diffraction peak intensity in a crystal irradiated by highly intense femtosecond pulses of an XFEL is investigated theoretically on the basis of rate equations for bound electrons and the Boltzmann equation for the kinetics of the unbound electron gas that plays an essential role in the time evolution of the electron density of a crystal. The photoionization, Auger process, electron-impact ionization, electron--electron scattering, and three-body recombination have been implemented in the system of rate equations. An algorithm for the numerical solution of the rate equations was simplified by incorporating analytical expressions for the cross sections of all the electron configurations in ions within the framework of the effective charge model. Using this approach we evaluate the time dependence of the inner shell population and electronic kinetic energy during the time of XFEL pulse propagation through the crystal for photon energies between 3 and 12 keV and a pulse width of 40 fs considering a flux of 10^12 ph/pulse (focusing on a spot size of ~ 1 mum^2, this flux corresponds to a fluence ranging between 0.6 and 1.6 mJ/mum^2). The time evolution of the atomic scattering factor and its fluctuation is numerically analyzed for the case of a Silicon crystal taking into account the decrease of the bound electron density during the pulse propagation. The time integrated intensity drops dramatically if the fluence of the XFEL pulse exceeds 1.6 mJ/mum^2.

physics.atom-ph↗