arXiv subjects

A. A. Maznev

Publications and source records attributed to A. A. Maznev.

15 recordsLinked to original sources

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↗

arXiv2022

Extreme ultraviolet transient gratings: A tool for nanoscale photoacoustics

Collective lattice dynamics determine essential aspects of condensed matter, such as elastic and thermal properties. These exhibit strong dependence on the length-scale, reflecting the marked wavevector dependence of lattice excitations. The extreme ultraviolet transient grating (EUV TG) approach has demonstrated the potential of accessing a wavevector range corresponding to the 10s of nm length-scale, representing a spatial scale of the highest relevance for fundamental physics and forefront technology, previously inaccessible by optical TG and other inelastic scattering methods. In this manuscript we report on the capabilities of this technique in the context of probing thermoelastic properties of matter, both in the bulk and at the surface, as well as discussing future developments and practical considerations.

cond-mat.mtrl-sci↗

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↗

arXiv2017

Contact-based and spheroidal vibrational modes of a hexagonal monolayer of microspheres on a substrate

We study acoustic modes of a close-packed hexagonal lattice of spheres adhered to a substrate, propagating along a high-symmetry direction. The model, accounting for both normal and shear coupling between the spheres and between the spheres and the substrate, yields three contact-based vibrational modes involving both translational and rotational motion of the spheres. Furthermore, we study the effect of sphere-substrate and sphere-sphere contacts on spheroidal vibrational modes of the spheres within a perturbative approach. The sphere-substrate interaction results in a frequency upshift for the modes having a non-zero displacement at the contact point with the substrate. Sphere-sphere interactions result in dispersion of spheroidal modes turning them into propagating waves, albeit with a small group velocity. Analytical dispersion relations for both contact-based and spheroidal modes are presented and compared with results obtained for a square lattice.

physics.class-ph↗

arXiv2016

Upholding the diffraction limit in the focusing of light and sound

The concept of the diffraction limit put forth by Ernst Abbe and others has been an important guiding principle limiting our ability to tightly focus classical waves, such as light and sound, in the far field. In the past decade, numerous reports have described focusing or imaging with light and sound "below the diffraction limit". We argue that the diffraction limit defined in a reasonable way, for example in terms of the upper bound on the wave numbers corresponding to the spatial Fourier components of the intensity profile, or in terms of the spot size into which at least 50% of the incident power can be focused, still stands unbroken to this day. We review experimental observations of "subwavelength" or "sub-diffraction-limit" focusing, which can be principally broken down into three broad categories: (i) "super-resolution", i.e. the technique based on the modification of the pupil of the optical system to reduce the width of the central maximum in the intensity distribution at the expense of increasing side bands; (ii) solid immersion lenses, making use of metamaterials with a high effective index; (iii) concentration of intensity by a subwavelength structure such as an antenna. Even though a lot of interesting work has been done along these lines, none of the hitherto performed experiments violated the sensibly defined diffraction limit.

physics.optics↗

arXiv2015

What is the Brillouin Zone of an Anisotropic Photonic Crystal?

The concept of the Brillouin zone (BZ) in relation to a photonic crystal fabricated in an optically anisotropic material is explored both experimentally and theoretically. In experiment, we used femtosecond laser pulses to excite THz polaritons and image their propagation in lithium niobate and lithium tantalate photonic crystal (PhC) slabs. We directly measured the dispersion relation inside PhCs and observed that the lowest bandgap expected to form at the BZ boundary forms inside the BZ in the anisotropic lithium niobate PhC. Our analysis shows that in an anisotropic material the BZ - defined as the Wigner-Seitz cell in the reciprocal lattice - is no longer bounded by Bragg planes and thus does not conform to the original definition of the BZ by Brillouin. We construct an alternative Brillouin zone defined by Bragg planes and show its utility in identifying features of the dispersion bands. We show that for an anisotropic 2D PhC without dispersion, the Bragg plane BZ can be constructed by applying the Wigner-Seitz method to a stretched or compressed reciprocal lattice. We also show that in the presence of the dispersion in the underlying material or in a slab waveguide, the Bragg planes are generally represented by curved surfaces rather than planes. The concept of constructing a BZ with Bragg planes should prove useful in understanding the formation of dispersion bands in anisotropic PhCs and in selectively tailoring their optical properties.

physics.optics↗

arXiv2015

Boundary scattering of phonons: specularity of a randomly rough surface in the small perturbation limit

Scattering of normally incident longitudinal and transverse acoustic waves by a randomly rough surface of an elastically isotropic solid is analyzed within the small perturbation approach. In the limiting case of a large correlation length $L$ compared with the acoustic wavelength, the specularity reduction is given by $4η^2k^2$, where $η$ is the RMS roughness and $k$ is the acoustic wavevector, which is in agreement with the well-known Kirchhoff approximation result often referred to as Ziman's equation [J. M. Ziman, Electrons and Phonons (Clarendon Press, Oxford, 1960)]. In the opposite limiting case of a small correlation length, the specularity reduction is found to be proportional to $η^2k^4L^2$, with the fourth power dependence on frequency as in Rayleigh scattering. Numerical calculations for a Gaussian autocorrelation function of surface roughness connect these limiting cases and reveal a maximum of diffuse scattering at an intermediate value of $L$. This maximum becomes increasingly pronounced for the incident longitudinal wave as the Poisson's ratio of the medium approaches 1/2 as a result of increased scattering into transverse and Rayleigh surface waves. The results indicate that thermal transport models using Ziman's formula are likely to overestimate the heat flux dissipation due to boundary scattering, whereas modeling interface roughness as atomic disorder is likely to underestimate scattering.

cond-mat.other↗

arXiv2015

Extraordinary focusing of sound above a soda can array without time reversal

Recently, Lemoult et al. [Phys. Rev. Lett. 107, 064301 (2011)] used time reversal to focus sound above an array of soda cans into a spot much smaller than the acoustic wavelength in air. In this study, we show that equally sharp focusing can be achieved without time reversal, by arranging transducers around a nearly circular array of soda cans. The size of the focal spot at the center of the array is made progressively smaller as the frequency approaches the Helmholtz resonance frequency of a can from below, and, near the resonance, becomes smaller than the size of a single can. We show that the locally resonant metamaterial formed by soda cans supports a guided wave at frequencies below the Helmholtz resonance frequency. The small focal spot results from a small wavelength of this guided wave near the resonance in combination with a near field effect making the acoustic field concentrate at the opening of a can. The focusing is achieved with propagating rather than evanescent waves. No sub-diffraction-limited focusing is observed if the diffraction limit is defined with respect to the wavelength of the guided mode in the metamaterial medium rather than the wavelength of the bulk wave in air.

physics.class-ph↗

arXiv2014

Lattice swelling and modulus change in a helium-implanted tungsten alloy: X-ray micro-diffraction, surface acoustic wave measurements, and multiscale modelling

Using X-ray micro-diffraction and surface acoustic wave spectroscopy, we measure lattice swelling and elastic modulus changes in a W-1%Re alloy after implantation with 3110 appm of helium. A fraction of a percent observed lattice expansion gives rise to an order of magnitude larger reduction in the surface acoustic wave velocity. A multiscale elasticity, molecular dynamics, and density functional theory model is applied to the interpretation of observations. The measured lattice swelling is consistent with the relaxation volume of self-interstitial and helium-filled vacancy defects that dominate the helium-implanted material microstructure. Molecular dynamics simulations confirm the elasticity model for swelling. Elastic properties of the implanted surface layer also change due to defects. The reduction of surface acoustic wave velocity predicted by density functional theory calculations agrees remarkably well with experimental observations.

cond-mat.mtrl-sci↗

arXiv2013

Detection of shorter-than-skin-depth acoustic pulses in a metal film via transient reflectivity

The detection of ultrashort laser-generated acoustic pulses at a metal surface and the reconstruction of the acoustic strain profile are investigated. A 2 ps-long acoustic pulse generated in an SrRuO$_{3}$ layer propagates through an adjacent gold layer and is detected at its surface by a reflected probe pulse. We show that the intricate shape of the transient reflectivity waveform and the ability to resolve acoustic pulses shorter than the optical skin depth are controlled by a single parameter, which is determined by the ratio of the real and imaginary parts of the photoelastic constant of the material. We also demonstrate a Fourier transform-based algorithm that can be used to extract acoustic strain profiles from transient reflectivity measurements.

cond-mat.mtrl-sci↗

arXiv2013

Interaction of a Contact Resonance of Microspheres with Surface Acoustic Waves

We study the interaction of surface acoustic waves (SAWs) with the contact-based, axial vibrational resonance of 1 μm silica microspheres forming a two-dimensional granular crystal adhered to a substrate. The laser-induced transient grating technique is used to excite SAWs and measure their dispersion. The measured dispersion curves exhibit "avoided crossing" behavior due to the hybridization of the SAWs with the microsphere resonance. We compare the measured dispersion curves with those predicted by our analytical model, and find excellent agreement. The approach presented can be used to study the contact mechanics and adhesion of micro- and nanoparticles as well as the dynamics of microscale granular crystals.

cond-mat.mes-hall↗

arXiv2012

Reciprocity in reflection and transmission: what is a "phonon diode"?

The newly popular topic of "phonon diodes" is discussed in the context of a broader issue of reciprocity in reflection/transmission (R-T) of waves. We first review a theorem well known in electromagnetism and optics but underappreciated in acoustics and phonon physics, stating that the matrix of R-T coefficients for properly normalized amplitudes is symmetric for linear systems that conform to power conservation and time reversibility for wave fields. It is shown that linear structures proposed for "acoustic diodes" in fact do obey R-T reciprocity, and thus should not strictly be called diodes or isolators. We also review examples of nonlinear designs violating reciprocity, and conclude that an efficient acoustic isolator has not yet been demonstrated. Finally, we consider the relationship between acoustic isolators and "thermal diodes", and show that ballistic phonon transport through a linear structure, whether an acoustic diode or not, is unlikely to form the basis for a thermal diode.

cond-mat.other↗

arXiv2012

Lifetime of sub-THz coherent acoustic phonons in a GaAs-AlAs superlattice

We measure the lifetime of the zone-center 340 GHz longitudinal phonon mode in a GaAs-AlAs superlattice excited and probed with femtosecond laser pulses. By comparing measurements conducted at room temperature and liquid nitrogen temperature we separate the intrinsic (phonon-phonon scattering) and extrinsic contributions to phonon relaxation. The estimated room temperature intrinsic lifetime of 0.95 ns is compared to available calculations and experimental data for bulk GaAs. We conclude that ~0.3 THz phonons are in the transition zone between Akhiezer and Landau-Rumer regimes of phonon-phonon relaxation at room temperature.

cond-mat.mes-hall↗

arXiv2012

Direct Measurement of Room Temperature Non-diffusive Thermal Transport Over Micron Distances in a Silicon Membrane

The "textbook" phonon mean free path (MFP) of heat carrying phonons in silicon at room temperature is ~40 nm. However, a large contribution to the thermal conductivity comes from low-frequency phonons with much longer MFPs. We present a simple experiment demonstrating that room temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances. Absorption of crossed laser pulses in a freestanding silicon membrane sets up a sinusoidal temperature profile that is monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the heat transport distance within the range ~1-10 μm. At small distances, we observe a reduction in the effective thermal conductivity indicating a transition from the diffusive to the ballistic transport regime for the low-frequency part of the phonon spectrum.

cond-mat.mtrl-sci↗

arXiv2011

Onset of non-diffusive phonon transport in transient thermal grating decay

The relaxation of a spatially sinusoidal temperature perturbation in a dielectric crystal at a temperature comparable to or higher than the Debye temperature is investigated theoretically. We assume that most phonons contributing to the specific heat have mean free path (MFP) much shorter than the thermal transport distance and can be described by the thermal diffusion model. Low-frequency phonons that may have MFP comparable to or longer than the grating period are described by the Boltzmann transport equation. These low-frequency phonons are assumed to interact with the thermal reservoir of high frequency phonons but not with each other. Within the single mode relaxation time approximation, an analytical expression for the thermal grating relaxation rate is obtained. We show that the contribution of "ballistic" phonons with long MFP to the effective thermal conductivity governing the grating decay is suppressed compared to their contribution to thermal transport at long distances. The reduction in the effective thermal conductivity in Si at room temperature is found to be significant at grating periods as large as 10 microns.

cond-mat.mtrl-sci↗