Laser ultrasonics in a multilayer structure: Semi-analytic model and simulated examples.

Laser-generated elastic waves have been the subject of numerous experimental, theoretical, and numerical studies to describe the opto-acoustic generation process, involving electromagnetic, thermal, and elastic fields and their couplings in matter. Among the numerical methods for solving this multiphysical problem, the semi-analytic approach is one of the most relevant for obtaining fast and accurate results, when analytic solutions exist. In this paper, a multilayer model is proposed to successively solve electromagnetic, thermal, and elastodynamic problems. The optical penetration of the laser line source, as well as thermal conduction and convection, are accounted for. Optical, thermal, and mechanical coupling conditions are considered between the upper and lower media of the multilayer. The simulation of laser-generated ultrasounds in multilayer structures is of interest for the development of nondestructive evaluation methods of complex structures, such as bonded assemblies in aeronautics [as discussed in Hodé et al., J. Acoust. Soc. Am. 150, 2076 (2021)]. The developed Python code is provided for free at https://doi.org/10.5281/zenodo.4301720.

Laser ultrasonics in a multilayer structure: Plane wave synthesis and inverse problem for nondestructive evaluation of adhesive bondings.

A laser ultrasonic method is proposed for the nondestructive evaluation of bonded assemblies based on the analysis of elastic plane waves reflected from the bonding interface. Plane waves are numerically synthesized from experimentally detected cylindrical waves. Several angles of incidence with respect to the bonding interface are achieved by varying the delay in the synthesis step. An inverse problem using these plane waves is then solved to identify the normal and transverse interfacial stiffnesses that model the mechanical coupling between two bonded media. The semi-analytic model developed and detailed in Hodé et al. [J. Acoust. Soc. Am. 150, 2065 (2021)] is used to create the database that contains simulated laser-generated ultrasounds required to solve the inverse problem. The developed method is first validated with semi-analytic simulated input data where Gaussian noise has been added. Next, the method is applied using signals acquired on an aluminum alloy plate and on assemblies (with and without adhesion defects) made of two aluminum alloy plates bonded by an aeronautical structural epoxy adhesive film. Differences between the identified values of interfacial stiffnesses distinguish the three samples and obtain quantitative values to characterize the adhesive bonding.

All optical control of comb-like coherent acoustic phonons in multiple quantum well structures through double-pump-pulse pump-probe experiments.

We present an advancement in applications of ultrafast optics in picosecond laser ultrasonics - laser-induced comb-like coherent acoustic phonons are optically controlled in a In0.27Ga0.73As/GaAs multiple quantum well (MQW) structure by a high-speed asynchronous optical sampling (ASOPS) system based on two GHz Yb:KYW lasers. Two successive pulses from the same pump laser are used to excite the MQW structure. The second pump light pulse has a tunable time delay with respect to the first one and can be also tuned in intensity, which enables the amplitude and phase modulation of acoustic phonons. This yields rich temporal acoustic patterns with suppressed or enhanced amplitudes, various wave-packet shapes, varied wave-packet widths, reduced wave-packet periods and varied phase shifts of single-period oscillations within a wave-packet. In the frequency domain, the amplitude and phase shift of the individual comb component present a second-pump-delay-dependent cosine-wave-like and sawtooth-wave-like variation, respectively, with a modulation frequency equal to the comb component frequency itself. The variations of the individual component amplitude and phase shift by tuning the second pump intensity exhibit an amplitude valley and an abrupt phase jump at the ratio around 1:1 of the two pump pulse intensities for certain time delays. A simplified model, where both generation and detection functions are assumed as a cosine stress wave enveloped by Gaussian or rectangular shapes in an infinite periodic MQW structure, is developed in order to interpret acoustic manipulation in the MQW sample. The modelling agrees well with the experiment in a wide range of time delays and intensity ratios. Moreover, by applying a heuristic-analytical approach and nonlinear corrections, the improved calculations reach an excellent agreement with experimental results and thus enable to predict and synthesize coherent acoustic wave patterns in MQW structures.

Acousto-Optics 2017: introduction to the feature issue.

The editors of Acousto-Optics 2018 provide an overview of the featured papers in the context of the 13th School of Acousto-Optics and Applications from which the articles were drawn.

Nonlinear Hysteretic Torsional Waves.

We theoretically study and experimentally report the propagation of nonlinear hysteretic torsional pulses in a vertical granular chain made of cm-scale, self-hanged magnetic beads. As predicted by contact mechanics, the torsional coupling between two beads is found to be nonlinear hysteretic. This results in a nonlinear pulse distortion essentially different from the distortion predicted by classical nonlinearities and in a complex dynamic response depending on the history of the wave particle angular velocity. Both are consistent with the predictions of purely hysteretic nonlinear elasticity and the Preisach-Mayergoyz hysteresis model, providing the opportunity to study the phenomenon of nonlinear dynamic hysteresis in the absence of other types of material nonlinearities. The proposed configuration reveals a plethora of interesting phenomena including giant amplitude-dependent attenuation, short-term memory, as well as dispersive properties. Thus, it could find interesting applications in nonlinear wave control devices such as strong amplitude-dependent filters.

Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy.

The origin of the martensitic transition in the magnetic shape memory alloy Ni-Mn-Ga has been widely discussed. While several studies suggest it is electronically driven, the adaptive martensite model reproduced the peculiar nonharmonic lattice modulation. We used femtosecond spectroscopy to probe the temperature and doping dependence of collective modes, and scanning tunneling microscopy revealed the corresponding static modulations. We show that the martensitic phase can be described by a complex charge-density wave tuned by magnetic ordering and strong electron-lattice coupling.

Slow relaxation and aging phenomena at the nanoscale in granular materials.

Granular matter exhibits a rich variety of dynamic behaviors, for which the role of thermal fluctuations is usually ignored. Here we show that thermal fluctuations can pronouncedly affect contacting nanoscale asperities at grain interfaces and brightly manifest themselves through the influence on nonlinear-acoustic effects. The proposed mechanism based on intrinsic bistability of nanoscale contacts comprises a wealth of slow-dynamics regimes including slow relaxations and aging as universal properties of a wide class of systems with metastable states.

Picosecond opto-acoustic interferometry and polarimetry in high-index GaAs.

By means of a metal opto-acoustic transducer we generate quasi-longitudinal and quasi-transverse picosecond strain pulses in a (311)-GaAs substrate and monitor their propagation by picosecond acoustic interferometry. By probing at the sample side opposite to the transducer the signals related to the compressive and shear strain pulses can be separated in time. In addition to conventional monitoring of the reflected probe light intensity we monitor also the polarization rotation of the optical probe beam. This polarimetric technique results in improved sensitivity of detection and provides comprehensive information about the elasto-optical anisotropy. The experimental observations are in a good agreement with a theoretical analysis.

Optimization of choline administration regimen for correction of cognitive functions in rats after brain injury.

Choline diet promotes improvement of the brain cognitive functions in rats with moderate-to-severe traumatic brain injury. In previous studies, the rats received choline being standard (0.2%) or choline-supplemented (2%) diet for 2 weeks prior to and 2 weeks after experimental brain injury. To the end of the experiments (in 4 weeks), the post-traumatic disturbances in the cognitive functions were observed in both groups, although they were less pronounced than in the rats kept on the choline-supplemented diet. Based on original mathematical model, this paper proposes a method to calculate the most efficient use of choline to correct the brain cognitive functions. In addition to evaluating the cognitive functions, the study assessed expression of α7 nicotinic acetylcholine receptors, the amount of consumed food and water, and the dynamics of body weight.

Surface plasmon-based detection for picosecond ultrasonics in planar gold-dielectric layer geometries.

Longitudinal acoustic modes in planar thin gold films are excited and detected by a combination of ultrafast pump-probe photoacoustic spectroscopy and a surface plasmon resonance (SPR) technique. The resulting high sensitivity allows the detection of acoustic modes up to the 7th harmonic (258 GHz) with sub-pm amplitude sensing capabilities. This makes a comparison of damping times of individual modes possible. Further, the dynamics of the real and imaginary part of the dielectric function and the film's thickness variation are separated by using the dependence of the amplitudes of the acoustic modes on the detection angle and the surface plasmon resonance. We find that longitudinal acoustic modes in the gold films mainly affect the real part of the dielectric function and highlight the importance to consider thickness related effects in acousto-plasmonic sensing.

Linear and nonlinear Biot waves in a noncohesive granular medium slab: transfer function, self-action, second harmonic generation.

Experimental results are reported on second harmonic generation and self-action in a noncohesive granular medium supporting wave energy propagation both in the solid frame and in the saturating fluid. The acoustic transfer function of the probed granular slab can be separated into two main frequency regions: a low frequency region where the wave propagation is controlled by the solid skeleton elastic properties, and a higher frequency region where the behavior is dominantly due to the air saturating the beads. Experimental results agree well with a recently developed nonlinear Biot wave model applied to granular media. The linear transfer function, second harmonic generation, and self-action effect are studied as a function of bead diameter, compaction step, excitation amplitude, and frequency. This parametric study allows one to isolate different propagation regimes involving a range of described and interpreted linear and nonlinear processes that are encountered in granular media experiments. In particular, a theoretical interpretation is proposed for the observed strong self-action effect.

Dynamics of the response to variations in dietary choline content in rats.

A mathematical model was developed based on the analysis of body response to variations in choline content in the ration of growing rats. The peculiarities of choline metabolism and the expression of α7-nicotinic receptors in the brain were studied. The information obtained can be useful for comparative analysis of choline administration for correction of certain cerebral pathologies.

Glass transition of nanometric polymer films probed by picosecond ultrasonics.

The possibility to measure the glass transition temperature in poly(methyl methacrylate) (PMMA) films by picosecond ultrasonics with thicknesses ranging from 458 nm to 32 nm is demonstrated. A shift of the longitudinal acoustic eigenmodes towards lower frequencies with temperature is observed accompanied by a change in the temperature-frequency slopes at the glass transition temperature. The contributions to the frequency shift from changes in film thickness and sound velocity are discussed and the latter is extracted below the glass transition temperature. Finally, the advantages and disadvantages of the current approach in a comparison to other methods based on acoustic measurements in the GHz regime are reviewed.

Experimental evidence of rotational elastic waves in granular phononic crystals.

A generalized theory of elasticity, taking into account the rotational degrees of freedom of point bodies constituting a continuum, was proposed at the beginning of the twentieth century by the Cosserat brothers. We report the experimental observation of coupled rotational-translational modes in a noncohesive granular phononic crystal. While absent in the classical theory of elasticity, these elastic wave modes are predicted by the Cosserat theory. However the Cosserat theory fails to predict correctly the dispersion of the elastic modes in granular crystals even in the long-wavelength limit.

Doppler backscattering diagnostic with dual homodyne detection on the Globus-M compact spherical tokamak.

This article considers a four-frequency microwave Doppler backscattering (DBS) system in the compact spherical tokamak Globus-M. The hardware was adequate for the purposes of studying the peripheral plasma in the tokamak. The multichannel DBS system is based on duplication of a dual homodyne detection circuit for four incident Ka-band frequencies. The ray tracing results for a spherical torus are described, and specific requirements for the antenna tilt adjustment are defined. Some new experimental results are given for using DBS diagnostics on the Globus-M tokamak in order to illustrate its efficiency.

Vibrations of microspheres probed with ultrashort optical pulses.

We use ultrashort optical pulses to excite and detect vibrations of single silica spheres with a diameter of 5 microm placed at the surface of an acoustically mismatched substrate. In addition to the photoelastic detection of picosecond longitudinal acoustic pulses propagating inside the bulk, we detect gigahertz acoustic resonances of the sphere through probe beam defocusing. The mode frequencies are in close accord with those calculated from the elastic vibrations of a free sphere. We also record a resonant enhancement in the amplitude of specific modes of two touching spheres.

Nonlinear effects for coda-type elastic waves in stressed granular media.

Experimental results and their interpretations are presented on the nonlinear acoustic effects of multiple scattered elastic waves in unconsolidated granular media. Short wave packets with a central frequency higher than the so-called cutoff frequency of the medium are emitted at one side of the statically stressed slab of glass beads, and received at the other side after multiple-scattering and nonlinear elastic effects. Typical signals are strongly distorted compared to their initially radiated shape both due to nonlinearity and scattering. It is shown that acoustic waves with a deformation amplitude much lower than the mean static deformation of the contacts in the medium can modify the elastic properties of the medium. This addresses the problem of acoustic wave action on granular matter during and after acoustic excitation, which is necessary to understand in the nondestructive testing of the elastic properties of granular media by acoustic methods. Coda signal analysis is shown to be a powerful time-resolved tool in monitoring slight modifications in the elastic response of an unconsolidated granular structure.

Tomography diagnostic of plasma radiated power on the spherical tokamak Globus-M.

A diagnostic system for plasma radiated power tomography on Globus-M was upgraded with a new linear array based on silicon photodiodes. The hardware of the diagnostic system based on a 256-channel tangential matrix array and a 24-channel linear array is described. The algorithm for 2D tomographic reconstruction of plasma emissivity is presented. It uses the Tikhonov regularization method and anisotropic diffusion functional as a side constraint. Simulated plasma emissivity profiles were used to test and assess the capabilities of the developed algorithm. Data analysis was carried out in ohmic and neutral beam heated discharges to demonstrate the performance of the tomography diagnostic in plasmas with different parameters. The presented results illustrate the proper functioning of the tomography system that allowed to reveal and study various plasma phenomena, including magnetohydrodynamic mode.

Interface Adhesion and Structural Characterization of Rolled-up GaAs/In0.2Ga0.8As Multilayer Tubes by Coherent Phonon Spectroscopy.

We present a detailed experimental and theoretical study of the acoustic phonon modes in rolled-up multilayers with thickness of the layers in the nanometre and diameters in the micrometre range. We compare our results to planar, unrolled multilayers grown by molecular beam epitaxy. For the planar multilayers the experimentally obtained acoustic modes exhibit properties of a superlattice and match well to calculations obtained by the Rytov model. The rolled-up superlattice tubes show intriguing differences compared to the planar structures which can be attributed to the imperfect adhesion between individual tube windings. A transfer matrix method including a massless spring accounting for the imperfect adhesion between the layers yields good agreement between experiment and calculations for up to five windings. Areas with sufficient mechanical coupling between all windings can be distinguished by their acoustic mode spectrum from areas where individual windings are only partially in contact. This allows the spatially resolved characterization of individual tubes with micrometre spatial resolution where areas with varying interface adhesion can be identified.