Thomas Pezeril

High strain rate deformation of layered nanocomposites

Insight into the mechanical behaviour of nanomaterials under the extreme condition of very high deformation rates and to very large strains is needed to provide improved understanding for the development of new protective materials. Applications include protection against bullets for body armour, micrometeorites for satellites, and high-speed particle impact for jet engine turbine blades. Here we use a microscopic ballistic test to report the responses of periodic glassy-rubbery layered block-copolymer nanostructures to impact from hypervelocity micron-sized silica spheres. Entire deformation fields are experimentally visualized at an exceptionally high resolution (below 10 nm) and we discover how the microstructure dissipates the impact energy via layer kinking, layer compression, extreme chain conformational flattening, domain fragmentation and segmental mixing to form a liquid phase. Orientation-dependent experiments show that the dissipation can be enhanced by 30% by proper orientation of the layers.

New Concept for Magnetization Switching by Ultrafast Acoustic Pulses

It is shown theoretically that a single acoustic pulse, a few picoseconds long, can reverse magnetization in a magnetostrictive material Terfenol-D. Following giant magnetoelastic changes of free energy density, the magnetization vector is ejected from a local in-plane energy minimum and decays into another minimum. For an acoustic pulse duration significantly shorter than magnetization precession period τac≪Tprec, the switching threshold is determined by the acoustic pulse area, i.e., pulse integral in the time domain, similar to coherent phenomena in optics. Simulation results are summarized in a magnetoacoustic switching diagram and discussed in the context of all-optical magnetization switching by circularly polarized light pulses.

Mechanical spectra of glass-forming liquids. II. Gigahertz-frequency longitudinal and shear acoustic dynamics in glycerol and DC704 studied by time-domain Brillouin scattering

This paper presents and discusses the temperature and frequency dependence of the longitudinal and shear viscoelastic response at MHz and GHz frequencies of the intermediate glass former glycerol and the fragile glass former tetramethyl-tetraphenyl-trisiloxane (DC704). Measurements were performed using the recently developed time-domain Brillouin scattering technique, in which acoustic waves are generated optically, propagated through nm thin liquid layers of different thicknesses, and detected optically after transmission into a transparent detection substrate. This allows for a determination of the frequency dependence of the speed of sound and the sound-wave attenuation. When the data are converted into mechanical moduli, a linear relationship between longitudinal and shear acoustic moduli is revealed, which is consistent with the generalized Cauchy relation. In glycerol, the temperature dependence of the shear acoustic relaxation time agrees well with literature data for dielectric measurements. In DC704, combining the new data with data from measurements obtained previously by piezo-ceramic transducers yields figures showing the longitudinal and shear sound velocities at frequencies from mHz to GHz over an extended range of temperatures. The shoving models prediction for the relaxation times temperature dependence is fairly well obeyed for both liquids as demonstrated from a plot with no adjustable parameters. Finally, we show that for both liquids the instantaneous shear modulus follows an exponential temperature dependence to a good approximation, as predicted by Granatos interstitialcy model.

Photoexcitation of gigahertz longitudinal and shear acoustic waves in BiFeO3 multiferroic single crystal

Using femtosecond laser pulses, coherent GHz acoustic phonons are efficiently photogenerated and photodetected in BiFeO3 (BFO) multiferroic single crystal. Due to the crystal lattice symmetry, longitudinal as well as two transverse acoustic modes are generated and detected, and the corresponding sound velocities are determined. This provides the opportunity to experimentally evaluate the elastic coefficients of the multiferroic compound BiFeO3 that have been estimated so far only through ab initio calculations. The knowledge of the elastic properties of BFO is highly desired for BFO integration in nanoelectronic devices. Moreover, our findings highlight also that BFO may be a good candidate for light-controlled coherent acoustic phonons sources.

Direct visualization of laser-driven focusing shock waves.

Direct real-time visualization and measurement of laser-driven shock generation, propagation, and 2D focusing in a sample are demonstrated. A substantial increase of the pressure at the convergence of the cylindrical acoustic shock front is observed experimentally and simulated numerically. Single-shot acquisitions using a streak camera reveal that at the convergence of the shock wave in water the supersonic speed reaches Mach 6, corresponding to the multiple gigapascal pressure range ∼30 GPa.

Narrow-band acoustic attenuation measurements in vitreous silica at frequencies between 20 and 400 GHz

Acoustic attenuation rates in vitreous silica in the 20–400 GHz frequency range have been measured using a multiple-pulse optical technique for generation of tunable multicycle acoustic waves that are detected interferometrically after traversal of the sample. The results connect the frequency ranges of several measurement methods, yielding a consistent description of the acoustic behavior.

Picosecond acoustics in p-doped piezoelectric semiconductors

We demonstrate by experiment and theoretical analysis that the presence of built-in electric fields near the (111) and (1¯1¯1¯) surfaces of p-doped GaAs causes efficient generation of acoustic waves due to the laser-induced inverse piezoelectric effect. At the same time, the generation efficiency from the electron-hole-phonon deformation potential is shown to be reduced. The polarity of the acoustic pulse is inverted when changing the laser irradiated surface from (111) to (1¯1¯1¯). The results have ramifications for optically controlled piezoelectric ultrasound transducers.

Transient Grating Spectroscopy in Magnetic Thin Films: Simultaneous Detection of Elastic and Magnetic Dynamics

Surface magnetoelastic waves are coupled elastic and magnetic excitations that propagate along the surface of a magnetic material. Ultrafast optical techniques allow for a non-contact excitation and detection scheme while providing the ability to measure both elastic and magnetic components individually. Here we describe a simple setup suitable for excitation and time resolved measurements of high frequency magnetoelastic waves, which is based on the transient grating technique. The elastic dynamics are measured by diffracting a probe laser pulse from the long-wavelength spatially periodic structural deformation. Simultaneously, a magnetooptical measurement, either Faraday or Kerr effect, is sensitive to the out-of-plane magnetization component. The correspondence in the response of the two channels probes the resonant interaction between the two degrees of freedom and reveals their intimate coupling. Unraveling the observed dynamics requires a detailed understanding of the spatio-temporal evolution of temperature, magnetization and thermo-elastic strain in the ferromagnet. Numerical solution of thermal diffusion in two dimensions provides the basis on which to understand the sensitivity in the magnetooptic detection.

Jones matrix formalism for the theory of picosecond shear acoustic pulse detection

A theoretical analysis of the transient optical reflectivity of a sample by a normalized Jones matrix is presented. The off-diagonal components of the normalized matrix are identified with the complex rotation of the polarization ellipse. Transient optical polarimetry is a relevant technique to detect shear acoustic strain pulses propagating normally to the surface of an optically isotropic sample. Moreover, polarimetry has a selective sensitivity to shear waves, as this technique cannot detect longitudinal waves that propagate normally to the sample surface.

Ultrafast acousto-plasmonics in gold nanoparticle superlattices

We report the investigation of the generation and detection of GHz coherent acoustic phonons in plasmonic gold nanoparticles superlattices (NPS). The experiments have been performed from an optical femtosecond pump-probe scheme across the optical plasmon resonance of the superlattice. Our experiments allow to estimate the collective elastic response (sound velocity) of the NPS as well as an estimate of the nano-contact elastic stiffness. It appears that the light-induced coherent acoustic phonon pulse has a typical in-depth spatial extension of about 45 nm which is roughly 4 times the optical skin depth in gold. The modeling of the transient optical reflectivity indicates that the mechanism of phonon generation is achieved through ultrafast heating of the NPS assisted by light excitation of the volume plasmon. These results demonstrate how it is possible to map the photon-electron-phonon interaction in subwavelength nanostructures.