Nonlinear Acoustics at GHz Frequencies in a Viscoelastic Fragile Glass Former

Using a picosecond pump-probe ultrasonic technique, we study the propagation of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the viscoelastic fragile glass former DC704. We observe an increase of almost ten percent in acoustic pulse propagation speed of its leading shock front at the highest optical pump fluence which is a result of the supersonic nature of nonlinear propagation in the viscous medium. From our measurement we deduce the nonlinear acoustic parameter of the glass former in the GHz frequency range across the glass transition temperature.Comment: 5 pages, 3 figure

Time-domain Brillouin scattering assisted by diffraction gratings

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches coherent compression/dilatation acoustic pulses in directions of different orders of acoustic diffraction. Their propagation is detected by the delayed laser pulses, which are also diffracted by the metallic grating, through the measurement of the transient intensity change of the first order diffracted light. The obtained data contain multiple frequency components which are interpreted by considering all possible angles for the Brillouin scattering of light achieved through the multiplexing of the propagation directions of light and coherent sound by the metallic grating. The emitted acoustic field can be equivalently presented as a superposition of the plane inhomogeneous acoustic waves, which constitute an acoustic diffraction grating for the probe light. Thus, the obtained results can also be interpreted as a consequence of probe light diffraction by both metallic and acoustic gratings. The realized scheme of time-domain Brillouin scattering with metallic grating operating in reflection mode provides access to acoustic frequencies from the minimal to the maximal possible in a single experimental configuration for the directions of probe light incidence and scattered light detection. This is achieved by monitoring of the backward and forward Brillouin scattering processes in parallel. Applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of the samples with a single direction of possible optical access.Comment: 21 pages, 4 figures, 1 tabl

Nonlinear acousto-magneto-plasmonics

We review the recent progress in experimental and theoretical research of interactions between the acoustic, magnetic and plasmonic transients in hybrid metal-ferromagnet multilayer structures excited by ultrashort laser pulses. The main focus is on understanding the nonlinear aspects of the acoustic dynamics in materials as well as the peculiarities in the nonlinear optical and magneto-optical response. For example, the nonlinear optical detection is illustrated in details by probing the static magneto-optical second harmonic generation in gold-cobalt-silver trilayer structures in Kretschmann geometry. Furthermore, we show experimentally how the nonlinear reshaping of giant ultrashort acoustic pulses propagating in gold can be quantified by time-resolved plasmonic interferometry and how these ultrashort optical pulses dynamically modulate the optical nonlinearities. The effective medium approximation for the optical properties of hybrid multilayers facilitates the understanding of novel optical detection techniques. In the discussion we highlight recent works on the nonlinear magneto-elastic interactions, and strain-induced effects in semiconductor quantum dots.Comment: 30 pages, 12 figures, to be published as a Topical Review in the Journal of Optic

Interferometric analysis of laser-driven cylindrically focusing shock waves in a thin liquid layer

Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted. The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of materials under shock compression.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-13-D-0001

Additive Laser Excitation of Giant Nonlinear Surface Acoustic Wave Pulses

The laser ultrasonics technique perfectly fits the needs for non-contact, non-invasive, non-destructive mechanical probing of samples of mm to nm sizes. This technique is however limited to the excitation of low-amplitude strains, below the threshold for optical damage of the sample. In the context of strain engineering of materials, alternative optical techniques enabling the excitation of high amplitude strains in a non-destructive optical regime are seeking. We introduce here a non-destructive method for laser-shock wave generation based on additive superposition of multiple laser-excited strain waves. This technique enables strain generation up to mechanical failure of a sample at pump laser fluences below optical ablation or melting thresholds. We demonstrate the ability to generate nonlinear surface acoustic waves (SAWs) in Nb:SrTiO$_3$ substrates, at typically 1 kHz repetition rate, with associated strains in the percent range and pressures close to 100 kbars. This study paves the way for the investigation of a host of high-strength SAW-induced phenomena, including phase transitions in conventional and quantum materials, plasticity and a myriad of material failure modes, chemistry and other effects in bulk samples, thin layers, or two-dimensional materials

Ultrafast tunable modulation of light polarization at terahertz frequencies

Controlling light polarization is one of the most essential routines in modern optical technology. Since the demonstration of optical pulse shaping by spatial light modulators and its potential in controlling the quantum reaction pathways, it paved the way for many applications as a coherent control of the photoionization process or as polarization shaping of terahertz (THz) pulses. Here, we evidenced efficient non-resonant and noncollinear chi((2))-type light-matter interaction in femtosecond polarization-sensitive time-resolved optical measurements. Such nonlinear optical interaction of visible light and ultrashort THz pulses leads to THz modulation of visible light polarization in bulk LiNbO3 crystal. Theoretical simulations based on the wave propagation equation capture the physical processes underlying this nonlinear effect. Apart from the observed tunable polarization modulation of visible pulses at ultrahigh frequencies, this physical phenomenon can be envisaged in THz depth-profiling of materials

Ultrafast acousto-optic mode conversion in optically birefringent ferroelectrics

The ability to generate efficient giga-terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics

GÉNÉRATION ET DÉTECTION<br /><br />D'ONDES ACOUSTIQUES TRANSVERSES PICOSECONDES :<br /><br />THÉORIES ET EXPÉRIENCES

Picosecond laser ultrasonics applies opto-acoustic and acousto-optic transformations for the excitation and detection, respectively, of the hypersonic pulses with the duration shorter than hundreds of picoseconds, which are commonly applied for the non-destructive testing of the nanostructures. Starting the pioneering experiments for more than 20 years most of the research has been conducted with longitudinal acoustic mode only. The excitation of quasi-plane acoustic waves is controlled by focusing the femtosecond pump laser pulse on the front surface of the sample to the spot with the diameter significantly exceeding both the depth of the substrate heated region of the material and the distance of sound propagation during the pulsed laser action. Due to this, the diffraction length of the acoustic pulses generated significantly exceeds the characteristic spatial scales involved in echo monitoring of the nanostructures. Unfortunately the layers of the nanostructures are usually elastically isotropic (microcrystalline) or anisotropic (monocrystalline) but with symmetry axis orientation along the normal to the layers. This prevents by symmetry principles excitation of plane shear acoustic waves. The shear mode generation in picosecond ultrasonics is of fundamental issue. One of clear perspectives in the application of coherent shear hypersound is the diagnostics of solid-solid and solid-liquid interfaces and Kapitza resistance by shear phonons. In this thesis, the thermoelastic generation of shear picosecond acoustic waves is analytically treated and experimentally investigated in the case of anisotropic opto-acoustic transducers made of metallic substrate cut off-axis of symmetry. The problematic concerned the thermoelastic generation and the acousto-optic detection of the picosecond shear acoustic waves. First, the thermoelastic generation theory in anisotropic materials is developed to characterize the efficiency of generation and to fully understand the physical processes. That merged on the description of a new volumetric but non-local mechanism of plane shear waves excitation through thermoelasticity which is operative even in crystals with isotropic thermal expansion (in cubic crystals). The theory of the acousto-optic detection in such anisotropic material is also developed. The using of the metallic substrate as a transducer and sensor of picosecond shear acoustic pulses is opportune. Second, the picosecond acoustic experiments are realized with monocristallines substrates of zinc cut off-axis of symmetry. Thanks to the theoretical predictions, the signals can be interpreted and analyzed. Experimentally, the efficiency of the thermoelastic generation is proved. The full numerical computation of the reflectivity signals gave a set of the zinc photoelastic coefficients. Finally, the extension to a zinc polycristalline sample, natively without grain preferenced orientation (001), also proved to be efficient.L'acoustique picoseconde est basée sur une technique pompe-sonde optique nécessitant l'emploi d'un laser pulsé subpicoseconde permettant la génération et la détection d'impulsions acoustiques ultrabrèves. L'onde acoustique picoseconde générée est par nature longitudinale lorsque le transducteur opto-acoustique est isotrope. Ainsi, jusqu'à présent, dans la plupart des cas, seul le mode acoustique longitudinal a pu être employé au diagnostic mécanique de structures submicroniques. La mise en oeuvre d'une source acoustique picoseconde d'ondes transverses ouvrirait la voie à de nouvelles perspectives expérimentales. Dans ce travail de thèse, la génération d'ondes acoustiques transverses picosecondes par le mécanisme de génération thermoélastique est envisagée analytiquement et exploitée expérimentalement à partir de transducteurs opto-acoustiques anisotropes, le cas échéant, des substrats métalliques désorientés. La problématique concerne tant la génération thermoélastique que la détection élasto-optique des ondes acoustiques transverses picosecondes.En premier lieu, la théorie de la génération thermoélastique dans des matériaux anisotropes est présentée afin de caractériser la potentialité de la génération thermoélastique et de concevoir les mécanismes propres à la génération des ondes transverses. Puis la théorie de la détection élasto-optique est exposée afin d'étayer l'opportunité d'utilisation du substrat métallique anisotrope en tant que capteur d'ondes transverses picosecondes. Le rôle bivalent du substrat en tant que transducteur et capteur d'ondes transverses picosecondes est opportun.En second lieu, les expériences picosecondes réalisées avec des substrats monocristallins de zinc désorientés sont présentées et interprétées grâce aux prédictions théoriques. Les expériences attestent de l'efficacité de la génération in situ des ondes acoustiques transverses picosecondes et de la génération par réflexion-conversion de modes des ondes acoustiques longitudinales picosecondes. Les simulations numériques concernant les résultats expérimentaux permettent la mesure des coefficients photo-élastiques du zinc. Enfin, l'extrapolation à des substrats polycristallins métalliques permet de démontrer l'efficacité de la génération thermoélastique à partir de cristallites non texturés (001)