Coherent Acoustic Pulses Research Articles

High transmission efficiency of intense sub-THz coherent phonons in strongly correlated VO2/TiO2 heterojunction

High-frequency coherent acoustic phonons hold immense value in characterizing the coupling between magnetic, lattice, and electronic properties, offering nanometer-scale spatial resolution within the ultrafast timescale. However, efficiently propagating intense sub-THz coherent acoustic phonons across diverse materials remains a formidable challenge. Here, we demonstrate that using vanadium dioxide (VO2) as a transducer can induce enhanced coherent acoustic pulses that propagate efficiently (∼90%) into TiO2 due to excellent acoustic impedance matching and minor lattice interface mismatch compared with traditional metals such as Pt, Au, and Cu. Employing time-resolved pump–probe reflectivity spectroscopy, we observe pronounced coherent phonon oscillations reaching up to 0.164 THz from the longitudinal acoustic mode along the c axis in VO2/TiO2. Furthermore, the temperature and pump fluence dependence of the coherent phonon oscillation signals suggest that the metallic state of VO2 responds to these large coherent acoustic phonons.

Time-domain Brillouin scattering theory for probe light and acoustic beams propagating at an angle and acousto-optic interaction at material interfaces

A theory has been developed to interpret time-domain Brillouin scattering (TDBS) experiments involving coherent acoustic pulse (CAP) and light pulse beams propagating at an angle to each other. It predicts the influence of the directivity pattern of their acousto-optic interaction on TDBS signals when heterodyne detection of acoustically scattered light is in backward direction to incident light. The theory reveals relationships between the carrier frequency, amplitude and duration of acoustically induced ”wave packets” in light transient reflectivity signals, and factors such as CAP duration, widths of light and sound beams, and their interaction angle. It describes the transient dynamics of these wave packets when the light and CAP encounter material interfaces, and how the light scattering by the incident CAP transforms into scattering by the reflected and transmitted CAPs. The theory suggests that single-point TDBS experiments can determine not only depth positions of buried interfaces but also their local inclinations/orientations.

Coherent acoustic pulse emission by ensembles of plasmonic nanoparticles

Coherent acoustic pulse emission by ensembles of plasmonic nanoparticles

3D characterization of individual grains of coexisting high-pressure H2O ice phases by time-domain Brillouin scattering

Time-domain Brillouin scattering (TDBS) uses ultrashort laser pulses to (i) generate coherent acoustic pulses of picoseconds duration in a solid sample and (ii) follow their propagation in order to image material inhomogeneities with the axial resolution that can be deeply sub-optical, to nm-scale, and the lateral one down to the optical diffraction limit (half the optical wavelength of the probe laser). TDBS permits highly resolved 3D-imaging of grains in polycrystalline transparent samples with unlimited lateral sizes and thicknesses of at least 10 μm also when samples are orientationally textured and/or located in devices permitting access along one direction and from one side only. This optical technique presents, accordingly, clear advantages compared to any x-ray based computed tomography (neither back-projection algorithm nor multiple viewpoints of the sample are needed) and classical spectroscopic methods. Here, we applied TDBS to the 3D-imaging of a sample of polycrystalline water ice containing two high-pressure phases. The imaging, accomplished via a simultaneous detection of quasi-longitudinal and quasi-shear waves, provided shape, coordinates, phase content, and crystallographic orientation of resolved crystallites in a common coordinate system. Monitoring of acoustic pulses simultaneously propagating in two neighboring grains provided a new tool for the localization of grain boundaries.

Contra-intuitive features of time-domain Brillouin scattering in collinear paraxial sound and light beams

Time-domain Brillouin scattering is an opto-acousto-optical probe technique for the evaluation of transparent materials. Via optoacoustic conversion, ultrashort pump laser pulses launch coherent acoustic pulses in the sample. Time-delayed ultrashort probe laser pulses monitor the propagation of the coherent acoustic pulses via the photo-elastic effect, which induces light scattering. A photodetector collects both the acoustically scattered light and the probe light reflected by the sample structure for the heterodyning. The scattered probe light carries information on the acoustical, optical and acousto-optical parameters of the material for the current position of the coherent acoustic pulse. Thus, among other applications, time-domain Brillouin scattering is a technique for three-dimensional imaging. Sharp focusing of coherent acoustic pulses and probe laser pulses could increase lateral spatial resolution of imaging, but could potentially diminish the depth of imaging. However, the theoretical analysis presented in this manuscript contra-intuitively demonstrates that the depth and spectral resolution of the time-domain Brillouin scattering imaging, with collinearly propagating paraxial sound and light beams, do not depend on the focusing/diffraction of sound. The variations of the amplitude of the time-domain Brillouin scattering signal are only due to the variations of the probe light amplitude caused by light focusing/diffraction. Although the amplitude of the acoustically scattered light is proportional to the product of the local acoustical and probe light field amplitudes, the temporal dynamics of the time-domain Brillouin scattering signal amplitude is independent of the dynamics of the coherent acoustic pulse amplitude.

Optical generation and detection of gigahertz shear acoustic waves in solids assisted by a metallic diffraction grating

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches in the material both compression/dilatation (longitudinal) and shear coherent acoustic pulses in directions of different orders of acoustic diffraction. The propagation of the emitted acoustic pulses can be monitored by measuring the variation of the optical reflectivity of the time-delayed ultrashort probe laser pulses. The direction of probe light incidence and its polarization relative to the sample surface as well as the orientation of the metallic grating should be specifically chosen for efficient Brillouin scattering of the probe light from shear phonons propagating in the elastically isotropic materials. As theoretically predicted, the obtained experimental data contain multiple frequency components which are due to a variety of possible Brillouin scattering angles for both shear and compression/dilatation coherent acoustic waves. All these different frequency components are explained through multiplexing the propagation directions of probe light and coherent sound by the metallic diffraction grating. Our experimental scheme of time-domain Brillouin scattering with metallic gratings operating in reflection mode provides access to monitoring the shear acoustic waves launched in the direction of the first diffraction order by backward Brillouin scattering process. Applications include simultaneous determination of several different acoustic mode velocities and optical refractive index and, potentially, measurements of the acoustic dispersion of samples with a single direction of possible optical access.

Advances in applications of time-domain Brillouin scattering for nanoscale imaging

Time-domain Brillouin scattering is an all-optical experimental technique based on ultrafast lasers applied for generation and detection of coherent acoustic pulses on time durations of picoseconds and length scales of nanometers. In transparent materials, scattering of the probe laser beam by the coherent phonons permits imaging of sample inhomogeneity. The transient optical reflectivity of the sample recorded by the probe beam as the acoustic nanopulse propagates in space contains information on the acoustical, optical, and acousto-optical parameters of the material under study. The experimental method is based on a heterodyning where weak light pulses scattered by the coherent acoustic phonons interfere at the photodetector with probe light pulses of significantly higher amplitude reflected from various interfaces of the sample. The time-domain Brillouin scattering imaging is based on Brillouin scattering and has the potential to provide all the information that researchers in materials science, physics, chemistry, biology, etc., get with classic frequency-domain Brillouin scattering of light. It can be viewed as a replacement for Brillouin scattering and Brillouin microscopy in all investigations where nanoscale spatial resolution is either required or advantageous. Here, we review applications of time-domain Brillouin scattering for imaging of nanoporous films, ion-implanted semiconductors and dielectrics, texture in polycrystalline materials and inside vegetable and animal cells, and for monitoring the transformation of nanosound caused by nonlinearity and focusing. We also discuss the perspectives and the challenges for the future.

Acoustic waves undetectable by transient reflectivity measurements

In laser ultrasonics, ultrashort light pulses generate coherent acoustic pulses of picosecond duration via multiple possible physical mechanisms, involving optoacoustic conversion processes. These wide-band GHz acoustic signals are optically detected at the sample surfaces by ultrafast time-delayed probe light pulses. When the coherent acoustic pulses in GaAS are detected via the Brillouin scattering of probe light pulses of 400 nm wavelength, certain spectral components of the acoustic pulses remain invisible. The theoretical analysis relates this observation to the existence of zeros in the spectral transformation function of acousto-optic conversion. The phenomenon of zero sensitivity of the optical reflectivity to coherent acoustic phonons of particular frequencies is rather general and depends on both the probe light wavelength and the evaluated material. These findings substantially contribute to picosecond ultrasonics and laser-based nondestructive testing.

Picosecond laser ultrasonics for imaging of transparent polycrystalline materials compressed to megabar pressures

Picosecond laser ultrasonics is an all-optical experimental technique based on ultrafast high repetition rate lasers applied for the generation and detection of nanometric in length coherent acoustic pulses. In optically transparent materials these pulses can be detected not only on their arrival at the sample surfaces but also all along their propagation path inside the sample providing opportunity for imaging of the sample material spatial inhomogeneities traversed by the acoustic pulse. Application of this imaging technique to polycrystalline elastically anisotropic transparent materials subject to high pressures in a diamond anvil cell reveals their significant texturing/structuring at the spatial scales exceeding dimensions of the individual crystallites.

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.

Coherent gigahertz phonons in Ge2Sb2Te5 phase-change materials

Using 40 fs ultrashort laser pulses, we investigate the picosecond acoustic response from a prototypical phase change material, thin Ge2Sb2Te5 (GST) films with various thicknesses. After excitation with a 1.53 eV-energy pulse with a fluence of 5 mJ cm−2, the time-resolved reflectivity change exhibits transient electronic response, followed by a combination of exponential-like strain and coherent acoustic phonons in the gigahertz (GHz) frequency range. The time-domain shape of the coherent acoustic pulse is well reproduced by the use of the strain model by Thomsen et al 1986 (Phys. Rev. B 34 4129). We found that the decay rate (the inverse of the relaxation time) of the acoustic phonon both in the amorphous and in the crystalline phases decreases as the film thickness increases. The thickness dependence of the acoustic phonon decay is well modeled based on both phonon-defect scattering and acoustic phonon attenuation at the GST/Si interface, and it is revealed that those scattering and attenuation are larger in crystalline GST films than those in amorphous GST films.

Revealing sub-μm and μm-scale textures in H2O ice at megabar pressures by time-domain Brillouin scattering.

The time-domain Brillouin scattering technique, also known as picosecond ultrasonic interferometry, allows monitoring of the propagation of coherent acoustic pulses, having lengths ranging from nanometres to fractions of a micrometre, in samples with dimension of less than a micrometre to tens of micrometres. In this study, we applied this technique to depth-profiling of a polycrystalline aggregate of ice compressed in a diamond anvil cell to megabar pressures. The method allowed examination of the characteristic dimensions of ice texturing in the direction normal to the diamond anvil surfaces with sub-micrometre spatial resolution via time-resolved measurements of the propagation velocity of the acoustic pulses travelling in the compressed sample. The achieved imaging of ice in depth and in one of the lateral directions indicates the feasibility of three-dimensional imaging and quantitative characterisation of the acoustical, optical and acousto-optical properties of transparent polycrystalline aggregates in a diamond anvil cell with tens of nanometres in-depth resolution and a lateral spatial resolution controlled by pump laser pulses focusing, which could approach hundreds of nanometres.

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 10% in acoustic pulse propagation speed 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 gigahertz frequency range across the glass transition temperature.

Picosecond ultrasound spectroscopy for studying elastic modulus of thin films: a review

This article introduces an advanced acoustic method for measuring the out-of-plane longitudinal-wave modulus of thin films using picosecond ultrasound. The ultrafast light pulse is focused on the film surface to excite the coherent acoustic pulse, which propagates in the thickness direction, and then, the time delayed probing light pulse irradiates the specimen for detecting the acoustic waves. For opaque thin films, the pulse echoes within the film or the thickness resonance frequency of the film is measured to determine the modulus. For transparent films, Brillouin oscillations from the film are observed, and their frequencies yield the modulus. The film thickness and refractive index are measured by X-ray reflectivity and ellipsometry, respectively. This method was applied to various thin films, providing important knowledge about the elasticity of thin films. Most thin films are softer than corresponding bulk materials, but some thin films are significantly stiffer.

Nanoacoustic waves in nanomaterials

We discuss the formation and detection of acoustic soliton trains in crystal slabs. High-amplitude amplitude strain pulses are generated by impact of intense femtosecond optical pulses on a metallic film and injected into the crystal slab. Nonlinear acoustic propagation leads to the formation of shock fronts (N-waves), that, in absence of viscous damping and by virtue of dispersion, may develop into soliton trains at the leading edge and high frequency tails at the trailing edge. Interferometric pump-probe optical experiments are discussed to directly detect the ultrafast surface displacements when a soliton is reflected at the surface of the crystal slab. Finally, we experimentally prove that these acoustic solitons are capable of impulsively exciting THz transitions in electronic centers in solids. The coherent terahertz acoustic pulse trains are applied to manipulate the optical response of two-dimensional excitons in a III-V quantum well on the ultrafast timescale. By virtue of the deformation potential, the coherent exciton emission becomes strongly "chirped" when the acoustic pulse train passes the quantum well. This yields the prospect to perform pump-probe terahertz acoustic experiments with nanoacoustic waves in semiconductor nanomaterials.

Phonons rectification in picosecond laser ultrasonics

In standard ultrafast acoustic experiments very short coherent acoustic pulses are generated by the absorption of a femtosecond laser pulse in a thin metallic transducer deposited on the sample. Subsequently the acoustic pulses and the heat generated in the transducer cross the sample and are partially transmitted in the underlaying substrate. At low temperature, heating of the metallic transducer gives rise to the emission of incoherent phonons wave packets which propagate ballistically over large distances in the substrate. We report on a series of experiments which demonstrate the acoustic rectification [1-3] of these wave packets as they propagate through large GaAs or Si substrates. [1] B. Perrin, J. de Phys. C8 (1979) 216. [2] S. M. Avanesyan, V. E. Gusev, Solid State. Commun. 54 (1985) 1065. [3] B. Perrin, E. Péronne, L. Belliard, Ultrasonics, 44 (2006) 1277.

Long-lived coherent traveling acoustic pulses induced by femtosecond laser pulses

In this paper, we systematically study the generation and propagation of coherent acoustic pulses in a metal–dielectric system using a two-color femtosecond pump–probe technique at different probe angles. A long-lived acoustic oscillation is observed in a borosilicate glass coated with gold and shows different attenuations and amplitude at different probe wavelengths. Our study demonstrates that the two-color optical pump–probe technique can be used as a noninvasive tool to study acoustic properties of dielectric materials.

Generation and detection of incoherent phonons in picosecond ultrasonics

In picosecond ultrasonics experiments the absorption of a femtosecond laser pulse in a thin metallic transducer is used to generate very short acoustic pulses. These pulses are made of coherent longitudinal waves with a frequency spectrum that can reach 100–200 GHz. The laser pulse absorption gives rise to a heating of the film of a few Kelvin within a typical time of 1 ps. Later on, the heat goes in the substrate through an interface thermal resistance and is diffused by thermal conduction. At very low temperature and in pure crystals the thermal phonons emitted by the heated metallic film can propagate ballistically over large distances and produce a so-called heat pulse. We report on the experimental evidence of the coexistence of the coherent acoustic pulse and the incoherent heat pulse generated and detected by laser ultrasonics.

Ultrashort shear acoustic pulse excitation via laser-induced electrostrictive effect (abstract)

In principle there are several opportunities to achieve excitation of ultrashort shear pulses by laser radiation. One of them is to use the anisotropy of material properties such as elasticity and thermal expansion (in the case of the thermoelastic mechanism of the optoacoustic transformation) or the electron–phonon deformation potential (in the case of sound excitation via electron–hole plasma photogeneration). Another is related to application of the laser-induced inverse (converse) piezoelectric effect. Laser-generated electron–hole plasma creates an electric field due to the electron–hole spatial separation (Dember field) or can screen an electric field (built-in or induced externally) pre-exsisting near the crystal surface. In piezoelectric materials the induced ultrafast transients in electric field distribution in the proximity of the crystal surface should cause excitation of ultrashort acoustic pulses of various modes. Experimentally achieved excitation of plane TA pulses in piezoelectric crystals by nanosecond laser pulses, and also excitation of both optical phonons and ultrashort electromagnetic pulses (THz radiation) by femtosecond laser pulses all support the possibility of exciting ultrashort TA, as well as LA, pulses. The idea of using the inverse piezoelectric effect is also supported by recent observations of extremely high efficiency of quasi-monochromatic LA coherent oscillations generation by laser-induced screening of built-in electric fields in strained multiple quantum wells of piezoelectric semiconductors. We analyze one more opportunity for ultrashort TA pulse excitation in optoacoustic transformation via the action of laser-induced electrostrictive stress on surface of optically transparent crystals. A simple theory is developed and numerical estimates are presented to demonstrate the possibility of exciting ultrashort acoustic pulses in optically transparent materials by laser-induced electrostrictive surface mechanical stresses. Both longitudinal and shear coherent acoustic pulses can be excited. The conditions for which the polarization of ultrashort shear pulses is controlled by the polarization of an incident laser pulse are established. The profiles and duration of longitudinal and shear acoustic strain pulses are shown to coincide with laser pulse intensity envelope. Picosecond shear pulses might be very useful in diverse applications related to measurements of shear rigidity or shear viscosity, including ultrafast solid–liquid phase transitions, ultrafast tribology, and diagnostics of liquids in confined geometry.

On the possibility of ultrashort shear acoustic pulse excitation due to the laser-induced electrostrictive effect

A simple theory is developed and numerical estimates presented to demonstrate the possibility of exciting ultrashort acoustic pulses in optically transparent materials by laser-induced electrostrictive surface mechanical stresses. Both longitudinal and shear coherent acoustic pulses can be excited. The conditions for which the polarization of ultrashort shear pulses are controlled by the polarization of an incident laser pulse are established. The profiles and durations of longitudinal and shear acoustic strain pulses are shown to coincide with laser pulse intensity envelope.