Acoustic Strain Pulses Research Articles

Tomographic reconstruction of picosecond acoustic strain pulses using automated angle-scan probing with visible light

By means of an ultrafast optical technique, picosecond acoustic strain pulses in a transparent medium are tomographically visualized at GHz frequencies. The strain distribution in BK7 glass is reconstructed from time-domain reflectivity changes of 415-nm probe light as a function of the optical incidence angle with 1 ps temporal and 120 nm spatial resolutions, enabled by automated angle scanning. The latter resolution is achieved owing to the commensurate acoustic wavelength. Applications include imaging strain, carrier and temperature distributions on ultrashort timescales.

Photoinduced insulator-to-metal transition and coherent acoustic phonon propagation in LaCoO3 thin films explored by femtosecond pump-probe ellipsometry

We have studied ultrafast dynamics of thin films of ${\mathrm{LaCoO}}_{3}$ and ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_{3}$ with femtosecond pump-probe ellipsometry in the energy range of 1.6--3.4 eV. We have observed a large pump-induced transfer of spectral weight in ${\mathrm{LaCoO}}_{3}$ that corresponds to an insulator-to-metal transition. The photoinduced metallic state initially relaxes via a fast process with a decay constant of about 200 fs. Both ${\mathrm{LaCoO}}_{3}$ and ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_{3}$ exhibit a significant secondary peak in the 1--30 ps range. Results of measurements on films with different thicknesses demonstrate that it corresponds to a propagation of an acoustic strain pulse. On timescales longer than 100 ps, heat diffusion to the substrate takes place that can be modeled with a biexponential decay.

Direct time-domain determination of electron-phonon coupling strengths in chromium

We report the results of an ultrafast, direct structural measurement of optically pumped phonons in a Cr thin film using ultrashort x-ray pulses from a free-electron laser. In addition to measuring and confirming the known long-wavelength dispersion relation of Cr along a particular acoustic branch, we are able to determine the relative phase of the phonons as they are generated. The Cr sample exhibits two generation mechanisms for the phonons: the releasing of a preexisting charge density wave at higher frequencies, and the creation of an acoustic strain pulse via laser heating that dominates at lower frequencies. For the latter mechanism, we are able to measure the frequency dependence of the time required to generate the phonons. To explain the observed magnitude and slope of the delays, we perform first-principles simulations in the framework of density functional perturbation theory and ab initio molecular dynamics to fit anharmonic phonon models. These results show that the wave-vector dependence of the electron-phonon coupling is the driving mechanism behind the delay times: Phase-space limitation leads to higher times near the zone center. The absolute magnitudes of the delay times measured are found to be much shorter than the equilibrium electron-phonon coupling times we compute, indicating that the coupling strength is greatly enhanced when the electronic system is out of equilibrium with the lattice, as has been seen in bismuth and other systems.

Generation of exchange magnons in thin ferromagnetic films by ultrashort acoustic pulses

We investigate generation of exchange magnons by ultrashort, picosecond acoustic pulses propagating through ferromagnetic thin films. Using the Landau-Lifshitz-Gilbert equations we derive the dispersion relation for exchange magnons for an external magnetic field tilted with respect to the film normal. Decomposing the solution in a series of standing spin wave modes, we derive a system of ordinary differential equations and driven harmonic oscillator equations describing the dynamics of individual magnon mode. The external magnetoelastic driving force is given by the time-dependent spatial Fourier components of acoustic strain pulses inside the layer. Dependencies of the magnon excitation efficiencies on the duration of the acoustic pulses and the external magnetic field highlight the role of acoustic bandwidth and phonon-magnon phase matching. Our simulations for ferromagnetic nickel evidence the possibility of ultrafast magneto-acoustic excitation of exchange magnons within the bandwidth of acoustic pulses in thin samples under conditions readily obtained in femtosecond pump-probe experiments.

Picosecond acoustic-excitation-driven ultrafast magnetization dynamics in dielectric Bi-substituted yttrium iron garnet

Using femtosecond optical pulses, we have investigated the ultrafast magnetization dynamics induced in a dielectric film of bismuth-substituted yttrium iron garnet (Bi-YIG) buried below a thick Cu/Pt metallic bilayer. We show that exciting the sample from Pt surface launches an acoustic strain pulse propagating into the garnet film. We discovered that this strain pulse induces a coherent magnetization precession in the Bi-YIG at the frequency of the ferromagnetic resonance. The observed phenomena can be explain by strain-induced changes of magnetocristalline anisotropy via the inverse magnetostriction effect. These findings open new perspectives toward the control of the magnetization in magnetic garnets embedded in complex heterostructure devices.

Ultrafast transient dynamics in composite multiferroics

We investigate theoretically the dynamic multiferroic (MF) response of coupled ferroelectric (FE)/ferromagnetic (FM) composites upon excitation by a photo-induced acoustic strain pulse. Two magnetoelectric (ME) mechanisms are considered: interface strain- and charge-mediated ME couplings. The former results in demagnetization, depolarization and repolarization within tens of picoseconds via respectively magnetostriction and piezoelectricity. Charge ME interaction affects the FE/FM feedback response leading to magnetization recovery. Experimental realization based on time-resolved x-ray diffraction is suggested. The findings indicate the potential of composite MF for photo-steered, high-speed, multi-state electronic devices.

Gigahertz Modulation of Femtosecond Time-Resolved Surface Sum-Frequency Generation Due to Acoustic Strain Pulses

We study the properties of aqueous solution/air interfaces with time-resolved surface sum-frequency generation (SFG) spectroscopy. Using this technique, we monitor the change in SFG signal following the excitation of the water hydroxyl stretch vibrations with a strong ultrashort infrared pump pulse. We observe that, in addition to the excitation-induced changes in the signal resulting from vibrational excitation and relaxation on a time scale of picoseconds, the infrared excitation also induces a modulation of the SFG signal with a period of ∼180 ps. This modulation is caused by the interference between the SFG field directly radiated away from the air/liquid interface and the SFG field reflected or generated at the acoustic strain wavefront that is created by the infrared excitation pulse. The modulation is observed to be much stronger for aqueous salt solutions than for pure water, and the phase of the oscillations depends on the nature of the dissolved salt. This dependence of the phase can be explained from the differences in surface propensities of the ions of the different salts, giving rise to different net orientation of the interfacial water species.

Femtosecond nonlinear ultrasonics in gold probed with ultrashort surface plasmons

Fundamental interactions induced by lattice vibrations on ultrafast time scales have become increasingly important for modern nanoscience and technology. Experimental access to the physical properties of acoustic phonons in the terahertz-frequency range and over the entire Brillouin zone is crucial for understanding electric and thermal transport in solids and their compounds. Here we report on the generation and nonlinear propagation of giant (1 per cent) acoustic strain pulses in hybrid gold/cobalt bilayer structures probed with ultrafast surface plasmon interferometry. This new technique allows for unambiguous characterization of arbitrary ultrafast acoustic transients. The giant acoustic pulses experience substantial nonlinear reshaping after a propagation distance of only 100 nm in a crystalline gold layer. Excellent agreement with the Korteveg-de Vries model points to future quantitative nonlinear femtosecond terahertz-ultrasonics at the nano-scale in metals at room temperature.

Depth-profiling of elastic and optical inhomogeneities in transparent materials by picosecond ultrasonic interferometry: Theory

The theoretical backgrounds for the depth-profiling of the optically transparent materials by picosecond ultrasonic interferometry are developed. The mathematical description of the light reflection from inhomogeneous transparent films or coatings is proposed. The inhomogeneity can be caused both by the film synthesis (intrinsic stationary inhomogeneity) and by the short acoustic transients launched in the film (time-dependent inhomogeneity). The theory indicates that the measurements of the complex optical reflectivity time evolution, caused by acoustic strain pulse propagation in such films, offer various possibilities to extract the depth profiles of intrinsic inhomogeneous distributions of mechanical/acoustical, optical, and acousto-optical parameters of the films. In particular it is proposed how the measurements of the transient complex optical reflectivity by the femtosecond optical interferometers, operating with light of different polarizations and probing the tested samples at different angles of light incidence, can be used. The spatial resolution of the method is limited by the acoustic spatial scale which, for picosecond acoustic pulses, is much shorter than optical wavelength.

Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)

The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of (Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect (MOKE). A transient MOKE signal followed by low amplitude oscillations was evidenced, with a strong dependence on applied magnetic field, temperature and ps-ASP amplitude. Careful interferometric measurement of the layer's thickness variation induced by the ps-ASP allowed us to model very accurately the resulting signal, and interpret it as the strain modulated reflectivity (differing for $\sigma_{\pm}$ probe polarizations), independently from dynamic magnetization effects.

Femtosecond laser generation and detection of high-frequency acoustic phonons in GaAs semiconductors

The experimental detection of short picosecond photoacoustic response induced by femtosecond laser pump radiation deeply penetrating in GaAs and generating long acoustic strain pulse is reported. It is demonstrated that it is possible, in this case, to achieve high-frequency coherent acoustic phonon monitoring in semiconductors as efficiently as in the case of metals where the penetration depth of pump radiation is shorter and the generated acoustic strain pulse is short itself. The physical origin of such monitoring of high-frequency acoustic phonons is discussed thanks to a detailed analysis of the spectral transformation functions of both the generation process (opto-acoustic transformation) and the detection process (acousto-optic transformation). We show that it is possible to tune the detection process from a narrowband to a broadband spectrum detection process. In particular, the broadband detection process is achieved with a proper choice of the probe wavelength permitting efficient coupling between the acoustic field and the probe electromagnetic wave only in a nanometric thin region near semiconductor surface. Efficient broadband acousto-optic detection results in the short pulsed photoacoustic responses in transient optical reflectivity even when long acoustic pulses are generated, because of sufficient sensitivity to high-frequency coherent phonons even weakly contributing to the total acoustic field. These results indicate the opportunity to broaden the range of the substrate materials and pump laser frequencies that can be used in high-frequency coherent phonon spectroscopy of the materials and nanostructures.

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.

On generation of picosecond inhomogeneous shear strain fronts by laser-induced gratings

The processes leading to excitation of inhomogeneous, plane compression/dilatation bulk acoustic strain pulses and shear bulk acoustic strain fronts following the creation of a transient laser interference pattern at a mechanically free surface of an elastically isotropic medium are described. It is shown that the characteristic frequencies of these acoustic disturbances can be much higher than those corresponding to the spatial period of the laser-induced grating. As well, it is shown that the inherent dispersive nature of these bulk acoustic eigenmodes provides plausible explanation for the reported observations of acoustic echo transformation in thin film, ultrafast laser ultrasonics experiments.

Tomographic reconstruction of picosecond acoustic strain propagation

By means of an ultrafast optical technique, picosecond acoustic strain pulses in a transparent medium are tomographically visualized. The authors reconstruct strain pulses in Au-coated glass from time-domain reflectivity changes as a function of the optical angle of incidence, with ∼1ps temporal and ∼100nm spatial resolutions.

Direct measurement of ultrafast surface displacement in laser picosecond acoustics

Picosecond acoustic pulses generated in solids with ultrashort optical pulses can be detected through the reflectance or phase changes of optical probe pulses. The detection relies on acousto-optic coupling resulting from a combination of the photoelastic effect and transient surface displacements. The acoustic strain pulse shape gives access to the ultrafast dynamics of the electrons that influence the generation process. This pulse shape is directly related to the contribution to the probe light modulation resulting from surface displacement. Here we present a method that allows the discrimination between the photoelastic and surface displacement contributions using a novel oblique optical incidence geometry, allowing the strain pulse shape to be directly obtained in a single measurement.

Laser picosecond acoustics with oblique probe light incidence

Laser picosecond acoustics involves the excitation and detection of picosecond acoustic strain pulses in thin films with ultrashort light pulses. The use of oblique probe light incidence permits a greater degree of freedom in choosing the optical probing conditions and thereby should allow the extraction of more information about the profile of the propagating acoustic strain pulses. Here, we present a theory for the modulation of light reflected at oblique incidence from a solid containing an acoustic strain distribution. The theory can account for the real and imaginary parts of the reflectance variation, and involves both the effect of the transient surface motion and of the photoelastically modulated dielectric constants in the solid. We show, in particular, how the theory can be applied to extract the contribution to the reflectance change arising from the surface motion in an opaque isotropic solid, thereby allowing direct access to the shape of the acoustic strain pulse propagating therein.

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.

Ultrafast generation of acoustic waves in copper

The ultrafast generation of acoustic waves in copper films is investigated with a femtosecond optical pump and probe technique. By studying the generation at times before the electrons and the lattice come into equilibrium, the strength of their interaction can be measured and the dynamics of ultrafast electron diffusion can be studied. The acoustic strain pulses observed are bipolar in shape with exponential tails that are much broader than expected from simple thermoelastic stress generation. This can be explained by the supersonic diffusion of electrons over distances larger than the optical skin depth. The nonequilibrium diffusion equations governing stress generation are nonlinear, and are solved numerically. Using a linearized formulation, we also solve them analytically to a good approximation. The acoustic strain profile provides a 'snapshot' of the initial spatial temperature distribution of the lattice, thus allowing a sensitive probe of the nonequilibrium dynamics of the diffusion. The electron-phonon coupling constant can be estimated directly from the acoustic pulse duration, provided that the sound velocity and thermal conductivity are known. In general, the relaxation and diffusion of carriers is specific to the sample in question, whether metal or semiconductor, suggesting the use of this method for thin film characterization. >

Nanosecond acoustic strain pulses from CdS phonon masers

Observations of nanosecond strain pulses generated by a mode-locked CdS phonon maser are reported. Peak strains exceeding 5 × 10 -5 have been measured. An analogy is drawn between the observed mode-locked operation and a repetitive pulse generator.