Picosecond Acoustic Pulses Research Articles

Ultrafast acoustics for imaging at the nanoscale

In this paper we present a series of experiments which show that 2-D and possibly 3-D imaging with sub-micron resolution is possible by means of ultrafast acoustic techniques. Optical pulses from a Ti:sapphire laser are used to generate picosecond acoustic pulses on one side of a ~1 mm thick Si wafer. The 1 mm distance is sufficient for the acoustic waves to diffract to the far field before they are detected by time-delayed probe pulses from the Ti:sapphire laser. The acoustic waves are either generated by a surface nanostructure or scattered from a buried nanostructure, and an image of that nanostructure is reconstructed through an analysis of the detected acoustic waves.

Generation and detection of plane coherent shear picosecond acoustic pulses by lasers: Experiment and theory

Hypersound generation and detection by laser pulses incident on the interface of an opaque anisotropic crystal are theoretically and experimentally investigated in the case where the symmetry is broken by a tilt of its axis of symmetry relative to the interface normal. A nonlocal volumetric mechanism of plane shear sound excitation is revealed and a modification of the temporal shape of the reflectivity signal with variation in probe light polarization is observed, both attributed to asynchronous propagation of the acoustic eigenmodes. Experiments and theory demonstrate the possibility of using polycrystalline materials with an arbitrary distribution of grain orientations for the generation and the detection of picosecond shear ultrasound.

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.

Soliton-like propagation modes of picosecond acoustic pulses in a paramagnetic crystal

The nonlinear propagation of picosecond acoustic pulses at an arbitrary angle to an external magnetic field is studied in an elastically isotropic paramagnetic crystal at low temperatures. Various soliton-like propagation modes arising due to spin-phonon interaction and acoustic anharmonicity are revealed, and the stability of these modes with respect to transverse perturbations is analyzed. In the case of defocusing cubic nonlinearity, the crystal can support the propagation of compression pulses, which undergo defocusing, and rarefaction pulses can propagate in the self-channeling mode. In the case of focusing cubic nonlinearity, only compression pulses can propagate if the conditions of stability with respect to self-focusing are satisfied.

In situ monitoring of the growth of ice films by laser picosecond acoustics

Ultrashort optical pulses are used to excite and probe picosecond acoustic pulses in a sample consisting of an opaque material upon which ice is continuously deposited from the vapor phase at ∼110K. By analysis of the ultrasonic propagation and reflection inside the submicron ice film and taking into account the scattering of the probe light by the acoustic waves, the thickness, sound velocity, refractive index, ultrasonic attenuation, and photoelastic constant of the ice film are derived. This method should find applications for the in situ monitoring of thin transparent films during growth.

Generation and detection of acoustic solitons in crystalline slabs by laser ultrasonics

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, α-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.

Generation and Propagation of a Picosecond Acoustic Pulse at a Buried Interface: Time-Resolved X-Ray Diffraction Measurements

We report on the propagation of coherent acoustic wave packets in (001) surface oriented Al0.3Ga0.7As/GaAs heterostructure, generated through localized femtosecond photoexcitation of the GaAs. Transient structural changes in both the substrate and film are measured with picosecond time-resolved x-ray diffraction. The data indicate an elastic response consisting of unipolar compression pulses of a few hundred picosecond duration traveling along [001] and [001] directions that are produced by predominately impulsive stress. The transmission and reflection of the strain pulses are in agreement with an acoustic mismatch model of the heterostructure and free-space interfaces.

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.

Generation of picosecond acoustic pulses using a p-n junction with piezoelectric effects

We demonstrate the generation of picosecond acoustic pulses using a piezoelectric-semiconductor-based p-n junction structure. This p-n junction picosecond ultrasonic experiment confirms that the piezoelectric effect dominates the thermal expansion and deformation-potential coupling in the generation of picosecond acoustic pulses. The characteristics of the p-n initiated acoustic pulses are determined by the width and the field strength inside the depletion region. Our study indicates the future possibility to electrically control the acoustic pulse characteristics if we could apply an external bias to modulate the depletion region width.

The Influence of Transverse Perturbations on the Propagation of Picosecond Acoustic Pulses in a Paramagnetic Crystal

The influence of transverse perturbations on the dynamics of a picosecond soliton-like acoustic pulse in a paramagnetic crystal in an external magnetic field is investigated. The nonlinear and dispersion effects are governed by the intrinsic properties of the crystal and the spin-phonon interaction. The effect of different nonlinear mechanisms and an external magnetic field on the stability against transverse perturbations is analyzed. It is shown that, in the absence of paramagnetic impurities, there can exist only a compression pulse that propagates in a defocusing regime. In the presence of paramagnetic ions in the crystal, there can arise rarefaction pulses that, under specific conditions, can propagate in self-focusing and self-channeling regimes.

Picosecond acoustic phonon pulse propagation in silicon

We report experimental and theoretical investigations of the behavior of coherent acoustic phonon pulses after propagation through millimeter-scale distances in crystalline Si. An ultrafast optical pump and probe technique is used to generate and detect the phonon pulses. The geometry of our experiment is such that we can make this measurement in either the far field or the near field of the acoustic source, allowing studies of diffraction effects in addition to dispersion and nonlinearity in the Si. We also present a rigorous derivation of the Korteweg--de Vries equation, which describes the behavior of these acoustic pulses in the nonlinear regime in one dimension. This one-dimensional (1D) model is combined with a 3D analysis of diffraction effects in anisotropic media in order to analyze far-field data.

Erratum: Picosecond acoustic phonon pulse generation in nickel and chromium [Phys. Rev. B67, 205421 (2003)

Erratum: Picosecond acoustic phonon pulse generation in nickel and chromium [Phys. Rev. B<b>67</b>, 205421 (2003)

Imaging nanostructures with coherent phonon pulses

We demonstrate submicron resolution imaging using picosecond acoustic phonon pulses. High-frequency acoustic pulses are generated by impulsive thermoelastic excitation of a patterned 15-nm-thick metal film on a crystalline substrate using ultrafast optical pulses. The spatiotemporal diffracted acoustic strain field is measured on the opposite side of the substrate, and this field is used in a time-reversal algorithm to reconstruct the object. The image resolution is characterized using lithographically defined 1-micron-period Al structures on Si. Straightforward technical improvements should lead to resolution approaching 45nm, extending the resolution of acoustic microscopy into the nanoscale regime.

Nondestructive evaluation of micrometric diamond films with an interferometric picosecond ultrasonics technique

Synthetic diamond thin or thick films are suitable materials for many applications because of their weak friction coefficient and their large hardness, Young modulus, and thermal conductivity. A series of diamond thin films of variable quality obtained by plasma-assisted chemical vapor deposition lying on titanium alloy substrates has been studied by picosecond ultrasonics. Femtosecond laser pulses allow the generation and detection of picosecond acoustic pulses that can be used to study the elastic properties of thin micrometric and submicrometric films. Acoustic fields generated in the substrate and propagating in the transparent diamond film are detected by an interferometric technique; they give rise to oscillations and abrupt step-like changes in the transient reflectivity variations. An analysis of these different features allows the determination of various characteristics of these films, such as longitudinal elastic constant, thickness, and roughness in terms of the diamond quality.

P2-42 Imaging of picosecond longitudinal acoustic pulses in a transparent medium(Short presentation for poster)

P2-42 Imaging of picosecond longitudinal acoustic pulses in a transparent medium(Short presentation for poster)

Integrable models of transverse and longitudinal acoustic pulses evolution in a paramagnetic crystal with spin [formula omitted] impurities

We study the interaction of transverse and longitudinal acoustic pulses propagating in a surface or crystal layer containing ion impurities with an effective spin S= 1 2 . We derive an evolution equations and show that coherent dynamics of acoustic pulses propagated under the condition of their unidirectional propagation is described by a new integrable system of evolution equations. The derived model describes the picosecond acoustic pulses dynamics corresponding to so called “a few cycle pulses”. To investigate an integrable reduction of the common model the inverse scattering transform is used. It is found that strong interaction of two components of the acoustic waves leads to formation of coupled soliton–antisoliton pairs.

Transverse acoustic soliton in anisotropic paramagnetic crystal

We derive a new integrable system of evolutions describing the dynamics of the transverse strain waves propagating in a bulk crystal with ion impurities possessing the spin-1/2. This system describes the evolution of the picosecond acoustic pulses corresponding to 'a few cycle pulses'. The Lax representation for this system and its particular cases are presented. Soliton solutions corresponding to the common and particular cases are found.

Gouy phase shift of single-cycle picosecond acoustic pulses

Ultrafast laser pulses are used to generate single-cycle picosecond acoustic pulses in thin metal films on silicon. For small initial excitation spot sizes, propagation of the acoustic pulses across a 485-μm Si crystal leads to significant diffraction effects. The temporal reshaping of the acoustic wave form due to diffraction is investigated, and we demonstrate that the acoustic far field can be reached.

Phonon avalanches in paramagnetic impurities with spinS=12

We theoretically study the dynamics of transverse-and-longitudinal acoustic waves propagating parallel to an external magnetic field in a crystal containing ion impurities with an effective spin S=1/2. Corresponding evolution equations describing the coherent pulse evolution are derived. These equations are used to study the phonon avalanches arising due to decay of an initially unstable state of the spin systems for different geometries of interaction. It is found that the coherent dynamics of acoustic pulses propagated in one direction is described by a pair of integrable systems of evolution equations. The picosecond acoustic pulses governed by these systems are "a few-cycle" pulses. By using a modified set of equations of the inverse scattering transform, it is found that the strong interaction of three or two components of the acoustic waves with the spin system is asymptotically described by the quasi-self-similar solutions. Physical applications of the obtained results are discussed.

Picosecond acoustic phonon pulse generation in nickel and chromium

The generation mechanisms for picosecond acoustic phonon pulses excited by ultrashort optical pulses in metals with strong electron-phonon coupling are elucidated. By means of an interferometric probing technique to extract changes in optical phase and amplitude, the acoustic pulse shapes in the transition metals Ni and Cr are shown to be governed by both thermal and nonequilibrium electron diffusion. Quantitative comparison of the results with a model that takes into account the nonequilibrium electron dynamics and ultrasonic attenuation gives excellent agreement.