Acoustic Phonon Propagation Research Articles

Development of five-dimensional scanning transmission electron microscopy.

By combining the scanning transmission electron microscopy with the ultrafast optical pump-probe technique, we improved the time resolution by a factor of ∼1012 for the differential phase contrast and convergent-beam electron diffraction imaging. These methods provide ultrafast nanoscale movies of physical quantities in nano-materials, such as crystal lattice deformation, magnetization vector, and electric field. We demonstrate the observations of the photo-induced acoustic phonon propagation with an accuracy of 4ps and 8nm and the ultrafast demagnetization under zero magnetic field with 10ns and 400nm resolution, by utilizing these methods.

Evaluation of optoacoustic response with a combined Monte Carlo optical and acoustic simulation approach

Optoacoustic, or photoacoustic, imaging combines the penetration capabilities of ultrasound imaging with the contrast mechanism of optical absorption related to the photoacoustic effect. To enable modeling of photoacoustic measurements and imaging applications, the problem can be divided into modeling of the optical photon propagation and the resulting acoustic wave propagation. In highly scattering media such as soft tissues, Monte Carlo (MC) methods are used to evaluate photon propagation, absorption and the subsequent generation of a heat source that produces the photoacoustic effect. Acoustic wave equations are then used to model the propagation of the sound in the medium. However, if the appropriate physics could be modeled with a single tool, there would be advantages of consistency in spatial and temporal domains as well as easier integration of the optical and acoustic simulations. We propose using MC methods for optical photon propagation and for the acoustic wave propagation. The simulations are split into two stages for the optical photon propagation and the acoustic phonon propagation. We describe how the same MC framework is used to model photon propagation and acoustic phonon propagation. We will then demonstrate this new combined MC framework in models of photoacoustic problems in homogeneous and heterogeneous cases. These results will be compared with results from using k-Wave models for the acoustic wave propagation. The correspondence between k-Wave the acoustic MC model are shown to be in good agreement.

Phonon dispersion in hypersonic two-dimensional phononic crystal membranes

We investigate experimentally and theoretically the acoustic phonon propagation in two-dimensional phononic crystal membranes. Solid-air and solid-solid phononic crystals were made of square lattices of holes and Au pillars in and on 250 nm thick single crystalline Si membrane, respectively. The hypersonic phonon dispersion was investigated using Brillouin light scattering. Volume reduction (holes) or mass loading (pillars) accompanied with second-order periodicity and local resonances are shown to significantly modify the propagation of thermally activated GHz phonons. We use numerical modeling based on the finite element method to analyze the experimental results and determine polarization, symmetry, or three-dimensional localization of observed modes.

Elasticity of an Assembly of Disordered Nanoparticles InteractingviaEither van der Waals-Bonded or Covalent-Bonded Coating Layers

Tailoring physical and chemical properties at the nanoscale by assembling nanoparticles currently paves the way for new functional materials. Obtaining the desired macroscopic properties is usually determined by a perfect control of the contact between nanoparticles. Therefore, the physics and chemistry of nanocontacts are one of the central issues for the design of the nanocomposites. Since the birth of atomic force microscopy, crucial advances have been achieved in the quantitative evaluation of van der Waals and Casimir forces in nanostructures and of adhesion between the nanoparticles. We present here an investigation, by a noncontact method, of the elasticity of an assembly of nanoparticles interacting via either van der Waals-bonded or covalent-bonded coating layers. We demonstrate indeed that the ultrafast opto-acoustic technique, based on the generation and detection of hypersound by femtosecond laser pulses, is very sensitive to probe the properties of the nanocontacts. In particular, we observe and evaluate how much the subnanometric molecules present at nanocontacts influence the coherent acoustic phonon propagation along the network of the interconnected silica nanoparticles. Finally, we show that this ultrafast opto-acoustic technique provides quantitative estimates of the rigidity/stiffness of the nanocontacts.

Monte Carlo modeling of phonon transport in nanodevices

Heat transport in nanostructured semiconductor devices has been investigated. Numerical simulations of acoustic phonon propagation through a dedicated Monte Carlo model in nanodevices have been performed. Material dispersion curves are taken into account and the Boltzmann collisional term is considered under the relaxation time approximation. The method allows the calculation of temperature fields at small time and space scales. Furthermore, confining effects on thermal conductivity due to boundary scattering in nanowires and nanofilms are displayed for several thicknesses and temperatures.

Dynamical model of Cosserat nanotubes

A Cosserat shell model of a single-wall carbon nanotube is developed based a recent static Cosserat rod model with microstructure. The new dynamical theory is used to study longitudinal acoustic phonon propagation.

Acoustic-phonon propagation in rectangular semiconductor nanowires with elastically dissimilar barriers

We have theoretically studied acoustic phonon spectra and phonon propagation in rectangular nanowires embedded within elastically dissimilar materials. As example systems, we have considered GaN nanowires with AlN and plastic barrier layers. It has been established that the acoustically mismatched barriers dramatically influence the quantized phonon spectrum of the nanowires. The barriers with lower sound velocity compress the phonon energy spectrum and reduce the phonon group velocities in the nanowire. The barriers with higher sound velocity have an opposite effect. The physical origin of this effect is related to redistribution of the elastic deformations in the acoustically mismatched nanowires. In the case of the “acoustically slow” barriers, the elastic deformation waves are squeezed in the barrier layer. The effect predicted for the nanowires embedded with elastically dissimilar materials could be used for reengineering phonon spectrum in nanostructures.

Nanoscale and Mesoscale Properties of Nanostructured Carbon Films

A multi‐scale investigation of nanostructured carbon films has been performed by means of inelastic light scattering (Raman and Brillouin scattering). Carbon films with different nano‐ and mesostructure have been deposited from supersonic cluster beams in a low energy deposition regime by exploiting aerodynamic focusing effects. Acoustic phonon propagation in the porous amorphous structure, where disorder acts as a damping factor, is investigated by Brillouin scattering. Depending on the nano‐ and meso‐structure, acoustic phonons can either propagate along the medium, which acts as an elastic continuum at the meso‐scale (i.e., hundreds of nm), or turn to overdamped oscillations localized by the structural disorder. Nevertheless, we show that it is always possible to measure the elastic constants of thin and porous films, when other techniques (e.g., nano‐indentation) become critical. At the nano‐scale, Raman scattering measurements show the typical structure of an amorphous carbon, where the structural disorder is affected by the primeval cluster mass distribution. The synthesis of cluster‐assembled carbon films and the in situ Raman characterization in a UHV system allowed to observe the presence of a relevant fraction of sp 1‐hybridized carbon chains (also known as carbynoid structures) embedded in the sp 2 amorphous network.

Nonequilibrium Acoustic Phonons in ZnTe at Liquid-Helium Temperatures: Effect of Impurities and Structural Defects on the Phonon Mean Free Path

ZnTe crystallites isolated from a druse of coarse-grained polycrystalline ZnTe, prepared by chemical vapor synthesis at a temperature of ∼650°C, were studied using x-ray diffractometry, luminescence, and the heat pulse technique. The crystallites are stacked in {110}-oriented macroscopic layers with a common twin system. The {111} twin planes separated at a distance of 50–100 μm are perpendicular to the (110) growth layers. Acoustic phonon propagation was studied using the heat pulse technique. A comparison of the responses to the arrival of differently polarized phonons in a given sample with the responses obtained on high-purity coarse-grained ZnTe with randomly distributed twin systems with a separation of 5–10 μm and on twin-free single-crystal ZnTe suggests that twins radically affect the scattering of acoustic phonons. The mean free paths of LA, FTA, and STA phonons were determined by comparing the experimental responses with Monte Carlo calculations.

Acoustic phonon propagation in nanowire superlattices

Vibrational states in a nanowire superlattice consisting of an alternate stacking of two cylindrical layers are theoretically studied. For azimuthally symmetric torsional modes, the expressions for phonon dispersion relations and transmittance are derived analytically. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Brillouin light scattering investigation of cluster-assembled carbon films: acoustic phonon propagation and elastic properties

Phonon propagation and elastic properties of cluster-assembled carbon films were investigated by surface Brillouin scattering. Films of different thicknesses, produced with different precursor cluster mass distributions have been studied. The propagation of surface and bulk acoustic phonons was strongly affected by the film structure at a typical scale of hundreds of nanometers. Absence of phonon propagation and localization effects have been observed in films assembled with large clusters due to surface roughness, whereas small clusters produced smooth films showing well defined Brillouin peaks. The determination of the material density by X-ray reflectivity, made the evaluation of the elastic properties of nanostructured carbon films possible even in presence of a high surface roughness. Numerical calculation of the scattering cross-section led to the estimation of the elasto-optic constants.

Acoustic phonon propagation and elastic properties of nano-sized carbon films investigated by Brillouin light scattering

Carbon films having a structure at the nanometer scale are of paramount interest. Experimental tools for the mechanical characterization of such materials are still an open issue, due to the relevance of mesoscopic scales. Brillouin light scattering by acoustic phonons of sub-micrometric wavelength is among the few techniques sensitive at the appropriate length scale. Elastic properties can be derived from measured acoustic properties, if the film thickness and mass density are independently measured, typically by X-ray reflectivity. Applications to two very different types of films are discussed. Tetrahedral amorphous carbon (ta-C) films of high density and stiffness can be deposited with thickness down to a few nanometers. We show that combining Brillouin scattering and X-ray reflectivity the elastic properties of films can be measured for thicknesses down to 2 nm. We also measure the dependence of stiffness on thickness, finding that the high stiffness of thicker ta-C films is reached for thicknesses of approximately 10 nm. Films deposited by low energy cluster beam deposition are characterized by granularity and porosity at scale lengths ranging from nanometers to micrometers. We show that Brillouin scattering discriminates well among films compact enough to support the propagation of acoustic phonons and films in which such phonons are confined and/or over-damped. When acoustic propagation is supported the elastic properties can be obtained, at wavelengths of hundreds of nanometers. It is thus shown that Brillouin scattering has a peculiar potential for the characterization of films having structures of nanometric size.

Projecting picosecond lattice dynamics through x-ray topography

A method for time-resolved x-ray diffraction studies has been demonstrated. As a test case, coherent acoustic phonon propagation into crystalline InSb is observed using a laser plasma x-ray source. An extended x-ray topogram of the semiconductor’s surface was projected onto a high spatial resolution x-ray detector and acoustic phonons were excited by rapidly heating the crystal’s surface with a femtosecond laser pulse. A correlation between the spatial position on the x-ray detector and the time of arrival of the laser pulse was encoded into the experimental geometry by tilting the incident laser pulse with an optical grating. This approach enabled a temporal window of 200 ps to be sampled in a single topogram, thereby negating the disadvantages of pulse-to-pulse fluctuations in the intensity and spectrum of the laser-plasma source.

Acoustic phonon propagation and elastic properties of cluster-assembled carbon films investigated by Brillouin light scattering

Acoustic phonon propagation and elastic properties of cluster-assembled carbon films investigated by Brillouin light scattering

Picosecond–milliångström lattice dynamics measured by ultrafast X-ray diffraction

Fundamental processes on the molecular level, such as vibrations and rotations in single molecules, liquids or crystal lattices and the breaking and formation of chemical bonds, occur on timescales of femtoseconds to picoseconds. The electronic changes associated with such processes can be monitored in a time-resolved manner by ultrafast optical spectroscopic techniques1, but the accompanying structural rearrangements have proved more difficult to observe. Time-resolved X-ray diffraction has the potential to probe fast, atomic-scale motions2,3,4,5. This is made possible by the generation of ultrashort X-ray pulses6,7,8,9,10, and several X-ray studies of fast dynamics have been reported6,7,8,11,12,13,14,15. Here we report the direct observation of coherent acoustic phonon propagation in crystalline gallium arsenide using a non-thermal, ultrafast-laser-driven plasma — a high-brightness, laboratory-scale source of subpicosecond X-ray pulses16,17,18,19. We are able to follow a 100-ps coherent acoustic pulse, generated through optical excitation of the crystal surface, as it propagates through the X-ray penetration depth. The time-resolved diffraction data are in excellent agreement with theoretical predictions for coherent phonon excitation20 in solids, demonstrating that it is possible to obtain quantitative information on atomic motions in bulk media during picosecond-scale lattice dynamics.

Modelling of Hot Acoustic Phonon Propagation in TwoDimensional Layers

The transport of confined acoustic phonons in a flee‐standing quantum well has been studied by solving the quantum kinetic equation for phonons. The phonon decay rate has been numerically calculated for GaAs flee‐standing quantum well. Phonon interaction with electrons through the deformation potential makes the major contribution to the acoustic phonon decay.

Dynamical electron and phonon processes in the excited state of LGO: Cr3+ crystals

In pseudo-Stark effect studies on the R, R' lines of LGO: Cr3+ the C1 point symmetry of both types of Cr3+ centers in the low-temperature ferroelectric (C2v) phase as well as evidence for local compensation of Cr3+ (Ge4+) extra charge in LGO lattices were established. In direct photoelectric studies it was found that Cr3+ photoionization takes place in a two-quantum process via the intermediate 2E excited state. With the help of fluorescent phonon detection the diffusive regime of THz acoustic phonon propagation in LGO at T = 2 K was observed; the anharmonic phonon decay times were measured for frequencies 55 cm−1 (150 ns) and 63 cm−1 (75 ns).

Acoustic phonon propagation in superlattices: Group velocities and angular dependence of stop bands

The field of group velocities in systems of elastically isotropic layers is calculated from dispersion surfaces. Numerical results are presented for the a-Si:H/a-siN x:H superlattice investigated in Ref. 1. The magnitude of the group velocity is used to classify modes. Furthermore we obtained, for different types of vibrations, the dependence of the frequency intervals corresponding to nonpropagating modes (stop bands) on the direction of incidence. It is shown that the stop band widths can be adjusted by introducing different outoffs of the group velocity. Finally, the dependence of the direction of propagation of frequency is derived for different angles of incidence.

Acoustic-phonon propagation in superlattices

Bragg reflection of acoustic phonons propagating through a periodic superlattice is explored in detail. We examine Bragg reflection for a single vibrational mode and relate it to gaps, or stop bands, in the phonon dispersion relation at the center and boundary of the folded Brillouin zone. Bragg reflection due to the coupling of different polarization modes is similarly linked to intrazone stop bands. We also calculate phonon transmission rates, and explore in depth the spatial distribution of the transmitted phonon intensity. These results are compared with experimental phonon images that provide a two-dimensional map of the acoustic-phonon transmission as a function of propagation direction.

Dynamic Spin Hamiltonian Formalism and Mössbauer Study of Spin—Lattice Relaxation of Paramagnetic Fe3+ (6S) Ions

AbstractIn a crystal field potential the 6S state of a paramagnetic 57Fe3+ (3d5, 6S) ion splits into three Kramers spin doublets Sz = ± 5/2, ± 3/2, and ± ½ and each doublet produces its own Mössbauer magnetic hyperfine spectrum at low temperatures as a result of the slow spin relaxation rate. These Mössbauer hyperfine spectra are used to study the temperature dependence of Fe3+ spin lattice relaxation rate falling in the region 106 to 109 s−1. The dynamic spin Hamiltonian formalism is utilised to explicitly calculate the spin—lattice relaxation rate of the three Kramers doublets for one‐phonon and two‐phonon processes in different temperature regions. It is further assumed that the various modes of acoustic phonon propagation are direction independent, the phonon spectrum is of Debye type, and the dynamic crystal field parameters are very similar to the static ones. It is found that for Fe3+ ions the one‐phonon relaxation process is most dominant and the theoretical results are in a very good agreement with the experimental observations. These calculations may be equally applicable to other S‐state paramagnetic ions.