Al. A. Kolomenskii

Mechanical and elastic properties of amorphous hydrogenated silicon films studied by broadband surface acoustic wave spectroscopy

Mechanical and elastic properties of a-Si∶H films were measured by broadband Surface Acoustic Wave Spectroscopy (SAWS). In the frequency range achieved, the SAW dispersion curves extend to 300 MHz, which allowed the density, Youngs modulus, and Poissons ratio to be evaluated for films grown by laser CVD or plasma CVD with different hydrogen concentrations. The films deposited by either method have the best mechanical and elastic properties. at a hydrogen concentration of about 10 at. %. For this material, a density of (2300±20) kg/m3 and Youngs modulus of (134±5) GPa was determined. The network structures of amorphous silicon are discussed by applying the constraint-counting model to estimate the mean coordination number.

Interaction of laser-generated surface acoustic pulses with fine particles: Surface cleaning and adhesion studies

The mechanical forces associated with the surface acceleration in high-amplitude surface acoustic waves (SAWs) detach the particles from the surface. The removal of micron sized particles with a nanosecond SAW pulse excited by a focused laser beam in a silicon wafer was quantitatively investigated. Both vertical and horizontal particle displacements have been observed. It is shown that for nanosecond SAW pulses the limit of the surface acceleration of about 1010 m/s2 is set by the fracture of the material and corresponds to the removal of particles larger than about 0.05 μm. In addition, the nonlinear transformation of the excited SAW pulses results in an increase of the surface acceleration and contributes to the cleaning process extending it to even smaller particle dimensions. The technique is applicable in vacuum and improves the energetic effectiveness of the cleaning due to the removal of particles not only in the irradiated region, but also in the wider area covered by the SAW pulse propagation. It...

Laser diagnostics of C60 and C70 films by broadband surface acoustic wave spectroscopy

Abstract A novel method is used for the determination of mechanical and elastic properties of thin films such as film thickness, density, Youngs modulus, and Poissons ratio. In this technique short laser pulses (nanosecond to picosecond) are used to excite a surface acoustic wave (SAW) pulse and laser probe beam deflection or a piezoelectric foil detector are employed for time-resolved detection of the resulting surface displacements. Fourier transformation of the oscillatory signals detected at distances of several millimeters to centimeters yields the dispersion of the phase velocity, which can be used for the accurate determination of film properties. Fullerite films (C 60 and C 70 ) with thicknesses of 0.7–2.1 μ m on silicon and quartz-glass substrates were investigated. The nonlinear dispersion curves were obtained experimentally with a maximum value of the product of film thickness and SAW wave number of γ max = 0.32 for the C 60 films and γ max = 0.18 for the C 70 films. The frequency bandwidth was limited by the attenuation of the surface acoustic waves in the fullerite films. The film parameters were evaluated by fitting the measured dispersion curve to a theoretical model. For the C 60 films a density of 1.67 ± 0.02 g/cm 3 , Youngs modulus of 10 ± 2 GPa, and Poissons ratio of 0.25 ± 0.08 were found. For the C 70 films the corresponding values were 1.64 ± 0.02 g/cm 3 , 4 ± 1 GPa, and 0.35 ± 0.1.

Effect of melting on the excitation of surface acoustic wave pulses by UV nanosecond laser pulses in silicon

The influence of melting on the excitation of Surface Acoustic Wave (SAW) pulses in silicon is studied both theoretically and experimentally. The developed theory of Rayleigh-type SAW laser-induced thermoelastic excitation in a structure composed of a liquid layer on a solid substrate predicts that the SAW is predominantly generated in the solid phase due to the absence of shear rigidity in a liquid. The characteristic changes in the SAW pulse shape as well as the saturation and even the decrease of the SAW pulse amplitude observed above the melting threshold are explained theoretically to be a result of the decrease of the heat flux into the solid phase as well as due to the decrease of the volume of the solid phase caused by melting. Although the heat flux into the solid phase is decreased both as a consequence of the reflectivity increase and the additional energy losses (latent heat of melting) at the phase transition, it is demonstrated that the influence of reflectivity changes on the SAW pulse is negligible in comparison with the effect of melt-front motion. For laser pulses of 7 ns duration at 355 nm, the threshold value of laser fluence for meltingFm=0.23±0.04 J/cm2 and for the ablationFa=1.3±0.2 J/cm2 were determined experimentally as the points of characteristic changes in the observed SAW pulses.

Nonlinear compression of giant surface acoustic wave pulses

Abstract The nonlinear propagation of very high-amplitude surface acoustic wave (SAW) pulses in polycrystalline aluminum and copper was studied. A nonlinear compression and an increase of the SAW pulse amplitude have been observed. SAW pulses were numerically simulated with a nonlinear evolution equation including local and nonlocal nonlinear terms.

Focal transformation and the Gouy phase shift of converging one-cycle surface acoustic waves excited by femtosecond laser pulses

We studied the changes of the pulse shape and the phase of the spectral components in converging-surface acoustic wave pulses. These pulses were excited with a femtosecond laser by a thermoelastic mechanism. To produce converging acoustic pulses, the laser beam was focused with an axicon in a circle on the surface of an aluminum sample. During propagation through the focus, the shape of the pulses of the normal surface velocity changed from two to three polar. The absolute value of the phase of the spectral components experienced a change close to pi/2 rad (Gouy phase shift) after passage of the focal region. These observations were confirmed by analytical and numerical calculations based on the two-dimensional wave equation for surface acoustic waves.

Nonlinear surface acoustic wave pulses in solids: Laser excitation, propagation, interactions (invited)

Laser techniques enabled generation of very high amplitude pulses with acoustic Mach numbers about 0.01. Such waves drive the medium into the nonlinear elastic regime and shock fronts can be formed during their propagation. As an intense surface acoustic wave (SAW) propagates, the temporal evolution of the wave shape provides information on the nonlinear acoustic parameters and the nonlinear elastic constants of the material. The nonlinear propagation of SAW pulses exhibits different types of nonlinear behavior depending on nonlinear acoustic constants. Changes of a SAW pulse shape were calculated using a nonlinear evolution equation. Measurements of SAW pulses in polycrystalline stainless steel have demonstrated that a compression of the pulse takes place in this material corresponding to a positive parameter of the local nonlinearity, which was evaluated by fitting the parameters of the evolution equation to the experimental data. Numerical simulations of a dispersive propagation of nonlinear SAWs in a ...

Propagation of laser‐generated surface acoustic waves visualized by shake‐off of fine particles

Shake‐off of micron‐sized alumina particles by nanosecond laser‐generated surface acoustic wave (SAW) pulses is used to visualize SAW beam propagation on surfaces of semiconductor crystals. Various phenomena in SAW propagation such as reflection from a sample edge, anisotropic diffraction, and beam steering are demonstrated.

Determination of linear and nonlinear elastic parameters from laser experiments with surface acoustic wave pulses

Abstract Time resolved laser techniques enabled the study of the nonlinear evolution of surface acoustic wave (SAW) pulses of very high-amplitudes with acoustic Mach numbers of approximately 0.01. In such waves even shock fronts can be formed during their propagation. Changes of the shape of intense SAW pulses provide information on the nonlinear acoustic parameters and the nonlinear elastic constants of the material. Measurements in polycrystalline stainless steel have shown that a compression of the nonlinear SAW pulse takes place in this metal material yielding a positive parameter of the local nonlinearity. The changes of the SAW pulse shape were calculated using a nonlinear evolution equation and the nonlinear acoustic parameters were determined by fitting the evolution equation to the experimental data. The attenuation of SAWs was determined by measuring low amplitude pulses. In addition, the velocities of longitudinal and shear waves were obtained by registering the precursors of bulk waves at the surface.

Storage and light scattering of microparticles in a ring-type electrodynamic trap

We employ a Paul-Straubel ring-type electrodynamic trap for studies of single microparticles. Such a trap provides ready access for laser beams to a stored species and is especially suited for scattering and spectroscopic studies of fine particles. We derive the pseudopotential for such a trap and determine the stability regions for confinement of charged particles considering also the viscous force of a buffer medium and the force of gravity. The dynamics of microparticles in such a trap is numerically simulated. The diffraction pattern of light scattered on a polystyrene particle of about 10μm diameter was registered. For measuring Raman spectra from a single dipicolinic acid microparticle, we used excitation at 488nm and detection with a fiber optics spectrometer. To improve the collection of light, the trap with the stored particle was placed inside an elliptical mirror.