S. N. Jerebtsov

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 ...

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.

Lifetime and anisotropy decay of excited Coumarin 30 measured by a femtosecond pump–probe technique

We measured the lifetime of Coumarin 30 dye molecules excited with 400 nm in ethanol solution with the pump–probe stimulated emission fluorescence technique using femtosecond laser pulses that provided high temporal resolution of about 50 fs. Both fluorescence and stimulated emission signals were observed. The stimulated emission was induced at 550 nm. The lifetime of the excited state was determined to be (1.9± 0.3) ns and the rotational depolarization time was measured to be (120± 20) ps. Our results are in agreement with data obtained previously by a different technique.

Propagation of Surface Acoustic Pulses Generated by a Femtosecond Laser in Thin Films on Solid Substrates

The technique of phase velocity dispersion measurements of surface acoustic waves in a thin film-substrate system was demonstrated. The excitation of surface acoustic waves (SAWs) was quite efficient with femtosecond laser pulses, and the damage of the surface was minimized. The measurements were performed with films of Al deposited on silicon wafers. The errors in the determination of the phase velocity and absorption were analyzed. The temperature changes in the propagation velocity on bare Si wafers were also measured. The data obtained permitted estimation of the accuracy of the temperature determination from measurements with SAW pulses.

Generation of surface acoustic waves in silicon with energetic femtosecond laser pulses

The generation of surface acoustic wave (SAW) pulses in Si single crystal by femtosecond laser pulses with high energy of up to 30 mJ is studied. The generation is rather efficient even at a free silicon surface. In this case the shape of the observed SAW pulses corresponds to the thermoelastic mechanism despite the fact that a well developed ablation takes place. Excitation through a liquid layer is also studied and provides longer SAW pulses. SAW gratings are generated by focusing the laser beam through a mask and the changes in the spectrum of the pulse due to nonlinear interactions are observed.