Shape Of Acoustic Pulse Research Articles

Femtosecond photodeflection spectroscopy in thin monocrystalline plates of germanium

A highly sensitive photodeflection spectroscopy technique with femtosecond time resolution has been developed. Using this technique, we have measured accurately the shape of acoustic pulses, which were generated from the ultrafast phonon emission in a germanium plate. Supersonic expansion of photoexcited electron-hole plasma was observed. The characteristic velocity of plasma diffusion was evaluated. It exceeded the longitudinal sound velocity in germanium by a factor of 4.0.

Acoustic pulse propagation in an outdoor turbulent atmosphere

In a series of short-range experiments, attempts were made to relate the prevailing meteorological conditions to significant wave shape changes in a 2 ms acoustic pulse propagating over distances from 0.5 to 16 m. Unlike the findings in long-range studies involving time and spatial averages, no meaningful correlations between wind speed and acoustic pulse shape parameters were obtained. Pulse delays could be predicted provided detailed measurements of the wind speed were obtained. Observations showed that two adjacent paths could experience quite different propagation regimes. The geometry and pulse nature of the experiments described here restrict the scale of turbules in any cascade suggesting that a single turbule model may be applicable. Indeed using one or two effective turbules was capable of explaining many of the observations. One model approach used typical parameter values obtained from the literature or from measurements and by locating a single turbule at an appropriate distance from the source receiver axis, correctly generated the observed variety of wave forms. Alternatively, relying on measured data from one distorted pulse and the spatial wind profile, the model was used to determine subsequent pulse distortions. This was successful in predicting behavior in 76% of cases.

Dispersion and soliton formation from longitudinal acoustic phonons in crystalline solids

Experiments are described in which the picosecond ultrasonic technique is used to study the effects of dispersion and elastic nonlinearity on the shape of longitudinal acoustic pulses propagating in crystalline solids. In these experiments, a subpicosecond light pulse (pump pulse) is used to generate an acoustic pulse at one surface of the sample. After propagation through the sample, the shape of this acoustic pulse is modified. When the amplitude of the pump pulse is sufficiently low, nonlinear effects can be neglected and a measurement of the acoustic pulse shape can be used to determine the phonon dispersion. This measured phonon dispersion is compared with various lattice dynamical models. When the pump pulse intensity is high, the nonlinear effects become strong enough to balance the dispersion, and acoustic solitons emerge from the original acoustic pulse. Measurements of the characteristics of these solitons are in agreement with the results of computer simulations based on the Korteweg-de Vries equation.

Studies of soliton formation of longitudinal acoustic phonons in crystalline solids

We have used the picosecond ultrasonics technique to study the effects of dispersion and elastic non-linearity on the shape of longitudinal acoustic pulses propagating in a silicon crystal. In these experiments, a sub-picosecond light pulse is used to generate an acoustic pulse at one surface of the sample. After propagation through the sample, the shape of this acoustic pulse is modified. When the pump pulse intensity is high, the non-linear effects become strong enough to balance the dispersion, and an acoustic soliton emerges from the original acoustic pulse. The form of the soliton is in good agreement with results obtained from a computer simulation based on a non-linear dispersive wave equation.

SOUND GENERATION BY BREAKDOWN PLASMA FORMATIONS INITIATED BY SOLID AEROSOL PARTICLES UPON EXPOSURE TO LASER RADIATION

The results of experimental investigations into the characteristics of acoustic signals generated by breakdown plasma formations initiated by a solid aerosol particle or an ensemble of monodisperse aerosol particles upon exposure to laser radiation with a wavelength of 1.06 μm are presented as functions of the materials and sizes of particles and of the acting laser energy density. It is demonstrated that the acoustic signal amplitude and duration depend linearly on the effective sizes of particles initiating plasma formations with the proportionality coefficient depending on the particle material. The coefficient of laser energy conversion into the acoustic energy is estimated. The acoustic pulse shape is satisfactorily approximated by a blast N-wave for particles of micron sizes. A comparison of the obtained results with the data available from the literature indicates their good agreement.

Generation of deformation waves in the processes of photoexcitation and recombination of nonequilibrium carriers in silicon

The shape of acoustic pulses excited in crystalline silicon during absorption of Nd3+ YAG laser radiation was studied as a function of incident light intensity. It was revealed that the amplitude of the dilatation wave is saturated while the duration of the compression pulse is shortened. A theoretical model is suggested which explains the above experimental facts by a decrease in the time of Auger recombination for nonequilibrium carriers with an increase in their concentration for higher intensity of the optical excitation. The value of the Auger constant obtained from the experiment is ∼5×10−31 cm6s−1.

Experimental study of laser‐induced underwater sound

Experiments have been carried out to characterize the sound generated when a CO2 “TEA” (pulsed) laser, operated without nitrogen, illuminates a free water surface. Energy densities ranging from below to above the value needed for rapid surface evaporation have been used. Acoustic pulse risetimes, as measured with very broadband detectors, are typically less than 100 ns at shallow depths. The acoustic pulse shape changes during propagation. The exact nature of the change depends upon the laser energy density. Generally a pulse stretches and the trailing edge steepens so that it changes from a nearly sinusoidal shape into an “N” shape, often with the rarefaction having a larger amplitude than the initial compression by the time it has traveled downward 5 cm from the surface. Although nonuniformities (“hot spots”) in the laser intensity make it difficult to determine the exact relation, it appears that the peak pressures are roughly proportional to the energy density (J/cm2) of the laser pulse. An approximat...

Estimates of the intensity of sound generated during propagation of modulated laser beams in the atmosphere and influence of this sound on thermal defocusing of beams

The amplitude and shape of acoustic pulses, generated in the course of propagation of laser beams in the atmosphere, are calculated. The possibility of measuring these pulses is considered. An analysis is made of changes in the focusing properties of inhomogeneities of the refractive index which form in the atmosphere as a result of thermal interaction with a modulated laser beam. It is shown that self-focusing of a part of the beam is possible in the case of spatial and temporal modulation.