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The stiffness coefficients of anisotropic materials were determined through a scanned point source-point receiver method, based on laser generation and detection of acoustic waves. A silicon crystal which is very sensitive to acoustic wave focusing induced by anisotropy was examined first. Four coefficients of the stiffness tensor were recovered with good reliability by scanning a symmetry plane.

Recently, the coherent generation of GHz acoustic waves using ultrashort laser pulses has demonstrated the ability to probe the sound velocity in vegetal cells and in cell-mimicking soft micro-objects with micrometer resolution, opening tremendous potentialities for single-cell biology. However, manipulating biological media in physiological conditions is often a technical challenge when using a laser-based setup. In this article, we present a new opto-acoustic bio-transducer composed of a thin metal film sputtered on a transparent heat sink that allows reducing importantly the laser-induced cellular stresses, and offers a wide variety of optical configurations. In particular, by exploiting the acoustic reflection coefficient at the sample-transducer interface and the photoacoustic interaction inside the transparent sample, the density and compressibility of the sample can be probed simultaneously. Using an ad hoc signal analysis based on Hilbert and wavelet transforms, these quantities are measured accurately for a reference fluid. Similar analysis performed in a single vegetal cell also suggests high sensitivity to the state of the transducer-cell interface, and notably to the presence of the plasma membrane that encloses the cell vacuole. [ABSTRACT FROM AUTHOR]

The acoustic displacements excited by a laser line source under either the ablation or thermoelastic regime are calculated for a two-layered cylinder made of homogeneous and isotropic materials. Two samples made by welding tin in two different copper tubes are considered. Experimental displacements are observed by the laser ultrasonic technique, and corresponding theoretical waveforms are calculated. Good agreement is found in the arrival time, shape, and relative amplitude of various longitudinal and shear bulk waves propagating in the two-layered cylinders, and the waves reflected by the interface are further identified by the ray trajectory analysis. The longitudinal wave reflected at the interface may be used for the nondestructive evaluation of the interface quality. [ABSTRACT FROM AUTHOR]

An optical grating has been introduced in a picosecond ultrasonics experiment, in order to vary continuously the duration of the laser beam pulse from 0.1 to 150 ps. The evolution of the measured signal has been observed and analyzed through the comparison with a theoretical approach based on a two-temperature model. The latter allows matching the acoustic echoes together with the thermal background and the coincidence peak, for each pulse duration and at any time scale. The broadening of the acoustic echoes and the disappearing of its Brillouin component, along with the diminishing of the thermal coincidence peak, have been demonstrated when increasing the pulse duration. For a constant incident pulse energy, the efficiency of acoustic generation is optimum for the shortest pulses. Nevertheless, for longer pulses designed to obtain thermal conditions below the ablation threshold, acoustic generation could be enhanced. [ABSTRACT FROM AUTHOR]

A noncontact laser–ultrasonic technique is described that allows determination of material properties of anisotropic platelike samples. The method requires access to only one side of the sample. Plane waves are synthesized by scanning the surface with a laser line source at judiciously chosen intervals. By doing so, the intricacies associated with the reflection of an acoustic ray off an interface of an anisotropic medium are avoided. The plane-wave reflections off the opposite free surface of the plate are simply governed by Snell's law. The identification of the stiffness-tensor components is based on the inversion of the phase velocities of these reflected plane waves. The procedure developed is presented and applied to both simulated and experimental signals. The method is easy to implement, and provides a means for noncontact measurement of elastic properties where there is limited access to test specimens. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]