Oblique laser incidence to select laser-generated acoustic modes

Abstract : The purpose of this paper is to study the effect of a non-normal optical penetration due to an obliquely incident laser source. It is shown that the loss of symmetry due to such a penetration influences specific bulk acoustic modes. For a given detection position, increase in shear wave amplitude is obtained by orienting the incident laser source. 1. Introduction Since the first quantitative approaches in the 60's [1], a large number of studies has been conducted to better understand the generation of laser-generated acoustic waves. Models for the acoustic generation in the thermoelastic regime, taking into account the optical penetration of the source [2] or the effect of source width [3], were developed. A more general approach based on Green's function formalism also included thermal diffusion in the medium [4]. These works have dealt with the modeling of a circular spot of laser illumination. However, modeling of the acoustic field generated by a line source is of interest, since the signal-to-noise ratio may be increased in this experimental situation [5]. All the works cited above assume a normal incidence of the laser beam with respect to the illuminated surface of the sample. The purpose of this paper is to analyse the effects of oblique incidence on acoustic waves generated by a line-focused laser source. We present a theoretical model accounting for the effects of optical penetration, finite width of the beam and pulse duration of the laser source. The absorbed energy density is first calculated by solving Maxwell's equations. Then the thermoelastic equations are solved to obtain the displacement field through a semi-analytical calculation based on a double Fourier transform in space and time. It is shown that oblique incidence increases shear wave amplitude. Changes of the longitudinal waveforms are also discussed.
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Journal of Physics: Conference Series, IOP Publishing, 2011, 278, 〈10.1088/1742-6596/278/1/012030〉
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Soumis le : mardi 9 octobre 2018 - 10:20:27
Dernière modification le : lundi 10 décembre 2018 - 14:54:03

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S. Raetz, T. Dehoux, B. Audoin. Oblique laser incidence to select laser-generated acoustic modes. Journal of Physics: Conference Series, IOP Publishing, 2011, 278, 〈10.1088/1742-6596/278/1/012030〉. 〈hal-01889414〉

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