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Génération et détection par réseaux photo-induits d'impulsions acoustiques picosecondes de cisaillement

Abstract : The picosecond acoustic technique which allows studies at picosecond characteristic time scale and nanometer space scale is a powerful tool for the diagnostic of elastic properties of sub-micrometer films. high frequency acoustic phonons (up to 1 thz) are generated and detected by the use of ultra-short femtosecond laser pulses. due to isotropy of most of opto-acoustic transducers, shear acoustic waves can be thermo-elastically excited only by mode conversion of longitudinal acoustic waves.this work suggests using optical gratings as inhomogeneous excitation laser source for the generation of shear acoustic phonons in isotropic media. the processes leading to excitation of inhomogeneous plane bulk compression/dilatation and shear acoustic modes following the creation of a transient laser interference pattern at an interface between an opaque and a transparent elastically isotropic media are analyzed. the developed theory predicts the excitation of shear acoustic grating by mode conversion of longitudinal waves obliquely incident to the interface. it is shown that only the association of 1d pump grating and 2d probe grating can allow selective and heterodyne detection of the excited shear acoustic gratings. in this configuration the incident probe is depolarized through the photo-elastic interaction by the shear acoustic gratings. this is experimentally realized by positioning the 2d probe grating perpendicularly to 1d pump grating plane. furthermore, the implementation and adaptation of high sensitive optical methods, particularly polarimetry and ellipsometry has permitted us to evaluate and to optimize the sensitivity of shear acoustic waves detection.
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Submitted on : Tuesday, September 25, 2012 - 9:12:13 AM
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  • HAL Id : tel-00734973, version 1

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Mansour Kouyaté. Génération et détection par réseaux photo-induits d'impulsions acoustiques picosecondes de cisaillement. Autre [cond-mat.other]. Université du Maine, 2012. Français. ⟨NNT : 2012LEMA1006⟩. ⟨tel-00734973⟩

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