THEORY AND POSSIBLE REALIZATION

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LASER-INDUCED PIEZOEXCITATION OF LOVE WAVES: THEORY AND POSSIBLE REALIZATION a

V. GUSEV & P. HESS

b

a

Permanently with International Laser Center, Moscow State University , Moscow, 119899, Russia b

Department of Physical Chemistry , University of Heidelberg , D-69120, Heidelberg, Germany Published online: 16 May 2007.

To cite this article: V. GUSEV & P. HESS (1994) LASER-INDUCED PIEZOEXCITATION OF LOVE WAVES: THEORY AND POSSIBLE REALIZATION, Nondestructive Testing and Evaluation, 11:6, 313-340, DOI: 10.1080/10589759408956412 To link to this article: http://dx.doi.org/10.1080/10589759408956412

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Nondestr. Test. Eval., 1994, Vol. 11, pp. 313-340 Reprints available directly from the publisher Photocopying permitted by license only

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LASER-INDUCEDPIEZOEXCITATIONOF LOVE WAVES: THEORY AND POSSIBLE REALIZATION v. GUSEV* and P. HESS Department ofPhysical Chemistry, University ofHeidelberg, D-69120Heidelberg, Germany; *Permanently with InternationalLaser Center, Moscow State University, 119899 Moscow, Russia (Received 25 February 1994) A full theoretical' analysis ,' of the optical excitation of Love waves is presented. The explicit formulae relating, the frequency, spectrumofthe Love modes to the space-time modulation 'of laser radiation are . derived. The photoacoustic transformation is assumed to be caused by the inverse piezoeffect in the. electric field induced by, the spatial separation of laser-generated electrons, and holes. It .' is" demonstrated that information on the parameters of the-film can be extractedfrom the measurements of 'the "relative amplitudes of"the different order Love modes. As the structure 'of the Love modes is simpler tlian that ofpseudoRayleigh waves, we consider, optical monitoring of the Love modes to be a promising, method for, remote diagnostics of thin coatings" when the film or the substrate, or both, "are piezoelectrics, . ,, KEY WORDS:, ," Love waves, .optical. excitation, photoacoustics, thin coatings, laser-induced ultrasound.

1 INTRODUGTION In.recent years in the field of surface acoustic wave (SAW) excitation by laser sources a shift has occurred away from traditional investigations of Rayleigh-type SAW generation at the surface of the homogeneous half-space" to an analysis of the SAW excitation in more complicated heterogeneous systems. The investigation of the ,laser excitation "of A" -"e....uuvp~"

(".. o.'V~o.....,.r1QA\ D~1l71o;n-h n1.n1.a", 1" f;l....,.,,,, rnnllntj3.r1 I"'\n V.l \;A ...\;lJU\;U J ~'-U] u ... ,.I.5.1.1. vv a v ""'~ 1..1.1 .I..I..I..l.l.l.~ .l.l..I.VU.l..I. ..."'''''" '-'.1.1.

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be mentioned first"The excitation of both the zero-order pseudo-Rayleighmodel" (i.e. 1 quasi-Rayleigh wave) and 'thehigherorder modes (i.ev Sezawa wa.vesi ,12 has been achieved in experiments. The interest in these investigations is' based on the prospect of contactless nondestructive laser-SAW -based diagnosticsof "thin coatings in real tiime. " •11-12 . ' It should be pointed out thatin most experiments carriedout td'date7~13 the excitation of the pseudo-Rayleigh waves by laser rradiation was through the thermoelastic mechanism, which is known to'be a very ineffective one." As a result sigmficant transient heating of the surface is required, which can damage the polymeric":" or 10 irrnorphous , films underinvestigation. 'To avoid excessiveheating and damage the Sample sh?uid be translat,e~ duriIlg data .collectionand toavoid.cumulative' dan,tage fast data collection times areimportant, 11,12 To reduce' transient heating the phase velocity scanning of a laser beam can be used." An alternative approach' is" the application of other (nonthermal) mechanisms of optoacoustic transformation such as the mechanism of the electrornholej-phonon deformation potential or the inverse piezoeffect, which are

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V. GUSEV ANDP.·HESS

more effective at high frequencies.v":" Recently the direct experimental comparison of the efficiencies of piezo- and thermo-excitation of Rayleigh-type SAWs has been demonstrated" in pulsed-laser action on a piezo-semiconductor. The application of an exrematctecrnc .fieldandthe traIlsition" from ,. the .thermoelasticmechanism to, sound

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eXcita:ti~n, 1:>Y the inverse p~ezoeffebthave. made.it possible to reducethe ener,gy of the

laser ptils~s,':'by:' more 'than four orders' of magnitude. Forisome industrial and acoustoelectronic applications it may be important that, as a result of the anisotropic nature of the piezoeffect (which-leads directly to the 'non-potentiality of the laser-induced distributed sources otacoustic, 'raves), the laser-induced piezoexcitation of.purely shear acoustic waves ..is possible for some crystallographic orientations of the crystals. Specifically the excitation of Gulyaev-Bleusteinsurface electro-acoustic waves at the free surface of a semi-infinite piezocrystal" and shear-horizontal (SH) waveguide modes in the plates" can be achieved. In other words, the laser initiation of the inverse piezoeffect leadsto theprospect of the .excitation of acoustic waves.which are still quite novel in 'optoacoustics ·and which can be' used .in nondestructive evaluation of .materialsaswell.i"' ..,. ' '" ".",,' In Jqe presentpublication we proposeatheoretical description of thelaser~induced .piezoe~citation"of.,·~Love waves, i.e. shear .surface elastic. waves ·in coatings. The mechanical displacement vectorin the Love wave is perpendicular to the .saggital plane and the direction of SAW propagation.v" Love-type waves' are applied to signal processing technology24,2~as. well as to the testing of inhomogeneous surface layers of solids." Optical interferometric techniques for measurements of the in-plane ultrasonic displacements also exist,26,27 All these factors confirm the possibility of the application of laser-based Love waves for remote diagnostics of films on a 'substrate.

2 'THEORY

2 .lFundamental equations"and: Laplace-transform formalism The excitationofan .acoustic wave by a laserisdescribed ,in . the .general case by the inhomogeneous waveequations (i.e., by the wave equations with the right-band side related to the transient spatially distributed laser-induced sources of acoustic disburbance), The, .most .straightforward mathematical approach 628 . . to solving .., ',".,' these '. equations is the method of integral transforms." Specifically, the application of the Laplace transform in.the depth coordinate is especially fruitful in the problems, pf.SAW laser excitationas .it, is well suited. to -. take into account the trapping of the 'acoustic disturbancesnear thesurface. is usually in the. case , .".", 61528 Also, .' ' theLaplace .,' ... ' . transform . '. .applied ..' , , of a semi-infinite space." In the present analysis we extend the formalismof the ltapla,.ce integral transform tothe investigation of a layered structure. The important advantage of. the Laplace-transform method lies. in the fact that this formalism leads directly to the evaluation of th~· mechanical displacenients and' strains at the interfaces, Thus theintermediate evaluation of the depth distribution for these quantities (see, for ~x~ple,thea.naiy~is of the pseudo-Rayleigh waves in Reference 12) is avoided. ,. '-Pirst let us,investigate the SH w~ve piezoexcitation by the electricpotential qJ, which ,

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