Basic equations of elastic waves

As in the previous chapter our discussion will be restricted mostly to the fields satisfying wave equations. However, in the last section I will outline the basic principles of nonlinear elastic inversion. [Pg.467]

The preceding discussion is based on the assumption that the incident laser is a plane wave. If the laser is a focused Gaussian beam, with a beam size o comparable to or smaller than the film thickness, transverse correlation effects will arise. Molecules situated outside the laser beam will exert torques on molecules inside the beam conversely, molecules inside the beam could also exert torques on those on the outside. The result is that the transverse dependence of the reorientation profile is not the same function as the transverse profile of the incident laser beam (e g., Gaussian). Put in another way, one may recognize that Equation (8.53) is basically a diffusion equation, where the elastic term plays the role of the diffusive mechanism. As a result of this diffusive effect, as in marty other physical processes, the spatial profile of the response is not the same as the excitation profile. [Pg.205]

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