Generation and Detection of Shear Acoustic Waves in Metal Submicrometric Films with Ultrashort Laser Pulses

Abstract

We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.

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“…These techniques make use of very short stress pulses, generated by thermoelastic deformation of the surface irradiated by an ultrashort optical pulse, that propagate across the specimen and are subsequently detected by another optical probing pulse after a controlled delay. The detection principle is usually the change in the specimen optical reflectivity (Thomsen et al 1986;Rossignol et al 2005;Vollmann et al 2002), although the change in the surface slope (OBD) (Wright and Kawashima 1992) and interferometric detection (Hurley and Wright 1999) have also been reported. The fine achievable temporal resolution (of the order of picoseconds), lateral resolution (of the order of micrometers), and the specific and expensive equipment needed make these techniques especially adequate for characterizing microstructures and thin films in laboratory, but not well-suited for macroscopic industrial inspection where the temporal and spatial scales are much larger.…”

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“…1,16 Nevertheless, in the case of point generation and point detection, when pump and probe are at the same position, experimental signals have demonstrated the existence of shear waves in polycrystalline aluminum. 15 Yet, the latter cannot be generated directly by a thermoelastic point source in an infinite isotropic material. The presence of the surface allows launching directly of shear waves T. However, for symmetry reasons, they can propagate in any direction but k 2 = 0.…”

“…The amplitude of the reflected light is barely affected, but its phase is significantly modified. In this case, the interferometric response of an opaque material can be represented by the normal displacement of the free surface of the medium in the direction x 1 , 15 and the acousto-optic interaction can be neglected. However, if the absorption is weak, the probe beam is sensitive to the modulation of optical properties created by the in-depth strain ͑2͒ .…”

“…13,14 Lately, increasing the magnification of the microscope objective, in order to focus the laser beam to a smaller spot on the surface, has allowed shear wave observation by interferometric measurements in submicrometric films. 15 In strongly absorbing materials, the probe light is almost entirely reflected at the free surface, and only its phase is modified by the surface displacement. Thus, to describe interferometric signals in opaque materials, only the displacement is required, 16 and one can solve the equation of motion for a three-dimensional geometry, without considering any interaction with the probe beam.…”

“…Thus, to describe interferometric signals in opaque materials, only the displacement is required, 16 and one can solve the equation of motion for a three-dimensional geometry, without considering any interaction with the probe beam. 15,17,18 As the optical penetration increases, the probe beam senses the in-depth optical index variation due to the strain propagation. Both phase and amplitude of the reflected light are affected.…”