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

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.

Figure 1

Calculated phase variations of the reflectivity of the first acoustic echoes in two isotropic aluminum plates ($h=0.54$ and $0.9\mu \mathrm{m}$) for three different focalizations of the laser pulses ($a=5$, 2, and $1\mu \mathrm{m}$).

Figure 2

Measured phase variations of the acoustic echoes in two polycrystalline aluminum plates ($h=0.54$ and $0.9\mu \mathrm{m}$) for three different focalizations of the laser pulses ($a=5$, 2, and $1\mu \mathrm{m}$).