  This paper investigates the dependence on Poisson's ratio of local plate
resonances in low attenuating materials. In our experiments, these resonances
are generated by a pulse laser source and detected with a heterodyne
interferometer measuring surface displacement normal to the plate. The laser
impact induces a set of resonances that are dominated by Zero Group Velocity
(ZGV) Lamb modes. For some Poisson's ratio, thickness-shear resonances are also
detected. These experiments confirm that the temporal decay of ZGV modes
follows a $t^{-0.5}$ law and show that the temporal decay of the thickness
resonances is much faster. Similar decays are obtained by numerical simulations
achieved with a finite difference code. A simple model is proposed to describe
the thickness resonances. It predicts that a thickness mode decays as
$t^{-1.5}$ for large times and that the resonance amplitude is proportional to
$D^{-1.5}$ where $D$ is the curvature of the dispersion curve $\omega(k)$ at
$k=0$. This curvature depends on the order of the mode and on the Poisson's
ratio, and it explains why some thickness resonances are well detected while
others are not.
