  Optimization of Lamb modes induced by laser can be achieved by adjusting the
spatial source distribution to the mode wavelength ($\lambda$). The
excitability of Zero-Group Velocity (ZGV) resonances in isotropic plates is
investigated both theoretically and experimentally for axially symmetric
sources. Optimal parameters and amplitude gains are derived analytically for
spot and annular sources of either Gaussian or rectangular energy profiles. For
a Gaussian spot source, the optimal radius is found to be $\lambda_{ZGV}/\pi$.
Annular sources increase the amplitude by at least a factor of 3 compared to
the optimal Gaussian source. Rectangular energy profiles provide higher gain
than Gaussian ones. These predictions are confirmed by semi-analytical
simulation of the thermoelastic generation of Lamb waves, including the effect
of material attenuation. Experimentally, Gaussian ring sources of controlled
width and radius are produced with an axicon-lens system. Measured optimal
geometric parameters obtained for Gaussian and annular beams are in good
agreement with theoretical predictions. A ZGV resonance amplification factor of
2.1 is obtained with the Gaussian ring. Such source should facilitate the
inspection of highly attenuating plates made of low ablation threshold
materials like composites.
