Theory and Applications of Laser Generated Zero-group Velocity Lamb Mode Resonance

Lamb modes exhibit a resonant behavior at frequencies where the group velocity vanishes while the phase velocity remains finite. This type of zero group velocity (ZGV) point exists in most isotropic materials at the minimum frequency-thickness product of the first order symmetric (S1) Lamb mode. Laser sources couple efficiently into this resonance and a sharp peak is observed with source and receiver on epicentre. A model for the laser generation of ultrasound in thin plates using an amplitude modulated laser source is presented. Lamb wave displacement spectra are measured using an amplitude modulated laser source for excitation and a Michelson interferometer coupled to an RF lockin amplifier for detection. The displacement spectra of Lamb waves generated in micron scale plates show good agreement with theoretical predictions. Experimental results are presented demonstrating excitation of the S1 ZGV resonance in a 4μm thick membrane using a high frequency (700 MHz) amplitude modulated laser source. Subsurface features below the membrane are detected by tracking the S1 ZGV resonance peak as the sample is scanned.