Simulation and measurement of the optical excitation of the S1 zero group velocity Lamb wave resonance in plates

Recent reports on the thermoelastic generation of Lamb waves in isotropic elastic plates show that a laser source efficiently excites a resonance that occurs at the minimum frequency of the first order symmetric (S-1) Lamb mode. The group velocity of the Lamb wave goes to zero at this frequency while the phase velocity remains finite, and the resonance is referred to as the S-1 zero group velocity (S-1 ZGV) resonance. The S-1 ZGV resonance can be employed for the nondestructive evaluation of the elastic properties of plates or plate thickness. A model for the generation of elastic waves in plates using an intensity-modulated continuous wave laser source is developed and used to study the behavior of the S-1 ZGV resonance. The effects of the laser source parameters on the generation of the S-1 ZGV resonance are explored, and the spatial distribution of the displacement produced at the resonance frequency is determined. The predicted displacement spectrum of Lamb waves generated in micron scale plates is found to compare well with experimental measurements. In addition, experimental measurements demonstrate that the S-1 ZGV resonance can be used to map subsurface features in thin (4 mu m) membranes at high ultrasonic frequencies (700 MHz).