Laser impulse generation and interferometer detection of zero group velocity Lamb mode resonance

In this letter, we describe experiments on,the generation of the first order symmetric (SI) Lamb mode by a pulsed yttrium aluminum garnet laser. The vibration of the plate is detected at the same point by a heterodyne interferometer. The acoustic signal is dominated by the resonance at the point of the dispersion curve where the group velocity vanishes. The time decay of the signal leads to the local attenuation coefficient of the material. The spectrum exhibits a very sharp peak, the frequency of which is sensitive to the plate thickness. For a 0.49-mm-thick Duralumin plate, thickness variations as small as 0.1 mu m have been detected. Moving the detection point away from the source allows us to record the standing wave pattern resulting from the interference between the S-1 and S-2b Lamb waves having opposite wave vectors at the zero group velocity point. (c) 2006 American Institute of Physics.