Optical evaluation of the flexural rigidity and residual stress in thin membranes: Picosecond transient grating measurements of the dispersion of the lowest Order Lamb acoustic waveguide mode

This paper describes convenient, non-contacting technique for determining the flexural rigidity and residual stress in thin membranes. The method is based on transient grating photoacoustics, also known as impulsive stimulated thermal scattering. Crossed laser pulses provide a coherent source of wavelength tunable acoustic modes in the membranes. Diffraction of a probe laser reveals the time dependence of these motions. The mesured dispersion of the lowest order Lamb mode at small acoustic wavevectors, coupled with classical plate theory, allows the flexural ridigity and the residual stress to be determined. The technique, its accuracy and spatial mapping capabilities are demonstrated through analysis of membranes designed for SCALPEL masks.