Real-time imaging of surface acoustic waves propagating on opaque substrates (abstract)

Abstract

All-optical techniques are well-suited for the nondestructive imaging of acoustic waves, but until recently it was not possible to image high frequency surface acoustic wave (SAW) propagation in real time on crystals. We are at present successfully developing such a method for the imaging of SAW with frequencies in the 100 MHz to 1 GHz range, propagating on transparent substrates coated with thin opaque films, using an optical pump and probe technique with picosecond temporal and micron spatial resolutions. [O. B. Wright, Y. Sugawara, O. Matsuda, and V. E. Gusev (these proceedings)]. Although this method is very effective for the investigation of SAW and for the evaluation of the elastic properties of thin films and substrates, it is limited to samples with transparent substrates. In the present study we demonstrate how to image propagating SAW in real time on samples with opaque substrates. We present in particular results for semiconductor wafers of commercial importance such as Si. We will show an image of the surface acoustic waves excited by pulses of light, a few picoseconds long, of wavelength 415 nm focused to a ∼4 μm diameter spot on the (100) surface of a Si single crystal coated with a thin film of gold of thickness 50 nm. The image represents the optical phase difference between two probe light pulses of wavelength 830 nm spaced closely in time (∼500 ps), effectively giving an image of the out-of-plane surface particle velocity at a given moment in time. The surface acoustic wavefronts, excited every 12.5 ns, are not circular because of the cubic symmetry of Si. This method should lead to a variety of applications for characterizing the elastic and mechanical properties of thin film structures.