Wave propagation in inhomogeneous media, phenomena and potential applications

Intermetallic diffusion leads to microstructures having gradually varying sound velocities. Such diffusion zones represent inhomogeneous wave guides. Depending on the ratio between the wave length and the thickness of the diffusion zone, bulk waves reaching the zone are dominantly transmitted or dominantly reflected. Thus a continuum having a variable sound velocity can be considered as an acoustic filter. The partial differential wave equation is solved numerically for various velocity-versus-propagation direction functions and the results are discussed. A first series of experimental results obtained by a short-pulse-laser-acoustic technique is presented. Mechanical waves are excited and detected using laser pulses of 70 fs duration at a wave length of 810 nm. An improved pump-probe set-up is developed in order to detect the acoustic echoes of very thin layers. Experimental results are compared with simulations and potential applications are proposed. Future directions of the on-going research project are discussed.