Mathias Fink par Lab. Ondes et Acoustique, Universite Paris 7, E.S.P.C.I., 10 rue Vauquelin, 75231 Paris Cedex 05, France
In previous works, the capacity of time-reversal mirrors (TRM) has been presented to optimize ultrasonic focusing on a pointlike source. However, the evaluation of the focusing quality has so far been limited to fluid media. In this paper, the first theoretical approach is presented to determine the capacity of the TRM to focus on a point situated in a homogeneous isotropic solid. P and S waves emitted by a pointlike source in the solid yield the generation of transmitted pressure waves in a fluid that limits the solid via a plane boundary. A TRM placed in the fluid behind the interface measures both incoming wavefronts and is hence able to reverse them one by one. The reversed pressure fields backpropagate to the interface, resulting in P and S waves in the solid. It is shown that, in spite of the losses at the interface, both kinds of waves focus at the location of the initial source. Numerical simulations are carried out, showing that the time reversal is more favorable using the S waves than the P waves, in terms of the width of the focal spot as well as in terms of displacement amplitude. The results are compared with experiments carried out with a laser source working in the ablation regime.