Towards thin film complete characterization using picosecond ultrasonics

Picosecond ultrasonics is an ultrafast time-resolved technique which offers an unique way of measuring elastic properties in thin films and multi-layers. In the conventional setup only longitudinal waves can be excited by the laser. But in order to have a complete characterization, we need in-plane informations. Here, we show that using a nanostructured aluminum film as a transducer, we can excite longitudinal and high-frequency surface waves using a standard setup. By measuring longitudinal and surface velocities, we can deduce the Young modulus and the Poisson ratio which complete the characterization of any isotropic materials. Here we applied this technique to AlN, SiO₂ and Si₃N₄.