Measuring bulk acoustic waves excited in thin films or micro-structures with ultra-short laser pulses is a powerful method for accurate and non-destructive evaluation as well as characterization of material properties.

Optical techniques like the pump-probe laser-based acoustic method generate bulk acoustic waves in a thermo-elastic way by absorbing the pump laser pulses at the surface of the specimen. The acoustic waves have very small wavelength in the order of 10nm. The acoustic waves are partly reflected at any discontinuity of the impedance. Back at the surface the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses.

The measurement technique and the excitation of the acoustic pulses is explained for the case of metallic thin films on sapphire and silicon substrates. The influence of the film thickness and the manufacturing method of the thin films on the bulk wave speed is shown.

For the first time this technique is used for measuring the bulk wave propagation in very thin membranes. The bulk acoustic wave propagation in freestanding aluminium-silicon nitride multi-layer membranes with total thickness in the order of several hundred nanometers is measured and compared with one-dimensional thermo-elastic models. The measurements of the freestanding membranes are compared with measurements of the supported case. The technique presented in this work can also be applied for the characterization of material or geometrical properties of thin film bulk acoustic wave resonators. The advantage of the method lies in its non-destructive and non-contact approach, which is necessary for ultra-thin and brittle structures.

Future directions of the on-going research-project are presented.