Size dependent elastic properties of GaAs pillar-resonators
Résumé
Mechanical micro- and nanoresonators show intriguing size dependent properties regarding their elastic behavior. The elastic modulus derived for wire- and pillar-resonators from dynamic flexural mode measurements often deviates significantly from their bulk counterpart and thus makes them interesting for a wide range of applications. This behavior has been mainly assigned to the increasing influence of surface stresses on the resonance frequency when resonators become smaller. However, despite showing also considerable different elastic moduli, some experiments on pillar resonators do not show a distinct size dependence. In order to test different models and the actual origin of the observed deviations, we started to combine nanomechanical flexural mode measurements with picosecond ultrasonics to obtain not only information about the effective elastic modulus but spatially resolve the local elastic properties inside pillar resonators. The pillars under investigation have been recently used to study mechanical coupling phenomena between nanoresonators and have previously shown strong deviations from the bulk modulus in flexural mode measurements. Resolving the elastic modulus along the pillars long axis becomes possible utilizing time resolved Brillouin scattering. Here, a pump-probe approach tracks a moving acoustic pulse inside the pillars via the interference caused by the light portions reflected off the interfaces and the moving acoustic pulse. This allows to monitor the local sound velocity inside the pillar and subsequent comparison with the results from flexural mode measurements become possible.