Probing elastic properties at small scales becomes crucial for the control of nanostructures. For this, hypersonous acoustic sources (GHz-THz) must be available to achieve the nanometric resolution. This type of sources exists since 30 years thanks to the advent of femtosecond lasers. In this work, we investigated the elastic response of colloidal and nanostructured materials in the 1-10 GHz domain. The samples studied are deposits of silica nanoparticles having a porosity of 55%. The interconnection of these particles is modified by a post-processing transforming Van der Waals-type bonds into covalent and hydrogen bonds. The hypersonic waves are emitted into the material studied via a transducer which is a thin metallic film on which the silica nanoparticles are deposited. This transducer is excited optically by a femtosecond laser. By mechanical transfer of these acoustic waves into the thin layer of silica, this layer resonates. The elastic response of this resonator/colloid system is presented as a function of the post-treatment ammonia time
antireflective, hardening, picosecond acoustics, non destructive testing
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