Visualizing coherent phonon propagation in the 100 GHz range: A broadband picosecond acoustics approach

The behaviour of high-frequency acoustic excitations in disordered materials is one of the most provocative and less understood aspects of glass science. The frequency dependence of sound damping, in particular, is the result of the interplay of several physical mechanisms crucial for many theoretical models. The emerging picture suggests the existence of a frequency crossover in sound attenuation identifying a transition from a macroscopic continuum-like behavior, due to anharmonic damping effects, to a microscopic regime in which the dynamics is dominated by topological disorder, characteristic of the amorphous phase. The very nature of this hypothetic crossover, its frequency position and its relation to other anomalies of the amorphous state is highly debated. In an effort to address this puzzling scenario, we developed a novel broadband setup for interferometric picosecond acoustics, which allows accessing in a single measurement a nearly octave-spanning range of the acoustic frequency wave-packet with unprecedented sampling efficiency.