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Phononic Surfaces
Propagation of surface acoustic waves (SAW) has relevant interest in both fundamental research and technological applications. Starting with the pioneering works of Lord Rayleigh (1885) a large number of experimental and theoretical studies have been published on SAW science. During the last 20 years, there was a great deal of interest for periodic elastic structures,which are called since that time phononic crystals,by analogy with their optical counter part the photonic crystals. The propagation of acoustic waves in phononic crystals shows many peculiar phenomena that open the possibility for the realization of acoustic metamaterials. A clear example is the creation of phononic band gaps (i.e a frequency intervals over which the propagation of sound is forbidden), that enables a unique control on the propagation of sound. Other phenomena are connected with the acoustic processes characterized by frequencies and wave-vectors at the band edge, where the folding and bending of the acoustic bands take place.
We studied SAW propagation on a 1D phononic surface (PS) by mean of an heterodyne-detected transient reflecting grating experiment. We excited and detected coherent stationary SAWs characterized by variable wave-vectors. The measured SAW frequencies enables the characterization of the band diagram of this PS sample beyond the first Brillouin zone. Four different SAW frequencies have been revealed, whose band diagram show articulated dispersion phenomena. In order to address the nature of the investigated SAWs, the experimental results are compared with a numerical simulation of elastic modes based on a finite element
model. The observed SAWs are addressed to four Bloch waves characterized by different frequencies and surface energy localization. Moreover, we measured the SAW propagation on a flat non-phononic part of the sample surface and compared it with results from the PS.
The phononic surface investigated and the experimental optical configuration to perform transient grating experiment on surfaces.
The band diagram of the acoustic waves measured in our experiments and the simulations. An acoustic band gap and the folding at the Brilluoin zone is evident.
Moreover, using a suitable experimental configuration, we excited traveling SAWs with variable wave numbers and the measurement of their propagation with temporal and spatial resolution. We studied the dispersion properties of the SAWs as the wave number approaches the Brillouin zone edge. SAW-packet group velocities show a clear slowing-down process approaching the BZ edge, with a measurable minimum velocity of about 140m/s.
The probing area of SAW is shifted respect to the excitation area by a variable distance, this enable a direct measurement of the propagating SAW group velocity. We investigated the SAW properties on both the Flat Surface (FS) and the Phononic Surface (PS).
Blue full circles are group velocities in the FS sample obtained by direct measurements using the pump and probe displaced configuration (X-dependent). Red squares are the group velocities in the PS sample, full squares for X-dependent experiments. Dashed and continuous lines are the group velocities indirectly obtained as the derivative of the SAW dispersion functions for the FS and PS, respectively. A clear slowing down of the SAW velocity is present in the PS.
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Malfanti, I., Taschin, a., Bartolini, P., Bonello, B., & Torre, R. (2011). Propagation of acoustic surface waves on a phononic surface investigated by transient reflecting grating spectroscopy. Journal of the Mechanics and Physics of Solids, 59(11), 2370–2381. doi:10.1016/j.jmps.2011.07.010
Malfanti, I., Taschin, A., Bartolini, P., & Torre, R. (2012). Evidence of slow acoustic surface waves on a 1D phononic surface by a pulsed laser spectroscopic technique resolved in time and space. EPL (Europhysics Letters), 97(4), 44010. doi:10.1209/0295-5075/97/44010
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