^{1}, Jingfei Liu

^{2}, Nico F. Declercq

^{2}and Vincent Laude

^{3}

^{1}Institut FEMTO-ST, Centre National de la Recherche Scientifique / Georgia Tech-CNRS,

Georgia Institute of Technology.

^{2}Georgia Tech-CNRS, Georgia Institute of Technology.

^{3}Institut FEMTO-ST, Centre National de la Recherche Scientifique.

Phononic Crystals (PnC) have been a topic of strong interest for the last 20 years. They are two- or three-dimensional periodic structures that are made of at least two materials with different mechanical properties. PnC can exhibit complete band gaps, i.e., finite continuous frequency regions where energy propagation is forbidden for all possible wave directions or conversely where only evanescent waves are allowed. Band-gap ranges and widths mainly depend on the materials employed in the crystal construction, and on the lattice geometry, the size, and the shape of any inclusion. In the field of phononic crystals, in general, diffraction is considered as an undesirable effect. Diffraction is here understood as the generation of new waves from an incident plane wave by multiple reflection or diffusion on a periodic grating. This phenomenon appears when the incident wavelength is of the same order as a grating pitch. There are actually few works which consider phononic crystals in the diffraction regime, and when considered it is generally for frequency regions for which the phononic crystal is “transparent” for a given incident wave. Recently, we have demonstrated [1] that the angles of propagation of diffraction orders in the surrounding medium are governed by the periodic boundaries between the incident medium and the PnC, in accordance with the grating law. It is well known that blazed optical diffraction gratings can significantly increase the diffraction efficiency of plane waves for a selected angle of incidence.

In this study we introduce a new type of phononic crystal grating that is blazed in order to maximize diffracted energy. We show that by combining blazing with a phononic band gap, diffraction efficiency approaching 100% can be achieved for acoustic waves. Experiments were performed using a Polar/C-scan system, a water tank 1-m wide, and a short pulse transducer. The transducer was used as both an emitter and a receiver, by switching the mode of operation after the emission of the pulse. The angle of incidence of the sound beam was varied. Pressure waveforms are collected for all angles of incidence. Frequency analysis is then performed by computing Fourier transform of waveforms in order to determine frequency as a function of angle. Finally, we obtain experimentally 98% diffraction efficiency with a two-dimensional phononic crystal of rotated steel rods of square cross-section immersed in water. This result opens the way toward the design of efficient phononic crystal gratings.