Experimental investigation of elastic modes localized within a defect in a phononic slab

Abstract

Heterostructures with periodic variations of both their optical refractive index and their elastic properties may induce band gaps for both electromagnetic and acoustic waves. As a consequence of the expected enhancement of the acousto-optic interactions, these artificial materials, called “PhoXonic crystals”, are of primary interest for new sensing applications. In this context, we have experimentally studied the localization of elastic energy within a defect. The heterostructures consist in arrays of voids periodically drilled throughout silicon plates (graphite symmetry) and featuring a vacancy, or a line of vacancies. An all-optical experimental technique allows for both the generation and the detection of the elastic guided waves. The non-contact probing allows one to monitor the displacements field inside the defects. First, we have measured the dispersion of broad band elastic waves guided in between the free surfaces of the sample. Then narrow band elastic guided waves, whose central frequency corresponds to resonance modes of the cavity are generated. The optical probe allows for the measurement of the out-of-plane displacements associated to the elastic modes localized within the cavity or transmitted through the phononic structure. The spatial distribution of elastic energy inside the cavity is measured and compared with numerical predictions. This work is supported by the European Community through the FET-Open project “TAILPHOX” (Grant No. 233883).