Our activities in phononic crystals are characterized by a strong involvement in research contracts, combined with an increased collaboration with experimental groups at both national and European levels. In 2008, we demonstrated the possibility of an absolute band gap among the Lamb modes in different type of solid-solid and air-solid two-dimensional phononic crystal plates . We investigated for the first time the band structure of a phononic crystal of finite thickness constituted of a periodical array of pillars deposited on a thin plate. We showed the possibility of a low-frequency gap, as compared to the acoustic wavelengths in the constituent materials, as in the case of locally resonant structures . These structures display different functionalities for the confinement, wave guiding and filtering of elastic waves. For example, we reported, both theoretically and experimentally in collaboration with INSP (Paris), on the spatio-temporal evolution of elastic wave confinement within a single cavity in a phononic crystal slab . Due to recent advances in nanofabrication techniques, submicron phononic crystals can reach the hypersonic (GHz) regime and allow for the interaction of light and sound in the same structure. We studied, both theoretically and experimentally using Brillouin Light Scattering (BLS) in collaboration with G. Fytas from Max Planck Institute of Mainz, hypersonic band features of AAO/polymer nanocomposites such as phonon localization, anisotropic sound propagation, effect of polymer solid-liquid transition . We also reported on the full control of the band diagram in new organic/inorganic periodic stacks .
Figure 1: Modulations of TM photonic modes frequencies by phononics modes .
In the frame of the FP7 project Tailphox, we investigated for the first time the existence of dual phononic/photonic band gaps, confined and slow modes in Si membranes  as well as in 1D nano-structured strip waveguides . The optical wavelengths are chosen in the telecom range of 1550 nm. The acousto-optic coupling, based on both photo-elastic and opto-mechanical mechanisms, were calculated in 2D infinite phoXonic crystal containing a single cavity  as well as in slabs and strips.
We investigated the propagation and filtering of Surface Plasmon Polaritons (SPPs) in metal-insulator-metal waveguides. In particular, we showed that inserting a cavity inside or just beside the waveguide respectively gives rise to peaks and dips in the transmission spectrum .
Figure 2: Transmission (solid line) and reflection spectra (dashed line) as a function of the wavelength for a straight waveguide with rectangular cavities ; magnetic field distribution for zero transmission (λ = 646 nm) .
For the purpose of biosensing applications, we studied, both theoretically and experimentally in collaboration with an IRI group, the localized surface plasmon resonance (LSPR) of an array of gold nanostructures on a substrate coated with a dielectric layer of variable thickness. We elucidated the origin of the thickness-dependent shift of the absorption maximum in a normal incidence transmission experiment.
Figure 3: Evolution with the silica layer thickness d of the position of the plasmon mode for an ITO/Au NS/SiOx/air system .
The oscillatory behavior of this shift originates from an interaction between the Perot-Fabry modes in the coating layer and the plasmon resonance of the nanoparticles. From the practical point of view, we demonstrated a high refractive index sensitivity to bio-chemical liquids for sensing applications .
 Absolute forbidden bands and waveguiding in two-dimensional phononic crystal plates, Vasseur J. et al., Phys. Rev. B, 77, 085415-1-15 (2008).
 Low-frequency gaps in a phononic crystal constituted of cylindrical dots deposited on a thin homogeneous plate, Pennec Y. et al., , Phys. Rev. B 78, 104105 (2008).
 Dynamics of confined cavity modes in a phononic crystal slab investigated by in situ time-resolved experiments, R. Marchal et al., Phys. Rev. B 86, 224302 (2012).
 Tuning and Switching the Hypersonic Phononic Properties of Elastic Impedance Contrast Nanocomposites, A. Sato et al., ACS Nano 10, 3471 (2010).
 Engineering the Hypersonic Phononic Band Gap of Hybrid Bragg Stacks, D. Schneider et al, Nano Letters 12, 3101 (2012).
 Simultaneous existence of phononic and photonic band gaps in periodic crystal slabs, Y. Pennec et al., Optics Express 18, 14301 (2010).
 Band gaps and cavity modes in dual phononic and photonic strip waveguides, Y. Pennec et al., AIP Advances 1, 041901 (2011).
 Acousto-optic couplings in two-dimensional phoxonic crystal cavities, Q. Rolland et al., APL 101, 061109 (2012).
 Nanoscale plasmon waveguide including cavity resonator, A. Noual et al., J. Phys.: Condens. Matter 21, 375301 (2009).
 Plasmonic Nanoparticles Array for High-Sensitivity Sensing: A Theoretical Investigation, O. Saison-Francioso et al., J. Phys. Chem. C, 116, 17819 (2012).