A non contact technique using Zero-Group Velocity (ZGV) Lamb modes is developed to probe the bonding between two solid plates coupled by a thin layer. The layer thickness is assumed to be negligible compared with the plate thickness and the acoustic wavelength. The coupling layer is modeled by a normal and a tangential spring to take into account the normal and shear interfacial stresses. Theoretical ZGV frequencies are determined for a symmetrical bi-layer structure and the effect of the interfacial stiffnesses on the cut-off and ZGV frequencies are evaluated. Experiments are conducted with two glass plates bonded by a drop of water, oil, or salol, leading to a few micrometer thick layer. An evaluation of normal and shear stiffnesses, is obtained using ZGV resonances locally excited and detected with laser ultrasonic techniques.PACS numbers: 43.20. Gp, 43.35.Zc, 62.30.+d, 68.60.Bs, 79.20.Ds The increasing use of adhesive bonding in the industry has been motivated by the need of stronger and lighter structures. This technique is more suitable for providing continuous adhesion properties and easier to process than other ones like welding, riveting or screwing. The bonding between two solid plates can be probed by various ultrasonic techniques: longitudinal and shear waves reflection or transmission at the interface, 1,2 thickness resonances, 3 ultrasonic guided waves propagating along the interface. 4-8 All these methods require a modelisation of the ultrasonic wave interaction with the interface supposed homogeneous. 9 Recently it was shown that Zero-Group Velocity (ZGV) Lamb modes associated with laser ultrasonic techniques, allow a local and non contact measurement of mechanical properties of isotropic or anisotropic plates and shells. 10,11 These ZGV modes, corresponding to a minimum frequency of dispersion curves, also exist in layered plate structures. 12 Local ZGV resonances have been used to image the lack of adhesive bond between two plates. 13In this letter a non contact method, based on the measurement of ZGV resonance frequencies is proposed to probe interfacial stiffnesses between two plates. An interfacial behavior model 9,16 is used to calculate the dispersion curves and Zero-Group Velocity Lamb modes in a symmetrical structure composed of two plates coupled by a thin layer. The ultrasonic wave interaction with the interface is described by spring boundary conditions. Firstly, the effect of longitudinal and transverse stiffnesses on cut-off and ZGV frequencies is studied. Secondly, experimental results with liquid or solid compliant layers are obtained by laser ultrasonic techniques. Then, the values of spring stiffnesses, estimated from ZGV resonance frequencies, are discussed.Theoretical model -The structure is composed of two identical, isotropic and homogeneous plates with a coupling layer in-between. 7 The plate thickness is denoted h and lateral dimensions are supposed infinite. Their mass density is denoted ρ and their bulk wave velocities c l and c t . The coupling layer thickness d is a...

Zero-group-velocity (ZGV) waves have the peculiarity of being stationary, and thus locally confining energy. Although they are particularly useful in evaluation applications, they have not yet been tracked in two dimensions. Here we image gigahertz zero-group-velocity Lamb waves in the time domain by means of an ultrafast optical technique, revealing their stationary nature and their acoustic energy localization. The acoustic field is imaged to micron resolution on a nanoscale bilayer consisting of a silicon-nitride plate coated with a titanium film. Temporal and spatiotemporal Fourier transforms combined with a technique involving the intensity modulation of the optical pump and probe beams gives access to arbitrary acoustic frequencies, allowing ZGV modes to be isolated. The dispersion curves of the bilayer system are extracted together with the quality factor Q and lifetime of the first ZGV mode. Applications include the testing of bonded nanostructures.

Experiments with an all-optical method for the study of the nonlinear acoustics of cracks in solids are reported. Nonlinear acoustic waves are initiated by the absorption of radiation from a pair of laser beams intensity modulated at two different frequencies. The detection of acoustic waves at mixed frequencies, absent in the frequency spectrum of the heating lasers, by optical interferometry or deflectometry provides unambiguous evidence of the elastic nonlinearity of the crack. The high contrast in crack imaging achieved by remote optical monitoring of the nonlinear acoustic processes is due to the strong dependence of the efficiency of optoacoustic conversion on the state of the crack. The highest acoustic nonlinearity is observed in the transitional state of the crack, which is intermediate between the open and the closed ones.

Zero-Group Velocity (ZGV) Lamb waves are studied in a structure composed of two plates bonded by an adhesive layer. The dispersion curves are calculated for a Duralumin/epoxy/Duralumin sample, where the adhesion is modeled by a normal and a tangential spring at both interfaces. Several ZGV modes are identified and their frequency dependence on interfacial stiffnesses and on the bonding layer thickness is numerically studied. Then, experiments achieved with laser ultrasonic techniques are presented. Local resonances are measured using a superimposed source and probe. Knowing the thicknesses and elastic constants of the Duralumin and epoxy layers, the comparison between theoretical and experimental ZGV resonances leads to an evaluation of the interfacial stiffnesses. A good agreement with theoretical dispersion curves confirms the identification of the resonances and the parameter estimations. This non-contact technique is promising for the local evaluation of bonded structures.

We extend time-domain imaging in acoustic metamaterials to gigahertz frequencies. Using a sample consisting of a regular array of ∼1 μm diameter silica microspheres forming a two-dimensional triangular lattice on a substrate, we implement an ultrafast technique to probe surface acoustic wave propagation inside the metamaterial area and incident on the metamaterial from a region containing no microspheres, which reveals the acoustic metamaterial dispersion, the presence of band gaps and the acoustic transmission properties of the interface. A theoretical model of this locally resonant metamaterial based on normal and shear-rotational resonances of the spheres fits the data well. Using this model, we show analytically how the sphere elastic coupling parameters influence the gap widths.

We present a minimally-invasive endoscope based on a multimode fiber that combines photoacoustic and fluorescence sensing. From the measurement of a transmission matrix during a prior calibration step, a focused spot is produced and raster-scanned over a sample at the distal tip of the fiber by use of a fast spatial light modulator. An ultra-sensitive fiber-optic ultrasound sensor for photoacoustic detection placed next to the fiber is combined with a photodetector to obtain both fluorescence and photoacoustic images with a distal imaging tip no larger than 250 µm. The high signal-to-noise ratio provided by wavefront shaping based focusing and the ultra-sensitive ultrasound sensor enables imaging with a single laser shot per pixel, demonstrating fast two-dimensional hybrid in vitro imaging of red blood cells and fluorescent beads.

Using an ultrafast optical technique with enhanced frequency control, we image surface-acoustic whispering-gallery-like modes in a microscopic disk at various frequencies up to 1 gigahertz (GHz), allowing experimental determination of their dispersion. This is made possible by intensity-modulated optical pumping and probing with a periodic femtosecond light source. Spatiotemporal Fourier transforms of the two-dimensional acoustic fields measured to micron resolution allow us to isolate individual whispering-gallery modes of first and second radial order as well as their mode patterns and Q factors to unprecedented frequency resolution. We thereby demonstrate arbitrary-frequency ultrafast control and imaging of a micro-acoustic system with an optical time-resolved technique. Applications include quality control of surface acoustic wave filters in telecommunications.

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