We use picosecond ultrasonics to image animal cells in vitro-a bovine aortic endothelial cell and a mouse adipose cell-fixed to Ti-coated sapphire. Tightly focused ultrashort laser pulses generate and detect GHz acoustic pulses, allowing three-dimensional imaging (x, y, and t) of the ultrasonic propagation in the cells with similar to 1 mu m lateral and similar to 150 nm depth resolutions. Time-frequency representations of the continuous-wavelet-transform amplitude of the optical reflectivity variations inside and outside the cells show GHz Brillouin oscillations, allowing the average sound velocities of the cells and their ultrasonic attenuation to be obtained as well as the average bulk moduli

Control of sound in phononic band-gap structures promises novel control and guiding mechanisms. Designs in photonic systems were quickly matched in phononics, and rows of defects in phononic crystals were shown to guide sound waves effectively. The vast majority of work in such phononic guiding has been in the frequency domain, because of the importance of the phononic dispersion relation in governing acoustic confinement in waveguides. However, frequency-domain studies miss vital information concerning the phase of the acoustic field and eigenstate coupling. Using a wide range of wavevectors k, we implement an ultrafast technique to probe the wave field evolution in straight and L-shaped phononic crystal surface-phonon waveguides in real- and k-space in two spatial dimensions, thus revealing the eigenstate-energy redistribution processes and the coupling between different frequency-degenerate eigenstates. Such use of k-t space is a first in acoustics, and should have other interesting applications such as acoustic-metamaterial characterization.

By means of an ultrafast optical technique, we track focused gigahertz coherent phonon pulses in objects down to sub-micron in size. Infrared light pulses illuminating the surface of a single metal-coated silica fibre generate longitudinal-phonon wave packets. Reflection of visible probe light pulses from the fibre surface allows the vibrational modes of the fibre to be detected, and Brillouin optical scattering of partially transmitted light pulses allows the acoustic wavefronts inside the transparent fibre to be continuously monitored. We thereby probe acoustic focusing in the time domain resulting from generation at the curved fibre surface. An analytical model, supported by three-dimensional simulations, suggests that we have followed the focusing of the acoustic beam down to a ~150-nm diameter waist inside the fibre. This work significantly narrows the lateral resolution for focusing of picosecond acoustic pulses, normally limited by the diffraction limit of focused optical pulses to ~1 μm, and thereby opens up a new range of possibilities including nanoscale acoustic microscopy and nanoscale computed tomography.

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.

By means of an ultrafast optical technique, picosecond acoustic strain pulses in a transparent medium are tomographically visualized. The authors reconstruct strain pulses in Au-coated glass from time-domain reflectivity changes as a function of the optical angle of incidence, with ϳ1 ps temporal and ϳ100 nm spatial resolutions. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2432238͔ Laser picosecond acoustics is well adapted to the investigation of thin films and nanostructures. 1-5 Subpicosecond light pulses are typically used to generate and detect ϳ10-1000 GHz longitudinal acoustic strain pulses. The acoustic strain pulse shape depends on the optoacoustic generation and the acoustic propagation, related, respectively, to the electron and phonon scattering mechanisms. 1,4,6,7 Measuring the strain pulse shape is therefore useful for studying these mechanisms. In some materials it is possible to measure the strain pulse shape near a free surface by monitoring either surface displacements through beam deflection or changes in the phase of the optical reflectance. 2,8 However, no existing method can continuously monitor picosecond strain pulse shapes during propagation. Stimulated by progress in lower frequency acoustic tomography, 9 we present a method to achieve this.Our sample consists of a thin metal film ͑with complex dielectric constant 2 ͒ deposited on a transparent substrate ͑with real 1 ͒, both materials being isotropic. For s-polarized probe light incidence at angle from the substrate side, the optical amplitude reflection coefficient r 0 ͑also termed the optical reflectance͒ can be expressed as 10 r 0 = cos − ͱ 2 / 1 − sin 2 cos + ͱ 2 / 1 − sin 2 . ͑1͒If 1 is slightly perturbed by a ͑anisotropic͒ longitudinal strain distribution 33 ͑z , t͒ ͑ϳ10 −5 here͒ in the depth ͑z͒ direction, the reflectance is changed: 11-13where variations in 2 and the effect of the finite substrate thickness have been ignored. Here, is the ͑central͒ wavelength of the probe light, P 12 ‫ץ=͑‬ 1 / ‫ץ‬ 33 ͒ is a photoelastic constant, and z = 0 at the 1-2 interface. Equation ͑2͒, which generalizes the formula derived in the past for reflectance changes at normal incidence, 1,11 is an example of an inhomogeneous Fredholm integral. 14 We calculate 33 ͑z , t͒ in the transparent substrate at a given time t from the observed reflectivity variation ␦R 0 / R 0 =2 Re͑␦r 0 / r 0 ͒ as a function of by solving the inverse problem ͑where R 0 = ͉r 0 ͉ 2 ͒. 14,15 ͑A similar approach involving optical phase variations should be feasible.͒ A polished 10 mm radius hemisphere of BK7 glass, whose flat surface is coated with a thin polycrystalline Au film of thickness of 600 nm, is mounted on a rotation stage to set ͑see Fig. 1͒. A photodetector is mounted on a separate coaxial rotation stage set to 2 . Visible s-polarized pump light pulses of duration of ϳ100 fs, repetition rate of 82 MHz, wavelength of 415 nm, and pulse energy of 1.1 nJ from the second harmonic of a Ti:sapphire mode-locked laser are used to illuminate an ϳ20 m diameter ͓full ...

High-frequency surface phonons have a myriad of applications in telecommunications and sensing, but their generation and detection have often been limited to transducers occupying micron-scale regions because of the use of two-dimensional transducer arrays. Here, by means of transient reflection spectroscopy we experimentally demonstrate optically coupled nanolocalized gigahertz surface phonon transduction based on a gold nanowire emitter arranged parallel to linear gold nanorod receiver arrays, that is, quasi-one-dimensional emitter–receivers. We investigate the response up to 10 GHz of these individual optoacoustic and acousto-optic transducers, respectively, by exploiting plasmon-polariton longitudinal resonances of the nanorods. We also demonstrate how the surface phonon detection efficiency is highly dependent on the nanorod orientation with respect to the phonon wave vector, which constrains the symmetry of the detectable modes, and on the nanorod acoustic resonance spectrum. Applications include nanosensing.

Surface acoustic wave band gaps in a diamond-based two-dimensional locally resonant phononic crystal for high frequency applications J. Appl. Phys. 111, 014504 (2012) Properties of defected one-dimensional terahertz plasmonic crystal films in a metal air-gap waveguide J. Appl. Phys. 110, 093101 (2011) Vibrational and thermodynamic properties of β-HMX: A first-principles investigation J. Chem. Phys. 134, 204509 (2011) Coupling of a surface plasmon with localized subwavelength microcavity modes Appl. Phys. Lett. 98, 021105 (2011) Evanescent modes in sonic crystals: Complex dispersion relation and supercell approximation

Real-time imaging of surface acoustic waves propagating on opaque substrates (abstract) Rev. Sci. Instrum. 74, 743 (2003);A high frequency amplitude-steered array for real-time volumetric imagingWe image gigahertz surface acoustic waves normally incident on a microscopic linear array of triangular holes-a generic "acoustic diode" geometry-with a real-time ultrafast optical technique. Spatiotemporal Fourier transforms reveal wave diffraction orders in k-space. Squared amplitude reflection and transmission coefficients for incidence on both sides of the array are evaluated and compared with numerical simulations. We thereby directly demonstrate acoustic rectification with an asymmetric structure. V C