Imaging phonon eigenstates and elucidating the energy storage characteristics of a honeycomb-lattice phononic crystal cavity.

We extend gigahertz time-domain imaging to a wideband investigation of the eigenstates of a phononic crystal cavity. Using omnidirectionally excited phonon wave vectors, we implement an ultrafast technique to experimentally probe the two-dimensional acoustic field inside and outside a hexagonal cavity in a honeycomb-lattice phononic crystal formed in a microscopic crystalline silicon slab, thereby revealing the confinement and mode volumes of phonon eigenstates-some of which are clearly hexapole in character-lying both inside and outside the phononic-crystal band gap. This allows us to obtain a quantitative measure of the spatial acoustic energy storage characteristics of a phononic crystal cavity.

Refraction, beam splitting and dispersion of GHz surface acoustic waves by a phononic crystal.

We exploit a time-resolved ultrafast optical technique to study the propagation of point-excited surface acoustic waves on a microscopic two-dimensional phononic crystal in the form of a square lattice of holes in a silicon substrate. Constant-frequency images and the dispersion relation are extracted, and the latter measured in detail in the region around the phononic band gap. Mode conversion and refraction at the interface between the phononic crystal and surrounding non-structured silicon substrate is studied at constant frequencies.

Gigahertz Optomechanical Photon-Phonon Transduction between Nanostructure Lines.

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.

Giant extraordinary transmission of acoustic waves through a nanowire.

Wave concentration beyond the diffraction limit by transmission through subwavelength structures has proved to be a milestone in high-resolution imaging. Here, we show that a sound wave incident inside a solid over a diameter of 110 nm can be squeezed through a resonant meta-atom consisting of a nanowire with a diameter of 5 nm equal to λ/23, where λ is the incident acoustic wavelength, corresponding to a transmission efficiency of 500 or an energy densification of ~14,000. This remarkable level of extraordinary acoustic transmission is achieved in the absence of ultrasonic attenuation by connecting a tungsten nanowire between two tungsten blocks, the block on the input side being furnished with concentric grooves.

Imaging gigahertz zero-group-velocity Lamb waves.

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.

Extraordinary transmission of gigahertz surface acoustic waves.

Extraordinary transmission of waves, i.e. a transmission superior to the amount predicted by geometrical considerations of the aperture alone, has to date only been studied in the bulk.

Optical tracking of picosecond coherent phonon pulse focusing inside a sub-micron object.

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.

Imaging arbitrary acoustic whispering-gallery modes in the gigahertz range with ultrashort light pulses.

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.

Broadband evolution of phononic-crystal-waveguide eigenstates in real- and k-spaces.

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.