Unidirectional devices that pass acoustic energy in only one direction have numerous applications and, consequently, have recently received significant attention. However, for most practical applications that require unidirectionality at audio and low frequencies, subwavelength implementations capable of the necessary time-reversal symmetry breaking remain elusive. Here we describe a design approach based on metamaterial techniques that provides highly subwavelength and strongly non-reciprocal devices. We demonstrate this approach by designing and experimentally characterizing a non-reciprocal active acoustic metamaterial unit cell composed of a single piezoelectric membrane augmented by a nonlinear electronic circuit, and sandwiched between Helmholtz cavities tuned to different frequencies. The design is thinner than a tenth of a wavelength, yet it has an isolation factor of >10 dB. The design method generates relatively broadband unidirectional devices and is a good candidate for numerous acoustic applications. Unidirectional acoustic devices only permit the flow of energy one way, but most implementations are large compared to acoustic frequencies. Popa and Cummer use a metamaterial approach to build such devices that are only a tenth of a wavelength thick but retain high acoustic isolation.

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Non-reciprocal and highly nonlinear active acoustic metamaterials.

Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action

Going beyond the canonical cavity-optomechanical system consisting of a Fabry–Perot cavity with a movable end mirror, here we explore a new paradigm in which phononic crystal waveguides are used to wire together local cavity elements to form interacting microcircuits of photons and phonons. Single cavity-waveguide elements, fabricated in the device layer of a silicon-on-insulator microchip, are used to optically excite and detect C-band (∼6 GHz) microwave phonons propagating in phononic-bandgap-guided acoustic waveguides. Interconnecting a pair of optomechanical cavities via a phonon waveguide is then used to demonstrate a tunable delay and filter for microwave-over-optical signals in the 1,500 nm wavelength band. Finally, we realize a tight-binding form of mechanical coupling between distant optomechanical cavities, leading to direct phonon exchange without dissipation in the waveguide. These initial demonstrations indicate the potential of cavity-optomechanical circuits for performing coherent signal processing as well as for realizing new modalities of optical readout in distributed micromechanical sensors.

Optical transduction and routing of microwave phonons in cavity-optomechanical circuits

The possibility of creating and probing a transient nonequilibrium electron population in metals with ultrashort laser pulses has been demonstrated by different groups.1,2 Because of the large electron densities, electron-electron interactions were assumed to be sufficiently fast to instantaneously thermalize the electron gas, although some deviations from Fermi-distribution behavior were observed dose to the Fermi surface.2 Recently, by using a photoemission technique, thermalization times as long as 600 fs were measured in gold films for large changes of the electron temperature (of the order of 400 K).3

Femtosecond investigation of electron thermalization in gold

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.

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

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 ∼1 μm lateral and ∼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.

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Three-dimensional imaging of biological cells with picosecond ultrasonics

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. Zero-group-velocity Lamb waves, which are surface waves with reduced losses and high Q factor, have many potential applications. The authors image such waves in 2 dimensions, and in the GHz range, with a bilayer using a time-resolved imaging technique with an ultra-short-pulse laser.

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Imaging gigahertz zero-group-velocity Lamb waves

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 ∼1ps temporal and ∼100nm spatial resolutions.

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Tomographic reconstruction of picosecond acoustic strain propagation

We 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.

Motonobu Tomoda

Papers

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

Three-dimensional imaging of biological cells with picosecond ultrasonics

Imaging gigahertz zero-group-velocity Lamb waves

Tomographic reconstruction of picosecond acoustic strain propagation

Real-time imaging of acoustic rectification