Johan Christensen

All-angle blockage of sound by an acoustic double-fishnet metamaterial

In this paper we design an acoustic metamaterial for broadband sound blockage that is easy to fabricate and presents tunable capabilities. Two adjacent holey plates are predicted to support a gap mode which is responsible of a forbidden band, displaying a negative effective bulk modulus. This acoustic metamaterial exhibits a weak dispersion with parallel momentum implying that strong attenuation appears for a broad range of angles of incident sound. Its bandwidth can be tailored at will by varying the separation between the two holey plates.

Confining and slowing airborne sound with a corrugated metawire

In this letter we present a theoretical study on the acoustic wave propagation along a periodically corrugated perfect rigid wire surrounded by air. It is shown how acoustic surface waves (ASWs) can be engineered with their propagation properties controlled by geometrical means. These highly localized ASWs give rise to strong acoustical field confinement along the wire, whereas the slowing down of sound decelerate the group velocity down to zero. What is believed to be a promising feature of these low-loss propagation properties is the ability to tune sensing and screening applications with good transducer coupling.

Parity-Time Synthetic Phononic Media.

In this presentation we show how active acoustic media can be engineered and tuned to work as a Parity-time (PT) synthetic phononic system. We demonstrate a feasible approach for the case of sound where the most important ingredients within synthetic materials, loss and gain, are achieved through electrically biased piezoelectric semiconductors. With colorful examples we demonstrate the rich physics involved and the broad landscape for applications associated with the PT properties.

Perfect imaging, epsilon-near zero phenomena and waveguiding in the scope of nonlocal effects

Plasmons in metals can oscillate on a sub-wavelength length scale and this large-k response constitutes an inherent prerequisite for fascinating effects such as perfect imaging and intriguing wave phenomena associated with the epsilon-near-zero (ENZ) regime. While there is no upper cut-off within the local-response approximation (LRA) of the plasma polarization, nonlocal dynamics suppress response beyond ω/vF, where vF is the Fermi velocity of the electron gas. Nonlocal response has previously been found to pose limitations to field-enhancement phenomena. Accounting for nonlocal hydrodynamic response, we show that perfect imaging is surprisingly only marginally affected by nonlocal properties of a metal slab, even for a deep subwavelength case and an extremely thin film. Similarly, for the ENZ response we find no indications of nonlocal response jeopardizing the basic behaviors anticipated from the LRA. Finally, our study of waveguiding of gap plasmons even shows a positive nonlocal influence on the propagation length.

Extraordinary optical transmission through nonlocal holey metal films

We investigate nonlocal electrodynamics based on the generalized hydrodynamic approach including electron diffusion in holey gold films, showing extraordinary optical transmission (EOT). Dramatic changes with respect to the local approximation for rather large film thicknesses t ≲ 100  nm impact both reflectance and absorbance at normal incidence. Beyond the familiar resonance blueshift with the decreasing film thickness, the interference of longitudinal pressure waves in the holey structure generates an unexpected oscillatory response with geometrical parameters. An increase in the transmittance relative to the local result of up to 33% (typically up to ± 6 % at resonance) was observed, but it can also be suppressed when absorption dominates. We expect that our findings will stimulate additional studies of EOT when taking into consideration nonlocal effects in holey films of experimentally accessible parameters in two-dimensional hole arrays.

Acoustic wave propagation and stochastic effects in metamaterial absorbers

We show how stochastic variations of the effective parameters of anisotropic structured metamaterials can lead to increased absorption of sound. For this, we derive an analytical model based on the Bourret approximation and illustrate the immediate connection between material disorder and attenuation of the averaged field. We demonstrate numerically that broadband absorption persists at oblique irradiation and that the influence of red noise comprising short spatial correlation lengths increases the absorption beyond what can be archived with a structured but ordered system.

Theory of resonant acoustic transmission through subwavelength apertures

A complete landscape is presented of the acoustic transmission properties of subwavelength apertures (slits and holes). First, we study the emergence of Fabry-Perot resonances in single apertures. When these apertures are placed in a periodic fashion, a new type of transmission resonance appears in the spectrum. We demonstrate that this resonance stems from the excitation of an acoustic guided wave that runs along the plate, which hybridizes strongly with the Fabry-Perot resonances associated with waveguide modes in single apertures. A detailed discussion of the similarities and differences with the electromagnetic case is also given.

Modelling the acoustical response of lossy lamella-crystals

The sound propagation properties of lossy lamella-crystals are analysed theoretically utilizing a rigorous wave expansion formalism and an effective medium approach. We investigate both supported and free-standing crystal slab structures and predict high absorption for a broad range of frequencies. A detailed derivation of the formalism is presented, and we show how the results obtained in the subwavelength and superwavelength regimes qualitatively can be reproduced by homogenizing the lamella-crystals. We come to the conclusion that treating this structure within the metamaterial limit only makes sense if the crystal filling fraction is sufficiently large to satisfy an effective medium approach.

Coalescence towards exceptional contours in synthetic phononic media

Parity-time symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create non-reciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where gain and loss are induced via the acousto-electric effect in electrically biased piezoelectric semiconductors. We study how wave attenuation and amplification can be tuned, and when combined, can give rise to phononic synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detections.

Minimal model for spoof acoustoelastic surface states

Similar to textured perfect electric conductors for electromagnetic waves sustaining artificial or spoof surface plasmons we present an equivalent phenomena for the case of sound. Aided by a minimal model that is able to capture the complex wave interaction of elastic cavity modes and airborne sound radiation in perfect rigid panels, we construct designer acoustoelastic surface waves that are entirely controlled by the geometrical environment. Comparisons to results obtained by full-wave simulations confirm the feasibility of the model and we demonstrate illustrative examples such as resonant transmissions and waveguiding to show a few examples of many where spoof elastic surface waves are useful.