Underwater acoustic surface waves on a periodically perforated metal plate.

Acoustic surface waves are supported at the surface of appropriately structured elastic materials. Here the excitation and propagation of the lowest-order surface mode supported by a square array of open-ended cavities on a metal plate submerged in water is demonstrated. This mode, which has a half-wavelength character in the cavity, arises due to inter-cavity interaction by evanescent diffraction of the pressure field, and forms a band from zero-frequency to an asymptotic limit frequency. The authors perform an acoustic characterization of the pressure field close to the surface of the perforated plate in the 60-100 kHz frequency range; sound is pulsed from a fixed point-like acoustic source, and the evolution of the acoustic field across the sample surface is measured as a function of time and space with a traversing detector. Using Fourier analysis, the dispersion is imaged between points of high-symmetry (Γ,X,M) and at planes in momentum-space at fixed frequencies. Beaming of acoustic energy on the surface over a narrow frequency band was observed, caused by the anisotropic mode dispersion of the acoustic surface wave on the square lattice. The measured dispersion shows good agreement with the predictions of a numerical model.

On the extraordinary optical transmission in parallel plate waveguides for non-TEM modes.

Extraordinary transmission has been recently measured in a parallel plate waveguide (PPWG) through a metal strip with a patterned 1-D periodic array of circular holes, the metal strip being embedded inside the PPWG. Wood's anomaly and the extraordinary transmission peak (EOT) were detected for transverse electric (TE) mode excitation at frequencies higher than those found for TEM mode excitation. In this paper we provide an explanation for this frequency shift by decomposing the problem of a TE mode impinging on the 1-D array of holes into two problems of plane waves impinging obliquely on 2-D periodic arrays of holes. By then solving the integral equation for the electric field on the surface of the holes, the origin of the frequency shift is proved both mathematically and physically in terms of the symmetries present in the system.

Designer surface plasmon dispersion on a one-dimensional periodic slot metasurface with glide symmetry.

In this Letter, we explore the dispersion of spoof surface plasmons supported by a single-layer glide-symmetric structure. This structure consists of an infinitely long double-notched slot perforated in a metal layer. The presence of a degeneracy of the two lowest-order modes at the Brillouin zone boundary, which have non-zero group velocity is explained and experimentally demonstrated. Further, the dependence of the band structure when glide-symmetric configuration is broken is also explored.

Demonstration and Control of "Spoof-Plasmon" Scattering from 3D Spherical Metaparticles.

Geometries that replicate the behavior of metal nanostructures at much lower frequencies via texturing surfaces so they will support a surface wave have been a central pillar of metamaterials research. However, previous work has focused largely on geometries that can be reduced to symmetries in one or two dimensions, such as strips, flat planes, and cylinders. Shapes with isotropic responses in three dimensions are important for applications, such as radar scattering and the replication of certain nanoscale behaviors. This work presents a detailed exploration of the scattering behavior of 3D spherical "spoof plasmonic" metaparticles, based on the platonic solids. Their behavior is compared to an effective medium model through simulation and experiment, and the vast range of behaviors that can be produced from a metal sphere of a given radius via tuning its internal structure is explored in detail.

Control of the stop band of an acoustic double fishnet.

The acoustic transmittance of two closely spaced solid plates, each perforated with a square array of cylindrical holes, exhibits a band of near-perfect acoustic attenuation originating from hybridization between a resonance in the gap separating the plates and pipe resonances in the holes. Displacement of one plate relative to the other, such that the holes are no longer aligned, or an increase in the plate separation leads to an increased center frequency of the stop band. This ability to easily tune the frequency of the stop band may prove advantageous.

Multi-resonant tessellated anchor-based metasurfaces.

In this work, a multi-resonant metasurface that can be tailored to absorb microwaves at one or more frequencies is explored. Surface shapes based on an 'anchor' motif, incorporating hexagonal, square and triangular-shaped resonant elements, are shown to be readily tailorable to provide a targeted range of microwave responses. A metasurface consisting of an etched copper layer, spaced above a ground plane by a thin (< 1/10th of a wavelength) low-loss dielectric is experimentally characterised. The fundamental resonances of each shaped element are exhibited at 4.1 GHz (triangular), 6.1 GHz (square) and 10.1 GHz (hexagonal), providing the potential for single- and multi-frequency absorption across a range that is of interest to the food industry. Reflectivity measurements of the metasurface demonstrate that the three fundamental absorption modes are largely independent of incident polarization as well as both azimuthal and elevation angles.

Light localization, photon sorting, and enhanced absorption in subwavelength cavity arrays.

A periodically patterned metal-dielectric composite material is designed, fabricated and characterized that spatially splits incoming microwave radiation into two spectral ranges, individually channeling the separate spectral bands to different cavities within each spatially repeating unit cell. Further, the target spectral bands are absorbed within each associated set of cavities. The photon sorting mechanism, the design methodology, and experimental methods used are all described in detail. A spectral splitting efficiency of 93-96% and absorption of 91-92% at the two spectral bands is obtained for the structure. This corresponds to an absorption enhancement over 600% as compared to the absorption in the same thickness of absorbing material. Methods to apply these concepts to other spectral bands are also described.

Surface plasmons on zig-zag gratings.

Optical excitation of surface plasmons polaritons (SPPs) on a 'zig-zag diffraction grating' is explored. The fabricated silver grating consists of sub-wavelength grooves 'zig-zagged' along their length, providing a diffractive periodicity to visible radiation. SPPs propagating in the diffraction plane and scattered by an odd number of grating vectors are only excited by TE polarized radiation, whereas for TM polarized light, which also induces surface charge, SPP excitation is forbidden by the grating's broken-mirror symmetry.

Slow waves on long helices.

Slowing light in a non-dispersive and controllable fashion opens the door to many new phenomena in photonics. As such, many schemes have been put forward to decrease the velocity of light, most of which are limited in bandwidth or incur high losses. In this paper we show that a long metallic helix supports a low-loss, broadband slow wave with a mode index that can be controlled via geometrical design. For one particular geometry, we characterise the dispersion of the mode, finding a relatively constant mode index of [Formula: see text] 45 between 10 and 30 GHz. We compare our experimental results to both a geometrical model and full numerical simulation to quantify and understand the limitations in bandwidth. We find that the bandwidth of the region of linear dispersion is associated with the degree of hybridisation between the fields of a helical mode that travels around the helical wire and an axial mode that disperses along the light line. Finally, we discuss approaches to broaden the frequency range of near-constant mode index: we find that placing a straight wire along the axis of the helix suppresses the interaction between the axial and high index modes supported by the helix, leading to both an increase in bandwidth and a more linear dispersion.

Confined acoustic line modes within a glide-symmetric waveguide.

Confined coupled acoustic line-modes supported by two parallel lines of periodic holes on opposite surfaces of a glide-symmetric waveguide have a hybrid character combining symmetric and anti-symmetric properties. These hybrid coupled acoustic line-modes have a near constant group velocity over a broad frequency range as no band gap is formed at the first Brillouin zone boundary. We show that the hybrid character of these confined modes is tuneable as a function of the spacing between the two surfaces. Further we explore how the band-gap reappears as the glide symmetry is broken.

Multi-modal transmission of microwaves through hole arrays.

The microwave transmission through hole arrays in thick metal plates for both large holes (cut-off below onset of diffraction) and small holes (cut-off above onset of diffraction) have been compared through both experiment and modelling. Enhanced transmission is in part mediated by the excitation of diffractively coupled surface waves. Large holes, with cut-off below the onset of diffraction (due to the hole periodicity), are able to support multiple modes in transmission when the depth of the holes is sufficient to support quantisation in the propagation direction. Small holes, with cut-off above the onset of diffraction however only support two coupled surface modes (symmetric and anti-symmetric) below diffraction.

Dark Mode Excitation in Three-Dimensional Interlaced Metallic Meshes.

Interlaced metallic meshes form a class of three-dimensional metamaterials that exhibit nondispersive, broadband modes at low frequencies, without the low frequency cutoff typical of generic wire grid geometries. However, the experimental observation of these modes has remained an open challenge, both due to the difficulties in fabricating such complex structures and also because the broadband mode is longitudinal and does not couple to free-space radiation (dark mode). Here we report the first experimental observation of the low frequency modes in a block of interlaced meshes fabricated through 3D printing. We demonstrate how the addition of monopole antennas to opposing faces of one of the meshes enables coupling of a plane wave to the low frequency "dark mode" and use this to obtain the dispersion of the mode. In addition, we utilize orthogonal antennas on opposite faces to achieve polarization rotation as well as phase shifting of radiation passing through the structure. Our work paves the way toward further experimental study into interlaced meshes and other complex 3D metamaterials.

Microwave transmission through a metal capped array of holes in a metal sheet.

The microwave response of a square array of "metal capped" holes in a metal sheet is explored both experimentally and numerically. Above each circular aperture are concentrically placed metallic discs, separated by a fraction of the wavelength, with discs having radii larger than the apertures. The volume bound by the overlap supports a family of resonances that mediate transmission with the fundamental resonant mode being a factor of ~2.3 lower in frequency than the bare aperture resonance.

Circuit modeling of the transmissivity of stacked two-dimensional metallic meshes.

This paper presents a simple analytical circuit-like model to study the transmission of electromagnetic waves through stacked two-dimensional (2-D) conducting meshes. When possible the application of this methodology is very convenient since it provides a straightforward rationale to understand the physical mechanisms behind measured and computed transmission spectra of complex geometries. Also, the disposal of closed-form expressions for the circuit parameters makes the computation effort required by this approach almost negligible. The model is tested by proper comparison with previously obtained numerical and experimental results. The experimental results are explained in terms of the behavior of a finite number of strongly coupled Fabry-Pérot resonators. The number of transmission peaks within a transmission band is equal to the number of resonators. The approximate resonance frequencies of the first and last transmission peaks are obtained from the analysis of an infinite structure of periodically stacked resonators, along with the analytical expressions for the lower and upper limits of the pass-band based on the circuit model.

Coupling and confinement of current in thermoacoustic phased arrays.

When a medium is rapidly heated and cooled, heat transfers to its surroundings as sound. A controllable source of this sound is realized through joule heating of thin, conductive films by an alternating current. Here, we show that arrays of these sources generate sound unique to this mechanism. From the sound alone, we spatially resolve current flow by varying the film geometry and electrical phase. Confinement concentrates heat to such a degree that the film properties become largely irrelevant. Electrical coupling between sources creates its own distinctive sound that depends on the current flow direction, making it unusually sensitive to the interactions of multiple currents sharing the same space. By controlling the flow, a full phased array can be created from just a single film.

Experimental characterisation of the bound acoustic surface modes supported by honeycomb and hexagonal hole arrays.

The Dirac point and associated linear dispersion exhibited in the band structure of bound (non-radiative) acoustic surface modes supported on a honeycomb array of holes is explored. An aluminium plate with a honeycomb lattice of periodic sub-wavelength perforations is characterised by local pressure field measurements above the sample surface to obtain the full band-structure of bound modes. The local pressure fields of the bound modes at the K and M symmetry points are imaged, and the losses at frequencies near the Dirac frequency are shown to increase monotonically as the mode travels through the K point at the Dirac frequency on the honeycomb lattice. Results are contrasted with those from a simple hexagonal array of similar holes, and both experimentally obtained dispersion relations are shown to agree well with the predictions of a numerical model.

Isotropic Backward Waves Supported by a Spiral Array Metasurface.

A planar metallic metasurface formed of spiral elements is shown to support an isotropic backward wave over a narrow band of microwave frequencies. The magnetic field of this left-handed mode is mapped experimentally using a near-field scanning technique, allowing the anti-parallel group and phase velocities to be directly visualised. The corresponding dispersion relation and isofrequency contours are obtained through Fourier transformation of the field images.

The acoustic phase resonances and surface waves supported by a compound rigid grating.

We study the radiative and bound acoustic modes supported by a rigid grating formed of three same-depth, narrow grooves per unit cell. One of the grooves is twice the width of the other two, forming a 'compound' grating. The structure supports so-called 'phase' resonances where the phase difference of the pressure field between the grooves on resonance varies by multiples of π. We explore the dispersion of these modes experimentally by monitoring the specularly reflected signal as a function of the angle of incidence. In addition, by near-field excitation, the dispersion of the non-radiative surface modes has been characterised. Our results are compared with the predictions of a finite element method model.

Experimental observation of photonic nodal line degeneracies in metacrystals.

Nodal line semimetals (NLS) are three-dimensional (3D) crystals that support band crossings in the form of one-dimensional rings in the Brillouin zone. In the presence of spin-orbit coupling or lowered crystal symmetry, NLS may transform into Dirac semimetals, Weyl semimetals, or 3D topological insulators. In the photonics context, despite the realization of topological phases, such as Chern insulators, topological insulators, Weyl, and Dirac degeneracies, no experimental demonstration of photonic nodal lines (NLs) has been reported so far. Here, we experimentally demonstrate NL degeneracies in microwave cut-wire metacrystals with engineered negative bulk plasma dispersion. Both the bulk and surface states of the NL metamaterial are observed through spatial Fourier transformations of the scanned near-field distributions. Furthermore, we theoretically show that the NL degeneracy can transform into two Weyl points when gyroelectric materials are incorporated into the metacrystal design. Our findings may inspire further advances in topological photonics.

Strong beaming of microwave surface waves with complementary split-ring-resonator arrays.

A thin copper sheet, populated by an array of complementary split ring resonators, presents strong surface wave beaming in orthogonal directions at two distinct frequencies. This simple array is significantly thinner than existing single frequency beaming surfaces. The observed beaming frequencies are associated with the two lowest resonance modes of the split rings, and the beams are subwavelength in width and approximately non-diverging. The beaming is analysed through comparison of near-field scans of the surface-normal electric fields with numerical simulations.