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This experiment demonstrates the generation of trapped acoustic surface waves excited by a turbulent flow source through the coupling of pressure fluctuations at the interface between an acoustic m...

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.

In this communication an efficient method of moments (MoM) code is used for the analysis of the extraordinary transmission (EOT) through a periodic array of rectangular slots in a conducting screen, in the case where the number of slots is finite in one direction and infinite in the orthogonal direction. The slots can be arbitrarily rotated within the periodic unit cell. Once the magnetic current density on the slots is obtained by means of MoM, both the transmission coefficient and the far-field radiated by the array of slots are computed. The onset of EOT turns out to be strongly dependent on the orientation of the slots with respect to the direction in which the array is infinite. If the slots are perpendicular to this direction, EOT appears for a single infinite chain of slots. However, tens of parallel chains of slots are required to reproduce the EOT response when the slots are aligned along the parallel chains direction. The obtained radiation patterns show the excitation of grating lobes as the number of slots grow in the direction where the arrays are finite. Engineering and Physical Sciences Research Council (EPSRC) U.K EP/L015331/1 Junta de Andalucía P12-TIC-1435 Ministerio de Economía, Industria y Competitividad TEC2017-84724-P

This is the final version. Available from the American Physical Society via the DOI in this record

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.

The superposition of resonances in a subwavelength particle can be used to achieve powerful scattering beyond the single channel limit and can also determine the directionality of scattered radiation. It has been proposed that by overlapping modes only with equivalent polarity in the far-field, a “superbackscattering” condition, where the total backscattered power is maximized, can be achieved. This effect can be observed through the simple geometry of a high permittivity, subwavelength sphere with a hollow core, and we demonstrate this experimentally by comparing the radar cross section (RCS) of such structures, attaining a doubling of the RCS compared to a solid particle. Furthermore, we show that several sets of modes can be overlapped at once, leading to a multiband, superbackscattering effect.

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.

In this work we study planar metasurfaces comprised of two layers of hexagonal arrays of circular metal patches. This two-layer geometry supports a bound surface wave that propagates along the $x$ direction. In our study, each metasurface is infinitely periodic in one direction ($x$) but only a few periods wide in the orthogonal direction ($y$). Through experiments and modeling we find evidence of a localized edge mode whose existence is dependent on the relative alignment of the two layers. This edge mode is used to guide the propagation of electromagnetic energy around both triangular and hexagonal shapes.

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.

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.

The propagation of surface waves supported by a finite array of slots perforated on a zero thickness perfect electrically conducting screen is studied both experimentally and theoretically. To generate numerical results, the integral equation satisfied by the electric field in the slots is efficiently solved by means of Galerkin's method, treating the metal as perfectly conducting. The finite size of the array along the direction of propagation creates a family of states of higher momentum and lower amplitude than the single mode for the corresponding infinite array. These modes are spaced in momentum with a periodicity inversely proportional to the length of the array. In addition, the finite width in the transverse direction produces a set of higher frequency modes due to this additional quantization. Both effects arising from finite sample length and width are explained by the theoretical model and validated experimentally. Engineering and Physical Sciences Research Council EP/L015331/1 Ministerio de Economía y Competitividad TEC2013-41913-P Junta de Andalucía P12-TIC-1435

Surface waves are supported by unstructured, or appropriately structured, elastic materials. Unstructured plates typically support a number of Surface Acoustic Waves (SAWs) that propagate at the boundary of the elastic materials—these can be bound or leaky. Whilst plates with appropriate structure can support bound Acoustic Surface Waves (ASWs) which propagate in the fluid at the fluid/solid boundary. In this talk, we present the underwater acoustic characterisation of both SAWs and ASWs supported by unstructured and structured aluminium plates. First, we measure the dispersion of the symmetric and antisymmetric leaky Lamb modes of an unstructured aluminium-alloy plate using a broadband ultrasound technique. Beaming of acoustic power over a narrow frequency band is observed. This is attributed to the first order symmetric leaky Lamb mode, which exhibits Negative Group Velocity (NGV) in a small region of its dispersion. Finally, we show that a perforated elastic plate supports a bound ASW. We characterize the dispersion of the supported modes using time-domain measurements of the surface wave pressure recorded as a function of position, and observe acoustic beaming of surface energy over a narrow frequency band. All results show close agreement with numerical models.

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.

A hexagonal metallic-wire mesh is fabricated and experimentally characterized to demonstrate graphene-physics in an electromagnetic analogue. In contrast to previous studies, our structure has a smaller ratio of out-of-plane to in-plane dimensions, more akin to real graphene. This allows for the development of a simple analytical treatment using equivalent electric circuit theory, and we demonstrate that the predicted dispersion curves of the supported eigenmodes agree well with those obtained from experimental measurements.

This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) Knowledge Transfer Account programme grant EP/H50012X/1, and by QinetiQ Ltd.

While we are all familiar with liquid crystal displays, an industry currently worth more than $60 billion yr −1 and growing rapidly, fewer people are aware of the breadth of the subject of liquid crystals—one that represents the study of the fourth state of matter. Liquid crystals are found as essential elements in biological systems, soaps and detergents, sensor technologies and in the manipulation of electromagnetic radiation of various wavelengths. This meeting was designed to highlight both the truly multidisciplinary nature of liquid crystal science and to feature those areas away from electro-optic displays; these issues are developed and summarized in more detail.

Excitation of bound surface waves on textured metallic structures can lead to strong resonant absorption of incident radiation at frequencies determined by the surface profile. In the present study however, attention is turned to the role of the surface structure in the enhancement of transmission through a circular, subwavelength-diameter aperture. Undertaking the experiment at microwave wavelengths allows for a precision of manufacture and optimization of the surface structure that would be difficult to replicate at optical frequencies, and demonstrates that transmission enhancement may be achieved with near-perfect metals. Further, the use of a finite element method computational model to study the electromagnetic response of the sample allows for the fields associated with transmission enhancement to be examined, thereby obtaining a better understanding of the role of the surface profile in the enhancement mechanism.

A broadband planar microwave Luneburg lens made of a three-layer metasurface with hexagonal symmetry is presented. This metasurface supports a transverse-electric mode confined to the surface that presents very low dispersion and a stable mode index across a broad frequency band. This enables the lens to operate in the range of frequencies between 8 GHz and 19 GHz. The lens was manufactured using a lithographic technique with three 25 μm copper-coated Mylar sheets. The resulting device is only 75 μm thick, flexible, and light-weight.

In this letter, we demonstrate that the dispersion properties associated with glide symmetry can be achieved in systems that only possess reflection symmetry by balancing the influence of two subla ...

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.

A microwave analogue of graphene comprised of cylindrical metallic rods arranged in a honeycomb array is fabricated. Dispersion curves of the bound electromagnetic eigenmodes of the system were experimentally determined by measuring the electric near-fields just above the surface. Two linear crossings are evident in these dispersion curves at each of the K and K′ points of the Brillouin zone, mimicking the well-celebrated Dirac cones in graphene.

This chapter reviews different techniques for physically investigating the biaxiality of nematic liquid crystal mesogens, including nuclear magnetic resonance (NMR), electron spin resonance (ESR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), Schlieren textures, conoscopic patterns, and electro-optic investigations. Keywords: biaxial nematic liquid crystals; Freederickz transition; cluster model; nuclear magnetic resonance (NMR); electron spin resonance (ESR); X-ray diffraction (XRD); differential scanning calorimetry (DSC); Schlieren textures; conoscopic patterns; electro-optic investigations

It is shown theoretically that a one-dimensional crystal with time-reversal and particle-hole symmetries is characterized by a topological invariant that predicts the existence or otherwise of edge states. This is confirmed experimentally through the construction and simulation of a photonic crystal analog in the microwave regime. It is shown that the edge mode couples to modes external to the photonic crystal via a Fano resonance.

We present a theoretical model of the optical response of a silver film having narrow-grooved zero-order gratings on both sides. It is found that incident p-polarized photons can resonantly tunnel through such a metal film via exciting standing-wave surface plasmon-polariton (SPP) modes localized in the grooves of the two opposite surfaces. This leads to strong transmission peaks in the visible and ultraviolet regions, while for s-polarized photons the film acts as a nearly perfect mirror.

The use of optical excitation of guided modes to study the director profile in thin liquid crystal layers is well established. The most commonly used coupling geometry is that of prism-coupling, the prism providing the required extra in-plane momentum to allow coupling between the external radiation and the guided modes in the liquid crystal layer. A second coupling arrangement involves the use of a grating to provide the extra in-plane momentum by diffraction. Optical modelling of grating structures with liquid crystal layers is by no means as easy as modelling the planar prism-coupled system. In this article we examine both experiments involved with, and the theoretical modelling of, grating coupling to guided modes in liquid crystals.

The grating studied here, produced by interferographic exposure of photoresist, comprises two gratings, one with twice the period of the other, aligned so that their Bragg vectors are separated by an angle of 0.02°. Thus, as one moves along the grooves, these two gratings locally vary their relative phase. Coating this grating with metal allows the optical characterization of this surface by the excitation of surface plasmon polaritons. This paper starts by demonstrating the inability of a profilometer to profile accurately the surface of this bi-grating because of the limitations imposed by the finite width of the stylus on the scale of the grating grooves. Angle-dependent reflectivity data are then used to characterize the metal-coated surface. The size and shape of the surface plasmon resonances in these data, once matched to a rigorous theoretical model, are shown to provide a quantification of the surface profile. This accords with both the profile expected from the limited profilometer stud...

Deep metallic surface relief diffraction gratings are shown to couple radiation to surface plasmon polaritons when transverse magnetic radiation is incident in a plane orthogonal to the grating wave vector. This sharp resonance for a deep sinusoidal surface profile is explored using modeling programs and its presence is confirmed experimentally for a silver grating in the visible part of the spectrum.

An experimental and theoretical modelling study of corrugated gold surfaces, where the refractive index of the upper medium is varied to change the effective wavelength of the incident radiation, is presented. For certain corrugation amplitudes we show that it is possible to convert nearly 100% of plane polarized light incident normal to the average plane of the grating surface into the orthogonal polarization. Investigations of the incident radiation's E -fields immediately above the corrugated surface show that the physical origin of this phenomenon rests with the phase retardation introduced by reflection. It is found that optimum polarization conversion occurs when the component of the optical E -field parallel to the grating wavevector is retarded by 180° with respect to the component parallel to the grooves.

We present experimental evidence for significant levels of surface plasmon-polariton mediated polarization conversion from a blazed surface relief diffraction grating, oriented in a geometry where the in-plane component of the incident wave-vector is normal to the grating Bragg vector. This is a configuration in which no polarization conversion would be expected from a diffraction grating with a profile exhibiting reflection symmetry in the plane normal to its surface. The experimental observations are shown to be consistent with model reflectivities calculated using rigorous conical diffraction theory. We go on to show how the presence of a gap in the surface plasmon-polariton dispersion curve perturbs the observed reflectivities, illustrating the splitting of the surface plasmon-polariton dispersion curve with reciprocal-space maps. Finally we use theoretical modelling codes to explore the effect of changing the blazed grating amplitude on the observed level of polarization mixing, also compari...

A test of a modelling theory for conical optical diffraction in which both real diffracted orders and evanescent fields are present is given. Data in the form of reflectivities from a metallic grating, as a function of angle of incidence for different azimuthal orientations and different radiation polarisations, is completely fitted by one simple model of the surface profile for a fixed complex metal permittivity. The form of the surface found from this fit to optical data is in extremely good agreement with the form found by mechanical profilometry.

We studied the microwave reflectivity of a structured, near perfectly conducting substrate that was designed to verify the existence of a theoretically proposed new class of surface mode. Measurements of the mode's dispersion curve show that it correctly approaches the predicted asymptotic frequency; the curve also agrees well with that derived from a computer simulation. Modeling of the field distribution on resonance provides evidence of strong localization of the electric field at the interface and substantial power flow along the interface, thus verifying the surface plasmon–like nature of the mode.

We present a review of recent advances in the molecular rectifier project. Using a donor - -bridge - acceptor compound, specifically designed to be a molecular rectifier, drastically improved rectifier characteristics were observed compared with a previously published -bridged compound. In particular the increased `rectification ratio' of the -bridged compound was attributed to more effective current blocking under reverse bias. Furthermore, we demonstrate that photodiode-like properties are possessed by films of the zwitterionic, -bridged compound, - , using a transparent electrode construction. Finally, we provide an explanation for the observation that all non-centrosymmetric Langmuir - Blodgett film samples exhibit an exponential current-density/voltage dependence.

Strongly enhanced transmission of microwave radiation (λ0∼5 mm) is observed through a single subwavelength circular aperture of diameter d=2.5 mm in a metallic plate. The phenomenon is caused by resonant excitation of electromagnetic surface waves supported by four concentric grooves surrounding the aperture on the illuminated side of the sample. It is also shown that similar surface patterning on the output face of the sample results in very strong angular confinement (directivity) of the transmitted beam. A finite element code is used to investigate the electromagnetic fields on both the illuminated and the exit side of the structure, the predictions from which show excellent agreement with the experimental results.

The surface plasmon modes supported by a nondiffracting 90° bigrating consisting of three grooves per repeat period with one slightly shallower than the other two are characterized by studying the reflectivity from the structure as a function of the angle of incidence and the incident wavelength (11.3

Surface waves at microwave frequencies are excited on a zigzag metasurface formed from unit cells comprised of a pair of identical, alternately oriented, rhombic-shaped metal tubes. This particular symmetry leads to diffractive excitation, with polarization selectivity of different orders, of a family of surface waves. Furthermore, the expected band gap at the first Brillouin zone boundary is absent.

The electromagnetic transmittance of a double layer of identical square arrays of square holes (mesh) in a perfectly conducting sheet is analytically modeled using a modal matching technique. The structure supports families of standing-wave modes together with surface modes that, close to the onset of diffraction, interact with each other. For frequencies below the onset of diffraction, it is the strength of this interaction mediated by evanescent diffraction in the near fields that dictates the electromagnetic response, which is studied as a function of mesh separation and the lateral misalignment between the meshes.

Remarkably enhanced transmission of microwave radiation through a single subwavelength slit in a thick metallic substrate surrounded by just a pair of parallel deep and narrow grooves is recorded. By also patterning the output face of the metal slab with two grooves there is strong exit beam confinement. There are no gratings in this structure and, hence, the transmission mechanism is not related to the conventional grating coupling of surface plasmons on the upper and lower surfaces of the substrate. Instead, the slit and the four grooves are all resonant, which is the essence of the functioning of the arrangement. The enhancement is due to the collective excitation of the Fabry–Perot mode in the slit and the cavity modes in the grooves. A finite-element modeling code is used to optimize the response of the structure, and to investigate the electromagnetic fields in the vicinity of the substrate.

The nonlinear I/V characteristics of a molecular rectifier structure of the form Au/20 layers of C16H33Q-3CNQ/Au have been explored from 8 to 300 K. At 8 K the voltage-controlled nonlinear conduction is explored in the absence of thermal effects. At the highest voltages (±15 V) at 8 K the rectification ratio was about 4 with current densities as high as 1000 A m−2 and log I varying as |V|0.5, indicating voltage-controlled hopping. The likely explanation for the complete I/V characteristics rests with the insulating aliphatic tails, which provide substantial electrical barriers within the structure.

A nondiffracting metallic lamellar grating formed from three equally spaced grooves per repeat period, with one being slightly shallower than the other two is examined at microwave frequencies. When filled with a slightly lossy dielectric, this structure supports a remarkably nondispersive surface plasmon polariton mode, which exhibits strong selective absorption of incident power. Measured reflectivities show excellent agreement with the results predicted by a rigorous coupled wave theory.

Using the Otto geometry, prism coupling of microwave radiation to a metamaterial surface that supports a bound transverse-electric-polarized surface mode is demonstrated. The dispersion of this surface mode is characterized experimentally for incident radiation beyond the critical angle of the prism, and its resonant fields are explored using a numerical model.

A zwitterionic molecule has been incorporated as a LangmuirBlodgett (LB) film between two electrodes. Current-voltage curves are presented which exhibit rectifier-like behaviour. By placing passive organic layers between the zwitterionic LB film and each electrode we show that Schottky contact and similar interfacial effects are not important and, therefore, that the rectifier-like behaviour is unequivocally associated with the zwitterionic molecular structure.