Superdirective Antennas of Coupled Helical Elements

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordUsing magnetically-coupled structures formed of subwavelength metallic helices, we demonstrate superdirective end-fire radiation in the low GHz frequency range. Numerical, experimental and analytical results are presented on superdirective dimers that are almost three times smaller compared to previously demonstrated dimers of split-ring-resonators (0.09λ compared with 0.25λ). Optimisation of such structures in terms of their size, directivity, efficiency and operational passband is demonstrated.Engineering and Physical Sciences Research Council (EPSRC

Microwave edge modes on a metasurface with glide symmetry

This is the author accepted manuscript. The final version is available from the American Physical Society via the DOI in this record.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.Engineering and Physical Sciences Research Council (EPSRC

Acoustic transmission through compound subwavelength slit arrays

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.The angular dependence of the transmission of sound in air through four types of 2D slit-arrays formed of aluminium slats is explored, both experimentally and numerically. For a simple, subwavelength periodic slit-array, it is well known that Fabry-Perot-like wave-guide resonances, supported by the slit-cavities, hybridising with bound acoustic surface waves, result in ‘Enhanced Acoustic Transmission’ at frequencies determined by the length, width and separation of each slit-cavity. We demonstrate that altering the spacing or width of some of the slits to form a compound array (i.e. an array having a basis comprised of more than one slit) results in sharp dips in the transmission spectra, that may have a strong angular dependence. These features correspond to ‘phase resonances’, which have been studied extensively in the electromagnetic case. This geometry allows for additional near-field configurations compared to the simple array, whereby the field in adjacent cavities can be out-of-phase. Several types of compound slit-array are investigated; one such structure is optimised to minimise the effect of boundary-layer loss mechanisms present in each slit cavity, thereby achieving a deep, sharp transmission minimum in a broad maximumThe authors would like to thank the UK Ministry of Defence’s Defence Science and Technology Laboratorty (DSTL) for their financial support and permission to publis

Isotropic Backward Waves Supported by a Spiral Array Metasurface.

This is the final version of the article. Available from Nature Publishing Group via the DOI in this record.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

A broadband metasurface Luneburg lens for microwave surface waves

This is the author accepted manuscript. The final version is available from AIP Publishing via the DOI in this recordA 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.We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). J.P.G. wishes to thank Flann Microwave Ltd (Bodmin, UK) for additional financial support of her studentshi

The waveguiding of sound using lines of resonant holes

This is the final version. Available on open access from Nature Research via the DOI in this recordThe dispersion of an acoustic surface wave supported by a line of regularly spaced, open ended holes in an acrylic plate, is characterised by precise measurement of its localised acoustic fields. We illustrate the robust character of this surface wave and show its potential for control of sound by the acoustic waveguiding provided by a ring of regularly spaced holes.Defence Science and Technology Laboratory (DSTL

Wiener-Hopf analysis of the scattering by a two dimensional periodic semi-infinite array of dipoles

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordWe present a rigorous solution of the scattering of plane waves by a truncated planar array of dipoles which is infinite and periodic in one direction and semi-infinite in the orthogonal direction, thus presenting an edge truncation. By applying the Wiener-Hopf technique to the Z-transformed system of equations derived from the electric field integral equation, the contributions to the current on the dipoles due to the scattering by the edge of the array and the excitation of surface waves are obtained rigorously.Engineering and Physical Sciences Research Council (EPSRC

Theoretical and experimental exploration of finite sample size effects on the propagation of surface waves supported by slot arrays (article)

This is the final version of the article. Available from American Physical Society via the DOI in this record.The dataset associated with this article is in ORE at: http://hdl.handle.net/10871/28223The 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 experimentallyThe authors wish to acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). R.R.B. and F.M. acknowledge financial support from the Spanish MINECO (Grant No. TEC2013-41913-P) and the Spanish Junta de Andalucía (Grant No. P12-TIC-1435)

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

This is the final version. Available from Nature Research via the DOI in this record. 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.Defence Science and Technology Laboratory (DSTL)Engineering and Physical Sciences Research Council (EPSRC

Theoretical and experimental exploration of finite sample size effects on the propagation of surface waves supported by slot arrays (article)

This is the final version of the article. Available from American Physical Society via the DOI in this record.The dataset associated with this article is in ORE at: http://hdl.handle.net/10871/28223The 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 experimentallyThe authors wish to acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). R.R.B. and F.M. acknowledge financial support from the Spanish MINECO (Grant No. TEC2013-41913-P) and the Spanish Junta de Andalucía (Grant No. P12-TIC-1435)