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.

Metamaterial Analogues of Strongly Coupled Molecular Ensembles.

The formation of polariton modes due to the strong coupling of light and matter has led to exciting developments in physics, chemistry, and materials science. The potential to modify the properties of molecular materials by strongly coupling molecules to a confined light field is so far-reaching and so attractive that a new field known as "polaritonic chemistry" is now emerging. However, the molecular scale of the materials involved makes probing strong coupling at the individual resonator level extremely challenging. Here, we offer a complementary approach based upon metamaterials, an approach that enables us to use cm-scale structures, thereby opening a new way to explore strong coupling phenomena. As proof-of-principle, we show that metamolecules placed inside a radio frequency cavity may exhibit strong coupling and show that near-field radio frequency techniques allow us, for the first time, to probe the response of individual metamolecules under strong coupling conditions.

Absorption modes of Möbius strip resonators.

In this work, the electromagnetic response of a mathematically interesting shape-a Möbius strip-is presented, along with a ring resonator for comparison. Both resonators consist of a central lossy dielectric layer bounded by perfectly conducting layers. For the case of the Möbius strips, the computational results show that there are a family of half-integer wavelength modes within the dielectric layer. These additional modes result in increased absorption, and a corresponding reduction in the radar cross section. Interestingly, rotational scans show that these modes can be excited over a large angular range. This investigation gives an understanding of the electromagnetic response of these structures, paving the way for future experiments on Möbius strip resonators.

Metamaterial Analogues of Molecular Aggregates.

Molecular aggregates are a fascinating and important class of materials, particularly in the context of optical (pigmented) materials. In nature, molecular aggregates are employed in photosynthetic light harvesting structures, while synthetic aggregates are employed in new generation molecular sensors and magnets. The roles of disorder and symmetry are vital in determining the photophysical properties of molecular aggregates, but have been hard to investigate experimentally, owing to a lack of sufficient structural control at the molecular level and the challenge of probing their optical response with molecular spatial resolution. We present a new approach using microwave analogues of molecular aggregates to study the properties of both individual meta-molecules and 1D molecular chains. We successfully replicate J- and H-aggregate behavior and demonstrate the power of our approach through the controlled introduction of structural symmetry breaking. Our results open a new area of study, combining concepts from molecular science and metamaterials.

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.

Absence of Anderson localization in certain random lattices.

We report on the transition between an Anderson localized regime and a conductive regime in a one-dimensional microwave scattering system with correlated disorder. We show experimentally that when long-range correlations are introduced, in the form of a power-law spectral density with power larger than 2, the localization length becomes much bigger than the sample size and the transmission peaks typical of an Anderson localized system merge into a pass band. As other forms of long-range correlations are known to have the opposite effect, i.e., to enhance localization, our results show that care is needed when discussing the effects of correlations, as different kinds of long-range correlations can give rise to very different behavior.

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.

Some considerations on the transmissivity of thin metal films.

As interest in plasmonics grows the optical properties of thin metal films becomes increasingly significant. Here we explore the transmissivity of thin metal films at normal incidence, from the ultraviolet to microwaves, and show how, contrary to simplistic treatments, the microwave transmissivity may be much less than the optical transmissivity for films which are well below the skin depth in thickness. This arises because the film is acting as a zero order Fabry-Perot with very high reflectivity at each interface. The skin depth then becomes irrelevant for thin metal films at microwave frequencies. We also note in passing that the expected exponential dependence on thickness at higher thicknesses has an asymptotic limit at zero thickness which may be as high as four times the input intensity.

Grating-coupled surface plasmon polaritons and waveguide modes in a silver-dielectric-silver structure.

A silver-dielectric-silver structure that supports both waveguide modes and surface plasmon polaritons is explored. The upper interface between the dielectric and the silver is periodically corrugated to allow coupling of visible photons to both types of mode. Such a metallic microcavity leads to plasmonic and waveguide self-interacting bandgaps at Brillouin zone boundaries. In addition there are found other bandgaps from mode crossings within the Brillouin zone. This results specifically in a very flat photonic band due to anticrossings between a surface plasmon polariton and waveguide modes. Characterization of the observed modes in terms of their resonant electromagnetic fields is achieved by using a multilayer, multishape differential grating theory.

Microwave transmission of a compound metal grating.

An array of subwavelength slits in a metallic substrate supports a series of Fabry-Perot-like resonances, where each harmonic results in a transmission peak. Addition of extra slits per period yields a compound grating with a structure factor associated with the basis. In this study each repeat period is comprised of a central slit flanked by a pair of narrower slits. It supports three resonances for every Fabry-Perot-like solution. New and useful insight into this phenomenon is gained by describing each of the modes in terms of the band structure of diffractively coupled surface waves.

Waveguide arrays as plasmonic metamaterials: transmission below cutoff.

Since the work of Ebbesen et al. [Nature (London) 391, 667 (1998)], there has been immense interest in the optical properties of subwavelength holes in metal layers. While the enhanced transmission observed is generally associated with surface plasmon polaritons (SPPs), theoretical predictions suggest a similar response with perfectly conducting materials. However, Pendry et al. [Science 305, 847 (2004)] proposed that, if textured on a subwavelength scale, even perfect conductors support surface modes. Here, using microwave radiation incident upon an array of metal waveguides, we observe peaks in the transmissivity below cutoff and confirm the crucial role of these SPP-like modes in the mechanism responsible.

Surface plasmon polaritons on narrow-ridged short-pitch metal gratings in the conical mount.

Recent investigations into high-aspect-ratio short-pitch metal grating structures have shown that it is possible to excite surface plasmon polaritons (SPPs) even in the zero-order region of the spectrum. The predominant reason this is possible is that extremely large bandgaps occur in the SPP dispersion curves, which are caused by the large depths, and heights, of the structures. The form of the resultant dispersion curves has also been found to be highly dependent on the shape of the grating profile. We present an extension to a previously published paper that described the nature of the SPPs excited on narrow-ridged short-pitch metal gratings in the classical mount by considering the case in which the radiation is incident at nonzero azimuthal angles (the conical mount). In particular, we consider the case of 90 degrees and 45 degrees azimuthal angles and discuss the coupling to the SPP modes and the way in which polarization conversion is evident on such structures.