Transmutation of singularities and zeros in graded index optical instruments: a methodology for designing practical devices.

We describe a design methodology for modifying the refractive index profile of graded-index optical instruments that incorporate singularities or zeros in their refractive index. The process maintains the device performance whilst resulting in graded profiles that are all-dielectric, do not require materials with unrealistic values, and that are impedance matched to the bounding medium. This is achieved by transmuting the singularities (or zeros) using the formalism of transformation optics, but with an additional boundary condition requiring the gradient of the co-ordinate transformation be continuous. This additional boundary condition ensures that the device is impedance matched to the bounding medium when the spatially varying permittivity and permeability profiles are scaled to realizable values. We demonstrate the method in some detail for an Eaton lens, before describing the profiles for an "invisible disc" and "multipole" lenses.This work was supported by the EPSRC (UK) through the QUEST project (ref: EP/1034548/1)

Waveguide-Mode-Enhanced Millimeter-Wave Photomodulators

This is the final version. Available from the American Physical Society via the DOI in this recordThe dataset associated with this article is available in ORE at https://doi.org/10.24378/exe.3823The modulation of millimeter-wave transmission through a silicon wafer upon photoexcitation is typically very inefficient for off-the-shelf low effective charge carrier lifetime wafers, typically requiring tens of kilowatts of photoexciting intensity to generate significant modulation. Here we demonstrate that increasing the light-matter interaction for the millimeter waves using diffractively coupled waveguide modes leads to an increase in photomodulation efficiency of greater than two orders of magnitude, while maintaining the switching speed, of the order of 10μs, of the bare wafer.Engineering and Physical Sciences Research Council (EPSRC)QinetiQ Lt

Electromagnetic interactions in a pair of coupled split-ring resonators

This is the author accepted manuscript. The final version is available from American Physical Society via the DOI in this record.Split-ring Resonators (SRRs) are a fundamental building block of many electromagnetic metamaterials. Typically the response of a metamaterial is assumed to be independent of inter-element interactions in the material. We show that SRRs in close proximity to each other exhibit a rich coupling that involves both electric and magnetic interactions. We study experimentally and computationally the strength and nature of the coupling between two identical SRRs as a function of their separation and relative orientation. We characterise the electric and magnetic couplings and find that, when SRRs are close enough to be in each other's near-field, the electric and magnetic couplings may either reinforce each other or act in opposition. At larger separations retardation effects become important.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)

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

Absence of Anderson localization in certain random lattices

This is the final version of the article. Available from American Physical Society via the DOI in this record.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.J.B. acknowledge support from the Leverhulme Trust's Philip Leverhulme Prize. I.R.H. acknowledges 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)

Absorption modes of Möbius strip resonators

This is the final version. Available on open access from Nature Research via the DOI in this record. 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.Engineering and Physical Sciences Research Council (EPSRC

Metamaterial analogues of strongly coupled molecular ensembles

This is the final version. Available on open access from the American Chemical Society via the DOI in this recordData in support of our findings are available at 10.6084/m9.figshare.16441584.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 f ield 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 complimentary 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 meta-molecules 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 meta-molecules under strong coupling conditions.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC)QinetiQ Ltd

Direct observation of defect modes in molecular aggregate analogs (article)

This is the final version. Available from the American Physical Society via the DOI in this recordThe dataset associated with this article is available in ORE at https://doi.org/10.24378/exe.2843In this work we investigate defect modes localized at the ends and within the bulk of 1D metamaterial analogs of molecular aggregates. The study is undertaken in the microwave regime, where the cm scale of the metamaterial analog provides an opportunity to directly probe the modes deep within their near fields in ways not easily achieved in molecular systems. To demonstrate the power of this approach we compare our observations to predictions from a simple Su, Schrieffer, and Heeger (SSH) model and find good qualitative agreement.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC

Covert Images Using Surface Plasmon-Mediated Optical Polarization Conversion

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordCovert optical signatures are a vital element in anticounterfeiting technologies. Plasmonic surfaces offer a means of manipulating the properties of light including the realization of colored pixels and images. In this work, concealed images with accurate color reproduction using plasmonic pixel arrays are demonstrated. The spectral and spatial control of optical polarization conversion is accomplished by tailoring the interaction of light with surface plasmons through the design and arrangement of surface nanostructures. The latent image is revealed using a polarization-sensitive optical system, which represents a means for the authentication of security features that can be created using these nanostructured devices. A red-green-blue color space is defined containing a wide gamut of chromaticities, enabling comprehensive full-color image capability. The device concept extends the functionality of a polarization-dependent plasmonic response to realize the encoding of a color image in covert form.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

Metamaterial analogues of molecular aggregates

This is the author accepted manuscriptMolecular 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.Engineering and Physical Sciences Research Council (EPSRC)QinetiQ LtdEuropean Research Counci