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

Massively sub-wavelength guiding of electromagnetic waves

Open Access journalRecently a new form of ultra-thin flexible waveguide consisting of a conducting comb-like structure with a thickness of the order of 1/600(th) of the operating wavelength was presented. However, whilst the thickness of the guide was massively sub-wavelength, the remaining dimensions (the height and period of the comb) were much longer. In this paper we propose, and experimentally verify, that a modified guiding geometry consisting of a chain of ultra-thin conducting spirals allows guiding of electromagnetic waves with wavelengths that are many times (40+) longer than any characteristic dimension of the guide, enabling super-sub-wavelength guiding and localisation of electromagnetic energy.Engineering and Physical Sciences Research Council (EPSRC

Polarization conversion from a thin cavity array in the microwave regime

Open Access journalLinearly polarized microwave radiation is shown to have its plane of polarization converted to the orthogonal state upon reflection from an ultrathin (λ/25) cavity array. The structure benefits from an uncomplicated design consisting of a metallic grating closely separated from a ground plane by a dielectric spacer. A single set of periodically spaced slits (monograting) exhibits polarization conversion when the normally incident electric field is aligned at 45° to the slits. Two orthogonal sets of slits (bigrating) allows this narrow-band effect to be broadened when the two orthogonal resonances are separated in frequency. We optimise the design and experimentally demonstrate near loss-less polarization conversion (95% of the incident intensity) across a 3.1 GHz frequency band. Finally, we study the dependence of the structure's performance on incident angle and slit width.DSTLEngineering and Physical Sciences Research Council (EPSRC

The effect of rotational disorder on the microwave transmission of checkerboard metal square arrays

Published onlineJournal ArticleResearch Support, Non-U.S. Gov'tThe effect of rotational disorder on the microwave transmission through thin metallic checkerboard arrays has been experimentally studied. Broad resonant features below the onset of diffraction, attributed to electromagnetic radiation coupling through the structure via the evanescent fields of bound surface waves, are found to be strongly dependent on the electrical connectivity of the surface. By applying rotational disorder to the elements comprising the arrays, with the lattice constant and element size unchanged, the electrical connectivity of the structure can be controlled whilst maintaining periodicity. The results show that rotational disorder can significantly affect transmission only when it changes the structure's connectivity. When the initial structure is just above the connectivity threshold (where the metallic occupancy is 50%), increasing disorder causes the resonant features in transmission to invert as the structure switches from a predominantly connected array to a disconnected array. When approximately half of the connections are broken, the resonant features are suppressed, with scattering loss shown to dramatically increase to as much as 40% of the incident power over a broad frequency range. The result is a thin, highly effective scatterer of microwaves.The authors acknowledge the financial support of DSTL. APH and JRS also acknowledge the support of EPSRC through the QUEST programme grant

A ferrite-filled cavity resonator for electronic article surveillance on metallic packaging (article)

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordThe dataset associated with this article is located in ORE at: https://doi.org/10.24378/exe.1924Conventional electronic article surveillance (EAS) tags are ineffective on metallic packaging. The component of RF magnetic field perpendicular to the surface of the packaging induces eddy currents that suppress the magnetic flux linking the inductive element of the tag. In this work an inductive quarter-wavelength planar cavity, formed by wrapping aluminum foil around a ferrite core, was extended by wrapping additional capacitive layers of foil/dielectric around the ferrite-filled central region. This so-called ‘wrapped tag’ exhibits the frequency, Q-factor, and read distance characteristics of existing EAS tags, but is instead driven by RF magnetic fields parallel to the surface of the metallic packaging. In this article we compare the observed frequency response of the wrapped tag with a simple LC-resonator model that takes account of the tag’s geometrical features, and use the model to describe how the design and construction of the tag can be optimized. Finite element method modeling is used to reveal how the current flows in the wrapped foil of the tag. Prototype tags show good reproducibility, demonstrating the potential of the design as a solution to the problem of tagging metallic packaging in the EAS industry.Engineering and Physical Sciences Research Council (EPSRC)QinetiQ Ltd

Experimental observation of photonic nodal line degeneracies in metacrystals

This is the final version of the article. Available from the publisher via the DOI in this record.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.This work was financially supported by the European Research Council Consolidator Grant (TOPOLOGICAL). S.Z. acknowledges support from the Royal Society, the Wolfson Foundation, Horizon 2020 Action Project No. 734578 (D-SPA), the Leverhulme Trust (RPG-2012-674), and the Engineering and Physical Sciences Research Council (EP/ J018473/1). B.Y. acknowledges support from the China Scholarship Council (201306110041). A.P.H. acknowledges financial support from EPSRC of the United Kingdom (Grant No. EP/L015331/1). Near-field scanning data were collected by VNA controlled with xyz-stage at G31 at the Department of Physics and Astronomy, University of Exeter, United Kingdo

Conformally coupled dark matter

Dark matter is obtained from a scalar field coupled conformally to gravitation; the scalar being a relict of Dirac's gauge function. This conformally coupled dark matter includes a gas of very light ($m\approx 2.25\times 10^{-34} eV$) neutral bosons having spin 0, as well as a time-dependent global scalar field, both pervading all of the cosmic space. The time-development of this dark matter in the expanding F-R-W universe is investigated, and an acceptable cosmological behaviour is obtained.Comment: LaTEX File 10 pages, no figure

Ideal Weyl points and helicoid surface states in artificial photonic crystal structures (article)

The dataset associated with this article is located in ORE at: http://hdl.handle.net/10871/30744This is the author accepted manuscript. The final version is available from AAAS via the DOI in this record.Weyl points are the crossing points of linearly dispersing energy bands in the Brillouin zone of three-dimensional crystals. Weyl points provide the opportunity to explore a variety of intriguing phenomena such as topologically protected surface states and chiral anomalies. However the lack of an ideal Weyl system poses a serious limitation to the further development of Weyl physics and potential applications. Here, by experimentally characterizing a microwave photonic crystal comprised of a three dimensional array of saddle-shaped metallic coils, we observe ideal Weyl points which are related to each other through symmetry operations. Topological surface states exhibiting helicoidal structure in the energy-momentum space have also been demonstrated, which serve as a direct manifestation of the chiral nature of the Weyl points.This work was financially supported by ERC Consolidator Grant (Topological) and Leverhulme Trust (RPG-2012-674). S. Z. acknowledges support from the Royal Society and Wolfson Foundation. B. Y. acknowledges support from China Scholarship Council (201306110041). Y. X. acknowledges support from the National Natural Science Foundation of China (Grant No. 61490713). L. E. B. and A. P. H. acknowledge financial support from EPSRC of the United Kingdom (Grant No. EP/L015331/1). C.F. was supported by the National Key Research and Development Program of China under grant No. 2016YFA0302400 and by NSFC under grant No. 11674370. L.L. was supported by the National key R&D Program of China under Grant No. 2017YFA0303800, 2016YFA0302400 and by NSFC under Project No. 11721404

Radiatively Induced Neutrino Masses and Large Higgs-Neutrino Couplings in the Standard Model with Majorana Fields

The Higgs sector of the Standard Model with one right-handed neutrino per family is systematically analyzed. In a model with intergenerational independent mixings between families, we can account for very light neutrinos acquiring Majorana masses radiatively at the first electroweak loop level. We also find that in such a scenario the Higgs coupling to the light-heavy neutrinos and to the heavy-heavy ones may be remarkably enhanced with significant implications for the production of these heavy neutrinos at high energy colliders.Comment: Making the text of an old paper electronically availabl

Are antenatal interventions effective in improving multiple health behaviours among pregnant women? A systematic review protocol.

BACKGROUND: Maternal behaviours in pregnancy associated with adverse pregnancy, birth and health outcomes include tobacco smoking, poor nutrition, alcohol consumption and low physical activity, collectively referred to as the SNAP risk factors. Due to the high prevalence, co-occurrence and possible interactive health effects of such health behaviours in pregnancy, antenatal interventions that support pregnant women to improve multiple SNAP behaviours have a greater potential impact on the health outcomes of women and their children than interventions addressing single behaviours. The objective of this review is to determine the effectiveness of interventions delivered as part of antenatal care that aim to improve multiple SNAP behaviours among pregnant women. METHODS: Seven electronic databases will be searched for potentially eligible studies. Eligible studies will include those where pregnant women are attending antenatal care. Studies that examine the effect of an intervention that addresses multiple SNAP behaviours (≥ 2 behaviours) during pregnancy and are delivered or instigated through antenatal care in a healthcare service will be included. Systematic reviews of randomised controlled trials (RCTs), RCTs, cluster RCTs, stepped-wedge RCTs and non-randomised control trials will be eligible. Studies that include a no-intervention control, wait-list control group, standard/usual care, or another active single behavioural intervention (e.g. addressing one behaviour only) will be considered. Two independent reviewers will conduct study screening, data extraction and risk of bias assessment. Discrepancies will be resolved by consensus or a third reviewer if required. A random effects model will be used to synthesise the results. Alternative synthesis methods will be investigated in instances where a meta-analysis is not appropriate, such as summarising effect estimates, combining P values, vote counting based on direction of effect, or synthesis in narrative form. DISCUSSION: The review will synthesise the evidence on the effect of interventions that address multiple SNAP behaviours in antenatal care and will help researchers, policy-makers and health services to develop and deliver best practice integrated models of antenatal care that have the potential to impact on both the short- and long-term health outcomes for women and their children. SYSTEMATIC REVIEW REGISTRATION: PROSPERO CRD42018095315