Ian J. Youngs

Dielectric relaxation in metal-coated particles: the dramatic role of nano-scale coatings

Insulating materials filled with conducting particles permit tailoring of electrical, electromagnetic and thermal properties of the resulting composite. When the filler particles are small and metallic, a dielectric relaxation due to interfacial polarization is commonly observed at optical or smaller wavelengths. Here, experimental results are presented in which the dielectric relaxation is shifted to microwave frequencies as a result of using metal-coated dielectric particles with a nano-scale coating thickness. The results are analysed in the context of effective medium theory adapted for multi-layer particles. Such a large shift in relaxation frequency, compared with that for a similar composite with solid metal filler particles, is shown to be a function of both the coating geometry and a thin-film-related reduction in the conductivity of the metal. The observed broadening of the relaxation peak is attributed to non-uniformity of the coating thickness and a consequent distribution of coating conductivity.

A magneto-electro-optical effect in a plasmonic nanowire material

Electro- and magneto-optical phenomena play key roles in photonic technology enabling light modulators, optical data storage, sensors and numerous spectroscopic techniques. Optical effects, linear and quadratic in external electric and magnetic field are widely known and comprehensively studied. However, optical phenomena that depend on the simultaneous application of external electric and magnetic fields in conventional media are barely detectable and technologically insignificant. Here we report that a large reciprocal magneto-electro-optical effect can be observed in metamaterials. In an artificial chevron nanowire structure fabricated on an elastic nano-membrane, the Lorentz force drives reversible transmission changes on application of a fraction of a volt when the structure is placed in a fraction-of-tesla magnetic field. We show that magneto-electro-optical modulation can be driven to hundreds of thousands of cycles per second promising applications in magneto-electro-optical modulators and field sensors at nano-tesla levels.

A Coordinate Transformation-Based Broadband Flat Lens via Microstrip Array

A conventional convex lens is compressed into a flat one based on a so-called discrete coordinate transformation technique. While maintaining a good performance of the original lens, such a transformed flat lens only requires a few blocks of isotropic dielectrics. Physical realization via multiple transmission lines is then demonstrated, and it shows an alternative approach to achieve the desired spatially varying dielectric constant across the lens aperture. The full-wave simulation shows that the proposed microstrip array mimics the original convex lens nicely and can function well over a broad frequency band while possessing the merits of a flat profile and small volume.

Toroidal circular dichroism

Optical activity is ubiquitous across natural and artificial media and is conventionally understood in terms of scattering from electric and magnetic moments. Here we demonstrate experimentally and confirm numerically a type of optical activity that cannot be attributed to electric and magnetic multipoles. We show that our observations can only be accounted for by the inclusion of the toroidal dipole moment, the first term of the recently established peculiar family of toroidal multipoles.

Reconfiguring photonic metamaterials with currents and magnetic fields

We demonstrate that spatial arrangement and optical properties of metamaterial nanostructures can be controlled dynamically using currents and magnetic fields. Mechanical deformation of metamaterial arrays is driven by both resistive heating of bimorph nanostructures and the Lorentz force that acts on charges moving in a magnetic field. With electrically controlled transmission changes of up to 50% at sub-mW power levels, our approaches offer high contrast solutions for dynamic control of metamaterial functionalities in optoelectronic devices.

The electromagnetic properties of nanoparticle colloids at radio and microwave frequencies

The aim of this study is to investigate the electromagnetic properties of nanoparticle colloids in the frequency range 1 MHz–20 GHz, with focus on the electromagnetic absorption mechanisms at microwave frequencies. A broad range of magnetic and dielectric properties are investigated and a number of mechanisms are highlighted for tailoring the electromagnetic performance. Results from the CoxNi1−x series, with particle sizes ranging from 25 to 200 nm, show particle size-related dielectric and magnetic properties, which aid the optimization of the resulting properties, in addition to conventional mechanisms, which are also demonstrated in colloidal form. A further reduction in particle size to below 20 nm leads to single magnetic domain particles, which also exhibit enhanced electromagnetic properties, as demonstrated with broadband magnetic performance achieved for Co ferrofluid with an average particle size of 5 nm.

Dielectric relaxation in composites containing electrically isolated particles with thin semi-continuous metal coatings

The dielectric relaxation due to interfacial charges in metal particle-filled composites occurs in the optical (high frequency) regime. However, many applications would benefit from a shift of the relaxation to a much lower frequency regime (e.g. radio or microwave range). This could be achieved by using a filler with reduced conductivity, but there is no continuum of conductivity in naturally occurring materials to allow engineers to readily achieve this aim or to have complete design freedom. Recently, it was demonstrated, experimentally and theoretically, how insulating particles with nano-scale metal coatings enable this relaxation to be shifted to a lower frequency regime. The current work investigates the microstructure of the metal coating, highlighting a semi-continuous coating structure. Therefore, the relevance of a two-dimensional (2D) percolation-based model to describe the effective dielectric properties of the coating is explored. It is demonstrated that a model in which the metal coating is assumed to be near the 2D percolation threshold can provide a reasonable fit to the experimental data. An improved fit is achieved when a distribution of metal area coverage is permitted.

Babinet's principle and the band structure of surface waves on patterned metal arrays

The microwave response of an array of square metal patches and its complementary structure, an array of square holes, has been experimentally studied. The resonant phenomena, which yield either enhanced transmission or reflection, are attributed to the excitation of diffractively coupled surface waves. The band structure of these surface modes has been quantified for both p-(transverse magnetic) and s-(transverse electric) polarized radiation and is found to be dependent on the periodicity of the electric and magnetic fields on resonance. The results are in excellent accord with predictions from finite element method modeling and the electromagnetic form of Babinet’s principle [Babinet, C. R. Acad. Sci. 4, 638 (1837)].

Nano- and micro-auxetic plasmonic materials

Plasmonic nanostructures with a negative Poisson ratio are demonstrated, having the unusual mechanical property of auxetics to expand laterally when being stretched. Using nanomembrane technology, auxetics are shrunk by orders of magnitude, giving simultaneous access to optical properties of plasmonic metamaterials, as well as auxetic mechanical properties on the nanoscale.

Resonantly inverted microwave transmissivity threshold of metal grids

The microwave transmission of arrays of square patches, each rotated by 45 from the axes of the square lattice on which they are positioned, has been experimentally studied as a function of metal occupancy. At low frequencies, the microwave transmissivity drops on passing through the connec- tivity threshold (50 per cent occupancy), as one would expect. However, quite counter-intuitively, near the onset of diffraction, resonant phenomena induce a complete reversal in the sense of this transmissivity switch, i.e. the transmission is seen to increase as the metal occupancy is increased.