We present a coupler-free, multi-mode refractive index sensor based on nanostructured split ring resonators (SRRs). The fabricated SRR structures exhibit multiple reflectance peaks, whose spectral positions are sensitive to local dielectric environment and can be quantitatively described by our standing-wave plasmonic resonance model, providing a design rule for this multi-mode refractive-index (MMRI) sensor. We further manifest that the lower-order modes possess greater sensitivity associated with stronger localized electromagnetic field leading to shorter detection lengths within five hundreds nanometers, while the higher-order modes present mediate sensitivity with micron-scale detection lengths to allow intracellular bio-events detection. These unique merits enable the SRR-based sensor a multi-functional biosensor and a potential label-free imaging device.

Au-Pd core-shell nanocrystals with cubic, truncated cubic, cuboctahedral, truncated octahedral, and octahedral structures have been employed to form micrometer-sized polyhedral supercrystals by both the droplet evaporation method and novel surfactant diffusion methods. Observation of cross-sectional samples indicates shape preservation of interior nanocrystals within a supercrystal. Low-angle X-ray diffraction techniques and electron microscopy have been used to confirm the presence of surfactant between contacting nanocrystals. By diluting the nanocrystal concentration or increasing the solution temperature, supercrystal size can be tuned gradually to well below 1 μm using the surfactant diffusion method. Rectangular supercrystal microbars were obtained by increasing the amounts of cubic nanocrystals and surfactant used. Au-Ag core-shell cubes and PbS cubes with sizes of 30-40 nm have also been fabricated into supercrystals, showing the generality of the surfactant diffusion approach to form supercrystals with diverse composition. Electrical conductivity measurements on single Au-Pd supercrystals reveal loss of metallic conductivity due to the presence of insulating surfactant. Cubic Au-Pd supercrystals show infrared absorption at 3.2 μm due to extensive plasmon coupling. Mie-type resonances centered at 9.8 μm for the Au-Pd supercrystals disappear once the Pd shells are converted into PdH after hydrogen absorption.

From a periodic array of commercially available zirconia cubes, we demonstrate artificial magnetic and electric dipoles due to the combination of displacement currents and Mie resonance. By scaling the size and periodicity of these dielectric resonators, the corresponding magnetic and electric responses are shifted to the desired frequencies. To further overlap the magnetic and electric resonances in the same frequency, we create a negative refractive index medium (NRIM) from single-dielectric resonators. Comparing with the conventional NRIM comprised of metallic or two-dielectric resonators, these single-dielectric structures present the low-loss and isotropic characteristics, and possess a further advantage to facilitate practical applications.

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Based on Maxwell's equations and Mie theory, strong sub-wavelength artificial magnetic and electric dipole resonances can be excited within dielectric resonators, and their resonant frequencies can be tailored simply by scaling the size of the dielectric resonators. Therefore, in this work we hybridize commercially available zirconia and alumina structures to harvest their individual artificial magnetic and electric response simultaneously, presenting a negative refractive index medium (NRIM). Comparing with the conventional NRIM constructed by metallic structures, the demonstrated all-dielectric NRIM possesses low-loss and high-symmetry advantages, thus benefiting practical applications in communication components, perfect lenses, invisible cloaking and other novel electromagnetic devices.

Abstract:The major issue regarding magnetic response in nature-"negative values for the permeability μ of material parameters, especially in terahertz or optical region" makes the electromagnetic properties of natural materials asymmetric. Recently, research in metamaterials has grown in significance because these artificial materials can demonstrate special and, indeed, extraordinary electromagnetic phenomena such as the inverse of Snell's law and novel applications. A critical topic in metamaterials is the artificial negative magnetic response, which can be designed in the higher frequency regime (from microwave to optical range). Artificial magnetism illustrates new physics and new applications, which have been demonstrated over the past few years. In this review, we present recent developments in research on artificial magnetic metamaterials including split-ring resonator structures, sandwich structures, and high permittivity-based dielectric composites. Engineering applications such as invisibility cloaking, negative refractive index medium, and slowing light fall into this category. We also discuss the possibility that metamaterials can be suitable for realizing new and exotic electromagnetic properties. OPEN ACCESSSymmetry 2011, 3 284

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