Metamaterials

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Metamaterials (from the Greek word μετά meta, meaning "beyond") are materials engineered to have a property that is not found in naturally occurring materials. They are made from assemblies of multiple elements fashioned from composite materials such as metals or plastics. In this field, optical metamaterials are artificially structured materials with nanoscale inclusions and strikingly unconventional properties at optical and near-infrared frequencies.
These materials can be treated as macroscopically homogeneous media and can exhibit a variety of unusual and exciting responses to light (V. Shalaev, Springer-2010).
Recently, metamaterials research has received tremendous significance in many potential applications for higher performances. The main core in all metamaterials comprises of fabricating a medium composed of unit cells of size far below the excitation wavelength. In contrast to existing optical metastructures in the visible and near-infrared wavelength regions, Hyperbolic Metamaterials (HMMs) are distinguished as promising class of metamaterials. 
Their unique properties enable the possibility to use them as extremely high performace bio-sensors.
Another class of metamaterials are Metastructures. In this class, the plasmonic response of Quasi-Crystals have been studied as the coupling effect between Surface Plasmon-Polaritons (SPPs) and Nano-Cavity Modes

Hyperbolic Metamaterials (HMMs)

Hyperbolic metamaterials (HMMs) are non-magnetic extremely anisotropic layered nanostructures with optical response that cannot be found in nature. They possess an open hyperboloid iso-frequency surface. This permits to support highly confined wavevector modes (high-k modes) in addition to surface plasmon modes within the structure due to hyperbolic dispersion.
Being characterized by hyperbolic dispersion, HMMs support surface plasmon polaritons (SPPs) as well as highly confined bulk plasmon polaritons (BPPs). A periodic metal/dielectric stack (HMM) supports bulk plasmon modes, which are the entire family gap plasmon modes of the multilayer. These high-k modes are conventionally known as volume plasmon polaritons (VPPs) or bulk Bloch plasmon polaritons (BPPs). BPPs are propagating waves inside the multilayer whereas decay exponentially outside the structure.

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Extremely Sensitive Bio-Sensors based on HMMs

Optical sensor technology offers significant opportunities in the field of medical research and clinical diagnostics, particularly for the detection of small numbers of molecules in highly diluted solutions. Several methods have been developed for this purpose, including label-free plasmonic biosensors based on metamaterials. However, the detection of lower-molecularweight (<500Da) biomolecules in highly diluted solutions is still a challenging issue owing to their lower polarizability.
In this context, we have developed a miniaturized plasmonic biosensor platform based on a hyperbolic metamaterial that can support highly confined bulk plasmon guided modes over a broad wavelength range from visible to near infrared. By exciting these modes using a grating-coupling technique, we achieved dierent extreme sensitivity modes with a maximum of 30,000nm per refractive index unit (RIU) and a record figure of merit (FOM) of 590. We report the ability of the metamaterial platform to detect ultralow-molecular-weight (244Da) biomolecules at picomolar concentrations using a standard affnity model streptavidin–biotin.

Extraordinary Effects in Quasi-Periodic Gold Nanocavities

Plasmonic quasi-periodic structures are well-known to exhibit several surprising phenomena with respect to their periodic counterparts, due to their long-range order and higher rotational symmetry. Thanks to their specific geometrical arrangement, plasmonic quasi-crystals offer unique possibilities in tailoring the coupling and propagation of surface plasmons through their lattice, a scenario in which a plethora of fascinating phenomena can take place. We investigate the extraordinary transmission phenomenon occurring in specifically patterned Thue−Morse nanocavities, demonstrating noticeable enhanced transmission, directly revealed by near-field optical experiments, performed by means of a scanning near-field optical microscope (SNOM). SNOM further provides an intuitive picture of confined plasmon modes inside the nanocavities and confirms that localization of plasmon modes is based on size and depth of nanocavities, while cross talk between close cavities via propagating plasmons holds the polarization response of patterned quasi-crystals.

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