Plasmonic metamaterials are metamaterials that exploit surface plasmons, which are produced from the interaction of light with metal-dielectric materials. Under specific conditions, the incident light couples with the surface plasmons to create self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons (SPPs). Once launched, the SPPs ripple along the metal-dielectric interface and do not stray from this narrow path. Compared with the incident light that triggered the transformation, the SPPs can be much shorter in wavelength.
Plasmonic metamaterials are tailor made composites - combinations of metallic and dielectric materials designed to achieve optical properties not seen in nature. The properties stem from the unique structure of the composites, with features smaller than the wavelength of light separated by subwavelength distances. By fabricating such metamaterials fundamental limits tied to the wavelength of light are overcome. Light hitting a metamaterial is transformed into electromagnetic waves of a different variety—surface plasmon polaritons, which are shorter in wavelength than the incident light. This transformation leads to unusual and counterintuitive properties that might be harnessed for practical use. Moreover, new approaches that simplify the fabrication process of metamaterials are under development. This work also includes making new structures specifically designed to enable measurements of the materials novel properties. Furthermore, nanotechnology applications of these nanostructures are currently being researched, including microscopy beyond the diffraction limit.
Plasmonic materials are metals or metal-like materials that exhibit negative real permittivity. Most common plasmonic materials are gold and silver. However, there are many other materials which show metal-like optical properties in the specific wavelength ranges. Various research groups are experimenting with different approaches to make plasmonic materials that exhibit lower-losses and tunable optical properties.
Negative index materials
Plasmonic metamaterials are incarnations of materials first proposed by Victor Veselago, a Russian theoretical physicist, in 1967. Also known as left-handed or negative index materials, the proposed materials were theorized to exhibit optical properties opposite to those of glass, air. These have been termed positive index—materials of our everyday world. In particular, energy is transported in a direction opposite to that of propagating wavefronts, rather than traveling in lockstep, as is the case in positive index materials. As a result, when juxtaposed with a positive index material, negative index materials were predicted to exhibit counterintuitive properties, like bending, or refracting, light in unnatural ways.
Normally, light traveling from, say, air into water bends upon passing through the normal (a plane perpendicular to the surface) and entering the water. In contrast, light beaming from air toward a negative index material would not cross the normal. Rather, it would bend the opposite way, and, as yet, not occurring in nature.
Negative refraction was first reported for microwaves and infrared radiation. In 2007, a collaboration team consisting of the Harry Atwater team at the California Institute of Technology, and the NIST reported narrow band, negative refraction of visible light in two dimensions.
To accomplish this a material platform that is a sandwich-like construction with exceedingly thin layers was fabricated. It consists of an insulating sheet of silicon nitride topped by a film of silver and underlain by gold. The critical dimension is the thickness of the layers, which taken together are only a fraction of the wavelength of blue and green light. By incorporating this metamaterial into integrated-optics on an IC chip, negative refraction was demonstrated over blue and green frequencies. The design exploits bulk materials properties of each component, but the collective result is a relatively significant response to light.
To create this response incident light couples with the undulating, gas-like charges normally on the surface of metals. This photon-plasmon interaction results in SPPs that generate intense, localized optical fields. The waves are confined to the interface between metal and insulator. This narrow channel serves as a transformative guide that, in effect, traps, squeezes, and compresses the wavelength of incoming light.
Graphene plasmonics and metamaterials
Recently, graphene has also shown to accommodate plasmons surface, observed via near field infrared optical microscopy techniques  and infrared spectroscopy. Potential applications of graphene plasmonics mainly addressed the terahertz to midinfrared frequencies, such as optical modulators, photodetectors, biosensors.
Three-dimensional optical materials
Computer simulations are designing plasmonic metamaterials with a negative index in three dimensions. The experimental composites will be made using a variety of fabrication methods, including multilayer thin film deposition, focused ion beam milling, and self-assembly. In addition, nanomechanical systems incorporating metamaterials are specifically designed to show one of the unusual predicted properties of metamaterials, and that is negative radiation pressure.
Light falling on conventional materials, with a positive index of refraction, exerts a positive pressure, meaning that it can push an object away from the light source. In contrast, illuminating negative index metamaterials should generate a negative pressure that pulls an object toward light.
Plasmonic negative-index metamaterials are also applicable to visible-light imaging of molecular and atomic scale objects. A theorized superlens could exceed the diffraction limit. The diffraction limit prevents positive-index lenses from resolving objects smaller than one-half of the wavelength of visible light. Because plasmonic materials can literally pinch light to a fraction of its original wavelength, a superlens would capture subwavelength spatial information that is beyond the view of conventional optical microscopes. There are several approaches to building a non-diffraction-limited optical microscope based on the superlens concept. The subwavelength domain can be applied to optical switches, modulators, photodetectors, and directional light emitters. These planned devices are also based on plasmonic metamaterials.
Gradient index plasmonics
Gradient index plasmonics works by placing a dielectric material on a metal substrate (composite material), along with electron beam lithography. Hence, a type of thermoplastic, known as a PMMA, is placed on a gold surface. This type of lithography is also used to apply three-dimensional surface topographies to computer chips. This method has resulted in a plasmonic type of Luneburg lens and Eaton lens.
Light waves propagate by employing transformation optics at surface plasmon scales. In other words, transformation optics is applied to the science of plasmonics. The Luneburg and Eaton lenses interact with surface plasmon polaritons rather than photons.
Possible applications are computers which will use light in place of electronic signals, enhanced optical microscopes, and carpet-cloaking devices.
Biological and chemical sensing
Other proof-of-concept applications that are being explored include high sensitivity biological and chemical sensing. This pertains to the development of optical sensors which exploit the confinement of surface plasmons within a certain type of Fabry-Perot nano-resonators. This tailored confinement will allow efficient detection of specific binding of target chemical or biological analyte molecules because of the strong spatial overlap between the optical resonator mode and the analyte ligands bound to the cavity sidewalls. Structures are optimized using finite difference time domain electromagnetic simulations, fabricated using a combination of electron beam lithography and electroplating, and tested using both near-field and far-field optical microscopy and spectroscopy.
- History of metamaterials
- Metamaterial cloaking
- Metamaterial antennas
- Nonlinear metamaterials
- Photonic metamaterials
- Photonic crystal
- Seismic metamaterials
- Split-ring resonator
- Acoustic metamaterials
- Metamaterial absorber
- Terahertz metamaterials
- Tunable metamaterials
- Negative index metamaterials
- Transformation optics
- Theories of cloaking
- Academic journals
- Metamaterials books
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- NIST researchers, Nanofabrication Research Group. "Three-Dimensional Plasmonic Metamaterials". National Institute of Science and Technology. Retrieved 2011-02-14.
- West, P. R.; Ishii, S.; Naik, G. V.; Emani, N. K.; Shalaev, V. M.; Boltasseva, A. (2010). "Searching for better plasmonic materials". Laser & Photonics Reviews 4 (6). doi:10.1002/lpor.200900055.
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- Lezec, H. J.; Dionne, J. A.; Atwater, H. A. (2007). "Negative Refraction at Visible Frequencies". Science 316 (5823): 430–2. Bibcode:2007Sci...316..430L. doi:10.1126/science.1139266. PMID 17379773.
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- "Metallic metamaterials and plasmonics". Philosophical Transactions of the Royal Society 369 (1950). ISSN 1471-2962. Theo Murphy Meeting Issue organized and edited by William L. Barnes.
- Plasmonic metamaterials - From microscopes to invisibility cloaks. Jan 21, 2011. PhysOrg.com.