Plasmonic metamaterial

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A plasmonic metamaterial is a

surface plasmon polaritons (SPPs). Once launched, the SPPs ripple along the metal-dielectric interface. Compared with the incident light, the SPPs can be much shorter in wavelength.[1]

The properties stem from the unique structure of the metal-dielectric composites, with features smaller than the wavelength of light separated by

subwavelength
distances. Light hitting such a metamaterial is transformed into surface plasmon polaritons, which are shorter in wavelength than the incident light.

Plasmonic materials

Plasmonic materials are

metals or metal-like[2] materials that exhibit negative real permittivity. Most common plasmonic materials are gold and silver. However, many other materials show metal-like optical properties in specific wavelength ranges.[3]
Various research groups are experimenting with different approaches to make plasmonic materials that exhibit lower losses and tunable optical properties.

Negative index

Plasmonic metamaterials are realizations of materials first proposed by Victor Veselago, a Russian theoretical physicist, in 1967. Also known as left-handed or negative index materials, Veselago theorized that they would exhibit optical properties opposite to those of glass or air. In negative index materials energy is transported in a direction opposite to that of

propagating wavefronts, rather than paralleling them, as is the case in positive index materials.[4][5]

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 reaching a negative index material through air would not cross the normal. Rather, it would bend the opposite way.

Negative refraction was first reported for

NIST reported narrow band, negative refraction of visible light in two dimensions.[4][5]

To create this response, incident light couples with the undulating, gas-like charges (plasmons) 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 and compresses the wavelength of incoming light to a fraction of its original value.[5]

Nanomechanical systems incorporating metamaterials exhibit negative radiation pressure.[8]

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.[8]

Three-dimensional negative index

Computer simulations predict plasmonic metamaterials with a negative index in three dimensions. Potential fabrication methods include multilayer thin film deposition, focused ion beam milling and self-assembly.[8]

Gradient index

PMMs can be made with a gradient index (a material whose refractive index varies progressively across the length or area of the material). One such material involved depositing a

electron beam lithography
.

Hyperbolic

Hyperbolic metamaterials behave as a metal when light passes through it in one direction and like a dielectric when light passes in the perpendicular direction, called extreme anisotropy. The material's dispersion relation forms a hyperboloid. The associated wavelength can in principle be infinitely small.[9] Recently, hyperbolic metasurfaces in the visible region has been demonstrated with silver or gold nanostructures by lithographic techniques.[10][11] The reported hyperbolic devices showed multiple functions for sensing and imaging, e.g., diffraction-free, negative refraction and enhanced plasmon resonance effects, enabled by their unique optical properties.[12] These specific properties are also highly required to fabricate integrated optical meta-circuits for the quantum information applications.

Isotropy

The first metamaterials created exhibit anisotropy in their effects on plasmons. I.e., they act only in one direction.

More recently, researchers used a novel self-folding technique to create a three-dimensional array of split-ring resonators that exhibits isotropy when rotated in any direction up to an incident angle of 40 degrees. Exposing strips of nickel and gold deposited on a polymer/silicon substrate to air allowed mechanical stresses to curl the strips into rings, forming the resonators. By arranging the strips at different angles to each other, 4-fold symmetry was achieved, which allowed the resonators to produce effects in multiple directions.[13][14]

Materials

Silicon sandwich

Negative refraction for visible light was first produced in a sandwich-like construction with thin layers. An insulating sheet of

IC chip, negative refraction was demonstrated over blue and green frequencies. The collective result is a relatively significant response to light.[4][5]

Graphene

Graphene also accommodates surface plasmons,

biosensors.[19]

Superlattice

A hyperbolic metamaterial made from titanium nitride (metal) and aluminum scandium nitride (dielectric) have compatible crystal structures and can form a superlattice, a crystal that combines two (or more) materials. The material is compatible with existing CMOS technology (unlike traditional gold and silver), mechanically strong and thermally stable at higher temperatures. The material exhibits higher photonic densities of states than Au or Ag.[20] The material is an efficient light absorber.[21]

The material was created using

magnetron sputtering. The material featured ultra-thin and ultra-smooth layers with sharp interfaces.[21]

Possible applications include a "planar

hyperlens" that could make optical microscopes able to see objects as small as DNA, advanced sensors, more efficient solar collectors, nano-resonators, quantum computing and diffraction free focusing and imaging.[21]

The material works across a broad spectrum from near-infrared to visible light. Near-infrared is essential for telecommunications and optical communications, and visible light is important for sensors, microscopes and efficient solid-state light sources.[21]

Applications

Microscopy

One potential application is

surface plasmon polaritons
rather than photons.

A theorized

photodetectors and directional light emitters.[22]

Biological and chemical sensing

Other proof-of-concept applications under review involve high sensitivity biological and

optical microscopy and spectroscopy.[4]

Optical computing

Optical computing replaces electronic signals with light processing devices.[23]

In 2014 researchers announced a 200 nanometer, terahertz speed optical switch. The switch is made of a metamaterial consisting of nanoscale particles of

vanadium dioxide (VO
2), a crystal that switches between an opaque, metallic phase and a transparent, semiconducting phase. The nanoparticles are deposited on a glass substrate and overlain by even smaller gold nanoparticles[24] that act as a plasmonic photocathode.[25]

Femtosecond laser pulses free electrons in the gold particles that jump into the VO
2 and cause a subpicosecond phase change.[24]

The device is compatible with current integrated circuit technology, silicon-based chips and

high-K dielectrics materials. It operates in the visible and near-infrared region of the spectrum. It generates only 100 femtojoules/bit/operation, allowing the switches to be packed tightly.[24]

Photovoltaics

Gold group metals (Au, Ag and Cu) have been used as direct active materials in photovoltaics and solar cells. The materials act simultaneously as electron [26] and hole donor,[27] and thus can be sandwiched between electron and hole transport layers to make a photovoltaic cell. At present these photovoltaic cells allow powering smart sensors for the Internet of Things (IoT) platform.[28]

See also

References

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  23. ^ Yarris, Lynn (2009-08-20). "GRIN Plasmonics…" (Online news release). U.S. Department of Energy National Laboratory Operated by the University of California. Retrieved 2011-02-15.
  24. ^ a b c "Nanoscale optical switch breaks miniaturization barrier". KurzweilAI. March 18, 2014. Retrieved 19 April 2015.
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  28. ^ "Peafowl Solar Power | MAKING ENERGY BEAUTIFUL".

Further reading

External links
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