David Kirkpatrick

April 22, 2010

New negative-index metamaterial for invisibility cloaks and more

Here’s news on a new artificial optical material with applications for invisibility cloaking tech and more.

From the first link:

Caltech-led team designs novel negative-index metamaterial that responds to visible light

Uniquely versatile material could be used for more efficient light collection in solar cells

IMAGE: Arrays of coupled plasmonic coaxial waveguides offer a new approach by which to realize negative-index metamaterials that are remarkably insensitive to angle of incidence and polarization in the visible range….

Click here for more information.

PASADENA, Calif.—A group of scientists led by researchers from the California Institute of Technology (Caltech) has engineered a type of artificial optical material—a metamaterial—with a particular three-dimensional structure such that light exhibits a negative index of refraction upon entering the material. In other words, this material bends light in the “wrong” direction from what normally would be expected, irrespective of the angle of the approaching light.

This new type of negative-index metamaterial (NIM), described in an advance online publication in the journal Nature Materials, is simpler than previous NIMs—requiring only a single functional layer—and yet more versatile, in that it can handle light with any polarization over a broad range of incident angles. And it can do all of this in the blue part of the visible spectrum, making it “the first negative index metamaterial to operate at visible frequencies,” says graduate student Stanley Burgos, a researcher at the Light-Material Interactions in Energy Conversion Energy Frontier Research Center at Caltech and the paper’s first author.

“By engineering a metamaterial with such properties, we are opening the door to such unusual—but potentially useful—phenomena as superlensing (high-resolution imaging past the diffraction limit), invisibility cloaking, and the synthesis of materials index-matched to air, for potential enhancement of light collection in solar cells,” says Harry Atwater, Howard Hughes Professor and professor of applied physics and materials science, director of Caltech’s Resnick Institute, founding member of the Kavli Nanoscience Institute, and leader of the research team

What makes this NIM unique, says Burgos, is its engineering. “The source of the negative-index response is fundamentally different from that of previous NIM designs,” he explains. Those previous efforts used multiple layers of “resonant elements” to refract the light in this unusual way, while this version is composed of a single layer of silver permeated with “coupled plasmonic waveguide elements.”

Surface plasmons are light waves coupled to waves of electrons at the interface between a metal and a dielectric (a non-conducting material like air). Plasmonic waveguide elements route these coupled waves through the material. Not only is this material more feasible to fabricate than those previously used, Burgos says, it also allows for simple “tuning” of the negative-index response; by changing the materials used, or the geometry of the waveguide, the NIM can be tuned to respond to a different wavelength of light coming from nearly any angle with any polarization. “By carefully engineering the coupling between such waveguide elements, it was possible to develop a material with a nearly isotopic refractive index tuned to operate at visible frequencies.”

This sort of functional flexibility is critical if the material is to be used in a wide variety of ways, says Atwater. “For practical applications, it is very important for a material’s response to be insensitive to both incidence angle and polarization,” he says. “Take eyeglasses, for example. In order for them to properly focus light reflected off an object on the back of your eye, they must be able to accept and focus light coming from a broad range of angles, independent of polarization. Said another way, their response must be nearly isotropic. Our metamaterial has the same capabilities in terms of its response to incident light.”

This means the new metamaterial is particularly well suited to use in solar cells, Atwater adds. “The fact that our NIM design is tunable means we could potentially tune its index response to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells,” he explains. “And the fact that the metamaterial has a wide-angle response is important because it means that it can ‘accept’ light from a broad range of angles. In the case of solar cells, this means more light collection and less reflected or ‘wasted’ light.”

“This work stands out because, through careful engineering, greater simplicity has been achieved,” says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

###

In addition to Burgos and Atwater, the other authors on the Nature Materials paper, “A single-layer wide-angle negative index metamaterial at visible frequencies,” are Rene de Waele and Albert Polman from the Foundation for Fundamental Research on Matter Institute for Atomic and Molecular Physics in Amsterdam. Their work was supported by the Energy Frontier Research Centers program of the Office of Science of the Department of Energy, the National Science Foundation, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek, and “NanoNed,” a nanotechnology program funded by the Dutch Ministry of Economic Affairs.

Visit the Caltech Media Relations website at http://media.caltech.edu.

April 20, 2010

Controlling the electronic properties of graphene

News from Physikalisch-Technische Bundesanstalt on plasmonics in graphene.

The release:

Graphene: What projections and humps can be good for

Investigators from Hanover and Braunschweig measure how the electronic properties of graphene can be controlled with purposefully used roughnesses

This release is available in German.

IMAGE: A residual interaction with the SiC substrate causes the formation of the six-fold satellite reflex structure.

Click here for more information.

At present, graphene probably is the most investigated new material system worldwide. Due to its astonishing mechanical, chemical and electronic properties, it promises manifold future applications – for example in microelectronics. The electrons in graphene are particularly movable and could, therefore, replace silicon which is used today as the basic material of fast computer chips. In a research cooperation, scientists of Leibniz University Hanover and of the Physikalisch-Technische Bundesanstalt (PTB) have now investigated in which way a rough base affects the electronic properties of the graphene layer. Their results suggest that it will soon be possible to control plasmons, i.e. collective oscillations of electrons, purposefully in the graphene, by virtually establishing a lane composed of projections and humps for them. The results were published in the current edition of the New Journal of Physics.

The structure of graphene itself is fascinating: It consists of exactly one single, regular layer of carbon atoms. To manufacture this incredibly thin layer absolutely neatly is a great challenge. A possible method to recipitate graphene extensively on an insulating substrate is epitaxy, i.e. the controlled growth of graphene on insulating silicon carbide. For this purpose, a silicon carbide crystal is heated in vacuum. Starting from a specific temperature, carbon atoms migrate to the surface and form a monoatomic layer on the – still solid – silicon carbide. An important question for later applications is, how defects and steps of the silicon carbide surface affect the electronic properties of the graphene grown on it.

Within the scope of a research cooperation between PTB and Leibniz University Hanover, the influence of defects in the graphene on the electronic properties has been investigated. During the investigations, special attention was paid to the influence of the defects on a special electronic excitation, the so-called plasmons.

By different sample preparation, first of all silicon carbide crystals with different surface roughness and, thus, with a different concentration of surface defects were investigated, on which, subsequently, graphene formed. The influence of the defects on the plasmon excitations was then investigated by means of low-energy electron diffraction (SPA-LEED) and electron loss spectroscopy (EELS).

The process revealed a strong dependence of the lifetime of plasmon on the surface quality. Defects, as they are caused on step edges and grain boundaries, strongly impede the propagation of the plasmons and drastically shorten their lifetime. Here it is remarkable that the other electronic properties of the plasmons, in particular their dispersion, remain largely unaffected.

This opens up interesting possibilities for the future technical application and use of plasmons (the so-called “plasmonics”) in graphene. By selective adjustment of the surface roughness, different graphene ranges could be generated in which the plasmons are either strongly dampened or can propagate almost unobstructedly. In this way, the plasmons could be conducted along “plasmon conductors” with low surface roughness specifically from one point of a graphene chip to another.

###

Original publication:
T. Langer, J. Baringhaus, H. Pfnür, H. W. Schumacher and C. Tegenkamp:
“Plasmon damping below the Landau regime: the role of defects in epitaxial graphene”.
New Journal of Physics 12, 033017 (2010).
http://iopscience.iop.org/1367-2630/12/3/033017/