Any material's physical properties are determined by what types of atoms it is made of and how those atoms are arranged.
In an ordinary solid material, the atoms are arranged in basic, roughly nanometer-size units of a few atoms each, repeated over and over in an extensive wallpaper-like pattern. Metamaterials are artificially structured materials with much larger repeating units that contain more atoms per unit. Since there are more possible ways to arrange larger sets of atoms than smaller sets, metamaterials have a greater possible range of physical properties than ordinary solids do.
Among the properties of greatest current research interest are metamaterials' effects on electromagnetic waves. These waves include visible light as well as waves of higher and lower frequency—ultraviolet light, x-rays, and gamma rays (with frequencies roughly 800 terahertz and above), and infrared light, microwaves, and radio waves (roughly 400 terahertz and below). Much of the current interest centers on far-infrared light and higher-frequency microwaves, whose frequencies are around one terahertz (1 THz). Few devices to observe and manipulate these waves can be made from natural materials. But metamaterials, with their greater range of possible internal structures, offer more ways to make optical devices such as modulators, polarizing filters, absorbers, and lenses that can manipulate those waves, as similar devices do for light of other frequencies, or even in new ways altogether.
A further point of interest is that some metamaterials can refract terahertz light in a way no known natural material can. Any material's refractive properties can be expressed as a refractive index that depends on the frequency of the refracted light. For every known natural material, the refractive indices have always been found to be positive. But the refractive indices of some metamaterials—particularly those whose repeating units include a component called a split-ring resonator[Wikipedia]—are negative.[Wikipedia] This suggests the possibility of quite a few new types of optical devices.
For these reasons and more (many of which are well explained in the introductory sections of two dissertations[Information Bridge] described below), many researchers are investigating different aspects of metamaterials, and the Department of Energy is sponsoring much of this work. Metamaterial research has led to greater understanding of how metamaterials work and what they might be capable of, characterization of actual metamaterials' behavior, the reduction or elimination of technical obstacles to their production and use, and new devices.
The report "Fabrication and Optical Measurements of Nanoscale Meta-Materials: Terahertz and Beyond",[Information Bridge] briefly describes simulating various metamaterials' responses to terahertz light as an aid to optimizing negative refractive-index properties. The report also describes experiments with a number of metamaterials constructed with split-ring resonator/metal strip arrays. The report shows an example comparison of one metamaterial's actual response to the simulation's prediction.
A more general look at how light waves interact with and propagate through metamaterials (as well as through another type of material) is presented in the dissertation "Wave propagation in photonic crystals and metamaterials: Surface waves, nonlinearity and chirality".[Information Bridge] The dissertation presents calculations from laws of electromagnetism of how light waves interact with both types of material, and shows how a negative index can result from chirality—a configuration of components not identical to its mirror image. (Incidentally, as mentioned before, this dissertation provides a useful introduction to the field of metamaterials, in section 1.3.)
The existence of material media with negative refractive index raises questions about whether optical processes besides refraction are also reversed in such media. Two such questions are examined in the reports "A Proposed Measurement of the Reverse Cherenkov Radiation Effect in a Metamaterial-Loaded Circular Waveguide"[Information Bridge] and "On the possibility of Casimir repulsion using Metamaterials".[Information Bridge]
Cherenkov radiation is to light what the bow wave generated by a fast-moving ship is to water waves: a wave stirred up in a medium by an object that moves faster through the medium than the wave can. The wave spreads out from the wake of the object. While no material object can move through empty space faster than light, energetic-enough material particles can outrun light waves in a material medium. The electromagnetic waves these particles stir up are known as Cherenkov radiation. In ordinary materials, the Cherenkov waves move forward, lagging behind the particle that produces them. "A Proposed Measurement of the Reverse Cherenkov Radiation Effect in a Metamaterial-Loaded Circular Waveguide"[Information Bridge] describes an experiment to check whether Cherenkov waves move backwards in negative-index media.
A different reversal is examined by calculation in the report "On the possibility of Casimir repulsion using Metamaterials".[Information Bridge] Ordinary conductors placed near each other alter the pattern of electromagnetic waves between them because only certain wavelengths will fit there; the result is less electromagnetic pressure in the space between the plates, so the plates are pushed toward each other by the remaining electromagnetic waves outside this space (the Casimir effect). [Wikipedia] Would metamaterials with negative refractive indices repel each other by a similar mechanism? The report shows that, if one particular model of magnetic permeability is applicable to the question, such repulsion would not occur.
As we saw above, though, negative refractive indices are not the only point of interest in metamaterials, or even in metamaterial optics; the possibility of controlling terahertz light is another. Different metamaterials have different effects, and different studies address these. "Asymmetric planar terahertz metamaterials"[Information Bridge] reports observations of resonance[Wikipedia] in different split-ring resonator metamaterials. In some of them, the split in the resonators is less symmetrically placed, near a corner of the rings instead of in their sides, allowing for an 85% modulation of the fundamental resonance and resulting in a new resonance not accessible with the more symmetrical rings.
Each of these metamaterials had a single response to terahertz light; for a different response, you'd need a different metamaterial. Other studies have shown how a single metamaterial can be tuned to give different responses according to its tuning.
One tuning method uses infrared-light pulses to induce charged particles to bridge the gaps in metamaterials' split-ring resonators, thereby shorting them out and turning off their resonance. Three reports from Los Alamos National Laboratory, one entitled "Terahertz metamaterials"[Information Bridge] and the others both entitled "Active terahertz metamaterials",[Information Bridge] describe one group's achievement in controlling planar metamaterials' resonance this way, thus providing unprecedented performance in terahertz spectroscopy and imaging. The group's metamaterials incorporate natural materials such as semiconductors as substrates or as critical regions of metamaterial elements. A fourth Los Alamos report, "Chiral THz metamaterial with tunable optical activity"[Information Bridge], is a computational study of how a similar metamaterial's effect on the polarization of terahertz light can be altered by the same basic mechanism.
A Sandia National Laboratories group investigated how the metamaterials they studied might be used as devices to alter beams of terahertz waves by filtering or modulating them. Such devices, described in "Microwave to millimeter-wave electrodynamic response and applications of semiconductor nanostructures: LDRD project 67025 final report"[Information Bridge], would allow terahertz waves to be used to transmit information: the message would be encoded as variations in the terahertz carrier beam, which could be read off and decoded by the receiver.
Other types of metamaterials were investigated at Los Alamos for tunability, and presented in "Superconductors and Complex Transition Metal Oxides for Tunable THz Plasmonic Metamaterials".[Information Bridge] According to this slide presentation, metal metamaterials were not found to be tunable, but superconducting metamaterials operating at high critical temperature Tc can be tuned by temperature adjustment. Furthermore, complex metal oxides can be used as tunable active substrates, but leave a problem of terahertz radiation losses. Experimental data is also presented in two other Los Alamos slide presentations, both entitled "Response of High-Tc Superconductor Metamaterials to High Intensity THz Radiation".[Information Bridge]
Metamaterials may make two new types of optical device possible. One is a new kind of lens (called "superlens"[Wikipedia]) that could overcome a significant resolution limitation of existing lenses. Experiments with metamaterial superlenses are described in "Studies on metal-dielectric plasmonic structures"[Information Bridge], a survey of Purdue University studies sponsored by Sandia National Laboratories. The other is a cloak that can render any object inside it invisible—at least not visible with light of the appropriate frequency—by diverting that light around the object so the object neither reflects that light from in front or obstructs it from in back. One of the many project plans for nuclear-fuel development described in the Idaho National Laboratory report "Update on INSIGHTS Development"[Information Bridge] is to shield a neutron detector with a gamma-ray cloak so it won't react to the gamma rays, but only to neutrons, thus making its signals clearer.
"Controlling terahertz waves with meta-materials and photonic bandgap structures"[Information Bridge] describes two devices, one of which is a terahertz beam absorber made of metamaterial. A different use of a beam absorber, to detect terahertz beams instead of control them, is described in the report "Room temperature array technology for the terahertz to far-infrared".[Information Bridge] In the experiments described in the latter report, a metamaterial absorbed electromagnetic waves of specific frequencies and transferred their energy as heat to a cantilever that moved in response; the cantilever's motion indicated the presence of the absorbed waves.
Devices that distinguish the different polarizations of light waves' electric and magnetic fields can also be useful. (A common example for visible light is found in polarizing sunglasses. These mainly block visible light waves whose electric fields vibrate horizontally, the predominant polarization of glare reflected from the horizontial surfaces of roads, automobiles, and bodies of water.) "Metamaterials for terahertz polarimetric devices"[Information Bridge] describes a metamaterial made to respond differently to terahertz light of different polarizations, and compares it with a different metamaterial that doesn't distinguish polarizations.
"Meandered-Line Antenna with Integrated High-Impedance Surface"[Information Bridge] shows how a particular composite positive/negative-index metamaterial can suppress extraneous electromagnetic waves near a miniature wireless device's antenna. Without the metamaterial, the antenna would have to be about four times larger and would require more space between it and other components of the device to function.
The report "A Spatial Light Modulator for Terahertz Beams"[Information Bridge] describes a device that could be used to produce changing images or otherwise imprint information on a wave front, by varying the amount of terahertz light that passes through the device at different points. The device described in "A metamaterial solid-state terahertz phase modulator"[Information Bridge] modulates waves in a different way. Waves passing through it still go through their back-and-forth vibration cycles at the same frequency, but the device abruptly shifts the phase of those cycles—in effect resetting their clocks. This is useful because of what happens when different waves are allowed to merge into the same space to form a new wave. In some directions, the merging waves will vibrate out of step with each other, and so cancel each other out, while in other directions the waves will vibrate in step and reinforce each other. Thus the merged wave will propagate only along directions in which the original waves vibrate in step. If the individual initial waves are resynchronized by phase modulators, their vibrations will be in and out of step in different directions from before, so phase modulators can be used to change the direction of the merged wave.
To make the metamaterial components functional, there are different kinds of technical problems to be overcome. Several reports address some of these.
The section "Left-Handed Nanoscale Meta-Materials: Towards the Optical Domain" in the Lawrence Berkeley National Laboratory report "Laboratory Directed Research and Development Program FY 2006"[Information Bridge] mentions one group's solving a problem of removing a thin base plating of gold without affecting the nanoscale structures in the metamaterial they were producing. (Further progress by the same group is described in LBNL's report "Laboratory Directed Research and Development Program FY 2007"[Information Bridge].)
The "Argonne National Laboratory annual report of Laboratory Directed Research and Development Program Activities FY 2009"[Information Bridge] contains a section "Three-Dimensional Metamaterials with Negative Refractive Index" that describes a different group's circumvention of another limitation. At the time of their work, the kind of metamaterials they describe were being made much as microelectronic devices were, resulting in planar, two-dimensional structures; but three-dimensional structures would aid the investigation of loss-reduction methods and extension of the frequency range that metamaterials could respond to, as well as open the possibility of applications like "invisibility cloaks, perfect lenses, reflectionless interfaces, novel nonlinear-optical effects, negative Doppler shifts, and actively modulated metamaterials". The group reported using colloidal synthesis to make the metamaterial subunits and beginning to develop methods of assembling them. A different method of assembling both two- and three-dimensional structures is proposed in "Levitated crystals and quasicrystals of metamaterials":[Information Bridge] namely, growing the metamaterial while floating in a dusty plasma on the International Space Station rather than supported on a substrate on Earth.
U. S. Patent 8,054,146 ("Structures with negative index of refraction")[Information Bridge] describes methods of making negative-index structures that are both easy to make and easy to experimentally characterize, thus providing a solution for two technical problems. But in another sense, characterizing a material's properties is not only a matter of measurement, but a matter of understanding—a different kind of technical problem. Misconceptions about two-dimensional metamaterials that were commonly found in the research literature in 2008 are addressed in "A Discussion on the Interpretation and Characterization of Metafilms/Metasurfaces: The Two-Dimensional Equivalent of Metamaterials".[Information Bridge]
Computational analyses of metamaterials' interactions with light demand significant use of computer memory and processor time. "GIFFT: A Fast Solver for Modeling Sources in a Metamaterial Environment of Finite Size"[Information Bridge] describes an algorithm designed to minimize these demands.
As pointed out above, metamaterials can be used as absorbers to control light. When you want the light absorbed, the light's tendency to lose energy in a metamaterial is an asset. But when you just want the light redirected in some way, energy loss is a problem. The dissertation "Reducing the losses of optical metamaterials"[Information Bridge] examines two different ways to design metamaterials so they remove less energy from the light waves that pass through them. (Like the dissertation "Wave propagation in photonic crystals and metamaterials: Surface waves, nonlinearity and chirality",[Information Bridge] this dissertation—in section 1.1—provides a useful introduction to electromagnetic metamaterials. Section 1.2 describes potential metamaterial uses, including superlenses and invisibility cloaks; this section also describes limitations on metamaterials, with emphasis on the dissertation topic of large energy losses.)
Prepared by Dr. William N. Watson, Physicist
DOE Office of Scientific and Technical Information