Metamaterials: Nature TransformedSeptember 2010
Topics: Nanoelectronics, Radar, Technological Innovations
Straight From Science Fiction
Metamaterials are fascinating new synthetic materials made by changing the structure of naturally occurring materials. This change in physical form alters the way the material reflects, transmits, and refracts electromagnetic waves, enabling the "bending" of specific frequencies. The ability to selectively direct radiation in this way may allow researchers to create smaller electronic components, conceal and protect assets from surveillance, and perhaps even develop invisibility cloaks previously thought only to exist in science fiction.
A metamaterial is created by shaping "cells" of natural materials, like copper or silicon, such that they are the same size or smaller than selected electromagnetic waves, allowing control of selected wavelengths. For example, silicon normally reflects all visible light. However, by fabricating cells spaced on the order of the wavelength of green light, the silicon metamaterial can be made to transmit green light while completely reflecting all the other colors, thus creating a green filter.
Two characteristics of metamaterials that researchers are learning to manipulate are the bandgap and the index of refraction.
The bandgap in a material is the gap between frequencies that are allowed to pass through that material. In other words, the material completely reflects any frequency that falls within the bandgap. By selectively choosing the size and spacing of the cells that comprise the metamaterial, engineers can position the bandgap to fall within designated frequencies, from the low frequencies of radar waves to the higher frequencies of visible light, thereby excluding electromagnetic waves in those regions from the material.
For frequencies higher than optical frequencies, nature has conveniently provided semiconductor materials with cells periodically spaced on the order of the wavelength of electrons. These naturally occurring lattice structures possess bandgaps appropriate for controlling the propagation of electrons, which enabled the semiconductor revolution of the last half century.
Now researchers like those at MITRE are learning to leverage metamaterials in the same way in order to control the propagation of longer waves like radar and even visible light. Two examples of types of devices that exploit the bandgap of metamaterials are electromagnetic bandgap (EBG) antennas and photonic crystals.
Perfect Antennas and Flawed Crystals
The Department of Defense has faced difficulties mounting antenna systems onto vehicles. The material and shape of a vehicle interfere with the operating bandwidth and radiation pattern of its antennas. To help solve this problem, MITRE fashioned an antenna mount from a metamaterial whose bandgap corresponds to the frequencies of the antenna, allowing the antenna to perform optimally regardless of the vehicle on which it is mounted.
Photonic crystals are metamaterials with electromagnetic bandgaps in the visible or infrared region of the electromagnetic spectrum. By introducing a "defect" into the crystals' structure, one can create extremely small regions in the material where electromagnetic radiation of the forbidden frequencies can be confined.
Using photonic crystals, researchers can manipulate visible light on a nanoscopic scale. The defects in a photonic crystal can be specifically engineered to create standard integrated optical devices like waveguides and resonators with sizes unattainable with standard techniques. Photonic crystal-based devices, like ultra-small lasers, waveguide modulators with very high speeds, and miniature sensors with very high sensitivity and specificity, have the potential to dramatically improve laser communication networks used by MITRE's sponsors.
Index of Refraction
A material's index of refraction is the ratio of the speed of light in the material to the speed of light in a vacuum. In other words, if a material has an index of refraction of 2, light will travel half as fast through the material as it would travel in a vacuum. Metamaterials can be created with the startling characteristic of possessing a negative index of refractionsomething that is not known to occur in nature.
Normally an electromagnetic wave travels in the same direction as the flow of energy, but in a material with a negative index of refraction, it flows in the opposite direction. Metamaterials constructed with a negative index of refraction can help researchers design valuable new devices such as super lenses and cloaking shields.
Super Lens and Invisibility Cloaks
While a conventional lens enables a viewer to see objects only as small as the wavelength of light being used, a lens with a negative index of refraction would allow a viewer to see objects much smaller than the wavelength. In 2000, Sir John Pendry published a design showing that the construction of such a "super lens" was theoretically possible. Five years later, two separate research groups experimenting with Pendry's predictions were able to image objects about 100 nanometers in sizeseveral times smaller than the wavelength of the light used. A super lens employed in optical lithography could enable the fabrication of incredibly small devices.
One of the most intriguing inventions that could be developed from metamaterials with a negative index of refraction is a real-world cloaking device. Similar to the way a rock in a river diverts the flow of water around it, a cloaking device works by redirecting electromagnetic waves around an object and bringing them back together on the other side so it appears to the viewer that the object is not there. A cloaking device operating at a single frequency in the near-infrared range was demonstrated by scientists at the University of California, Berkeley in 2008.
A New Revolution
Historically, chemical techniques were used to design and produce materials with desired optical properties. Metamaterials open up an entirely new way of controlling material properties. They have the potential to fuel an explosion of technological applications in the same way that semiconductors enabled the computer revolution. MITRE's continued research in this arena promises that its sponsors will reap the benefits as technologies from science fiction become reality.
by Jody Mandeville