Mass-Producing Infrared Metamaterials
Some metamaterials have a negative refractive index, a physical property not found in natural substances. This property could enable the production of telecommunication components with very low noise, or it could permit the fabrication of ultrahigh-resolution lenses for use in biomedical imaging systems.
The market for metamaterial-based infrared imaging systems and optical telecommunication components is expected to be large enough to warrant mass production; however, that has proved difficult because metamaterials are nanometer-size objects.
Researchers from Hewlett-Packard in Palo Alto, Calif., the University of California, Berkeley, and the University of Illinois at Urbana-Champaign have now made mid-infrared negative-index metamaterials using nanoimprint lithography, a relatively new method widely believed to be scalable to mass-production levels.
The investigators are working with infrared metamaterials because infrared wavelengths do less damage to biological samples and transmit with less attenuation and dispersion than other wavelengths, which is helpful for telecommunication.
They fabricated arrays of the metamaterials on a Si3N4 membrane supported by a silicon wafer because it is easier and more cost-effective to use the same machinery employed to make silicon chips to mass-produce metamaterials than it would be to develop new equipment.
For the metamaterials, the scientists used four L-shaped optical resonators whose rotational symmetry eliminated the undesirable bianisotropy of split-ring resonators, the first negative-index metamaterials developed. Bianisotropy would distort delivery of light through the metamaterial and decrease the resolution of imaging systems.
Wei Wu from Hewlett-Packard said that nanoimprint lithography essentially involves stamping a pattern on a nanometer-size mold. After creating the pattern, the researchers hardened the patterned material using UV light. Then they engraved the pattern using reactive-ion etching and deposited the metal into the sample. They fabricated about 30 samples that each contained several resonator arrays, with 100,000 resonators within each array.
They measured the reflection spectra of the resonators to demonstrate that they have negative permittivity and negative permeability in the mid-infrared, properties that give metamaterials a negative refractive index. Comparison of these results to theoretical predictions made with modeling software from Computer Simulation Technology of Darmstadt, Germany, showed that the scientists had successfully fabricated mid-infrared metamaterials using nanoimprint lithography.
Applied Physics Letters, Feb. 6, 2007, 063107.
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