Microwave Engineering Europe - November 2007 - (Page 12)

12 HF MATERIALS Metamaterials tackle communications wavelengths By R. Colin Johnson, EE Times T he world’s ﬁrst communicationswavelength metamaterial was recently demonstrated by a Princeton University design fabricated at Alcatel-Lucent. The clever semiconductor architecture sidesteps the need to craft nanoscale mechanical structures, as has been the universal approach until now, by alternating layers of indium gallium arsenide with layers of aluminum indium arsenide. “When we started out on this project, we thought we would need a complicated architecture like other research groups are pursuing. But as our work progressed, we found that we could achieve the same results with a much simpler and straightforward layered structure,” said Claire Gmachl, a Princeton University electrical engineering professor. More than a dozen groups worldwide are trying to craft optical metamaterials from nanoscale mechanical structures. For instance, earlier this year the Department of Energy’s Ames (Iowa) Laboratory showed a “ﬁshnet” design cast in two metal screens with 100-nanometer (nm) holes separated by an optically clear dielectric. Also this year, Purdue University announced a design that uses arrays of nano-needles, each measuring 10 nm in diameter. All these nanoscale mechanical structures were aimed at mimicking the physical properties of the seminal metamaterials that use arrays of subwavelength resonators that respond not only electrically but also magnetically to incident microwaves — achieved with arrays of split-ring resonators. For the much shorter optical wavelengths, most researchers have been seeking ways to shrink these mechanical structures down to sub-micron sizes. However, the extreme difﬁculty of fabricating such tiny mechanisms led the Princeton team to ﬁnd a better solution. “Our material’s architecture is deceptively simple — just two alternating layers of semiconducting material — aluminum indium arsenide and highly doped indium gallium arsenide,” said Gmachl. “We could have used two other semiconductors, but we chose to use those materials which are already well understood for optical communications.” The individual layers were 80-nm thick — much thinner than the wavelength of communications lasers that range from as low as 9 to as high as 20 microns. By using 100 alternating layers, the total thickness of the metamaterial was 8 microns. How does it work The mechanism by which metamaterials reverse the basic optical properties of natural materials — refracting electromagnetic radiation by bending it toward the angle of incidence, instead of away from the angle of incidence — is by introducing a resonant grid whose pitch is smaller than the wavelength to be transmitted through it. As a result, a ﬂat planar lens made from metamaterials can focus light the same way as a concave natural lens, but without the aberrations that make concave lenses imperfect. Theoretically, the negative index of refraction of such metamaterials should enable perfect lenses to be constructed — a feat impossible for natural materials. The trick is to fabricate subwavelength structures that can resonate both electrically and magnetically at the desired transmission frequency, thereby reversing the natural response of metamaterials. This is relatively easy at microwave wavelengths where millimeter-sized spitring resonators can easily be fabricated to respond both electrically and magnetically. But shrinking from the millimeter size of microwaves down to the micrometer and nanometer scale of communications and optical wavelengths, respectively, has proven exceedingly difﬁcult. Now the Princeton architecture sidesteps the issue by employing an equivalent mechanism in highly anisotropic materials whose lattice structure has unequal physical properties along different axes. “We found that it was relatively easy to achieve the necessary electrical resonance by doping appropriately,” said Gmachl. “But it is very difﬁcult to achieve a magnetic resonance at this scale, so instead we use anisotropy to substitute for the magnetic resonance.” The electrical resonance in the highly doped semiconductor layer, plus the layer with very high anisotropy, according to the researchers, conﬁnes the light in such a way that it has only one possible way to bend — in the opposite direction to normal. Next, the Princeton researchers and their colleagues at the University of Oregon (Corvalis), the University of Massachusetts (Lowell), Purdue University (West Lafayette, Ind.) and at Alcatel-Lucent (Murray Hill, N.J.) will attempt to craft applications of their new metamaterial for communications lasers and waveguides. “Our main focus now will be to use this new material to make better semiconductor lasers — making them smaller and cheaper — as well as to fabricate better waveguides,” said Gmachl. Members of the Princeton engineering team included doctoral candidates Anthony Hoffman, Leonid Alekseyev, Scott Howard and Kale Franz, as well as Council of Science and Technology fellow Dan Wasserman (now at the University of Massachusetts), and former Princeton electrical engineering professor Evgenii Narimanov, now at Purdue University. The research was performed at Princeton’s Mid-Infrared Technologies for Health and the Environment (MIRTHE) lab and the Princeton Center for Complex Materials, both sponsored by the National Science Foundation. The crystals were grown for Princeton by Alcatel-Lucent. This article has been reproduced courtesy of EE Times. Microwave Engineering Europe ● November 2007 ● www.mwee.com 012_MWEE.indd 12 25/10/07 15:47:42 http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - November 2007

Microwave Engineering Europe - November 2007 Contents News Comment Metamaterials: Metamaterials Tackle Communications Wavelengths Microwave Components — EM tools: Microwave Component Design Easier With New EM and EDA Tools Cover Feature: RF Testing for OFDMA in LTE Base-Stations Startup Eyes Battery-Free Wireless Sensor Nets High-speed ADC Technology Paves the Way for Software Defined Radios Planning a WiMAX network: Maximising the ROI by Using Advanced Optimisation Tools Transporting Video Over Wireless Networks Ultrawideband Under the Gun Specifying the Proper SAW Filter Products Product Feature: RF Test Solution Supports Emerging 4x4 MIMO as Well as Multiple Commercial Standards Calendar

Microwave Engineering Europe - November 2007

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