Metamaterials – Materials for the Near-Future By Dr. Khalid Z. Rajab, Research Associate • Queen Mary University of London
When designing an antenna for any wireless application, the selection of materials traditionally available allows the antenna engineer relatively limited flexibility in achieving performance goals. Dielectrics, for example, are commonly limited to either polymers or ceramics. Polymers, although lightweight and inexpensive, have low dielectric constants. Ceramics on the other hand, may have much higher permittivities, but are brittle, and become increasingly heavy and expensive as higher dielectric constants are required. If magnetic properties are desired, the range of available materials is further reduced.
In recent years, the term metamaterial has been coined to describe the range of designer materials that allow the antenna designer the possibility of increasing the range of different material properties available. A metamaterial has come to mean any composite material that is formed typically, but not exclusively, using periodically arranged inclusions, and designed to provide a set of electromagnetic properties not generally found in naturally occurring materials. Although the terminology may be relatively new, the underlying principles have been applied in antenna engineering for many years. In the 1940s for example, W. E. Kock designed novel lenses for antenna applications that were significantly lighter than their predecessors [1]. He took advantage of the physical effects caused by many periodic, sub-wavelength conductors, in effect creating an early artificial material.
With investment from the UK’s Technology Strategy Board, the partners in the AMULET (Advanced Materials for Ubiquitous Leading-edge Electromagnetic Technologies) project [2], Cobham Technical Services, Queen Mary University of London (QMUL) and the National Physical Laboratory (NPL), are designing a range of novel materials, using software tools such as Cobham Technical Services’ Concerto.
Researchers at QMUL have demonstrated a host of new metamaterials, including lightweight artificial magnetic and high dielectric constant materials that operate at RF frequencies, but that are composed only of inexpensive polymers and conductors. Other examples of metamaterials designed at QMUL include materials for antenna size reduction, high-impedance surfaces, anisotropic materials, wire-medium lenses, composite right-/left-handed transmission lines and cloaking materials.
In 2006, a cloaking material was proposed that could be used, in effect, to make an object invisible to microwave frequencies [3]. Despite apparent limitations in the design, including a narrow operating frequency band, high losses and a requirement for anisotropic dispersive materials with permittivities and permeabilities below unity, the concept nevertheless invoked imagery of cloaking shields that would render a user invisible – so popular in science fiction.
On a more down-to-earth level, such devices do have potential to improve the performance of some types of antenna systems. In one suggestion, it has been proposed that where multiple antennas are present, operating at different frequencies, antennas could be wrapped in these cloaking materials, designed to hide the antenna at the operating frequency of the other antennas. The cloak would be designed to guide the wave around the antenna, recombining on the other side, almost as if the antenna were not present. The cloaked antenna would then have a reduced effect on the radiation pattern performance of the other antennas.
The team at QMUL has performed electromagnetic simulations to investigate these cloaking material applications. Recently, a new class of cloaking material has been demonstrated that is composed entirely of regions of isotropic dielectric, with naturally occurring dielectric constants [4]. This is particularly interesting as it means the cloaking material is practically realisable using naturally occurring materials and dielectric mixtures. Furthermore, this particular class of cloaking material is operable over a broad band of frequencies, albeit, over a limited range of incidence angles.
Research currently being performed under the AMULET project is investigating the use of active devices to overcome the limitations of passive structures. As part of this, for instance, Cobham Technical Services and NPL have proposed, analyzed, designed and manufactured active metamaterials based on negative immitance converter (NIC) circuits. These can provide Non Foster’s components in the VHF and UHF bands [5,6], and together with QMUL, innovative concepts are being developed that promise low loss and wideband metamaterials for applications such as sub-wavelength imaging, as a superior method for suppression of surface waves in periodic arrays, as well as for improved cloaking materials.
In much of this work, full-wave simulations, and in particular time-domain simulations that account for the temporal response, have been extremely important in investigating the properties of metamaterials and the devices constructed from them. Results of example simulations are shown in the sidebar. These were produced by the Concerto finite-difference time-domain (FDTD) solver from Cobham Technical Services, which has been perfectly suited for this task, having the capability to handle novel materials, including dispersive materials with negative electrical and magnetic properties, and to simulate entire structures or periodic unit cells that include negative lumped loads. This has allowed the investigation of an entirely new class of artificial material that, with as yet almost entirely untapped potential, may lead to a whole host of useful applications.
References
[1] W. E. Kock, "Metal-Lens Antennas", Proc. IRE, November 1946, pp 828-836.
[2] Advanced materials for ubiquitous leading-edge electromagnetic technologies (AMULET), Technology Programme Project No: TP/8/ADM/6/I/Q2084L
[3] D. Schurig et al., “Metamaterial Electromagnetic Cloak at Microwave Frequencies”, Science 10 November 2006: Vol. 314. no. 5801, pp 977-980.
[4] E. Kallos, C. Argyropoulos, Y. Hao, "Ground-Plane Quasi-Cloaking for Free Space", Physical Review A, vol. 79, pp. 063825, 2009.
[5] J. Vazquez, D. Moore, M. Shelley, K. Rajab, C. Argyropoulos, Y. Hao and L. R. Arnaut, “AMULET Research into Theoretical Characteristics of Artificial Materials – Active Structures and Concepts”, U.K. Technology Strategy Board, Report IR-1000-2, ERA Technology, Apr. 2009.
[6] L. R. Arnaut, “Active self-adaptive metamaterials based on two-port impedance loaded dipole arrays”, Proc. “Metamaterials 2009” Int. Congress, 31 Aug - 4 Sep 2009, London, UK.
Khalid Rajab received the BS degree in Electrical Engineering from Pennsylvania State University in 2003, and an MA in Mathematics and a PhD in Electrical Engineering, both from the same university in 2008. Currently, he is a postdoctoral research associate at Queen Mary University of London. His present research interests include materials characterization at microwave and terahertz frequencies, computation electromagnetics and composite media including metamaterials. He can be reached at khalid.rajab@elec.qmul.ac.uk.
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Metamaterials – Materials for the Near-Future
In recent years, the term metamaterial has been coined to describe the range of designer materials that allow the antenna designer the possibility of increasing the range of different material properties available. A metamaterial has come to mean any composite material that is formed typically, but not exclusively, using periodically arranged inclusions, and designed to provide a set of electromagnetic properties not generally found in naturally occurring materials. Although the terminology may be relatively new, the underlying principles have been applied in antenna engineering for many years. In the 1940s for example, W. E. Kock designed novel lenses for antenna applications that were significantly lighter than their predecessors [1]. He took advantage of the physical effects caused by many periodic, sub-wavelength conductors, in effect creating an early artificial material.