Affiliated with the
Communication & Space
Sciences Laboratory

Novel Electromagnetic Metamaterials

Broadband Negligible Loss Metamaterials

Lining the interior walls of a rectangular horn antenna with a low-index metamaterial sets up the appropriate electromagnetic boundary conditions to create a tapered aperture field distribution, which radiates with lower sidelobes and backlobes, as shown by the following 3D radiation patterns for C-band horns.

 

We design the wire grid metamaterial by calculating the anisotropic surface impedances and the effective material properties, treating the wire grid as a homogeneous material. The surface impedance plot below shows that the wire grid approximates a soft-surface, and the refractive index plot below shows that the effective refractive index is well below unity and follows the desired Drude-like dispersion across the frequency band of interest. Also note that the intrinsic loss of the metamaterial, represented by n" in the index plot, is essentially zero across the band.

To test the theoretical predictions, we built the wire grid metamaterial on copper plates and then soldered the plates into a C-band horn antenna. The following photograph shows the horn prototype.

Measurements of the prototype agreed with simulations well; the plots below show the theoretical co-polarization (solid) and cross-polarization (dashed) in blue and the corresponding measured patterns in red. The insets on the upper right of each plot show the corresponding electric field distribution in the metahorn aperture. The minor differences and asymmetries in the cross-polarization patterns can be attributed to manufacturing imperfections. Future prototypes will incorporate metamaterials based on printed circuit boards, which allow for much more accuracy and precision in manufacturing than soldered wires, as well as lower cost.


We have also investigated metamaterials satisfying the balanced hybrid condition, to be used in a conical Ku-band horn antenna. For this design, the metamaterial is based on a printed circuit board with vias connecting patterned conductive patches on the top surface to the ground plane on the bottom surface of the board. The following figure summarizes the results of the theoretical study.

Note that the product of the surface impedances is approximately unity across the band; i.e. the metamaterial satisfies the hybrid-mode condition (upper left). Again, this is a dispersion-engineered low-index metamaterial with negligible loss (upper right). As one would expect from a hybrid-mode horn, both sidelobes and cross-polarization are extremely low (bottom two plots).

This performance can already be obtained over sub-octave bandwidths using corrugated horns, but this approach would provide the same performance over a comparable or broader bandwidth. Moreover, this approach would be significantly less expensive to manufacture and would require much less mass than corrugated horns, which is critical for satellite applications.

..: References :..

1-) An octave-bandwidth negligible-loss radiofrequency metamaterial
by Erik Lier, Douglas H. Werner, Clinton P. Scarborough, Qi Wu, and Jeremy A. Bossard
Nature Materials, Vol. 10, Issue 3, pp. 216-222, March 2011.

ABSTRACT: Metamaterials provide an unprecedented ability to manipulate electromagnetic waves and are an enabling technology for new devices ranging from flat lenses that focus light beyond the diffraction limit to coatings capable of cloaking an object. Nevertheless, narrow bandwidths and high intrinsic losses arising from the resonant properties of metamaterials have raised doubts about their usefulness. New design approaches seek to turn the perceived disadvantages of dispersion into assets that enhance a device's performance. Here we employ dispersion engineering of metamaterial properties to enable specific device performance over usable bandwidths. In particular, we design metamaterials that considerably improve conventional horn antennas over greater than an octave bandwidth with negligible loss and advance the state of the art in the process. Fabrication and measurement of a metahorn confirm its broadband, low-loss performance. This example illustrates the power of clever implementation combined with dispersion engineering to bring metamaterials into their full potential for revolutionizing practical devices.

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2-) Experimental demonstration of a broadband transformation optics lens for highly directive multibeam emission
by Zhi Hao Jiang, Micah D. Gregory, and Douglas H. Werner
Phys. Rev. B, 84, 165111/1-6, 2011.

ABSTRACT: Metamaterials have dramatically expanded the range of available optical properties, enabling an array of new devices such as superlenses, perfect absorbers, and ultrafast switches. Most research has focused on demonstrating negative- and high-index metamaterials at terahertz and optical wavelengths. However, far less emphasis has been placed on low-loss near-zero-index metamaterials that exhibit unique properties including quasi-infinite phase velocity and infinite wavelength. Here, we experimentally demonstrate a free-standing metallodielectric fishnet nanostructure that has polarization-insensitive, zero-index properties with nearly ideal transmission at 1.55 μm. This goal was achieved by optimizing the metamaterial geometry to allow both its effective permittivity and permeability to approach zero together, which simultaneously produces a zero index and matched impedance to free space. The ability to design and fabricate low-loss, near-zero-index optical metamaterials is essential for new devices such as beam collimators, zero-phase delay lines, and transformation optics lenses.

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3-) A Broadband Monopole Antenna Enabled by an Ultrathin Anisotropic Metamaterial Coating
by Zhi Hao Jiang, Micah D. Gregory, and Douglas H. Werner
IEEE Antennas and Propagation Letters, Vol. 10, pp. 1543-1546, 2011.

ABSTRACT: A new type of compact flexible anisotropic metamaterial (MM) coating is proposed, which greatly enhances the impedance bandwidth of a quarter-wave monopole to over an octave. The MM coating has a high effective permittivity for the tensor component oriented along the direction of the monopole. By properly choosing the radius and tensor parameter of the MM coating, another resonance at a higher frequency can be efficiently excited without affecting the fundamental mode of the monopole. Additionally, the similar current distributions on the monopole at both resonances make stable radiation patterns possible over the entire band. To experimentally verify the concept, an S-band MM coated monopole was designed, fabricated, and characterized, exhibiting a 2.14:1 bandwidth (2.15-4.6 GHz) with a VSWR of less than 2:1. The demonstrated MM coating has a radius of only λ/24 and extremely light weight, which renders it attractive for use in applications such as broadband arrays and portable wireless devices.





4-) Demonstration of enhanced broadband unidirectional electromagnetic radiation enabled by a subwavelength profile leaky anisotropic zero-index metamaterial coating
by Zhi Hao Jiang, Qi Wu, and Douglas H. Werner
Physical Review B, Vol. 86, pp. 125131/1-7, 2012.

ABSTRACT: In this paper, we report a technique for creating broadband unidirectional electromagnetic radiation with a grounded nonmagnetic anisotropic zero-index metamaterial coating. We demonstrate that the leaky modes supported by the finite sized coating structure can be exploited to create a unidirectional radiation device with a subwavelength thickness of only 0.12λ. Numerical simulations and experimental characterization of the metamaterial-enabled radiator confirm the physical mechanisms governing its performance, showing a stable directive beam with an over fivefold radiation power enhancement at broadside. Due to the resulting nonmagnetic material properties, this approach provides a new path to highly efficient, small footprint, low profile, and broadband directive radiation devices for microwave and potentially even optical wavelength applications.

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5-) A Low-Profile High-Gain Substrate-Integrated Waveguide Slot Antenna Enabled by an Ultrathin Anisotropic Zero-Index Metamaterial Coating
by Zhi Hao Jiang, Qi Wu, Donovan E. Brocker, Peter E. Sieber, and Douglas H. Werner
IEEE Transactions on Antennas and Propagation, Vol. 62, pp. 1173-1184, 2014.

ABSTRACT: A low-profile high-gain unidirectional antenna is proposed and demonstrated using both metamaterial (MM) and substrate-integrated waveguide (SIW) technologies. First, the leaky modes supported by a grounded anisotropic slab are studied. These investigations reveal that a grounded slab consisting of an anisotropic zero/low index material can provide an extremely low value for the real part of the propagation constant of the leaky mode, thereby facilitating stable unidirectional broadside radiation over a wide frequency range. The truncation effect of the slab is then investigated through full-wave simulations, which is found to be beneficial for a practical implementation of dispersive metamaterials. Finally, to validate the proposed concept, a subwavelength end-loaded dipole array is designed to realize the required anisotropic zero-index property and is applied to a SIW fed longitudinal slot antenna for the 5.8 GHz wireless local area network (WLAN) band. Measurements of the fabricated antenna prototype are shown to be in strong agreement with simulation results, thus confirming the proposed antenna design. The resulting antenna is only 0.12λ thick, all while accomplishing a broadside gain of more than 10 dBi and a front-to-back ratio larger than 26 dB, which is ~7 dB and ~10 dB higher than that of the SIW fed slot alone, respectively. The -10 dB impedance bandwidth is more than 9% both with and without the presence of the MM coating. The proposed technique offers a means for realizing low-cost and low-profile unidirectional antennas with moderate bandwidth.

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6-) Design Synthesis of Metasurfaces for Broadband Hybrid-mode Horn Antennas with Enhanced Radiation Pattern and Polarization Characteristics
by Q. Wu, C. P. Scarborough, D. H. Werner, E. Lier, and X. Wang
IEEE Transactions on Antennas and Propagation, Vol. 60, No. 8, pp. 3594-3604, 2012.

ABSTRACT: Metamaterial surfaces (metasurfaces) with a low effective index of refraction have been recently proposed for application in the design of hybrid-mode horn antennas, such as soft and hard horns. Here we explore designs of several metasurfaces and their use as liners for coating the interior walls of horn antennas. The design process combines the genetic algorithm optimization technique with a full-wave electromagnetic solver to create dispersion-engineered metamaterials that possess customized surface impedance properties. A metamaterial parameter extraction technique is developed and employed in the optimization process, which is based on the surface impedance expressions for a homogeneous slab backed by a perfectly conducting ground plane illuminated at near grazing incidence. The optimized metasurface is found to be equivalent to a low index metamaterial with a dispersion that can improve the performance of conventional horn antennas over the entire Ku -band while introducing negligible losses. We conclude with a numerical study of a conical horn antenna whose interior is lined with a low index metasurface. The far-field radiation patterns and aperture field distributions confirm hybrid-mode operation over a wide bandwidth, validating the proposed metasurface design methodology.

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7-) Broadband High Directivity Multi-Beam Emission Through Transformation Optics Enabled Metamaterial Lenses
by Z. H. Jiang, M. D. Gregory and D. H. Werner
IEEE Transactions on Antennas and Propagation, Vol. 60, No. 11, pp. 5063-5074, 2012.

ABSTRACT: A new broadband two- and three-dimensional, polarization independent coordinate transformation is introduced that is capable of mapping the radiation from an embedded omnidirectional source into any desired number of highly directive beams pointed in arbitrary directions. This transformation requires anisotropic materials, yet is spatially invariant and thereby can be readily implemented by currently existing metamaterial technologies. Moreover, the performance of the transformation is not sensitive to small material parameter variations, thus enabling a broad operational bandwidth. To validate the concept, a broadband 3-D coordinate transformation metamaterial lens fed by a simple monopole antenna was designed, fabricated and characterized, achieving a quad-beam radiation pattern over a 1.26:1 bandwidth with approximately 6-dB realized gain improvement in the H -plane. In addition, the near-field coupling between the monopole and the lens was carefully tuned to accomplish a remarkable 70% broadening of the impedance bandwidth compared to the monopole antenna operating alone. It is also shown from the field simulations that the realized metamaterial lens provides both near-field and far-field 3-D collimating effects.

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8-) Low Cost and Broadband Dual-Polarization Metamaterial Lens for Directivity Enhancement
by J. P. Turpin, Q. Wu, D. H. Werner, B. Martin, M. Bray, and E. Lier
IEEE Transactions on Antennas and Propagation, Vol. 60, No. 12, pp. 5717-5726, 2012.

ABSTRACT: Metamaterials have been used in many different configurations to enhance the radiation properties of antennas. However, the vast majority of these metamaterial applications only consider linearly polarized antennas. This paper discusses the theory, design, implementation, and measurements of a far-field collimating lens for use with a circularly-polarized crossed-dipole antenna constructed from a 3D-volumetric metamaterial slab. Zero-index materials (ZIM) and low-index materials (LIM) cause the magnitude and phase of the radiated field across the face of the lens to be distributed uniformly, increasing the broadside gain over the feed antenna alone. Full-wave simulations were used in design of the lens, and a prototype metamaterial lens (meta-lens) was constructed and measured to verify the theoretical predictions. The meta-lens was found to increase the measured directivity of a crossed-dipole feed antenna by more than 6 dB, in good agreement with numerical simulations.

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9-) Demonstration of an Octave-Bandwidth Negligible-Loss Metamaterial Horn Antenna for Satellite Applications
by C. P. Scarborough, Q. Wu, D. H. Werner, E. Lier, R. K. Shaw, and B. G. Martin
IEEE Transactions on Antennas and Propagation, Vol. 61, No. 3, pp. 1081-1088, 2013.

ABSTRACT: This paper reports on the detailed design and experimental demonstration of a metamaterial-enabled low-sidelobe horn antenna (metahorn) based on principles similar to those of earlier soft horn antennas. The target application is a linearly polarized feed horn in the super-extended C-band (3.4-6.725 GHz) for communication satellite reflector antennas. The paper describes the detailed design and manufacturing of the -plane metamaterial liner (metaliner) based on a freestanding wire grid without the need for a dielectric substrate material. The measured copolarized and cross-polarized antenna patterns from the feed horn demonstrate over an octave pattern bandwidth with negligible loss. The results show similar bandwidth with lower sidelobes and backlobes than those of the trifurcated horn that is currently used as the standard C-band feed for single linear polarization. This demonstration shows promise for lightweight metamaterial horns to replace heavy and expensive C-band corrugated horns.

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10-) A Ku-band Dual Polarization Hybrid-Mode Horn Antenna Enabled by Printed-Circuit-Board Metasurfaces
by Q. Wu, C. P. Scarborough, B. G. Martin, R. K. Shaw, D. H. Werner, E. Lier, and X. Wang
IEEE Transactions on Antennas and Propagation, Vol. 61, No. 3, pp. 1089-1098, 2013.

ABSTRACT: Metamaterials with properly engineered surface properties have been recently proposed for application in the design of broadband hybrid-mode horn antennas, such as soft and hard horns. In this paper, we present the design, fabrication, and measured results of a square dual-polarization horn antenna with thin metasurfaces lining the four walls, demonstrating broadband, negligible-loss hybrid-mode operation. By employing a powerful genetic-algorithm (GA) design optimization technique, we have dispersion-engineered low-index metaliners whose surface impedances satisfy the balanced hybrid condition across the Ku-band. The optimized metaliners were synthesized based on conventional printed-circuit board technology, leading to a lightweight and low-cost construction. To improve the cross-polarization response, a simple dielectric plug was placed in the throat of the horn to perform effective mode conversion. Measurements showed that the fabricated horn antenna prototype provided low sidelobes, low cross-polarization levels, and radiation patterns that are approximately independent of polarization. Excellent agreement was found between measured and simulated results across the entire band of operation. Both the far-field radiation patterns and the aperture field distributions confirm the hybrid-mode operation of the horn, validating the balanced metasurface design. This metamaterial-enabled antenna represents a low-cost alternative to other types of soft feed horns, such as corrugated horns.

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11-) Tailoring Dispersion for Broadband Low-loss Optical Metamaterials Using Deep-subwavelength Inclusions
by Z. H. Jiang, S. Yun, L. Lin, J. A. Bossard, D. H. Werner, and T. S. Mayer
Scientific Reports, Vol. 3, pp. 1571/1-9, 2013.

ABSTRACT: Metamaterials have the potential to create optical devices with new and diverse functionalities based on novel wave phenomena. Most practical optical systems require that the device properties be tightly controlled over a broad wavelength range. However, optical metamaterials are inherently dispersive, which limits operational bandwidths and leads to high absorption losses. Here, we show that deep-subwavelength inclusions can controllably tailor the dispersive properties of an established metamaterial structure thereby producing a broadband low-loss optical device with a desired response. We experimentally verify this by optimizing an array of nano-notch inclusions, which perturb the mode patterns and strength of the primary and secondary fishnet nanostructure resonances and give an optically thin mid-wave-infrared filter with a broad transmissive pass-band and near-constant group delay. This work outlines a powerful new strategy for realizing a wide range of broadband optical devices that exploit the unique properties of metamaterials.

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12-) Transformation Optics Inspired Multibeam Lens Antennas for Broadband Directive Radiation
by Q. Wu, Z. H. Jiang, O. Quevedo-Teruel, J. P. Turpin, W. Tang, Y. Hao, and D. H. Werner
IEEE Transactions on Antennas and Propagation, Vol. 61, No. 12, pp. 5910-5922, 2013.

ABSTRACT: Recent advancements in transformation optics (TO) and metamaterials have inspired tremendous interest in the electromagnetic community, creating a variety of novel antennas with enhanced performance, such as broad bandwidth, large gain, and high polarization efficiency. Although there could be infinitely many transformations for designing a given device, most of them result in rather complicated material compositions. This paper compares two recently introduced TO techniques, both of which lead to much simpler material requirements. In particular, a linear geometrical transformation or a quasi-conformal mapping was employed to design multi-beam collimating lenses, which possess either homogeneous or isotropic constituent materials. A systematic comparison is made for the first time between these two TO design approaches for a specific example of a quad-beam focusing lens, where the advantages and disadvantages of each method are clearly identified. Full-wave numerical simulations were performed to demonstrate the well-collimated beams produced by the TO lenses designed by either transformation. The characteristics of the two lens antennas, such as radiation pattern and bandwidth, were contrasted, providing valuable guidance on design tradeoffs for a specific application.

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13-) Bifunctional Plasmonic Metamaterials Enabled by Subwavelength Nano-notches for Broadband, Polarization-Independent Enhanced Optical Transmission and Passive Beam-steering
by Z. H. Jiang, L. Lin, J. A. Bossard, and D. H. Werner
Optics Express, Vol. 21, No. 25, pp. 31492-31505, 2013.

ABSTRACT: In this work, we present the design, numerical experiments, and analysis of a plasmonic metamaterial thin film based on subwavelength nano-notch loaded modified fishnet structures. The resulting device offers a simultaneous bandpass filtering functionality with a broad enhanced optical transmission window and a gapless negative-zero-positive index transition to enable polarization-independent passive beam-steering. This unique characteristic is made possible by the introduced subwavelength nano-notches, which provide fine tuning and hybridization of the external and internal surface plasmon polariton modes. This allows tailoring of the dispersive properties of the plasmonic metamaterial for broadband operation. Specifically, a multilayer nanostructured modified fishnet with feature sizes accessible by modern nanofabrication techniques is presented, exhibiting a broad passband at the mid-infrared wavelengths from 3.0 to 3.7 µm and stopbands elsewhere in the 2.5 ~4.5 µm window. The transmittance normalized to area is around 3 dB within the broad 20% bandwidth of the passband. Additionally, the effective index undergoes a smooth transition from negative unity through zero to positive unity with low loss within the passband. The physical mechanism and the angular dispersion of the metamaterial are analyzed in detail. Finally, full-wave simulations of a prism formed from this metamaterial are performed to demonstrate that the proposed structure achieves simultaneous polarization-insensitive passive beam-steering and filtering functionalities.

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14-) A Low-Profile High-Gain Substrate-Integrated Waveguide Slot Antenna Enabled by an Ultrathin Anisotropic Zero-Index Metamaterial Coating
by Z. H. Jiang, Q. Wu, D. E. Brocker, P. E. Sieber and D. H. Werner
IEEE Transactions on Antennas and Propagation, Vol. 62, No. 3, pp. 1173-1184, 2014.

ABSTRACT: A low-profile high-gain unidirectional antenna is proposed and demonstrated using both metamaterial (MM) and substrate-integrated waveguide (SIW) technologies. First, the leaky modes supported by a grounded anisotropic slab are studied. These investigations reveal that a grounded slab consisting of an anisotropic zero/low index material can provide an extremely low value for the real part of the propagation constant of the leaky mode, thereby facilitating stable unidirectional broadside radiation over a wide frequency range. The truncation effect of the slab is then investigated through full-wave simulations, which is found to be beneficial for a practical implementation of dispersive metamaterials. Finally, to validate the proposed concept, a subwavelength end-loaded dipole array is designed to realize the required anisotropic zero-index property and is applied to a SIW fed longitudinal slot antenna for the 5.8 GHz wireless local area network (WLAN) band. Measurements of the fabricated antenna prototype are shown to be in strong agreement with simulation results, thus confirming the proposed antenna design. The resulting antenna is only 0.12 λ thick, all while accomplishing a broadside gain of more than 10 dBi and a front-to-back ratio larger than 26 dB, which is ~ 7 dB and ~ 10 dB higher than that of the SIW fed slot alone, respectively. The -10 dB impedance bandwidth is more than 9% both with and without the presence of the MM coating. The proposed technique offers a means for realizing low-cost and low-profile unidirectional antennas with moderate bandwidth.

Link to Article



15-) Far-Zone Focusing Lenses Designed by Complex Coordinate Transformations
by B. Q. Lu, Z. H. Jiang, and D. H. Werner
IEEE Antennas and Wireless Propagation Letters, Vol. 13, pp. 1779-1782, 2014.

ABSTRACT: In this letter, a transformation optics approach involving complex coordinates is presented for designing far-zone focusing lenses fed by a single radiating electromagnetic source. In contrast to conventional coordinate transformation (CT) methods that tailor only the real parts of the material parameters, a spatial amplitude tapering function containing either gain or loss is introduced within the lens, which provides an extra degree of freedom in controlling both the amplitude and phase distributions at the aperture of the lens, resulting in a controllable far-field radiation pattern. The concept is validated by full-wave lens simulations with parametric studies carried out to investigate the impact of both the geometrical and material properties of the lens on the far-field patterns. It is then demonstrated that the lens designs can be further optimized to control the sidelobe levels (peak as well as minor lobes) while maintaining a relatively high gain. Finally, it is shown that the lens exhibits a far-field radiation pattern comparable to that of a linear array with uniform element spacing, but is unidirectional rather than bidirectional. By providing a means to control both the amplitude and phase distribution across the lens aperture, the proposed complex CT design approach provides a pathway to constructing directive emitting devices with radiation properties similar to conventional antenna arrays but with only a single feeding source.

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16-) Functionalized Metamaterials Enable Frequency and Polarization Agility in a Miniaturized Lightweight Antenna Package
by C. P. Scarborough, D. H. Werner, and D. E. Wolfe
Advanced Electronic Materials, pp. 201500295/1-7, 2016.

ABSTRACT: Electromagnetic metamaterials share a perceived disadvantage with miniature radio communication antennas: limited operating bandwidths. In conventional radio systems, tuning has been confined to the radio, requiring broadband antennas and materials. With the advent of software defined and digital radios, adding the antenna into the plethora of tunable radio subsystems can become a reasonable proposition, allowing miniaturized antennas with narrow instantaneous (channel) bandwidths to be tuned across entire communications bands, depending on the channel in use. Moreover, the antenna will provide an effective filtering stage before the signal reaches the radio. The tunable metamaterial presented herein enables an antenna showcasing this functionality and more. Dramatic size reductions are made possible by a tunable, lightweight, miniaturized metamaterial. Tuning the metamaterial and antenna in tandem provides a dynamic operating channel, with a tunable, nearly arbitrary polarization response as an added benefit. Finally, this antenna provides one of the first examples in the literature of a practical device improved by functionalized metamaterials, which has been tested on a real-world platform.

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