Realizing the Potential of Metamaterials
The subject of metamaterials has gained popularity over the past decade with regard to the benefits they may provide to various radio frequency (RF) and electro-optic (EO) applications employing antennas, lenses, guiding structures and electro-optic components. The term was introduced by Dr. Rodger Walser, formerly of the University of Texas, Austin, who offered the following definition: “Metamaterials are macroscopic composites having a man-made, three-dimensional periodic cellular architecture designed to produce an optimized combination, not available in nature, of two or more responses to specific excitation.” (See Figure 1.) An alternate definition (adopted by the Air Force Research Lab) states that metamaterials are a class of engineered materials exhibiting highly beneficial electromagnetic properties, which are neither naturally occurring nor common synthetic materials, that exhibit structure-dependent properties not obeying the “rule of mixtures” for composite materials. The rule of mixtures is a method used to estimate a composite material’s properties assuming that these properties are a simple volume-weighted average of the properties of the matrix and dispersed phases. Several features of these metamaterials definitions are worth noting. An engineered material can be a combination of different types of materials and/or structures used to obtain desired material properties. Examples of this are the use of periodic or aperiodic grid surfaces and/or the use of different applications of materials in a suitable combination. Synthetic materials can include ways of introducing vias, voids or cavities into a conventional dielectric with the voids/cavities potentially filled to include dielectric and/or magnetic materials with properties different from the surrounding bulk medium. Layering of such composite materials can then be adopted to yield a bulk material with the desired material properties.
The study of metamaterials was initiated with a scholarly article in 1968 by Vesalgo,1 who was studying electromagnetic transmission properties through a hypothetical medium with a refractive index that was assumed to be negative. Vesalgo predicted that some unusual wave mechanisms would emerge in such a medium, like refraction, wherein a light ray is bent in a negative direction as compared to the conventional bending of a ray in the positive direction for a positive index medium. Vesalago did not claim that such a material could be realized, however.
The work remained dormant until about 1996 when Pendry et al.2 demonstrated that through the clever use of periodic structures one could obtain a medium having negative permittivity. The use of split-ring resonators in a periodic grid was shown to offer a negative index of refraction, and with the additional work of Smith et al.,3 who demonstrated simultaneous negative permittivity and permeability through the use of periodic structures, a flood gate of extensive research opened up the possibility of using a variety of periodic surfaces as a way to provide unique material properties. It should be noted that, although these properties were demonstrated with periodic structures, periodicity is not required to create a metamaterial.
While headlining applications — such as cloaking and invisibility — gain much public fanfare, there are many practical but often overlooked challenges in applying metamaterials to real-world applications. Due to the resonant nature of most metamaterial solutions, the approaches for achieving broadband effective properties are challenging. The principle of loss, or dissipation of the electromagnetic energy through its interaction with the material, presents a significant barrier for applications requiring transparency or high-efficiency transmission. Additionally, the creation of manufacturable bulk materials, beyond a few stacked surfaces, can be a significant challenge for materials and manufacturing engineers. The difficulty of this process is amplified as metamaterial features are pushed down into the nano-scale required for optical frequencies. Ultimately, the success of metamaterials requires an emphasis on the ability to model, design and manufacture them for system applications.
Raytheon has a long legacy of expertise in employing electromagnetic technologies to meet the mission needs of our customers and the warfighter. A significant number of our products involve either the transmission or the sensing of electromagnetic radiation. These include radar, electro-optic and infrared (IR) surveillance, missile seekers, global positioning systems (GPS), lasers, directed energy, command and control, communications and remote sensing. Because of this, Raytheon is pursuing metamaterial technology, both internally and with our academic and industry partners. There are several ongoing developments that promise to add capability and reduce the size of our products from the RF through the visible part of the electromagnetic spectrum.
With reduced availability of real estate for antennas on aircraft, there is a demand for conformal structures with multi-function capability offering broad bandwidths, dual polarization and wide scan angles. Conventional technologies offer either broad-bandwidth capability over a narrow scan angle or wide-angle scanning capability with limited bandwidth. Metamaterials offer a new paradigm for synthesizing suitable structures for dual-band or broad band array applications. Unique conceptual metamaterial structures are currently being investigated to provide alternate solutions to the challenging problem of providing broad bandwidth, or multiple frequency bands, along with a wide scan angle.
Microstrip patch antenna arrays are a popular choice for conformal applications. Broadband, wide-angle scanning can be achieved with the use of low-dielectric-constant substrates, as they delay the generation of surface waves in the spatial and frequency domains. When dealing with certain applications where a low dielectric constant is not an option, the use of higher dielectric constant substrates results in an inherent trade between realizable bandwidth and scan angle performance of the array. Electronic bandgap structures (made up of periodic cells with grounded vias) offer the ability to suppress the surface waves that limit scan angle performance, and when integrated with the microstrip patch elements, these structures offer improved array bandwidth and scan angle performance.
The development of composite skins (Figure 2) that control surface reflectivity is an ongoing effort in collaboration with the University of Michigan. The composite skin uses a periodic array of metallic surface patterns (loops) with embedded varactor (voltage variable capacitance) diodes connected between them. Transmission as a function of frequency through the periodic frequency selective surface (FSS) is controlled by altering the voltage across varactor diodes. Alternatives to the use of varactor diodes, including thin films and ferromagnetic materials, are also being investigated.
Raytheon, in collaboration with the University of South Florida, is developing antennas that incorporate metamaterials for reduced-size dual-band GPS antenna applications. Most of the demonstrated benefits of metamaterial structures for antenna size reduction have been for narrowband applications. However, with proper design of the unit cell structure, dual-band applications are feasible. Due to the close proximity of antenna elements in an array, techniques are required to improve electrical isolation between elements. An example of a dual-band GPS antenna element is shown in Figure 3.
For EO applications, such as lenses, dielectric-based micro- and nanostructure designs are being investigated for the development of low-loss optical polaritonic metamaterials. Polaritonic metamaterials are compounds of at least two materials with very different values of dielectric permittivity (ε).
The design approach is different from conventional plasmonic design, which is based on metal patterns on a dielectric or semiconductor substrate — no metals are used in the polaritonic structure. This significantly reduces the dissipation (loss) associated with conductivity current. Dissipation is one of the key mechanisms of optical loss in plasmonic metamaterial, reaching levels of 10–100 dB/unit wavelength (λ). Such a huge loss makes even micrometer (mm)-thick metamaterial layers fully opaque and precludes many interesting practical applications for plasmonic materials.
In the polaritonic design, resonant electromagnetic oscillations, which are needed to change the refractive index of a material via an induced strong spectral dispersion, are supported by the displacement current of dielectric re-polarization. Displacement current oscillates as a mode pattern of a dielectric cavity formed by an elementary cell. This replaces the conductivity current oscillation mode in the electrical circuit of a plasmonic cell.
Dielectric metamaterials can be created in several different ways. Fine-patterning of a crystalline silicon carbide (SiC):4H wafer was selected as the manufacturing process of choice. This material is the only option among a family of high-e materials for which a transparency window covers a significant portion of the visible/IR spectrum (0.5 to 5 microns); i.e., the region where most applications are expected.
Raytheon, in partnership with Purdue University, is focused on modeling and designing optimal cell patterns of dielectric compounds and developing the processes required for producing such compounds. In parallel, a new method for characterizing metamaterial layers by using reflection spectra for a few different incidence angles is also being developed.
On the manufacturing side, Raytheon upgraded and optimized SiC patterning technology to produce micron-level feature sizes, providing a significant improvement over previous technologies (Figure 4). This advancement enables the production of SiC optical metamaterials for the mid-IR spectral range. The work continues with a goal of reaching even finer spatial resolution to access future IR and visible spectral ranges. Nano- and micro-patterned materials can be deliberately made to exhibit controlled spatial gradients of refractive index (n). This ability offers the potential for realizing novel ultra-light optical components. The idea is similar to creating gradient-index (GRIN) optics. However, a low-loss metamaterial layer provides a very high amplitude range in the index; about Δn ~ 1 compared to Δn ~ .01, which is typical for GRIN optics. Instead of using optical glass, an optical component (such as a lens) can be made into an extremely thin layer, or even a coating, while still providing a large optical path difference.
Figure 5 shows the results of a study by Purdue University to evaluate the potential for applying Raytheon’s SiC-based polaritonic metamaterial layer to create a micro-lens. The left portion of Figure 5 is a schematic of a metamaterial micro-lens design, a set of parallel groves etched into a SiC wafer surface (“cylindrical” version of the lens is shown). The grooves are specified by their depth, width and separation. The right portion of Figure 5 demonstrates that this fine pattern, which is less than 100 microns in overall size, provides a strong focusing effect at a distance of 100 microns. Light is incident from the bottom of Figure 5. A dielectric metamaterial is deliberately used to minimize insertion loss. Such focusing ability cannot be obtained without the metamaterial-based micro-lens array.
There are several advantages to such a micro-lens. First, from a packaging perspective, it is thinner and lighter, and it can also be written directly on a substrate of a detector or emitter matrix to reduce the number of parts for imaging arrays and displays. Second, it can add new functionality features because such a lens can be made spectrally selective to focus radiation for a specific narrow wavelength band, passing the rest of the background spectra unfocused. Third, it can improve device performance by increasing both the detector’s signal-to-noise ratio and the fill factor for a micro-lens array.
In addition to the work noted above, Raytheon is a partner in the newly formed National Science Foundation (NSF)-backed I/UCRC (Industry & University Cooperative Research Center) Program led by the City University of New York (CUNY), and is supported by partner universities including Clarkson University, the University of North Carolina and Western Carolina University. Technologists from across the corporation are engaging with the metamaterials community, which includes industry partners, academia, government and Federally Funded Research and Development Centers, with the intent of applying metamaterials to provide significant gain over the state of the art, striving to advance system capabilities for our customers.
1V. G. Veselago, The electrodynamics of substances with simultaneously negative values of e and m, Sov. Phys. Uspekhi, vol. 10, no. 4, pp. 509–514, 1968.
2J. B. Pendry, et al., Low frequency plasmons in thin wire structures, J. Phys. Condens. Matter, vol. 10, pp. 4785–4809, 1998.
3D. R. Smith, et al., Composite medium with simultaneously negative permeability and permittivity, Phys. Rev. Letter, vol. 84, no. 18, pp. 4184–4187, May 2000.