2011 IR&D Annual Report

Design, Modeling and Fabrication of Metamaterials, 14-R8008

Principal Investigators
A. Leigh Griffith
Diana L. Strickland
Michael A. Miller
Jeremy R. Pruitt

Inclusive Dates:  12/03/08 – Current

Background — Metamaterials are engineered structures consisting of the arrangement of artificial elements of different materials (usually metallic and dielectric) designed to exhibit specific and often unusual properties not found in natural materials when they interact with electromagnetic (EM) fields. The artificial elements consist of inclusions (structures much smaller than a wavelength) placed within a host background (e.g., polymer, ceramic or air). The EM properties of metamaterials are controlled by the spatial arrangement and shape of the inclusions, and by the selection of materials of which the host and inclusions are composed. Surface plasmons (SPs), which may also be known as surface plasmon polaritons, are intense surface-bound EM waves that propagate in a direction parallel to a metal/dielectric interface. They are often elicited in these structures and are key to the unusual properties exhibited by certain types of metamaterials.

Metamaterials have emerged recently as a subject of intense research by the physics, chemistry and materials science communities because they promise to become the building blocks for novel device applications, such as optical components not limited by diffraction, future-generation microprocessors based on the propagation of light or surface plasmons (instead of current), small radio-frequency antennas, high-sensitivity chemical sensors and optical cloaking, among many others. In this project, SwRI is developing theory, modeling tools and fabrication processes to explore high-consequence applications of metamaterials in two areas of interest: small, high-performance radio-frequency antennas and surface plasmon generation in structures to mediate chemical interactions for catalysis and for chemical sensing.

SwRI is employing metamaterials to reduce the size of electrically small antennas while maintaining an impedance match to their power source. Additionally, metamaterial surfaces or "metasurfaces" are being used to improve the performance of antennas by reducing lossy surface waves and also to improve gain, directivity and element isolation in arrays.

SwRI's interests are further directed toward the fundamental question of whether surface plasmons can be elicited from metamaterials at infrared (IR) frequencies to effect the binding, repulsion or chemical transformation of adsorbed molecules when the frequency of the surface plasmon is matched to molecular vibrations (i.e., resonance conditions).

Approach — A novel, electrically small patch antenna incorporating metamaterials as the key component was selected from the literature for the feasibility study. The goal was to determine whether metamaterials improve electrically small antennas and whether they are practical to implement. SwRI designed, built and tested this metamaterial antenna at several frequencies and compared its antennas to conventional, electrically small antennas. Additionally, we tested the effects of various metamaterial surfaces to improve antenna performance using a broadband simulation test bed.

In an effort to understand the fundamental requirements and potential limitations of mediating the outcome of surface-plasmon-molecule interactions, quantum mechanical and classical computational techniques were applied to study the resonant coupling between low-frequency SPs evinced from nano-scale artificial elements of metasurfaces and the vibrational harmonics of simple molecules. Experimental validation of plasmon-mediated chemical binding and catalysis effects was accomplished by establishing a framework for modeling the electromagnetic scattering properties of two- and three-dimensional periodic structures and exploring suitable techniques for fabricating the structures. A laser-induced, thermal desorption, mass spectrometry (LTDMS) technique was then refined to enable measurement of the interaction energies between these metasurfaces and small molecules (e.g., H2, CO) adsorbed on them.

Accomplishments — SwRI researchers found that antenna resonance can be tuned independently of antenna dimensions. Excellent impedance match was achieved for antennas as small as a 30th of a wavelength (λ/30). The types of metamaterials used to load the antenna included spiral ring resonators printed on circuit board, an array of barium strontium titanate cubes, and Sievenpiper (or mushroom-shaped) structures on printed circuit board. The SwRI team is one of the first to report results for this innovative antenna.

SwRI researchers were also able to reduce the mutual coupling between antenna array elements and improve the gain and bandwidth of individual antennas through the use of metasurfaces. These metasurfaces were printed on the same substrate as the antenna. This can be done on the same layer as the antenna itself, or on an inner layer of the substrate, depending on the desired results and end application.

In the plasmonics research, SwRI demonstrated via systematic computations that free-standing (three-dimensional) wire grids of cubic symmetry can be tailored to evince surface plasmons with infrared frequencies. It was further shown that the oscillating EM field of these plasmons may directly couple with the ground-state fundamental vibrations of adsorbed molecules. However, fabrication of these structures via advanced techniques such as proximity nano-patterning and optical phase-mask lithography was found to be exceedingly difficult and not commercially practical. In its place, two-dimensional devices consisting of simple, pad-like periodic structures surrounded by nanowires, as well as three-dimensional "log pile" devices, were designed to evince SP modes in the infrared near 60 THz. Under a Cooperative Research and Development Agreement with Sandia National Laboratories, Albuquerque, NM, these devices were successfully fabricated and are currently being evaluated using the LTDMS technique to measure the coupling strength between SPs and adsorbed molecules, such as carbon monoxide. If strong enough, this coupling may be exploited to control the structure of surface matter at the nano-scale, opening the door to important opportunities in nano-engineered devices for photocatalysis, quantum control of structure, optical sensors and, possibly, molecular levitation.

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 9 technical divisions.