2012 IR&D Annual Report

Design, Modeling and Fabrication of Metamaterials, 14-R8008

Principal Investigators
Michael Miller
Jeremy Pruitt
Diana Strickland
Jerry Helffrich
Leigh Griffith

Inclusive Dates:  12/01/08 – 05/01/12

Background — Metamaterials are engineered materials consisting of arranged components, usually metallic and dielectric, designed to exhibit specific and often unusual electromagnetic properties not found in natural materials. The components typically consist of inclusions, structures much smaller than a wavelength, placed within a host background (e.g., polymer, ceramic, or air). Surface plasmons 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 some metamaterials at infrared frequencies and above.

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, small and novel radio frequency components and antennas, high-sensitivity chemical sensors, and optical cloaking among many others. In this project, SwRI developed 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 staff employed metamaterials to reduce the size of electrically small antennas while maintaining an impedance match to their power source. Additionally, metamaterial surfaces were used to improve the performance of antennas by reducing lossy surface waves and also to improve gain, directivity and element isolation in arrays. SwRI staff also postulated the fundamental question and subsequently demonstrated that surface plasmons could be elicited from metamaterials at infrared frequencies to affect the binding of adsorbed molecules when the frequency of the surface plasmon is matched to molecular vibrations, the molecule's resonance conditions. This demonstration paves the way to engineered repulsion or chemical transformation of adsorbed molecules.

Approach — To determine whether metamaterials improved electrically small antennas and whether they were practical to implement, a novel, electrically small patch antenna incorporating metamaterials as the key component was selected from the literature for a feasibility study. SwRI staff designed, built and tested this metamaterial antenna at several frequencies and compared its performance to conventional, electrically small antennas. Additionally, the effects of various metamaterial surfaces were tested 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 surface plasmons 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 EM 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.

Figure 1. A copper patch antenna with metamaterial between the ground plane and patch.
Figure 1. A copper patch antenna with metamaterial between the ground plane and patch. a) A drawing with transparent patch, exposing a view of aligned spiral rings that compose the metamaterial. b) A top view of the fabricated antenna. c) Radiation pattern measurements. d) Return loss measurements.

Accomplishments — SwRI researchers found that antenna resonance could be tuned independently of antenna dimensions (Fig. 1). 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 as seen in Fig. 1, an array of barium strontium titanate cubes, and Sievenpiper 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 metamaterials. These metamaterials were printed on the same substrate as the antenna with conventional circuit board manufacturing methods.

Figure 2. A 3-D log pile structure designed to support surface plasmons at 60 THz.
Figure 2. A 3-D "log pile" structure designed to support surface plasmons at 60 THz. a) Scanning electron micrograph of a log pile sample. b) Simulation of surface plasmon generation in a log pile unit cell.

SwRI staff demonstrated via systematic computations that free-standing (3-D) wire grids of cubic symmetry could be tailored to evince surface plasmons with infrared frequencies. It was further shown that the oscillating EM field of these plasmons directly coupled 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, 2-D devices consisting of simple, pad-like periodic structures surrounded by nanowires, as well as 3-D "log pile" devices (as seen in Fig. 2), were designed to evince surface plasmon modes in the infrared near 60 THz. Under a Cooperative Research and Development Agreement with Sandia National Laboratories in Albuquerque, N.M., these devices were successfully fabricated and were evaluated using the LTDMS technique to measure the coupling strength between surface plasmons and adsorbed molecules, such as carbon monoxide. 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 10 technical divisions.