Work on structural composites with tunable chiral elements has produced electronically tunable overall chiral composites, mechanically tunable chiral composites, flat lenses with soft hyperbolic focusing due to indefinite overall permittivity, a tunable flat lens based on chiral elements with adjustable focal spot based on applied mechanical deformation, and a three-phase periodic composite that demon strates positive and negative refraction, depending on the input frequency and angle of incidence. A MATLAB code directly computes the group velocity and pass bands for a given set of wave vectors, and generates an intuitive plot for quick, but thorough analysis.

Schematic of the test configuration (top) and the actual test setup (bottom). Note: Schematic is not to scale.

A broadening application range has increased the demand for advanced RF control. Recent research has identified several metamaterials to provide this control. This work seeks to expand this idea through several novel metamaterials with enhanced electromagnetic properties. First, copper wires braided with Kevlar and nylon to form conductive coils were woven among structural fiber to create a fabric. This yielded a composite with all coils possessing the same handedness, producing a chiral material. The measured scattering parameters showed considerable chirality within the 5.5-8 GHz frequency band, agreeing with simulation results.

Simulation modeled the material using a full-wave adaptive solution for the 1-12 GHz frequency band with an incidence angle between 0 and 90 degrees. Experiments are underway with a three-layer sample consisting of an array of hollow glass tubes in a Rexolite matrix. Thicker samples may be tested through the addition of extra layers. The sample is placed in a polycarbonate-fiberglass test fixture that is adjusted for the desired angles of incidence. A 3D scanning robot scans the desired test volume, and the vector analyzer (VNA) sends and receives the field response in the form of S- parameters for the 7-12 GHz frequency band (see figure).

Electronic chirality tuning is investigated by integrating varactor diodes into an array of helical elements on a printed circuit board. Applying a varied reverse bias voltage across the sample effectively tunes the chiral behavior of the material. Chirality can be further tuned mechanically through the deformation of an array of conductive coils. Parallel, metallic helices embedded in a polyurethane matrix are subjected to mechanical stretching for pitch adjustment. This change in pitch directly affects the overall chirality of the composite. Repeatable elastic deformation is achieved up to 50% axial strain. Over the 5.5-12.5 GHz frequency range, an increase of 30% axial strain yields an ~18% change in axial chirality.

Hyperbolic microwave focusing is explored through an indefinite medium with anisotropic permittivity. An array of 12-gauge brass wires is embedded in Styrofoam and scanned over the 7-9 GHz frequency band to establish focusing patterns. A soft-focusing spot is observed at 7.6 GHz with a relative gain of ~7dB over averaged background.

Tests of the fixture with and without the sample(s) will be normalized with respect to air. The measured S-parameters will indicate the stop and pass bands in the frequency range, and will be correlated with the numerical predictions.

Applying an axial refractive gradient to a coil composite creates a lens capable of fine adjustment in the microwave range. The gradient required to achieve sharp focusing, and the extent of this effect, is calculated through an anisotropic ray-tracing analysis. A composite is created using coils of opposite handedness to minimize chiral effects. Through extension of these coils, the refractive index can effectively be fine-tuned to achieve the desired result. Measurements and full-wave simulations confirm a gain of 6-8 dB over averaged background at the predicted focal frequencies.

This work was done by Siavouche Nemat Nasser of the University of California San Diego for the Air Force Office of Scientific Research. AFOSR-0010


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This article first appeared in the December, 2015 issue of Aerospace & Defense Technology Magazine.

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