Circuit Analog RF-Visual Antireflection Coatings
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1223-EE02-03
Circuit Analog RF-Visual Antireflection Coatings Nicole L. Abueg1 and Thomas Delfeld1 1 The Boeing Company, P.O. Box 516, St. Louis, MO 63166, U.S.A. ABSTRACT The Boeing Company has developed a unique nanotechnology for antireflective coatings that can perform at near grazing angles (~80°), in the long wave infrared (LWIR). This technology has been validated through mathematical modeling and the fabrication and testing of small scale components. The coatings were designed to perform best at 10 micron wavelengths, and moderately well over the 8 to 12 micron region. The technology is based upon a circuit analog sheet (capacitive) buried within a dielectric, to produce a reflection that adds out of phase with respect to the face sheet reflection. Since the TE component of the reflection is highest near grazing, the sheet is designed to primarily affect that polarization (low impedance) while leaving the TM wave unaffected (high impedance). When desired, in order to improve the TM transmission as well as TE, two layers are used at different depths. This dual-layer approach has also been modeled (in closed form), fabricated and tested. Also, explored in this paper, are the impacts of a design that works well azimuthally at grazing angles. Until recently, the above solution was limited to RF frequencies, but with advances in fabrication it has become possible to fabricate very small nanostructures that operate in LWIR using traditional thin-film vacuum deposition techniques. It is envisioned that eventually such a concept could be used in the visual regime via self assembly. INTRODUCTION Effective antireflection at grazing angles, angles greater than 75 degrees, are difficult to achieve due to the need for low dielectric constant matching layers having low loss. In an attempt to reduce the reflection from a surface at grazing angles, and maximize transmission of energy, The Boeing Company modeled, built and tested circuit analog antireflective coatings at long-wave infrared (LWIR). This allowed for the verification of modeling and to systemically determine where failures in the manufacturing can occur. When electromagnetic energy is incident upon a dielectric interface at a shallow angle, very little power actually enters the dielectric, particularly for polarizations perpendicular to the plane of incidence (TE). For example, when a dielectric having a dielectric constant of 4.86 (ZnS) is used with incident electromagnetic (EM) energy striking the dielectric at 80° off normal, only 30% of the EM energy actually enters the dielectric. The remaining 70% of the energy is reflected. A traditional approach is to graduate the material properties from that of free space to that of the dielectric material being matched. This type of graduation provides transmission over a relatively broad band of wavelengths. A second traditional technique exists in which the thickness and dielectric constant of a matching layer is selected such that the reflection from the surface of the matching layer exactly cancels that from the
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