Design and fabrication of narrow band radar absorbing materials at terahertz frequencies

The technique of tailoring the complex refractive index of an artificial dielectric material has been developed at the University of Lowell Research Foundation (ULRF). Low reflection coatings, generally referred to as Dällenbach layers, have been designed for metal substrates using the artificial dielectric. The method of characterizing materials for the purpose of tailoring their dielectric properties at terahertz frequencies will be discussed. Results will be shown for a typical dielectric coated aluminum plate specifically designed for low reflection behavior at ≈ 0.6 terahertz. Design of optical and quasi-optical measurement systems operating at terahertz frequencies require a wide range of materials for component fabrication. Critical to implementation of these laboratory systems are far-infrared radiation absorbing material (FIRAM) which may be used to suppress unwanted stray radiation. One type of FIRAM, known at microwave frequencies as the Dällenbach layer, consists of a metal substrate coated with an evenly thick homogeneous lossy dielectric material.[1] See figure 1. Realization of the Dällenbach layer as FIRAM starts with ULRF's development of a particular class of submillimeter wavelength artificial dielectric, generically referred to as metallic paints. The paint, manufactured by Stainless Steel Coatings, Inc. of Littleton, Mass, consists of resins such as vinyl acetate, silicone or polyurethane uniformly loaded with stainless steel flakes. Since the metal flakes are dimensionally small compared to the wavelength, the paint exhibits optical properties of a homogeneous media which depend on the concentration of stainless steel flake. Measurements were performed on a series of vinyl acetate based paint samples, each with a different concentration of stainless steel flake. The material's complex refractive index, n – i k, was shown to be well behaved as a function of the metal flake loading ratio, ρ. An algorithm was developed to describe the behavior analytically and expressed as: n ≈ 1.73 x 10 0.0201 ρ and k ≈ 0.082 x 10 0.0504 ρ (1) for a loading ratio in the regime of 0 ≤ ρ ≤ 30 grams of stainless steel flake per liter of vinyl acetate binder. The technique of characterizing and tailoring the dielectric constant of metal loaded paints at terahertz frequencies had been previously demonstrated at ULRF.[2] When tied to theoretical modeling of the paint's reflectivity as a function of thickness, dielectric constant and frequency using the Fresnel equations, anechoic coatings can be designed to provide more than –25dB of radar cross section (RCS) reduction at any frequency in the terahertz …