Using in-house expertise in the technique of tailoring artificial dielectrics and the polarimetric characterization of materials, a variety of composite structures have been created as far-infrared radiation absorbing material (FIRAM™) . As demonstrated by Janz and co-workers in the millimeter wavelength regime, wedge and pyramidal–structured surface geometries improve a material's absorption properties by increasing the number of surfaces incident radiation must encounter before backscattering to the receiver occurs. Measurements performed by these and other researchers have shown that the reduction in reflectivity achieved may be expressed by:
Rs ≈ Rf(180°/θg)
where Rf is the material's front surface reflectivity and θg the structure's groove angle. Since this type of FIRAM™ is generally fabricated from homogeneous lossy dielectric materials which exhibit front surface reflectivities (Rf) of less than 10%, anechoic structures can be designed to provide more than -80dB of reflectivity reduction for a groove angle of 22.5°.
Using a silicone elastomer and electrically insulating siliceous filler, Prewer and Milner of Thorn EMI Technology Inc. fabricated the first samples of terahertz frequency anechoic in the form of pyramidal surface structured tiles. Measurements performed during July of 1989 at STL further documented their success when specular and diffuse reflectivity levels of better than -40dB were observed at 0.584 THz for almost all incident directions. However, wedge and pyramidal structured anechoic materials suitable for large-scale use at terahertz frequencies were not commercially available at that time.
Therefore, STL researchers initiated a project to design and fabricate silicone-based wedge-structured anechoic material for use with its submillimeter-wave measurement systems. A method of estimating refractive indices was used to characterize a variety of materials in search for a lossy dielectric exhibiting low front surface reflectivity. Using cost-efficient molding techniques, prospective dielectric materials such as the widely available plastics and elastomers promised to provide good anechoic properties if constructed with modified surface geometries.
Shown above are the design parameters of the FIRAM™'s wedge-type surface geometry. A -30dB reflectivity reduction or greater was achieved at normal incidence, however when appropriately oriented within quasi-optical measurement systems these materials reduce reflections due to unwanted stray radiation by more than -60dB. The geometry of the grooved surfaces shown below are manufactured in 2'x2' sheets to precise tolerances through a pressure injection molding process. Considerations such as manufacturablility and cost were addressed in choosing the methodology to implement the anechoic structures.
The original FIRAM-500, shown above on the right, is optimized for frequencies of 500 GHz and above. The FIRAM-160, shown above on the left, is optimized for use at 160 GHz and has proven to also work efficiently at frequencies of 500 GHz and above.
STL also has developed TERASORB™ which consists of interlocking 4"x4" (10cm x 10cm) tiles. Designed for applications at 500 GHz, TERASORB-500 provides 40 dB reduction in reflectivity at normal incidence and achieves reflectivity reduction levels of at least 60 to 80 db away from the normal. Designed for applications at 1.5 THz, TERASORB-1500 has similar performance characteristics.