Optically transparent metamaterial RF absorbers

Abstract

Recent advances in metamaterials have allowed to impart unique properties to flat RF absorbers including broadband absorption, low thickness (in terms of the longest operating wavelength) and polarization insensitiveness, all essential to high-performance absorbers. For these RF absorbers, introducing additional properties of high optical transparency (in the visible range) and mechanical ro-bustness opens up also stealth window applications. However, achieving all of these critical characteristics in a single design is a challenging task. In this thesis, to address this challenge, we propose and demonstrate an optically transpar-ent, broadband, and polarization-insensitive RF absorbing metamaterial that is extremely thin (thickness = 0.079λL; λL: longest operating wavelength). Our design consists of a single dielectric layer of polymethyl methacrylate (PMMA) sandwiched between the top and bottom indium tin oxide (ITO) films, altogether providing high optical transmission. The bottom ITO film acts as a ground plane, which reduces the RF transmission significantly. On the other hand, the top ITO film adorns a unique pattern that minimizes the RF reflection across a particular frequency range. Here we obtained these customized ITO patterns using a novel design methodology. We developed the fabrication process specific to the pro-posed RF structure and fabricated their prototypes. To validate numerical simu-lation results, we measured experimentally the RF absorption of these fabricated prototypes. The experimental results show that the proof-of-concept absorbers achieve over 90% absorption between 4.4-11.2 GHz and over 95% absorption between 4.8-10.6 GHz. Furthermore, we found the fabricated absorbers to be in-sensitive to polarization angles and preserve 90% absorption for oblique incidence angles of 60° for TM and 40° for TE polarizations in agreement with the numer-ical predictions. Also, besides RF characterizations, we optically recorded the transmittance in the visible range to be 65% on average for the tested absorbers. These findings indicate that the proposed single-dielectric-layered architecture of optically transparent, broadband, polarization-insensitive RF absorbers, featur-ing a record relative thickness of 0.079λL, holds great promise for use in stealth window applications.