Microwave Meta-Material Absorbers Utilizing Laser Micro-Machining Technology

Abstract

Recently, artificially structured electromagnetic (EM) materials have become an extremely active research area because of the possibility of creating materials which exhibit novel EM responses not available in nature. This includes negative refractive index (NRI), super-lens, cloaking, and more generally, coordinating transformation materials. For the most part, these composites, often called meta-materials (MTMs). The double negative (DNG) MTM structure was realized in 2000 by appropriately depositing SRRs and thin-wires on dielectric substrate. Since then, most of reported designs have a 1D or 2D geometry that responds only to one (two) electrical and magnetic components of the electromagnetic fields. Much of the work in MTM has been focused on the real parts of permittivity and permeability to enable the creation of a negative refractive index material. However, they can be manipulated to create a high performance absorber. According to effective medium theory, MTMs can be represented by the complex electric permittivity ┝eff (= ┝t + j┝u) and magnetic permeability ┤eff (= ┤t + j┤u). By varying the dimensions of electric and magnetic components, it is possible to adjust permittivity and permittivity independently. Additionally, by tuning the electric and magnetic resonances a MTM can be impedance matched to free space, resulting reflectivity R = 0. The additional multiple layers or metallic back-plate will also ensure transmission T = 0. As a result, 100 % absorbance A (= 1– R – T) is theoretically possible. The microwave absorbers are used in military application to reduce the radar cross-section (RCS) of a conducting object and electromagnetic (EM) interference among microwave components. One of the earliest approaches for the design of EM absorber structure is based on the use of Salisbury screen. This type of absorber needs the resistive sheet and a metallic ground plane. The metallic backing plays two main roles; 1) it is used to avoid power transmission on the other side of the absorber, 2) it cancels out a reflected component that combined with the impinging wave. Recently, advancement in absorber technology has been obtained by using artificially MTMs to create a high-performance absorber for the microwave and terahertz frequency regime. In practice, it is difficult to make the absorber’s electrical size small enough at low frequencies. For the design of compact microwave absorbers made by MTM complimentary pairs, we need to choose proper unit cell structures which are characterized by oppositely signed values of real parts of permittivity and permittivity. However, the absorbers are usually made with metallic backing plates in order