Broadband Metamaterial Absorbers for the Infrared
Introduction to Metamaterial Absorbers
A wide variety of applications exists for electromagnetic absorbers both at radiofrequency (RF) and optical regimes, ranging from spectroscopy [91, 108] and thermal imaging [23, 59] to signature control [19, 52, 58, 96]. This has motivated the development of an array of technologies for realizing absorbers with diverse functionality, including multiband  or broadband [7, 15, 21] performance as well as targeting specific angles  or polarizations  of light. Generally, electromagnetic absorbers rely on electric and/or magnetic losses in the constituent material properties in order to achieve energy dissipation in the form of heat. The typical approach is to employ some type of resonant structure that supports high currents or fields that amplify the absorption in the lossy material. Broadband absorption can be obtained using structures with multiple resonances, including multilayer structures with many patterned screens cascaded together . By contrast, the approach described in the following sections requires only a single patterned screen to achieve broadband absorption covering one or more octaves in bandwidth in the infrared (IR), producing designs that can be fabricated without the need for performing difficult registration between patterned layers.
Electromagnetic absorbers have long been a topic of interest, with metal diffraction gratings being identified several decades ago for producing high absorption at specific wavelengths and incidence angles . More recently, metallic gratings were explored for use as narrowband optical absorbers [11, 60, 92]. Similarly, another versatile absorber technology based on exploiting properties of electromagnetic bandgap (EBG) metamaterials was also first introduced for use at RF  and later adapted for THz and optical wavelength applications by employing micro- and nanofabrication techniques [7, 21, 38, 45, 65, 67, 104]. A more general effective medium approach involves designing structures with subwavelength inclusions that give rise to resonances in the effective permittivity or effective permeability of the medium [6, 9, 58, 90]. Near resonance, the effective parameters of such metamaterials can have large imaginary parts that produce high absorption as a wave passes through the medium. One other recent technique termed an optical black hole guides incident light to the core of the device, where an absorbing material can be used to attenuate the electromagnetic waves . Among these techniques, EBG structures are particularly appealing for applications at optical wavelengths because they offer straightforward nanofabrication.
Significant progress has been made at both THz and optical wavelengths to improve the performance of metamaterial absorbers (MMA), experimentally demonstrating single- and multiband devices with near-unity absorption, polarization-independent absorption, and good performance over wide fields of view (FOV) [6, 45, 65, 67, 104]. Efforts have also been made to realize absorbers with wide bandwidths by combining resonators of different sizes [7, 21] or by optimizing the structure [5, 97] to have a wide bandwidth. Recently, researchers were able to achieve an octave of bandwidth with nearperfect absorption across the band .
In the following sections, we describe a technique for synthesizing EBG-based MMAs designed to have excellent (>90%) absorptivity over octave and multi-octave bandwidths and a wide FOV in the near- to mid-IR. Recent material advances such as using Pd instead of conventional Au as the metal and introducing an impedance matching superstrate into the EBG structure are incorporated into the design , and a GA  is employed to synthesize the screen geometry and structure dimensions to meet the challenging performance requirements. Two super-octave MMA designs are presented utilizing Pd and Au screens with >90% absorption over the 2 |rm to 5 |rm band, and two multi-octave MMA designs are synthesized with >90% absorption over the 1 |rm to 5 |rm band and a ±40° FOV, demonstrating the effectiveness of the proposed design technique.