Design and characterization of metamaterial-based dual band absorber for infrared applications

Metamaterials are compounds of periodic sub-wavelength elements, whose properties are strongly influenced by the applied external electromagnetic (EM) waves within a typically narrow frequency band. Metamaterial structures could be designed to produce very thin films with high losses manifested by perfect absorption. This work discusses a metamaterial (MTM) based dual band absorber operating at mid infrared (IR) regime. The idea of this absorber originates from combining the absorption responses of the cross shaped and cut wire (CW) resonators. The strong coupling between the two resonators is enhanced by introducing a U-shaped resonator interdigitated between the CW and the cross-shaped resonators. This setup would improve the coupling between the neighboring cells and hence, reduce the overall dimensions. The simulated results are obtained using the full electromagnetic wave simulation package CST MWS. The absorber is designed to possess two distinct absorption bands centered at 9.57μm and 4 μm. The two bands are well separated with a first to second band ratio of ~2.39. The sub-wavelength period size is found to be 0.19λ and 0.45λ with respect to the first and second bands respectively. High absorption is also observed for wide range of incident angles. Moreover, circuit elements based on circuit approximation are calculated by analyzing the electric and magnetic fields to provide a physical meaningful circuit representation of the electromagnetic response, which can be very helpful in the design and optimization process. The extracted circuit model fully describes and simplifies the behavior of the periodically distributed physical structure and translates it into equivalent discrete entities. Therefore, it offers an approximate prediction for the resonance frequencies. Given that the size of the sub-wavelength elements are much smaller than the incident EM wave operating wavelength, the MTM compound behaves as a homogeneous medium, whose properties can be expressed by effective constitutive parameters. Effective constitutive parameters are fitted using Drude-Lorentz oscillators. This provides insight into the nature of the microscopic behavior of the absorber elements when they are excited by external fields. Finally, samples of this design were fabricated and tested. The measured response shows two absorption peaks with better than 85% absorption for both peaks. This design shows significant improvement over the existing designs, which makes it appealing for various planar applications in the mid-IR frequencies.