A dual band terahertz metamaterial absorber

We present the design, fabrication and characterization of a dual band metamaterial absorber which experimentally shows two distinct absorption peaks of 0.85 at 1.4 THz and 0.94 at 3.0 THz. The dual band absorber consists of a dual band electric-field-coupled (ELC) resonator and a metallic ground plane, separated by an 8 μm dielectric spacer. Fine tuning of the two absorption resonances is achieved by individually adjusting each ELC resonator geometry. (Some figures in this article are in colour only in the electronic version) Metamaterials consisting of artificially constructed electromagnetic (EM) materials have recently attracted considerable interest due to their ability to exhibit engineered exotic EM responses not available in nature, including negative refractive index, superlensing and cloaking [1–4]. Many of these ideas were initially implemented at microwave frequencies due to the simplicity in fabrication. Nevertheless, there has been substantial progress and successful demonstrations of metamaterials extending from the terahertz through the visible frequencies using micro/nano fabrication technologies during the past several years [5–7]. Development of metamaterials at terahertz frequencies is especially important and attractive since there is a strong need to create components to realize applications ranging from spectroscopic identification of hazardous materials to noninvasive imaging. However, it is difficult to find naturally existing materials with strong absorption coefficients that are also compatible with standard microfabrication techniques. Thus, metamaterials are a promising approach to manipulate terahertz waves and to fill this ‘terahertz gap’ [8, 9]. Sub-wavelength split ring resonators (SRRs) have been extensively employed to exhibit a magnetic or electric resonance response at terahertz frequencies [6–11]. Although SRR structures are bianisotropic, it is possible to eliminate the magnetoelectric coupling by making symmetrical structures such as electric-field-coupled (ELC) resonators with shared centre gap or shared centre bar. A pure electric resonance response was experimentally demonstrated at microwave frequency with the EM wave propagating along the substrate surface [12]. However, researchers have found that such ELC resonators could show similar resonance responses with the EM wave propagating normal to the substrate and the electric field aligned perpendicular to the resonators’ gap [10]. To date, due to the limitations of currently available fabrication and characterization techniques, most terahertz metamaterials were made by patterning periodic SRRs in a single planar form to show effective negative electric permittivity (ε̃) or negative magnetic permeability (μ̃). Additional layers or more complex structures need to be introduced for tailoring both ε̃ and μ̃, which is important to realize negative refractive index. Tailoring the effective ε̃ and μ̃ also provides the means to engineer the impedance. Recently, a narrow band microwave resonant metamaterial absorber with theoretical unity absorptivity realized through perfect impedance matching was demonstrated [13]. Progress has been made to extend the absorber design to terahertz frequencies by using double metamaterial layers [14, 15]. Although those metamaterial absorbers show decent absorptivity at terahertz frequencies, they are basically single band absorbers with high absorptivity at a specific frequency. Multi-band terahertz absorbers with two or more optimized absorption peaks have yet to be demonstrated, which could be important, for example, in the development of terahertz spectroscopic imagers/detectors [16, 17]. We experimentally demonstrate a dual band resonant metamaterial absorber with two distinct absorption peaks at 1.4 and 3.0 THz. We use an absorber design similar to [15]. 0022-3727/10/225102+05$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK & the USA J. Phys. D: Appl. Phys. 43 (2010) 225102 H Tao et al