Abstract
We propose a design for an universal absorber, characterized by a resonance frequency that can be tuned from visible to microwave frequencies independently of the choice of the metal and the dielectrics involved. An almost perfect absorption up to 99.8% is demonstrated at resonance for all polarization states of light and for a very wide angular aperture. These properties originate from a magnetic Fabry-Perot mode that is confined in a dielectric spacer of λ/100 thickness by a metamaterial layer and a mirror. An extraordinary large funneling through nano-slits explains how light can be trapped in the structure. Simple scaling laws can be used as a recipe to design ultra-thin perfect absorbers whatever the materials and the desired resonance wavelength, making our design truly universal.
Highlights
We propose a design for an universal absorber, characterized by a resonance frequency that can be tuned from visible to microwave frequencies independently of the choice of the metal and the dielectrics involved
Whatever the approaches considered based either on critical coupling or impedance matching effects, the targeted operating frequency usually imposes the choice of the materials constituting the absorbers and strongly constraints the design
Plasmonic absorbers have proven to be effective for visible and infrared radiations while metamaterials are preferably used from the terahertz to the microwaves[7,8,9,10,11,12,13]. Another approach consists in modifying the material property such as its plasma frequency according to the targeted operating frequency[14,15,16,17]
Summary
The absorber consists of a deeply subwavelength grating made of nanometers slits etched in a thin metallic slab which is separated from a metallic back mirror by a dielectric spacer, Fig. 1. The resonant wavelength of the gap plasmon mode is equivalently linked to the effective index and thickness by λs = 2nt With these definitions, the whole 1D absorber can be replaced by a much simpler equivalent system made of a dielectric spacer sandwiched between a back mirror assumed to be a perfect electric conductor (PEC) and an absorbing layer of complex index n corresponding to the grating layer, Fig. 1c. It turns out the RCWA allows to access this phase rigorously, by retrieving the actual reflection coefficient on the metamaterial layer (see Supplementary section C), and that such a computation totally confirms the analytical results Inserting this phase condition into Eq (6) and for nanometer slits (2δ f ), we get a simple expression for the resonant wavelength of the FP mode: Λ d f (9). Eqs (8) and (9) provide simple scaling laws for designing universal absorbers operating at arbitrary large wavelengths
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