Abstract
Metamaterial absorbers have been a topic of considerable interest in recent years, with a particular focus on Terahertz (THz) frequencies due to many natural materials having a weak interaction with THz light. Great efforts have aimed to expand such THz absorbers to cover a wide bandwidth whilst also being highly efficient. However, many of these require cascaded or stacked multilayer resonant elements, where even a small deviation in the alignment between layers is extremely detrimental to the performance. Here, we propose a bilayer metasurface absorber (thickness ∼ λ/6) that is immune to such layer misalignments capable of exceeding a fractional bandwidth (FWHM) of 100% of the central frequency. The design works due to a novel absorption mechanism based on Salisbury Screen and anti-reflection absorption mechanisms, using fractal cross absorbers to expand the bandwidth. Our work is of particular benefit to developing devices which require ultra-wide bandwidth, such as bolometric sensing and planar blackbody absorbers, with the extremely robust absorption responses being unaffected by any misalignments between layers - a limiting factor of previous absorbers.
Highlights
Metasurfaces are the two-dimensional, planar equivalent of the much-studied metamaterials – subwavelength structures arranged periodically to tailor the response of the transmitted or reflected incident light
A particular emphasis has been placed on advancing these devices in the THz regime, which is classed as being the frequency range of 0.1–10 THz, due to the low interactivity of THz radiation with normal materials; many applications can be utilized from THz radiation, such as spectroscopy of several interesting materials which have characteristic “fingerprints”, imaging of biological tissues due to its ability to penetrate visibly opaque media, and other non-destructive and security related examples [32,33,34,35,36,37,38,39,40,41,42,43,44]
We propose an alternative figure of merit for assessing how broadband an absorber is, by calculating the fractional bandwidth, which was stated in section 2 as being the ratio between the Full-Width at Half-Max (FWHM) and central frequency of the band
Summary
Metasurfaces are the two-dimensional, planar equivalent of the much-studied metamaterials – subwavelength structures arranged periodically to tailor the response of the transmitted or reflected incident light. The former approach has yielded impressive results [45], but suffers from fabrication issues – small deviations or imperfections in the alignment of the elements results in a much poorer performance, as was investigated by Chen et al [46], where it was found that a λ/10 misalignment reduces the absorbance from near 100% to 30% The latter approach of horizontally arranging resonant elements allows much more freedom and simplicity, by typically allowing a single-layer and fabrication step, but does not yet match the efficiencies and bandwidths achieved by stacked absorbers [47,48]. Such a design will be of particular benefit to the development of low form-factor blackbody THz absorbers and bolometric sensors, where the bilayer alignment insensitivity will be attractive for commercialization
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