Metasurfaces and metamaterials
Can one flat surface treat left and right circular waves differently?
Open access · cc by · source: Europe PMC
By making two resonators in each cell interfere constructively for one circular polarization and destructively for the other, a thin metasurface reflects one spin almost perfectly while absorbing the other.
Study at a glance
- Design
- Other — Analytic interference criterion for a two-split-ring meta-atom, FDTD simulations, and microwave anechoic-chamber measurements of two printed-circuit metasurfaces
- N
- Two fabricated metasurfaces (31x31 and 39x39 meta-atoms); no sample count
- Population
- Microwave reflective metasurfaces made of twisted split-ring resonators on a grounded dielectric
- Outcome
- Co-polarized reflection of left- vs right-circular waves, Bessel-beam field profiles, radar cross-section reduction, vortex beam patterns
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Key findings
Simulations gave an extinction ratio of 33.3 between the two spins at 9.5 GHz, with strong contrast kept up to large incidence angles. The Bessel-beam surface produced a needle-like beam for one spin over about 75 mm, with a half-power bandwidth of roughly 3.4 GHz, while for the other spin it cut the radar cross-section by up to 20.7 dB. The vortex surface produced four vortex beams at 10.5 GHz but only two at 9.5 GHz, where the spin-up beams were absorbed.
Methodology
The authors designed a unit cell of two split-ring resonators twisted 45 degrees relative to each other, and derived that a 90 degree propagation-phase difference plus this twist gives in-phase reflection for one circular polarization and out-of-phase cancellation for the other. They checked the cell with FDTD simulations, then built two microwave metasurfaces by rotating cells across the surface: one to make a non-diffracting Bessel beam and one to make multiple vortex beams carrying orbital angular momentum. Both were measured in an anechoic chamber.
Limitations
The design theory neglects coupling between the two resonators, and simulations deviate from it away from the centre frequency. Everything is demonstrated at microwave frequencies; operation in the visible or infrared is only argued, not shown. The spin selectivity works over a limited band, and alignment and feed imperfections cause measurement deviations.
How this study connects
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