Abstract
Metamaterials and metasurfaces provide a paradigm-changing approach for manipulating light. Their potential has been evinced by recent demonstrations of chiral responses much greater than those of natural materials. Here, we demonstrate theoretically and experimentally that the extrinsic chiral response of a metasurface can be dramatically enhanced by near-field diffraction effects. At the core of this phenomenon are lattice plasmon modes that respond selectively to the illumination’s polarization handedness. The metasurface exhibits sharp features in its circular dichroism spectra, which are tunable over a broad bandwidth by changing the illumination angle over a few degrees. Using this property, we demonstrate an ultra-thin circular-polarization sensitive spectral filter with a linewidth of ~10 nm, which can be dynamically tuned over a spectral range of 200 nm. Chiral diffractive metasurfaces, such as the one proposed here, open exciting possibilities for ultra-thin photonic devices with tunable, spin-controlled functionality.
Highlights
Contrary to the extrinsic chirality observed in previous investigations[21,22,23,24,25,26,27,28], the diffraction-assisted phenomenon reported here enables an extraordinary circular dichroism (CD) for illumination close to normal incidence, and exhibits a spectral response that is extremely sensitive to the illumination angle
Our experimental results demonstrate that extrinsic chirality in metasurfaces can be dramatically enhanced by near-field diffraction effects
For the metasurface at hand, this occurs because high-order resonances of the individual SRRs hybridize with grazing diffraction orders leading to the formation of lattice surface modes (LSMs) that selectively respond to the handedness of circularly polarized light
Summary
We report a new approach for chiral metasurfaces, which results in the first demonstration of a chiral structure with a broad, continuous spectral tunability. This approach is based on a fundamentally different manifestation of extrinsic chirality, where near-field diffraction effects play a key role. Contrary to the extrinsic chirality observed in previous investigations[21,22,23,24,25,26,27,28], the diffraction-assisted phenomenon reported here enables an extraordinary CD for illumination close to normal incidence, and exhibits a spectral response that is extremely sensitive to the illumination angle. We demonstrate experimentally that the proposed metasurface can be used as an ultra-thin circular-polarization spectral filter at near infrared wavelengths, achieving a tuning range of 200 nm with a linewidth of ~10 nm by varying the illumination angle over a range of only 16.5°
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