Abstract
Planar optical chirality of a metasurface measures its differential response between left and right circularly polarized (CP) lights and governs the asymmetric transmission of CP lights. In 2D ultra-thin plasmonic structures the circular dichroism is limited to 25% in theory and it requires high absorption loss. Here we propose and numerically demonstrate a planar chiral all-dielectric metasurface that exhibits giant circular dichroism and transmission asymmetry over 0.8 for circularly polarized lights with negligible loss, without bringing in bianisotropy or violating reciprocity. The metasurface consists of arrays of high refractive index germanium Z-shape resonators that break the in-plane mirror symmetry and induce cross-polarization conversion. Furthermore, at the transmission peak of one handedness, the transmitted light is efficiently converted into the opposite circular polarization state, with a designated geometric phase depending on the orientation angle of the optical element. In this way, the optical component sets before and after the metasurface to filter the light of certain circular polarization states are not needed and the metasurface can function under any linear polarization, in contrast to the conventional setup for geometry phase based metasurfaces. Anomalous transmission and two-dimensional holography based on the geometric phase chiral metasurface are numerically demonstrate as proofs of concept.
Highlights
Left and right circular polarizations are spin eigenstates of a photon
We proposed an all-dielectric planar chiral metasurface that exhibits near unity circular dichroism and asymmetric transmission for circularly polarized lights
The transmitted light from one particular handedness is efficiently converted to the opposite handedness, acquiring a geometric phase relying on the rotation angle
Summary
Left and right circular polarizations are spin eigenstates of a photon. As light propagates, the instant electric field vector traces out a helical trajectory, which is intrinsically chiral, meaning that its mirror image ‘cannot be brought to coincide with itself’ (Lord Kelvin). In chiral molecules ( called ’enantiomers’), both the real and imaginary parts (extinction coefficient) of the refractive index vary for left and right circularly polarized light, giving rise to chiroptical effects such as optical activity (circular birefringence) and circular dichroism (CD), respectively [2]. Twisted multi-layer cascaded structures are commonly used to achieve giant optical chirality [6,7,8,9,10,11] Most of these metal-based three dimensional chiral metamaterials demands sophisticated fabrication processes and exhibit unavoidable absorption losses [12]. We propose a low-loss planar-chiral all-dielectric metasurface that exhibits giant circular dichroism and asymmetric transmission value close to unity for circularly polarized lights, free from the 25% limit and the loss requirement. The concept is numerically verified with examples including anomalous transmission and a two-dimensional computer generated hologram with high efficiency of 0.6
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