All-dielectric planar chiral metasurface with gradient geometric phase

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.

Spin-preserving chiral photonic crystal mirror

“…It has been recently demonstrated that judicious design of nanopillars carrying in-plane geometrical chirality can potentially result in different coupling of opposite helicities to the waveguide-array modes which in turn leads to their differential response between left-and right-handed circularly polarized lights. 42 Ye et al 43 have shown that multi-mode interference can also arise in metasurfaces made of low-loss metallic nanostructures with finite thickness. Consequently, properly designed metallic nanoposts can support surface plasmon modes whose different interference schemes for opposite helicities yields a giant chiroptical effect.…”

Biomimetic ultra-broadband perfect absorbers optimised with reinforcement learning

“…Metamaterials that exhibit a wide range of functionalities have been designed over the past few decades [1][2][3][4][5][6]. Of these metamaterials, perfect absorbers offer numerous benefits over conventional absorbers such as increased effectiveness, miniaturisation and the ability to be designed for a specific narrow or broadband range [7][8][9][10].…”

Dielectric Chiral Metasurfaces for Second‐Harmonic Generation with Strong Circular Dichroism

“…Chiral metamaterials and metasurfaces [1][2][3][4][5] have recently attracted much attention in the community of nanophotonics due to their practical applications for circular dichroism (CD), [6][7][8][9][10][11] polarization conversion, [12] chiral biosensing and imaging, [13][14][15] polarimetric detection, [16] spin-controlled wavefront shaping, [17,18] and asymmetric transmission. [10,18,19] Spin-selective optical responses can also be obtained in intrinsically achiral metasurfaces due to extrinsic chirality by the experimental arrangement different from their mirror image [20] and even by the direct phase modulation depending on the handedness of circular polarization of the incident wave. [21] The above metamaterials and metasurfaces can be made of both plasmonic and dielectric materials, the latter of which are preferable due to lower optical losses and stronger CD.…”

“…[21] The above metamaterials and metasurfaces can be made of both plasmonic and dielectric materials, the latter of which are preferable due to lower optical losses and stronger CD. [9,10] Over the past decade, nonlinear nanostructures and metasurfaces [22] have extensively investigated for diverse practical applications, including efficient harmonic generation, [23][24][25][26][27][28] four-wave mixing, [29] high-order stimulated Raman scattering, [30] and nonlinear beam shaping. [31] In particular, nonlinear optical processes in chiral metasurfaces [32][33][34][35][36][37][38][39] have great importance for nonlinear chiral sensing and nonlinear metasurface holography.…”