Extrinsic chirality in metasurfaces: Traditional and unconventional experiments – INVITED

Plasmonic nanostructures with achiral, but asymmetric shapes can exhibit chiro-optical phenomena at the nanoscale, given that the nanostructure-light interaction symmetry is broken. Such behaviour is defined as extrinsic chirality, and it is induced by properly arranging the experimental set-up. We show measurement techniques for extrinsic chirality in low-cost, asymmetric samples with nanostructures organized in metasurfaces. We employ widely tuneable chiro-optical characterization of transmission and reflection, as well as the circular polarization degree of the transmitted signal; near-infrared range (680-1080nm) and oblique incidence allow for the detection of resonant features in extrinsic chirality. Other, unconventional experiments use photo-thermal consequences of chirality governed absorption in metasurfaces. Photo-acoustic spectroscopy directly gives circular dichroism as a differential absorption of the left and right circular polarizations exciting the sample. Photo-deflection spectroscopy gives additional information of diffraction phenomena governed by the extrinsic chirality. We showed that these techniques can monitor the extrinsic chiral behaviour of the hybrid plasmonic metamaterials. Moreover, they can be used in combination with fluorescence-detected circular dichroism to measure the emission properties of fluorescent materials

Characterization of output circular polarization degree in lowcost asymmetric metasurfaces

Addition of asymmetry in plasmonic nanostructures can lead to chiro-optical phenomena, usually monitored as different absorption of left and right polarization, i.e. circular dichroism. Moreover, interesting features arise when the nanostructure changes the polarization state of the input beam. In this work, we perform extrinsic chirality characterization in a widely tuneable near-infrared range, by monitoring both polarization of the input and of the transmitted beam. We characterize low-cost metasurfaces based on polystyrene nanospheres asymmetrically covered by Ag, by exciting them at different angle of incidence with left, right and linear polarization. We then resolve the circular polarization degree of the transmitted beam, demonstrating resonance-governed circular polarization degree in the output, showing the interplay of both intrinsic and extrinsic chirality

Experiments and simulations of chiro-optical response in lowcost nanohole arrays in silver

2D metasurfaces based on periodic nanoholes in metal have been proposed in various plasmonic platforms. Specifically, their resonant features have led to applications spanning in biosensing. Here we investigate additional degree of freedom in elliptical nanohole arrays with hexagonal geometry: chiro-optical effects. Namely, the in-plane asymmetry and a slightly elliptical shape of nanoholes were previously shown to differently extinct light of opposite handedness, even at normal incidence. We now fully characterize nanoholes in Ag, fabricated by low-cost nanosphere lithography. We first measure the dependence of the transmitted intensity for opposite handedness, in a broad spectral and angle of incidence range. We then resolve the circular polarization degree of the transmitted light when the nanohole array is excited with linear polarization. Finally, we numerically investigate the origin of the chiro-optical effect at the nanoscale. We believe that circular polarization resolving of the transmitted degree could be further adapted as a highly sensitive tool in chiral sensing