Extrinsic or pseudo-chiral (meta)surfaces have an achiral structure, yet they can give rise to circular dichroism when the experiment itself becomes chiral. Although these surfaces are known to yield differences in reflected and transmitted circularly polarized light, the exact mechanism of the interaction has never been directly demonstrated. Here we present a comprehensive linear and nonlinear optical investigation of a metasurface composed of tilted gold nanowires. In the linear regime, we directly demonstrate the selective absorption of circularly polarised light depending on the orientation of the metasurface. In the nonlinear regime, we demonstrate for the first time how second harmonic generation circular dichroism in such extrinsic/pseudo-chiral materials can be understood in terms of effective nonlinear susceptibility tensor elements that switch sign depending on the orientation of the metasurface. By providing fundamental understanding of the chiroptical interactions in achiral metasurfaces, our work opens up new perspectives for the optimisation of their properties.

In situ XRD and IR optical measurements demonstrate the coexistence of M₁ and R phases during the SMT transition of VO₂ thin films.

The optical limiting properties of a one-dimensional, transparent metallodielectric photonic band gap structure are studied. Due to light confinement in the structure that enhances the nonlinear response of the layers, a nonlinear transmission dependence on the incident light intensity is found. Experimental results are reported for a four period sample where the single period consists of ZnO and Ag layers 109 and 17 nm thick, respectively. The structure was designed to exhibit a transmission resonance at 532 nm. Under the action of a Q-switched frequency doubled Nd:yttrium–aluminum–garnet laser, a decrease in transmission of approximately 50% is obtained for a maximum incident light intensity of $\text{2\hspace{0.5em}}{\mathrm{GW/cm}}^2.$ These results are explained in terms of a dynamic change of the absorption coefficient due to the enhancement of the two-photon absorption process. These results suggest that the structure is suitable for optical limiting applications in the visible range.

Semiconductor nanostructures hybridized with metals have been known to offer new opportunities in nonlinear optics, plasmonics, lasing, biosensors; among them GaAs‐based nanowires (NWs) hybridized with gold can offer new functionalities, as chiral sensing and light manipulation, as well as circular polarization sources. This study investigates GaAs–AlGaAs–GaAs NWs fabricated by self‐catalyzed growth on Si substrates, and partially covered with gold, thus inducing the symmetry breaking and a potential chiral response. Three different samples are investigated, each of them with a different morphology, as the length and the overall diameter ranging from 4.6 to 5.19 µm and from 138 up to 165 nm, respectively. The samples are first characterized by measuring the absorption spectra by using a scattering‐free photoacoustic (PA) technique. Then, the circular dichroism (CD) is investigated by measuring PA absorption for circularly polarized light under different incident angles at 532 and 980 nm. An efficient CD is found for proper configurations, and results are in good agreement with extrinsic chiral theory predictions and numerical simulations. It is therefore proven that these samples exhibit chiral behavior, and can be further optimized. Moreover, PA technique can be used as an extremely sensitive and efficient technique to characterize their “extrinsic” chirality.

It is possible to obtain an optical response from metasurfaces that exhibit second-order circular dichroism, thanks to the recent successes in nanotechnology, opening the way to new functionalities on a spatial scale of a few hundred nanometers. The second-order nonlinear response of this kind of nanostructured material is an interesting and fascinating issue, as becomes clear from an overview of second harmonic generation and, in particular, the nonlinear circular dichroism raised by metasurfaces of different morphologies and material components, realized by means of self-assembling methods. In this article, we show that the second harmonic generation circular dichroism is very sensitive and useful in the field of nanotechnology characterization.

Nowadays, nanophotonics aims towards low-cost chip-scale devices that can tailor electromagnetic properties, one of which is the control of circular polarization at the nanoscale. Here, we demonstrate the chiral properties of metasurfaces produced by means of nanosphere lithography combined with tilted metal deposition. We apply the photo-acoustic technique to characterize the circular dichroism (CD) at 633 nm of polystyrene nanospheres covered by three different metals: Au- and Cr-covered samples show extrinsic chiral behavior, while the Ag-covered sample shows CD at normal incidence, characteristic of intrinsic chirality. As the experimental data are in good agreement with numerical predictions, we believe that such design can be optimized to get efficient circularly polarized detection at the nanoscale.