Here we report the experimental observation of circular dichroism in the second-harmonic field (800–400 nm conversion) generated by self-organized gold nanowire arrays with subwavelength periodicity (160 nm). Such circular dichroism, raised by a nonlinear optical extrinsic chirality, is the evident signature of the sample morphology. It arises from the curvature of the self-assembled wires, producing a lack of symmetry at oblique incidence. The results were compared, both in the optical linear and nonlinear regime, with a reference sample composed of straight wires. Despite the weak extrinsic optical chirality of our samples (not observable by our optical linear measurements), high visibility (more than 50%) was obtained in the second-harmonic generated field.

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.

Here we report the second harmonic emission properties of self-organized gold nanowires arrays supported on dielectric substrates with a sub-wavelength periodic pattern. The peculiar morphology of the nanowires, which are locally tilted with respect to the average plane of the substrate, allows to generate maximum second harmonic signal at normal incidence with a polarization direction driven by the orientation of the wires (perpendicular to the wires). The generation efficiency was increased by tailoring the growth process in order to tune the metal plasmon resonance close to the pump field frequency and also by increasing the local tilt of the nanowires.

We experimentally and theoretically investigate the second order nonlinear optical response of metallodielectric multilayer structures composed of Ag and ${\mathrm{Ta}}_2{\mathrm{O}}_5$ layers, deposited by magnetron sputtering. Second harmonic generation measurements were performed in reflection mode as a function of incidence angle, using femtosecond pulses originating from a Ti:sapphire laser system tuned at $\lambda =800\mathrm{nm}$. The dependence of the generated signal was investigated as a function of pump intensity and polarization state. Our experimental results show that the conversion efficiency from a periodic metallodielectric sample may be enhanced by at least a factor of 30 with respect to the conversion efficiency from a single metal layer, thanks in part to the increased number of active surfaces, pump field localization, and penetration inside the metal layers. The conversion efficiency maximum shifts from 70° for the single silver layer down to approximately 55° for the stack. The experimental results are found to be in good agreement with calculations based on coupled Maxwell-Drude oscillators under the action of a nonlinear Lorentz force term.