Measurements of difference in optical interactions between circularly polarized excitations of opposite handedness (circular dichroism) are highly important for both natural and artificial chiral structures. Here the photothermal deflection technique is proposed as a method to detect the optical chirality of a metasurface, analyzing the diffracted beams by the metasurface itself. Two metasurfaces are investigated, based on Au and Ag. The samples are fabricated by nanosphere lithography, with a tilted deposition of thin metal layer, which produces symmetry‐breaking. The unit cell periodicity of these metasurfaces allows for multiple order diffraction in the 450–520 nm range, which encompasses the emission lines of an Ar laser. The metasurfaces are placed on a mirror and excited by an Ar pump beam at different orientations, laser wavelengths, and circular polarization degree; the probe beam scans the absorption of the diffraction orders back‐reflected from the underlying mirror. In this way, the chiral investigation is simplified by placing the scanning of absorption‐induced thermal effect into the metasurface plane, thus avoiding the transmission/reflection measurements of a specific order, which are done by angular placement of detector. Theoretical and numerical approaches are further developed to reconstruct both thermal and optical behavior of the chirality at the nanoscale.

Here we investigated the asymmetric transmission and the anomalous refraction introduced by a metasurface of bent gold nanowires. The refraction follows the generalized Snell’s law that takes into account the resonant behavior of metallic nanostructures located at the interface between two dielectrics. Measurements performed in the linear optical regime reveal a large sensitivity to the subwavelength features of the gold nanostructures.

Arrays of nanoholes in metal are important plasmonic devices, proposed for applications spanning from biosensing to communications. In this work, we show that in such arrays the symmetry can be broken by means of the elliptical shape of the nanoholes, combined with the in-plane tilt of the ellipse axes away from the array symmetry lines. The array then differently interacts with circular polarizations of opposite handedness at normal incidence, i.e., it becomes intrinsically chiral. The measure of this difference is called circular dichroism (CD). The nanosphere lithography combined with tilted silver evaporation was employed as a low-cost fabrication technique. In this paper, we demonstrate intrinsic chirality and CD by measuring the extinction in the near-infrared range. We further employ numerical analysis to visualize the circular polarization coupling with the nanostructure. We find a good agreement between simulations and the experiment, meaning that the optimization can be used to further increase CD.

Optical circular dichroism (CD) is an important phenomenon in nanophotonics, that addresses top level applications such as circular polarized photon generation in optics, enantiomeric recognition in biophotonics and so on. Chiral nanostructures can lead to high CD, but the fabrication process usually requires a large effort, and extrinsic chiral samples can be produced by simpler techniques. Glancing angle deposition of gold on GaAs nanowires can (NWs) induces a symmetry breaking that leads to an optical CD response that mimics chiral behavior. The GaAs NWs have been fabricated by a self-catalyzed, bottom-up approach, leading to large surfaces and high-quality samples at a relatively low cost. Here, we investigate the second harmonic generation circular dichroism (SHG-CD) signal on GaAs nanowires partially covered with Au. SHG is a nonlinear process of even order, and thus extremely sensitive to symmetry breaking. Therefore, the visibility of the signal is very high when the fabricated samples present resonances at first and second harmonic frequencies (i.e., 800 and 400 nm, in our case).

Chirality, a lack of mirror symmetry, is present in nature at all scales; at the nanoscale, it governs the biochemical reactions of many molecules, influencing their pharmacology and toxicity. Chiral substances interact with left and right circularly polarized light differently, but this difference is very minor in natural materials. Specially engineered, nanostructured, periodic materials can enhance the chiro-optical effects if the symmetry in their interactions with circular polarization is broken. In the diffraction range of such metasurfaces, the intensity of diffracted orders depends on the chirality of the input beam. In this work, we combine a photothermal deflection experiment with a novel theoretical framework to reconstruct both the thermal and optical behavior of chiro-optical behavior in diffracted beams.

Optical enantioselectivity of chiral molecules could be enhanced by depositing them on suitable nanostructured substrates. Different kind of chiral substrates can be developed, but chiral features are in general difficult to fabricate or costly. Self-assembled approach allows realizing plasmonic metasurfaces with a low cost reliable procedure. In this case asymmetric fabrication parameters can induce chiral optical response of the realised substrate. Self-organized polystyrene spheres deposited on glass substrate, are utilised to produce asymmetric hole array on a metal thin film. In our case the spheres (518 nm in diameter) where reduced by selective reactive ion etching and then covered by gold (and other metals), that is evaporated at a glancing angle. After the removing of the spheres an elliptical-hole array is produced forming a circular-dichroic substrate. The circular dichroic response of light interacting with the substrate can be tuned by choosing proper incidence angle and excitation wavelength, while the flat nature of the metasurface it is very useful for easy molecular deposition processes. Two new enantiomers (right-handed and left-handed molecules) have been synthesized in order to present a good circular dichroism in the visible range and to be tested on the realized metasurfaces. Different tests were carried out on the samples, investigating the spectral optical properties of the structures with and without chiral molecules on top of them. The results are very promising due to the possibility of easily tuning and optimizing the optical response.