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 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.
In this work, we study chiral effects in well-known 2D plasmonic nanohole arrays with a triangular unit cell. The chirality can be induced by moving from circular to elliptical nanohole shape and tilting the ellipse away from the array symmetry. This symmetry breaking induces a different absorption of the circularly polarized light of opposite handedness, i.e. circular dichroism. We numerically study circular dichroism at normal incidence in elliptic nanoholes in Au in the spectral range 400-1000 nm. CD arises in transmission and absorption spectra in the same wavelength region of extraordinary optical transmission, indicating highly resonant light-metasurface coupling mechanisms. We focus on its dependence on the elliptic nanohole tilt and further proceed with the ellipse radii optimization. The optimized CD is on the order of 80%, and it is robust with respect to the radii and rotation angle variations. Moreover, such samples could be produced by means of low-cost nanosphere lithography, which makes them interesting for applications in enhanced sensing of chiral biomolecules.
Asymmetric nanostructures can mimic a chiral response when circular polarized light interacts with the structures under particular angle of incidence . This phenomenon is called ‘extrinsic chirality’ and usually is present under linear optical investigation with low visibility. Due to the fact that optical second harmonic generation is possible only in samples with some degree of asymmetry, this can be used in order to investigate the extrinsic chirality with a background free technique, thus inducing a high visibility of the artificial circular dichroism [2,3].
Here we present the second harmonic generation (SHG) measurements obtained on samples composed by GaAs nanowires grown on silicon.
The wires present resonant leaky modes around 800nm and at 400nm due to the high refractive index contrast ratio between wires and air, even if these wavelengths lie on the absorption band of GaAs . The measurements performed on this sample present good SHG signal due to the second order nonlinear term of GaAs, but did not present any circular dichroism (SHG-CD).
By coating the sample with a 20nm thin layer of gold deposited asymmetrically, by evaporating the metal only from one side of the nanowires, the symmetry of the structure is broken, thus induced high SHG-CD.
The SHG-CD is measured by shining the sample with circular polarized pump light at the fundamental wavelength of 800nm and by revealing the second harmonic signal at 400nm in s or p polarization, as a function of sample rotation.
Four samples were measured with GaAs wires of about 5 micron in length with different diameters ranging from 140nm to 200nm. In this case it is possible to explore different resonance conditions and different SHG-CD is revealed. For each sample, the measured were carried out before and after the asymmetric gold layer deposition, thus allowing direct comparison of the results.
 A. Belardini, M. C. Larciprete, M. Centini, E. Fazio, C. Sibilia, D. Chiappe, C. Martella, A. Toma, M. Giordano, and F. Buatier de Mongeot, Phys. Rev. Lett. 107, 257401 (2011).
 A. Belardini, M. Centini, G. Leahu, D. C. Hooper, R. Li Voti, E. Fazio, J. W. Haus, A. Sarangan, V. K. Valev, C. Sibilia, Sci. Rep. 2016, 6, 31796.
 G. Leahu, E. Petronijevic, A. Belardini et al., Adv. Optical Mater. 2017, 1601063 (2017).
 G. Leahu, E. Petronijevic, A. Belardini, M. Centini, R. Li Voti, T. Hakkarainen, E. Koivusalo, M. Guina, C. Sibilia. Sci. Rep. 7, 2833 (2017).
Electromagnetically induced transparency (EIT), a pump-induced narrow transparency window within the absorption region of a probe, had offered new perspectives in slow-light control in atomic physics. For applications in nanophotonics, the implementation on chip-scaled devices has later been obtained by mimicking this effect by metallic metamaterials. High losses in visible and near infrared range of metal-based metamaterialls have recently opened a new field of all-dielectric metamaterials; a proper configuration of high refractive index dielectric nanoresonators can mimick this effect without losses to get high Q, slow-light response. The next step would be the ability to tune their optical response, and in this work we investigate thin layers of phase change materials (PCM) for all-optical control of EIT-like all-dielectric metamaterials. PCM can be nonvolatively and reversibly switched between two stable phases that differ in optical properties by applying a visible laser pulse. The device is based on Si nanoresonators covered by a thin layer of PCM GeTe; optical and transient thermal simulations have been done to find and optimize the fabrication parameters and switching parameters such as the intensity and duration of the pulse. We have found that the EIT-like response can be switched on and off by applying the 532nm laser pulse to change the phase of the upper GeTe layer. We strongly believe that such approach could open new perspectives in all-optically controlled slow-light metamaterials.