Effective chiral behavior on self-assembled tilted gold nanowires metasurface by means of linear and nonlinear optical techniques

Even non-chiral objects can exhibit effective optical chiral response due to particular symmetry breaking of the investigating light and the sample morphology. Here we show linear and nonlinear optical measurements performed on a metasurface composed by self-assembled tilted golden nanowires on silicon substrate. The measurements are performed in three different schemes: optical reflectance, photoacoustic absorbance and second harmonic generation. In all these schemes circular polarized light was used in order to evidence the optical chiral behavior in different reciprocal disposition of the wires and light direction. The circular dichroism results to be present in all schemes when the three directions formed by i) the wires orientation, ii) the impinging light wave vector and iii) the normal to the metasurface forms a non-planar triad. Indeed non-planar triad of vectors represents a system that cannot be superposed to its mirror image, thus it is chiral system. We measured a sample obtained by vacuum evaporation of gold at glancing angle on a silicon substrate maintained at the temperature of 300K. The gold nanowires form a forest homogeneously distributed on 1 square inch substrate. Even if the chirality was detected both in linear and nonlinear optical measurements, the second harmonic generation process results to be more sensitive.