Plasmonic nanostructures with achiral, but asymmetric shapes can exhibit chiro-optical phenomena at the nanoscale, given that the nanostructure-light interaction symmetry is broken. Such behaviour is defined as extrinsic chirality, and it is induced by properly arranging the experimental set-up. We show measurement techniques for extrinsic chirality in low-cost, asymmetric samples with nanostructures organized in metasurfaces. We employ widely tuneable chiro-optical characterization of transmission and reflection, as well as the circular polarization degree of the transmitted signal; near-infrared range (680-1080nm) and oblique incidence allow for the detection of resonant features in extrinsic chirality. Other, unconventional experiments use photo-thermal consequences of chirality governed absorption in metasurfaces. Photo-acoustic spectroscopy directly gives circular dichroism as a differential absorption of the left and right circular polarizations exciting the sample. Photo-deflection spectroscopy gives additional information of diffraction phenomena governed by the extrinsic chirality. We showed that these techniques can monitor the extrinsic chiral behaviour of the hybrid plasmonic metamaterials. Moreover, they can be used in combination with fluorescence-detected circular dichroism to measure the emission properties of fluorescent materials.

Circular dichroism spectroscopy is a key probe of the structural and optical properties of chiral materials, however, commercial circular dichroism spectrometers are large, prohibitively expensive and rarely offer environmental control of the sample under test. Here we demonstrate two low-cost (<£2,000) and portable imaging systems controlled by our own bespoke open-source control software which are capable of spatially mapping the circular dichroism of chiral solid state films. By coupling these imaging systems with a temperature controlled stage, we show that we can rapidly identify the thermal processing conditions required to maximise circular dichroism in chiral solid state films by measuring circular dichroism in situ during thermal annealing of a sample under test. The accuracy and spatial resolution of these circular dichroism imagers are cross-compared against our previous studies using an existing circular dichroism imaging system at the Diamond Light Source and are shown to be in good agreement, with a sensitivity down to 250 mdeg and a spatial resolution of 100 μm.

Lattice resonances, collective electromagnetic modes supported by periodic arrays of metallic nanostructures, produce very strong and spectrally narrow optical responses. Recently, there has been significant effort devoted to exploring their chirality (dissymmetric response to right- and lefthanded circularly polarized light) in arrays built from complex unit cells. In this communication, we investigate the lattice resonances of square bipartite arrays in which the relative positions of the nanostructures can vary in all three spatial dimensions, i.e., 2.5- dimensional arrays. Despite the achirality of their unit cell, the lattice resonances supported by these systems can display an almost perfect chiral response and very large quality factors due to the constructive and destructive interference between the electric and magnetic dipoles induced in their nanostructures. Our results provide the fundamental understanding to achieve strong chiral lattice resonances in structurally achiral 2.5-dimensional periodic arrays.

Exotic light fields combining non-trivial spin and angular momentum may not be eigenstates of either the spin or orbital angular momenta operators. For these fields, it is relevant to define a Generalized Angular Momentum operator of which they are eigenvectors. Their associated eigenvalues can take, depending on the case, non-integer values.We report that this new quantity is conserved via non-linear phenomena, such as High Harmonic Generation.

Understanding high-order harmonic generation (HHG) from solid targets holds the key of potential technological innovations in the field of high-frequency coherent sources. Solids present optical nonlinearities at lower driving intensities, and harmonics can be efficiently emitted due to the increased electron density in comparison with the atomic and molecular counterparts. In addition, crystalline solids introduce a new complexity, as symmetries play a role in the anisotropic character of the optical response. An extraordinary playground is, therefore, the scenario in which solids are driven by vector beams, since crystal symmetries can be directly coupled with the topology of the driving laser beam. In this contribution we analyze the topological properties of the HHG radiation emitted by a single-layer graphene sheet driven by a vector beam. We show that the harmonic field is a complex combination of vortices, whose geometrical properties hold information about the details of the non-linear response of the crystal. We demonstrate, therefore, that the analysis of the topological structure of the harmonic field can be used as a spectroscopic measurement technique, paving the way of topological spectroscopy as a new strategy for the characterization of the optical response of macroscopic targets.