Photo-acoustic spectroscopy with widely tuneable laser enables circular dichroism mapping

Sensitive and reliable characterization of chirality in nanostructures and molecules is of great importance in multidisciplinary research combining physics, chemistry and nanotechnology, with potential applications in pharmaceutical and agrochemical industry. Chirality is connected to circular dichroism (CD) - the absorption difference when the chiral medium is excited with circular polarizations of opposite handedness. Hence, measuring chirality by direct absorption measurements is of great interest in nanophotonics and plasmonics community, where the nanostructured media can enhance chiro-optical effects. Here we present a recently constructed photo-acoustic spectroscopy (PAS) set-up, which offers many degrees of freedom in characterization. We use a laser which is widely tuneable in the near-infrared (680-1080 nm) and visible (340-540 nm) ranges. The laser output is modulated with a mechanical chopper, where its frequency defines the penetration depth of the thermal signal. The input polarization is controlled by a linear polarizer and a quarter-wave plate, and the laser can be focused before impinging on the sample in the tightly closed photo-acoustic cell. The cell is placed on translational and rotational stages, which allows for the spatial mapping and extrinsic chirality measurements. Finally, a sensitive microphone measures the pressure changes in the cell, enabling scattering-free measurement of absorption and CD.