Experimental control of optical helicity in nanophotonics

An analysis of light–matter interactions based on symmetries can provide valuable insight, particularly because it reveals which quantities are conserved and which ones can be transformed within a physical system. In this context, helicity can be a useful addition to more commonly considered observables such as angular momentum. The question arises how to treat helicity, the projection of the total angular momentum onto the linear momentum direction, in practical experiments. In this paper, we put forward a simple but versatile experimental treatment of helicity. We then apply the proposed method to the scattering of light by isolated cylindrical nanoapertures in a gold film. This allows us to study the helicity transformation taking place during the interaction of focused light with the nanoapertures. In particular, we observe from the transmitted light that the scaling of the helicity transformed component with the aperture size is very different to the direct helicity component. Light beams: helicity control Researchers in Australia have developed a way to control the helicity of a light beam in nanophotonics. Helicity – the projection of a beam's total angular momentum onto the direction of its linear momentum – is a useful parameter for explaining light-matter interactions. Nora Tischler and co-workers first passed collimated red laser light through a polarizer and a quarter-waveplate to create left-circularly polarized light with a helicity of +1. They then focused the light through a thin piece of gold-coated glass containing a nanoaperture of diameter 100–600 nm. Interaction with the nanostructure caused some of the light to transform into a beam with a helicity of −1, as analysed by polarization optics and a CCD camera. The amount converted depended on the diameter of the aperture.