We have built an optical set-up has enabled us to measure VCD. The optical set-up is a home-built microscope that uses circularly polarized vortex beams to probe nanoholes of diverse sizes. Our set-up is versatile and we can measure both the transmission and reflection at the same time. The set-up has a very precise control of the polarization of the beam in different parts of optical path, making sure that vortex beams used in the experiments are symmetric under duality, cylindrical and mirror symmetry. A key aspect of the optical set-up is making sure that the two left and right circularly polarized beams that are involved in the VCD measurement carry the same power. It is also important to make sure that the focusing and collecting microscope objectives that are on the optical path maintain the symmetries of the incident vortex beams.
In parallel, we have nanofabricated plasmonic circular nanoholes of different sizes with a focused ion beam. We have optimized the design of these nanoholes for a multilayer system made of 150 µm of glass, 5 nm of titanium, and 150 nm of gold. The diameters of the plasmonic nanoholes that we have fabricated range from 100 nm to 2 µm. These samples have been used in different trials as well as in the concluding experiment.
Once the optical set-up was up and running, we have performed many VCD measurements with different samples. The measuring protocol has been defined thinking about the repeatability of the measurements, as well as the comparison with the simulations. Due to the difficulty in programing a proper 3D positioning system with a vortex beam excitation, the 3D positioning has been actively done by us throughout the experiments. We have observed that the scalar/nature regime of light-matter interactions could be measured with VCD: a VCD ≈ 0 has been consistently found to be a trademark of scalar light-matter interactions, whereas it has been observed that the trademark behavior of vectorial light-matter interactions is a VCD signal clearly different from 0.
We have corroborated these experimental findings with numerical simulations. The numerical simulations have allowed us to confirm that a signal of VCD ≈ 0 is associated to the preservation of polarization in scattering, whereas a VCD significantly different from 0 is linked to great changes in polarization. Moreover, we have seen that the trend of VCD as function of the diameter size is almost the same for nanoholes and spheres. The simulations have also allowed us to study the VCD for vortex beams of different orders l. The result of these studies has consistently been that the scalar/vectorial regime is not an exclusive function of the focusing regime of the beam, or of the size of the nanostructure. Instead, it is a convolution of both parameters, thus making it difficult to be predictable. That is why quantifying it with a measuring technique is significant from the point of view of fundamental research.
Some of our results have been partially disseminated in different presentations and in social networks. Moreover, two publications have been written and they will be published soon.
