Metalenses in the visible spectrum: materials and nanofabrication processes

Metalenses are ultrathin optical components based on arrays of subwavelength nanostructures, offering novel ways to manipulate light within a compact platform. Their implementation in the visible spectrum, and in particular at 729 nm, a wavelength relevant for quantum technologies such as trapped-ion systems, presents both great opportunities and stringent challenges in terms of materials and nanofabrication. In this context, this thesis addresses the realization of a q-plate designed to operate at 729 nm, a polarization-dependent optical element that converts the circular polarization state of incident light and generates vortex beams through the Pancharatnam–Berry phase. Different dielectric materials, including silicon nitride and silicon oxide, were deposited by reactive magnetron sputtering and investigated in order to identify a suitable candidate for fabrication. For silicon oxide, the oxygen content plays a key role in determining the optical properties, and by tuning the deposition conditions a suitable compromise between absorption coefficient and refractive index was obtained, which enabled the fabrication of the q-plate using this material. The prototype was fabricated by direct-writing electron beam lithography, while UV-nanoimprint lithography was also investigated as a high-throughput replication technique, although it was not fully optimized in this work. Optical characterization was performed in order to confirm that the fabricated optic performs in agreement with the design, enabling vortex beam generation.