Silicon based metalenses for generation and control of orbital momentum endowed light beams

Metalenses are flat nanostructured devices capable of optically structuring light. Beam wavefronts can be shaped into helicoidal profiles, which provide light with orbital angular momentum (OAM). Here a spin multiplexing metalens is proposed. Unlikely common metasurfaces, this device transmits a different phase profile, an OAM wavefront and a constant phase respectively, according to the handedness of the input circular polarization of the beam. Its meta-unit structure is simulated with COMSOL software, and its design is realized by means of a customized MATLAB code. The metalens behaviour under gaussian beam illumination is then simulated, fulfilling the optical requirements. The device is fabricated through a master replica process adopting the nanoimprint lithography technique, after a nano-fabrication tuning procedure. The morphological features of the fabricated devices are analyzed via SEM imaging, showing a discrete adherence with the design. Eventually, the optical measurements confirm the presence of the double functionality: a gaussian and OAM beam can be independently generated according to the handness of the incoming light circular polarization, showing the possibility to control the light structure generated by bi-functional metalenses.