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Using e-beam lithography and reactive ion beam etching of a 120 nm amorphous silicon film, we fabricate a 30-µm spiral metalens that has a very short focal length that is equal to the incident 633 nm wavelength (numerical aperture about 1) and is composed of 16-sectored subwavelength binary gratings with a 220 nm period. Its unique feature is that the metalens generates a left-hand circularly polarized optical vortex with topological charge 2 when illuminated by a left-hand circularly polarized light but generates a cylindrical second-order vector beam when illuminated by linearly polarized light. Both for linearly and circularly polarized light, an inverse energy flow occurs near the strong focus of the metalens. Transverse near-focus intensity distributions measured with a scanning near-field optical microscope are found to be in qualitative agreement with those obtained via the FDTD-aided numerical simulation. Thus, the near-focus reverse energy flow is experimentally confirmed to occur.
Kotlyar, V.V., Stafeev, S.S., Nalimov, A.G., O’Faolain, L. and Kotlyar, M.V. (2021). A dual-functionality metalens to shape a circularly polarized optical vortex or a second-order cylindrical vector beam. Photonics and Nanostructures - Fundamentals and Applications, [online] 43, p.100898. doi:https://doi.org/10.1016/j.photonics.2021.100898.