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Published in Photonics and Nanostructures - Fundamentals and Applications, Vol 43, February 2021.


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.

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