Focusing of laser light by sectoral spiral metalens

A large number of scientific papers are currently devoted to the investigation of metasurfaces, based on the subwavelength gratings. Such subwavelength gratings are anisotropic – TE- and TM-polarized waves propagated through them have a different phase. Based on this effect it is possible to create analogues of the classical half-wave plates, which rotate the direction of polarization. In this work we proposed a spiral metalens, which simultaneously converts linearly polarized light into an azimuthally polarized vortex beam and focuses it. This metalens is a combination of a spiral zone plate with a topological charge m = 1 (focal length f = 633 nm) and a sector subwavelength grating (period of 220 nm, relief depth of 120 nm, illumination wavelength of λ = 633 nm). The metalens was fabricated using electron beam lithography and ion etching in 130-nm thick amorphous silicon film. Using FDTD-method it was numerically shown that the metalens illuminated by a plane wave with linear polarization forms a circular focal spot with dimensions smaller than the scalar diffraction limit: FWHMx = 0.435λ and FWHMy = 0.457λ. The focusing by the fabricated metalens was investigated experimentally using scanning near-field optical microscope (Ntegra Spectra, NTMDT).