Subwavelength grating-based spiral metalens for tight focusing of laser light

Abstract

In this paper, we investigate a 16-sector spiral metalens fabricated on a thin film (130 nm) of amorphous silicon, consisting of a set of subwavelength binary diffractive gratings and with a numerical aperture that is close to unity. The metalens converts linearly polarized incident light into an azimuthally polarized optical vortex and focuses it at a distance approximately equal to the wavelength of the incident light, λ = 633 nm. Using a scanning near-field optical microscope, it is shown experimentally that the metalens forms an elliptical focal spot with diameters smaller than the diffraction limit: FWHMx = 0.32λ (±0.03λ) and FWHMy = 0.51λ (±0.03λ). The experimental results are close to those of a numerical simulation using the FDTD method, with FWHMx = 0.37λ and FWHMy = 0.49λ. The technological errors due to manufacturing were taken into account in the simulation. This is the smallest focal spot yet obtained with a metalens.