Thin metalens with high numerical aperture

We designed, numerically simulated, fabricated and characterized a thin metalens in an amorphous silicon film. The lens has a diameter of $30 \mu m$, a $633$-nm focal length equal to the incident wavelength, a $220$-nm period of the subwavelength features, and a $110$-nm relief depth. The lens simultaneously controls the phase and polarization of the incident laser light, transforming linear polarization into the radial one and generating a sharp focus. The metalens was fabricated using electron lithography and reactive ion etching. The focal spot obtained from an incident linearly polarized Gaussian beam of wavelength $\lambda$ measured $0.49\lambda$ and $0.55\lambda$ on the $x$- and $y$-axes. This is a record-breaking focal spot ever obtained using metalenses. Experimental results agree well with the numerically simulated focal spot size of $0.46\lambda$ and $0.52\lambda$.