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 µm, 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 λ measured 0.49λ and 0.55λ 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 0.46λ and 0.52λ.