Modeling a high numerical aperture micrometalens with a varying number of sectors

Using the numerical solution of differential Maxwell’s equations, we show that a binary microlens with unit numerical aperture (NA = 1) manufactured in a thin-film amorphous silicon focuses the laser light into a near-surface subwavelength optical focal spot. The microlens contains sectored subwavelength diffraction gratings operating as half-wave plates. The incident light is a linearly polarized plane wave. The micrometalens is numerically shown to operate with near-same efficiency with the number of sectored grating varying from 3 to 16. It is shown that a 16-sector micrometalens generates a focal spot of size at the full-width at half-maximum intensity of FWHMx = $0.435\lambda$ and FWHMy = $0.457\lambda$ along the Cartesian axes, where $\lambda$ is the incident wavelength. A 4-sector microlens is numerically shown to focus light into a focal spot of size FWHMx = $0.428\lambda$ and FWHMy = $0.46\lambda$.