Abstract
Based on the finite-difference solution of Maxwell's equation, we study a unit-NA binary microlens fabricated in a thin-film amorphous silicon as an array of subwavelength diffraction gratings that operate as half-wave zone plates, numerically showing it to focus a linearly polarized plane incident beam into a near-surface circular focal spot. Focusing is shown to be performed with near-same quality for a varying number of polarization-rotation diffraction gratings, ranging from 3 to 16. The subwavelength focal spot is shown to measure, in terms of full-width at half-maximum (FWHM) intensity, FWHMx = 0.435λ and FWHMy = 0.457λ for 16-sector diffraction gratings, and FWHMx = 0.428λ and FWHMy = 0.460λ for 4-sector diffraction gratings, where λ stands for the incident wavelength.
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