Figure 2. Details of mask.

a, Schematic drawings of a metasurface y-phi mask composed of an array of a-Si nanoposts on a glass substrate. The elliptical-cross-section nanoposts behave as uncoupled birefringent scatterers with principal axis orientation along the ellipse diameters. b, Scanning electron micrographs of the top view (left), and tilted view (right) of a fabricated metasurface y-phi mask. c, Schematic illustration of a y-phi mask which converts azimuthally polarized light to y-polarized light. The mask locally functions as a half-wave plate whose principal axis orientation (shown by red dashed lines) gradually varies across the mask.