Figure 1. Schematic of the abnormal phase gradient induced by a curved slit in a metallic screen.

(a) Physical mechanism of the geometric phase in space-variant subwavelength apertures. The angle between the main axis of the anisotropic aperture and the instantaneous polarization state of the CPL determines the phase shift. (b) The phase of electromagnetic waves is changed via a curved subwavelength slit, which could be approximated as composed of space-variant anisotropic apertures. Here E_i and E_t denote the incident and transmitted electric fields.