  Extraordinary optical transmission and good focusing properties of a
two-dimensional scattering structure is presented. The structure is made of
Fresnel zone plates periodically arranged along two orthogonal directions. Each
plate consists of two ring-shaped waveguides supporting modes that match the
symmetry of a circularly polarized incident plane wave. High field
concentration at the focal plane is obtained with short transverse and long
longitudinal foci diameters. Optical vortex excitation in a paraxial region of
the transmitted field is also observed and analysed in terms of
cross-polarisation coupling. The structure presented may appear useful in
visualization, trapping and precise manipulations of nanoparticles.
