Generation of midfield concentrated beam arrays using periodic metal annular apertures

Generation of minimally diffracting beam arrays in the midfield region using periodic metal annular apertures is investigated. The relations between the patterns of the diffraction fields and the structural parameters of the periodic metal annular aperture are numerically analyzed. Material dependent transmission characteristics are also studied with finite difference time-domain simulation. The results reveal that the beam concentration efficiency and axial intensity uniformity have a trade-off restriction due to strong inter-aperture interference and surface plasmon mediates the transmission efficiency of the periodic annular apertures. The design criteria of the metal annular aperture to achieve the strong and uniform beam arrays are addressed.