Extraordinary optical transmission and vortex excitation by periodic arrays of Fresnel zone plates

Abstract

Abstract. 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 the 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.

Optical beam interactions with a periodic array of Fresnel zone plates

“…However, its influence on the amplitude and phase distributions of the field transmitted through the FZP cell is slight. This was confirmed by our comparison of the FZP arrays that are etched in silver and that are in a perfect electric conductor (PEC) [21]. For the case of the ELG beams at the silver structures, only the focal plane is slightly shifted, with respect to the case of the PEC structure.…”

“…Additionally, the increase of the field periodicity from 2Λ to 4Λ causes the shift of the focal plane from z ≅ 2500 nm to z ≅ 2800 nm in the case of the FZP structure. The latter value is close to the focal plane position z ≅ 2980 nm for the incident plane wave [21]. Both of these effects suggest only a slight influence of the neighbouring beams on the field distribution at the particular focal plane.…”

“…Moreover, a higher transmission diffraction efficiency (DE) was obtained in the case of the presented two-zone FZP array, illuminated with an ELG 0,1 beam (DE ≅ 0.53) in comparison with the DE of the eight-zone FZP illuminated with a plane wave (DE ≅ 0.4) [7] and with the DE of the twozone FZP with the plane wave (DE ≅ 0.41) [21]. On the other hand, the measured DE in the case of the highly focussed Gaussian beam incident on the coaxial structure (r 1 = 0.68λ, r 2 = 0.90λ) was around DE ≅ 0.5 for both polar polarizations [4].…”

“…For example, for the incidence of the circular polarizations, η ≅ 1.60, in comparison to η ≅ 1.45 for our hole array. Additionally, the EOT for a plane wave of circular incidence on the same FZP array was estimated in [21] at around η = 1.25. Thus, for the incident beams, and particularly for the narrow beams (here, kw w = 13.8, where w w = 2.2λ is the Gaussian beam waist radius), the normalised transmittance of the FZP structure is higher than this for the incident plane waves (1.60 vs 1.25).…”

“…A longitudinal component of the transmitted field under incidence of circular polarization is characterised by a narrow intensity peak of the diameter, which exceeds the resolving power of conventional optical systems. Moreover, the FZP structure also allows for an extraordinary optical transmission (EOT), which is much stronger than the EOT in the case of the plane wave incidence upon the similar structure [21]. The XPC, EOT and focussing effects will be graphically presented here with the use of numerical calculations based on the Rigorous Coupled Wave Analysis (RCWA) method [22][23][24][25].…”