META Conference, META'10

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Single-layer metal nanolenses with tight foci in far-field
Piotr Wróbel, Tomasz J. Antosiewicz, Jacek Pniewski, T. Szoplik

Last modified: 2009-10-31

Abstract


In a numerical experiment we analyse performance of plasmonic nanolenses made of a single metal layer. Due to radially polarized illumination lenses have foci of full widths at half maxima (FWHMs) better than half a wavelength and focal lengths of the order of a few wavelengths. We consider the nanolenses in two configurations. In the first, the nanolens is a free standing silver layer with no hole on the optical axis and double-sided concentric corrugations. We examine the following parameters of the nanolens: film thickness, diameter of an on-axis stop, number of double-sided concentric grooves, lattice constant of the grooves, as well as their depth and width. The parameters listed above influence the intensity transmission, FWHM of the focus and focal length. Transmission of light through the continuous metal layer with double sided narrow grooves reaches 30%. It is based on photon-plasmon and plasmon-photon coupling processes at both lens surfaces. High transmission is achieved under the condition of resonant tunneling of light through coupled grooves.

In the second configuration, the nanolens has a set of slits instead of grooves. This necessitates integrating the annular metal elements with a dielectric matrix. The parameters influencing the intensity transmission, focal length and FWHM are as follows: nanolens thickness, number of concentric slits, their lattice constant and width, as well as dielectric permittivities of substrate, slit filling medium and superstrate. These parameters influence the lens focusing properties and energy throughput. In this case, transmission of light exceeding 80% results from waveguided modes in slits, which form a concentric metal-dielectric-metal structure.

Tight focusing is possible due to radially polarized illumination with a Laguerre-Gauss (LG) intensity profile, what assures that the radial component of the electric field is perpendicular to metal edges. The axis of symmetry of corrugations and slits coincides with the optical axis of incident LG beam with radial beam profile of electric field E(r) = (r/R)´exp(-r2/2R2), where R is radius of maximum intensity. Matching the beam diameter with that of lens area with grooves or slits maximizes efficient use of light energy.

The nanolenses act like classical, high-numerical aperture, optical refractive systems. Simulations using body-of-revolution finite difference time domain method (BOR FDTD) illustrate their performance.

 

This work has been partially supported by the Polish Ministry of Science and Higher Education under the project N N507 445534 and the National Centre for Research and Development under the project N R15 0018 06. The authors are partners in COST Actions MP 0702 and MP 0803.

 

References:

1.            P. Wróbel, J. Pniewski, T.J. Antosiewicz, T. Szoplik, “Focusing radially polarized light by concentrically corrugated silver film without a hole” Phys. Rev. Lett. 102, 183902 (2009).

2.            P. Wróbel, T.J. Antosiewicz, T. Szoplik, “Silver nanolens with slits for focusing radially polarized light into far field,” Opt. Lett. submitted.

 

 

 


Keywords


nanooptics, plasmonics, surface plasmons, radial polarization