We report on the optimization of ultrasmall microlenses based on the diffraction of two parallel metallic nanowires. The Rayleigh–Sommerfeld integral is used in the visible range to simulate the near field diffraction patterns induced by single and paired planar silver wires. We demonstrate that the wire width w affects only the diffraction efficiency and the contrast of the diffraction pattern. The wire interdistance D controls the focal length and the depth of focus, which are equal and vary in the 0.1 to 10 μm range when D/λ increases from 1 to 8. The transversal FWHM increases from 200 to 700 nm, and a normalized intensity greater than 2.2 is obtained at the focal point when w is about 300 nm and D/λ=3. There is excellent agreement between these calculated properties and the experimental results obtained for single and paired parallel silver nanowires. We show that in our microsized geometry, the plasmon contribution is negligible with respect to pure diffraction effect. In addition, these nanowire microlenses have focusing properties similar to those of ideal refractive lenses limited by diffraction.
© 2012 Optical Society of AmericaPDF Article