Miniature optical components at the wavelength scale remain today a theoretically opened challenging problem of great technological interest. Appart from refractive micro-optics, plasmonics have been proposed to realize micro lenses with properly designed planar metallic nano-patterns. We show in this paper that efficient light focusing at the diffraction limit with higher transmission can be obtained with micro-structures much easier to fabricate than nano ones, such as a simple micro-slit studied here as an example. Optical properties are attributed to diffraction and a quantitative excellent agreement between experiment and theory is obtained.
We theoretically and experimentally demonstrate that the diffraction of microstructures based on silver nanowires leads to very efficient microfocusing effects. Pairs of parallel nanowires act as ultrasmall cylindrical microlenses with diffraction-limited resolution in the Fresnel region. This is a new diffraction scheme to make micron-sized optical lenses with higher transmittance than plasmonic microlens based on nano-aperture arrays. Calculations based on the scalar Rayleigh-Sommerfeld integral highlights the pure scalar diffractive contribution. Thus, the plasmon contribution is negligible in such micron-sized metallic geometry. We demonstrate that two-dimensional grids of nanowires can be used to fabricate dense arrays of microlenses, i.e. 10000x10000 DPI (dots per inch).
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.
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