Diffractive microstructures based on metallic nanowires: a low cost solution for optical focusing devices

In the present paper, we will show how diffractive microstructures can lead to efficient lenses which present several advantages with respect to other proposed solutions. Also, they only require wavelength-scale resolution and not very small nanostructuring. Nevertheless we obtain comparable performances as plasmonics lenses and we show that the diffraction phenomenon which is at the origin of the observed effects can indeed lead to efficient focusing in the Fresnel region. The structures we proposed in this paper consist of pairs of parallel metallic nanowires fabricated by direct laser writing technique. The fabrication set-up is based on a metallic photoreduction initiated by two photon absorption using a nanosecond Q-Switched Nd-YAG laser. We show for instance experimentally that this pair of metallic nanowires separated by 2 μm when irradiated with an unpolarized light (at λ=546 nm) lead to a focusing at 2 μm with a diffraction limited resolution and an intensity enhancement at the focusing point of about 2.2 times the incoming intensity. Two different theoretical models were used to corroborate our experimental measurement. The first one is the diffraction theory based on the Rayleigh-Sommerfeld integral and the second one is the well kwon FDTD simulations, which are in very good agreement with experiments and confirm the origin of the focusing process. In addition they show that, in the case of our microstructures, plasmonic effects do not contribute to the focusing process. Finally, we propose a 2D array of microlenses based on a grid of metallic nanowires separated by a distance D. This device has slightly the same lens characteristics as a pair of metallic nanowires but with an intensity enhancement higher than 5, and thus may present practical interest in view of applications.