Metallic planar lens formed by coupled width-variable nanoslits for superfocusing

Abstract

We report a metallic planar lens based on the coupled nanoslits with variable widths for superfocusing. The influence of the interaction between two adjacent nanoslits on the phase delay is systematically investigated using the finite-difference time-domain (FDTD) method. Based on the geometrical optics and the wavefront reconstruction theory, an array of nanoslits perforated in a gold film is optimally designed to achieve the desired phase modulation for light beaming. The simulation result verifies our design in excellent agreement and the realized metallic lens reveals the superfocusing capability of 0.38λ in resolution, well beyond the diffraction limit.

Robustly Efficient Superfocusing of Immersion Plasmonic Lenses Based on Coupled Nanoslits

“…In order to compare the focusing performance of the non-immersion metallic nanoslit-based plasmonic lens with that of the immersion versions, all the designed lenses have the same focal length of 0.3 μm with the metallic spacing of 30 nm in a 400-nm-thick gold film. The phase delay of a coupled nanoslit filled with the immersion materials depends not only on its own width but also the width of adjacent ones and the metallic spacings between them, similar to the non-immersion situation discussed in [18]. Also, when the influence of the aperiodicity of adjacent nanoslits on the phase delay is small, the phase delay of a nanoslit can be effectively predicted by Re(β)t, where t is the thickness of the metal film and β is the complex propagation constant in the nanoslit which is calculated based on the symmetric-mode dispersion relation in a periodic metallic waveguide array with the following Eq.…”

“…The wider nanoslit is beneficial for improving the optical transmission efficiency and alleviating the practical nanofabrication of the device. Furthermore, as the coupling effect is considered, the gold spacing can also be adopted to modulate the phase delay for a specific nanoslit, as demonstrated in [18]. As for an immersion metallic nanoslit-based plasmonic lens, the desired focusing performance cannot be implemented by directly immersing the non-immersion one into a highindex material due to the resulted poor match of the phase delay introduced by nanoslits.…”

“…The prediction approach for the phase delay introduced by a coupled nanoslit is not accurate for all the nanoslits, thus leading to a large deviation of the actual focal length from the desired value. In our latest research [18], the proposed approach was optimized and a plasmonic lens formed by coupled nanoslits operating in air was built with the successful realization of light focusing at the anticipated distance. Meanwhile, it has revealed the capability of superfocusing with the full-width at halfmaximum (FWHM) of λ /2.6.…”

“…The gold spacing between two adjacent nanoslits is 30 nm, and the thickness of the gold film is 400 nm Fig. 3 a The geometry of the optimized non-immersion metallic nanoslit-based planar lens, taken from [18]. b The oil immersion case for the same lens.…”

Recent Progress in Far-Field Optical Metalenses

“…Simultaneously, the field at the exit surface of the film is also modulated since the transmission of each nanoslit varies as a function of slit width. Finally, the influence of the interaction between two adjacent nanoslits on the phase delay has been systematically investigated using the finite-difference time-domain method [34].…”