Quadratic phase modulation and diffraction-limited microfocusing generated by pairs of subwavelength dielectric scatterers

Abstract

Diffractive approaches are needed when refractive microlenses reach their focusing limit at the micron-scale in visible light. Previously, we have reported on micron-sized optical lenses based on the diffraction of metallic nanowires. Here, we extend our study to lenses based on pairs of subwavelength dielectric scatterers. Using simulations by two-dimensional finite element method, we demonstrate that focusing holds for pair spacings as small as the wavelength-size. For pairs with distances between inner walls larger than about 1.2λ, the scattered waves generate a quadratic phase modulation on the total propagating field leading to a diffraction-limited focusing i.e. an effective optical lens effect with high numerical aperture. In addition, they have low sidelobe intensities, long depths of focus, and they have a low sensitivity with polarization. For pairs with inner wall distances smaller than about 1.2λ, the focusing phase modulation is accumulated during the propagation through the dielectric pair structure. In this work, we report only on the experimental demonstration for the case of larger wall separation to emphasize on the scattered wave effect on micro-focusing. A pair of parallel polymer lines (cylindrical lens), and a grid of polymer lines (square microlens array) with 2 μm-spacing were fabricated by two-photon induced polymerization. Their focal lengths are comparable to their separating distances, their spot-sizes are 0.37 μm and 0.28 μm at wavelength 530 nm, and their focusing efficiencies are 70% and 60%, respectively.

Time domain photonic hook beam for manipulating nanoparticles along curved trajectories

“…Recently, it has been shown that two identical rectangular dielectric rods to make a diffraction-limited focus 17 . Based on this results we have proposed and demonstrated a method of generating diffraction-limited PH by breaking the symmetry of these systems based on their individual scattering and diffraction properties through variations of their dimensions and refractive indices 18 .…”

3D Printing and Pyrolysis of Optical ZrO2 Nanostructures by Two‐Photon Lithography: Reduced Shrinkage and Crystallization Mediated by Nanoparticles Seeds

“…Previously, we have used the two‐photon polymerization of acrylates to demonstrate the fabrication of microlens arrays based on the diffraction of microgrids. [ 50 ] We obtained periods as small as 1.2 µm, that are needed for the continuous trend of downsizing the pixel size of CMOS sensors. [ 51 ] Due to the low refractive index of 1.5, the optimized line widths and heights, for high diffraction efficiency, were 0.4 and 0.8 µm, respectively.…”

Generating photonic hooks using dual-dielectric designs

“…

Near-field, sub-wavelength, and localized curved light beams, known as photonic hooks [1]-[3], discovered in 2015 [2], have become an area of interest due to their relatively simple design, compactness and potential to be used in a number of applications such as in sensing and optical trapping [2]. Recently, it has been shown how concepts to design focusing devices in the microwave and optical regimes using mesoscale particles (dimensions in the order of the operational wavelength) [4]-[10] such as two identical rectangular dielectric particles to make a diffraction-limited focus [11] can be exploited to produce photonic hooks by breaking the symmetry of these systems. This asymmetry can be introduced using several techniques such as asymmetric illumination (oblique incidence) or by changing the shape and dimensions of the particle [1].

…”