Abstract
Abstract Vector beams with phase modulation in a high numerical aperture system are able to break through the diffraction limit. However, the implementation of such a device requires a combination of several discrete bulky optical elements, increasing its complexity and possibility of the optical loss. Dielectric metalens, an ultrathin and planar nanostructure, has a potential to replace bulky optical elements, but its optimization with full-wave simulations is time-consuming. In this paper, an accurate and efficient theoretical model of planar metalens is developed. Based on this model, a twofold optimization scheme is proposed for optimizing the phase profile of metalenses so as to achieve subdiffraction focusing with high focusing efficiency. Then, a metalens that enables to simultaneously generate radially polarized beam (RPB) and modulate its phase under the incidence of x-polarized light with the wavelength of 532 nm is designed. Full-wave simulations show that the designed metalens of NA = 0.95 can achieve subdiffraction focusing (FWHM = 0.429λ) with high transmission efficiency (77.6%) and focusing efficiency (17.2%). Additionally, superoscillation phenomenon is found, leading to a compromise between the subdiffraction spot and high efficiency. The proposed method may provide an accurate and efficient way to achieve sub-wavelength imaging with the expected performances, which shows a potential application in super-resolution imaging.
Highlights
Vector beams with phase modulation in a high numerical aperture system are able to break through the diffraction limit
Compared to spatially uniform polarizations, such as linear and circular polarizations, radially polarized beam (RPB) with an annular filter can be focused into a shaper spot thanks to the significant strong longitudinal electric field component [5, 6]
Subdiffraction focal spots have been demonstrated by focusing RPB through binary phase pupil filters and a high-numerical aperture (NA) lens [13, 14]
Summary
For fixed height of nanoposts, H, the rotational angle θ accounts for polarization change as shown in Figure 2B while a combination of width, W, and length, L, of a nanopost for the phase delay of transmitted light. To achieve phase coverage of 2π for φy as well as high transmission, we sweep the length and width of a nanopost with fixed height (H = 900 nm), rotation angle (θ = 0) and lattice constant (a = 320 nm). Using these eight nanoposts, full 2π phase coverage can be achieved with the interval of π/4. The selected nanoposts should be arranged in such a way that the phase profile Φ allows the metalens to focus RPB into a subdiffraction spot
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