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Superresolution microscopy is of great interests in modern optics and photonics applications. Subdiffraction focusing lens, that has superresolution focal spot with local spatial frequency greater than the maximum Fourier component, has attracted intensive attentions in past decade, because of the significant capability of unlabeled far-field superresolution. Here, we propose a dielectric subdiffraction focusing metalens based on the depletion of Bessel beams. Different from traditional binary wavefront modulation elements, this metalens is designed according to the reverse engineering of subdiffraction focal field, which is constructed from the depletion of a fundamental Bessel beam. With the independent amplitude and phase control ability of dielectric metasurface, this metalens enables breaking the diffraction limit and can produce a focal spot with full width at half maximum of 0.3λ/NA. This work may provide a platform for the design of integrated and compact photonic devices to control the light propagation dynamics and spin angular momentum associated with circular polarization.

Binary Fresnel zone plates (ZP) are one of the most frequently used focusing elements of inplane optical schemes in micro- and nanophotonics. With a decrease in the diameter and focal distance of the ZP to meso-wavelength sizes, the parameters of the focusing region begin to be significantly influenced by features of the ZP design (material, thickness, relief depth). The spatial structure of the focal spot formed in the near-field is investigated by the numerical finite elements (FEM) simulations of the transmission of a plane optical wave through a mesoscale binary phase ZP. We show that there is a range of optimal etching depths of the ZP ridges and optimal thicknesses of the plate substrate, at which the best focusing of the incident optical wave is realized in terms of the maximum field intensity and the minimum size of the focal spot. In addition, a concept of a super-focusing binary phase ZP with an immersion layer in the form of a truncated cone fabricated of ZP material is proposed, which makes it possible to focus the circularly polarized light wave into a subdiffraction region with a half-width of about "lambda"/2n (n is the ZP refractive index).

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.