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.
Metagratings have been shown to form an agile and efficient platform for extreme wavefront manipulation, going beyond the limitations of gradient metasurfaces. Here, we theoretically show perfect unitary diffraction in...
Here, a metasurface‐based fiber‐to‐chip multiplexing coupler is presented that can realize flexible mode conversion and multiplexing coupling between few‐mode fibers and on‐chip single‐mode waveguides. The proposed approach makes use of the symmetrical characteristics of fiber eigenmodes and high freedom of light phase manipulation to obtain the functional phase distribution of metasurfaces. The use of cylindrical nanopillar as metasurface nanostructures ensures its polarization independent optical response. Several fiber‐to‐chip multiplexing couplers are demonstrated by using the angular spectrum methods and the finite‐difference time‐domain (FDTD) simulations. It is found that the two or three modes can be flexibly demultiplexed and then be coupled into the corresponding on‐chip waveguides with low crosstalk. Further, two possible fabrication schemes for multiplexing couplers are proposed and then the related fabrication tolerances and misalignment effects are evaluated and discussed. Compared with traditional on‐chip couplers, metasurface‐based couplers have advantages of ultracompact footprint and low crosstalk. Such study explores the application of metasurfaces in multiplexing coupling between few‐mode fiber and on‐chip single‐mode waveguides, which is expected to break through the bottleneck of current mode‐division multiplexing technologies in optical interconnection and meet the ever‐increasing demand for large data throughput of the photonic integrated chips.
We demonstrate a kind of grating coupler that generates a high quality flat-top beam with a small beamwidth from photonic integrated circuits into free-space. The grating coupler is designed on a silicon-on-insulator wafer with a 220-nm-thick silicon layer and consists of a dual-etch grating (DG) and a distributed Bragg reflector (DBR). By adjusting the structural parameters of DG and DBR, a pixel-level (6.6 µm) flat-top beam with a vertical radiation of −0.5 dB and a mode match of 97% at 1550 nm is realized. Furthermore, a series of high-efficiency grating couplers are designed to create a flat-top beam with different scales.
We developed a meta-objective with silicon nitride cascaded metalens. It can achieve sub-micrometer resolution (~ 0.8 μm) in wide (~ 100 μm) FOV. Combined with fiber bundle microscope system, the outline of biological cells can be clearly observed.
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