Polarization-insensitive broadband metalens integrated on photonic crystal fiber tip

Fiber-tip lens not only makes the optical system more flexible and compact, but also eliminates complex alignment process, which makes it widely used in the fields of spatial light-fiber coupling, laser direct writing and fiber optic imaging. Compared to traditional glass lens, metalens is easier to be processed and integrated on fiber tip because of its special planar structure and small footprint. Fiber-tip metalens with high efficiency achromatic focusing ability has great application potential in multimode endoscope and other optical fiber imaging systems, so we designed a metalens with Si nanopost array that can achieve broadband focusing in the wavelength range of 1.2 mu m - 1.6 mu m. For realizing broadband achromatic focusing, it is the crucial to meet the focusing phase and phase dispersion requirements simultaneously. Then we built a library of unit pillars with three kinds of cross-sectional geometries to provide diverse focusing phase (f) and phase dispersion (df) combinations and defined a phase-dispersion space composed of parameters. In order to find the optimal unit pillar at each cell, we regarded the minimum Euclid distance between the points from the library and the target point in the phase-dispersion space as the criterion. Photonic crystal fiber (PCF) is a suitable integration platform for metalens because of the large core diameter and good dispersion property, so the PCF guided mode and plane wave are selected to lunch into the metalens for comparison and verification in the present work. The simulation results demonstrated that the metalens had a good achromatic focusing performance in the target wavelength band and changed little at different polarization of the source, which showed a good polarization-insensitive property.