Nanophotonic devices, which consist of multiple cell structures of the same size, are easy to manufacture. To avoid the optical proximity effect in the ultraviolet lithography process, the cell structures must be maintained at a distance from one another. In the inverse design process, the distance is maintained by limiting the optimized range of the location. However, this implementation can weaken the performance of the devices designed during transmission. To solve this problem, a self-adjusting inverse design method based on the adjoint variable method is developed. By introducing artificial potential field method, the location of one cell structure is modified only when the distances between this cell structure and other cell structures are smaller than a threshold. In this case, the range of the location can be expanded, and thus the performance of the designed devices can be improved. A wavelength demultiplexer with a channel spacing of 1.6 nm is designed to verify the performance of the proposed method. The experiment reveals that the transmission of the designed devices can be improved by 20%, and the self-adjusting inverse design process is 100 times faster than the inverse-design process based on the genetic algorithm.

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 1.2μm -1.6μm. 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 (φ) and phase dispersion (δφ) 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.