Exceptional points, as degenerate points of non-Hermitian parity-time symmetric systems, have many unique physical properties. Due to its flexible control of electromagnetic waves, a metasurface is frequently used in the field of nanophotonics. In this work, we developed a parity-time symmetric metasurface and implemented the 2π topological phase surrounding an exceptional point. Compared with Pancharatnam-Berry phase, the topological phase around an exceptional point can achieve independent regulation of several circular polarization beams. We combined the Pancharatnam-Berry phase with the exceptional topological phase and proposed a composite coding metasurface to achieve reflection decoupling of different circular polarizations. This work provides a design idea for polarimetric coding metasurfaces in the future.

A multi-focus optical fiber lens is numerically demonstrated based on an all-dielectric metasurface structure. The metasurface consists of an array of rectangular silicon resonators with varying widths in order to obtain the required phase distribution. The core diameter of the multimode fiber is large enough to contain sufficient resonance units. The spatial distribution of the dielectric resonators is dictated by spatial multiplexing, including interleaving meta-atoms and lens aperture division, to achieve multi-focus properties. The proposed optical fiber metalens can produce two or three focal points along the longitudinal direction with high focusing efficiency. The size of every focal point is close to the diffraction limit, and the relative intensity on each focus can be controlled by adjusting the number of the respective resonators. The proposed optical fiber lens will have a great potential in the fields of integrated optics and multifunctional micro/nano devices.

We theoretically demonstrate a compact all-dielectric metasurface fiber-tip lens composed of sub-wavelength amorphous silicon on the end face of a multimode fiber. The full 2π phase control was realized by varying the widths of resonant units. The tunable focal length is achieved by using the thermal-optic effect of amorphous silicon. The focal length increases from 309 μm to 407 μm when the temperature changes by 300 K. The temperature controlled all-fiber integrated lens is compact and with high efficiency and provides an excellent platform of a fiber-tip lab. Meanwhile, the proposed fiber lens does not have any structural changes during dynamic tuning, which improves the durability and repeatability of the devices.