Generation of Bessel beam by manipulating Pancharatnam-Berry phase

Abstract

Bessel beam is one of diffraction-free beams and has some peculiar properties. Varieties of its applications have been found, such as microparticle manipulating, material processing and biological studies. In this work, we propose a method of creating a Bessel beam by manipulating Pancharatnam-Berry phase. Using femtosecond laser, nano waveplatelets are written on a fused silicon glass to form a metasurface. The optical axis of waveplatelets rotating in the radial direction can produce the space-varying Pancharatnam-Berry phase. The designed metasurface acts as a planar axicon to generate Bessel beams by replacing the traditional one. A Jones calculation is employed to analyze the transformation of the metasurface. The theoretical results indicate that a left-handed circularly polarized light passing through the planar axicon is convergent, while a right-handed circularly polarized one is divergent. The intrinsic physical reason is that Pancharatnam-Berry phase is spin-dependent. Therefore, Bessel beams are generated by the planar axicon only when a left-handed circularly polarized light inputs the system. It is notable that the maximum nondiffracting distance is determined by the rate of rotation of the metasurface microstructure. By reducing the rate of rotation, we can easily obtain a longer nondiffracting distance, thus avoiding the problem that the base angle of the traditional axicon is too small to fabricate. According to the Fresnel diffraction integral, we simulate the propagation of the field emerging from the planar axicon and obtain the intensity distributions behind the planar axicon with different distances. The results show that the intensity pattern remains unchanged in the propagating process and possesses the propagation properties of Bessel beam. It implies that approximate nondiffraction Bessel beams can be achieved by employing the planar axicon with metasurface. Finally, we set up an experimental system with the Pancharatnam-Berry phase metasurface with period d=1000 upm to verify the theoretical analysis. Theoretically, the maximum nondiffraction distance is 7.9 m. In the shaded region, we measure the intensity distributions at different distances. The experimental results are in good agreement with the simulation results, so the planar axicon based on Pancharatnam-Berry phase can be an effective Bessel beam generator. We believe that these results are helpful for developing more spin-dependent photonic devices.