Generation of focusing ring of metalens and its application in optical trapping of cold molecules

Abstract

Metasurface can precisely control degrees of freedom of the phase, polarization, and amplitude of the incident light field. It provides a new way to develop the next generation of the experimental platform of quantum-state manipulation on-chip, which has important application prospects. This paper proposes a new type of metasurface structure, that is, a metalens composed of silicon grating elements with different duty ratios that can form a focusing ring on the focal plane. The intensity distribution of the ring light field in the focal plane and the focusing characteristics of metalens with different numerical apertures are studied. An optical storage ring of magnesium fluoride (MgF) molecule is constructed by using this kind of metalens focusing ring. The optical potential and dipole force of the MgF molecule in the focused light field are calculated, and the dynamic process of MgF molecule motion in the storage ring is simulated by the Monte-Carlo method. The research results show that for the incident light of 1064-nm radially polarized light, the designed metasurface structure has good focusing characteristics, and the light field intensity of the focusing ring is 55.1 times stronger than that of the incident light. The focal length of the annular light field is 22 μm and the full width at half maximum of the light intensity distribution in the focal plane is 0.8 μm, and the numerical aperture of the hyperlens is 0.69. The maximum dipole potential of MgF molecules in the light field is 32 μK, which can realize the loading of MgF molecules and trap them in the surface storage ring.