Trapping and guiding of large-size particles in hollow beams produced by nonlinear crystals
Authors: Ning Xiao-Long  , Wang Zhi-Zhang  , Pei Chun-Ying  , Yin Ya-Ling  ,   School of Physics and Materials Science, State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
Published: December 2020

Since 1970, the trapping of the small objects in space by optical radiation pressure, such as nano particles and other atomic living cells, has been successfully developed and used in the applied physics, life sciences and other fields. As the optical radiation pressure is very weak, the use of radiation pressure on the particle will be strictly limited by the particle size. Also, the manipulated particles can move particle with only hundreds of microns. Therefore, it is not suitable for trapping and long-distance transporting particles with large size (micron). In recent years, with the development of the manipulation technology for large particles, a new control force-photophotetic force has gradually entered into people's vision field. Compared with the optical radiation pressure, the photophoretic force is much large under the same light intensity. Therefore, the photophoretic force makes it possible to manipulate and trap the large particles. With the development of laser beam-shaping technology, the species of laser beams become more and more abundant, which makes it more attractive to study particle manipulation based on the photophoretic force. For example, a hollow beam is used to capture and guide carbon nanoclusters in the air. A tapered optical fiber is used to trap, migrate and separate SiO2 particles. A Bessel Gaussian beam is used to trap and manipulate magnetic particles. An airy beam is used to trap glass carbon particles of absorption type. In this paper, a trapping and guiding scheme for large-size particles by using the photophoretic force of the hollow beams generated by nonlinear ZnSe crystals is proposed and analyzed theoretically. Our calculated results can be concluded as follows. 1) For the cases of two-dimensional particle trapping and one-dimentional particle guiding using a hollow beam generated by a single nonlinear ZnSe crystal, the magnitude of the longitudianl optical force is proportional to the ratio between particle size and hollow beam size to the fourth power and is proportional to the power of the hollow beam, and the direction is the same as that of the beam propagation. The closer to the hollow beam size the particle size, the greater the transverse optical force is. The results show that the photophoretic force can achieve the two-dimensional trapping of large-size particles, and a long distance (in a meter region) guiding. 2) For the case of three-dimensional particle trapping using a localized hollow beam generated by two nonlinear ZnSe crystals, the dependence of transverse photophoretic forceand that of longitudinal photophoretic force on the system parameters are similar to the scenario for the particles trapping in the hollow beam produced by a single nonlinear crystal. The difference is that under this condition, the direction of the longitudinal photophoretic force points to the center of the beam. So this scheme can achieve the effective three-dimensional trapping of large-size particles. Above all, the hollow beams generated by nonlinear ZnSe crystals can be used as an effective noncontact controlling tool for large-size particels, and might have potential applications in modern optics and biomedicine.