Generation Of Localized Hollow Laser Beam And Application In Particle Manipulation

Dark hollow laser beams (DHBs) are the ring beams with zero center intensity in the propagation direction, which are generated by laser shaping technology. DHBs have many novel and unique features, such as barrel-shaped intensity distribution, spiral wave front, center phase singularity, spin angular momentum, orbital angular momentum, propagation invariance, small dark-spot size and no-heating effect etc. So DHBs have many potential and extensive applications in laser cooling and trapping, optical tweezers, manipulation of the microscopic particles, including biological cells, atoms, molecules and dielectric particles etc. Thanks to these novel features, lots of researchers are interested in DHBs.Localized hollow laser beams (LHBs) have three-dimensional closed region with minimal or zero center intensity in the propagation direction, which are one special type of DHBs. The region is surrounded by high intensity distribution, which likes a special sealed bottle. In this paper, we propose three simple schemes to generate LHBs. The first scheme is to generate a submicron LHB by using the system of a single mode fiber, a circle binary phase plate and a micro lens. According to Rayleigh-Sommerfeld diffraction theory, we calculate the intensity distributions of the generated LHB near the focal position and its propagating properties in free space, and also study the dependences of the dark-spot size (DSS) of the LHB on the mode radius of single-mode fiber and the focal length of the micro lens. Our calculated results show that the intensity distribution of the LHB presents approximately symmetrical distribution near the focal position. In the center of the focal plane, the light intensity is zero and increases around. So a spherical closed light field (i.e. localized hollow laser beam) with a radius of 0.4μm has been generated. When LHB is blue-detuned, atoms will be trapped in the minimum light filed. If a re-pumping laser beam is applied, atoms will be cooled by the intensity-gradient Sisyphus cooling. In this paper, we form a device for the trapping and cooling of a single atom using the generated submicron localized hollow laser beam. We study the dynamical process of intensity-gradient cooling of a single Rb atom trapped in the localized hollow beam by Monte-Carlo methods. Our study shows that a single 8/Rb atom with a temperature of 120μK from a magneto-optical trap (MOT) can be directly cooled to a final temperature of 5.8μK.The second scheme is to generate LHB with a large scale based on nonlinear effect of the nonlinear crystal material ZnSe. We have studied the basic principle of generating LHB based on nonlinear effect and the dependences of the quality of the generated LHB on the basic parameters of the optical system. According to Huygens-Fresnel diffraction theory, we calculate the intensity distribution of the LHB along the z-axis at different distances and its propagating properties in free space. We study the dependences of the quality of LHB on the waist radius of the incident Gaussian laser beam. The calculated results show that the waist radius not only exerts an influence on the dark-spot size of the LHB, and also the depth of the potential well of the LHB. The LHB generated by this scheme provides a theoretical basis in the applications of atomic (molecular) trapping.The third method is to generate LHB based on circular aperture diffraction. Firstly, we analyze the theoretical basis of the circular aperture diffraction, and according to Rayleigh-Sommerfeld diffraction theory, we calculate propagating properties of the incident Gaussian laser beam after diffracting. The calculated results show that there is a series of intensity peaks and intensity troughs. Secondly, we verify the above theoretical scheme experimentally, and we find that the calculated results are consistent with the experimental results.We also introduce the main methods of particles manipulation and outline the main methods of neutral molecules trapping:electrostatic trapping, magnetostatic trapping, laser trapping and magneto-optical trapping. Then we discuss the LHB’s application in the cold molecular optics, which is generated by using the system of a single mode fiber, a circle binary phase plate and a micro lens. We take molecular optical trap as an example, and then we calculate O2 molecules’transverse distribution of optical potential at the focus of the LHB, and O2 molecules’transverse distribution of optical dipole force. The calculated results show that the dipole force of O2 molecules in the blue-detuned LHB is pointed to the minimum light field. O2 molecules moving in the blue-detuned light field, they are trapped in the three-dimensional LHB, so LHB can be used as a hollow optical trap.