Optical Trapping Of Spherical Gold Nanoparticle By Radially Polarized Beams

Different from traditional linearly polarized beams and circularly polarized beams, radially polarized beams have attracted more interests by symmetric polarization and tight focusing properties, using widely in particle acceler ation, fluorescence imaging, Raman spectra and optical trapping. With the improvement of light control efficiency and result and further enhancement of focusing properties of radially polarized beams, it is possible to obtain a light spot with smaller size and higher intensity,which is also an urgent problem to be solve d. One aim of this thesis is to obtain sharper focus,higher light distributions and superior theoretical foundation of optical trapping by means of changing the amplitude distributions of the incident beams and using plasmonic nanostructures excitated by metallic nanostructure to the light field.We study the modulation of the phase information on the LCOS of SLM and of the beams’ polarization state through two quarter-wave plates, and prepare the radially polarized beams experimentally. Also,we make detailed studies on the characters of tight focusing fields based on the Richards-Wolf vectorical diffraction theory.Moreover, we carry on some studies on different way of calculating the optical force on varisized particles in the light field, including the Rayleigh model, the MST method and the Ray optics model, which are used to analyse the optical force on small-size particles, middle-size particles and large-size particles in the light field, respectively.We study the optical potential well formed from radially polarized beams with the wavelength of 600 nm by using the vectorical diffraction theory model of tight focusing of radially polarized beams. The research shows that the optical potential well can trap dielectric particles with various sizes, however, stable three-dimensional trap is still a problem to metallic nanoparticles. To address this problem, we put forward a RTEM11* incident light distribution and make researches on the focusing field along with the change of beam parameters, and later we calculate the optical potential well and the optical force on gold nanoparticles in the light field. The result presents that when β is about 2,the bright focus spot is able to trap the gold nanoparticles with radius of 50 nm,while β goes to 1.3, we obtain a focusing light field which is capable to trap low-reflective with a dark hollow focus spot in the center.We adopt the axisymmetrical metal-insulator-metal nanostructure and take it as nanolens to focus the SPP excited by radially polarized beams. Compared with traditional linearly polarized beams, radially polarized beams can obtain the focal spot with smaller size and lager light intensity gradient, as a result, metallic nanoparticles can be trapped more effectively. We study the effect of the variation of excited metallic grating location to the focusing field and simulate the optical force on varisized particles, and finally it turns out that the focusing field has the ability to trap gold nanoparticles with lager radius,which also shows that the ability has been significantly enhanced.