Theoretical Studies On Focus Shaping Technology And Optical Trapping Properties Of Beams

Because of the wide application of laser technology, the research of the tightly focusing properties of beams, focus shaping technique and optical trapping properties becomes interesting and necessary in both theory and practice. To take full advantage of laser technology and related novel effects and phenomena, the key is to design and manipulate the spatial structure of the beam field. By designing the spatial structures of the polarization, amplitude and phase of incident beams, we can easily manipulate the focused field distribution and obtain many special beam patterns, such as "optical needle", "optical chain", "optical cage" and "flat-topped field" etc. These special patterns will find valuable applications in information technology, biomedicine, precision measurement, material processing, aerospace and even the national defense security, and so on. Furthermore, the capability of shaping the focal volume by using the focus shaping technique has many obvious advantages in particle trapping and manipulation. For example, the optical cage generated by cylindrical vector beams has the trapping capability of the low-refractive-index or light-absorbing particles. Thus, the research of the focus shaping technique is of great significance.Currently, the research of the focus shaping techniques is fresh and there remain many scientific and technological problems to be solved. Thus, this dissertation is mainly focused on the focus shaping techniques and the optical trapping properties of beams. The main contributions of this dissertation are as follows:(1) The research of the tightly focusing properties and optical trapping properties of the radially polarized hollow sinh-Gaussian beams. Based on the Richards-Wolf vector diffraction theory, the focal intensity distribution of the radially polarized hollow sinh-Gaussian beams with different values of beam order has been theoretically studied. According the Rayleigh scattering theory, the optical trapping forces acting on a Rayleigh metallic particle for different values of beam order in different locations have also been studied. By changing the relatively parameters of the trapping condition, the influence of each parameter on the trap stiffness is investigated in detail. The results show that we can improve the transverse trapping stability and increase the axial trap range using high-order radially polarized HSG beams. And finally, the particle trapping stability has also been studied in detail.(2) The research of the tightly focusing properties and focus shaping technique of the partially coherent circularly polarized vortex beams. Based on the vector diffraction theory and coherence theory, the three-dimensional (3D) focus shaping technique using the combination of partially coherent circularly polarized vortex beams with a binary diffractive optical element (DOE) is reported. It is found that the intensity distribution near the focus can be tailored in three dimensions by appropriately adjusting the parameters of the incident beams, numerical aperture of the objective lens and the design of the DOE. Numerical results show that partially coherent circularly polarized vortex beams can be used to generate several special beam patterns, such as optical chain, optical needle, optical dark channel, flat-topped field and 3D optical cage. Furthermore, compared with the ordinary 3D optical cage, this kind of 3D optical cage generated by our method has a controllable switch; that is, it can be easy to "open" and "close" by controlling the coherence length of the incident beams.(3) The tightly focusing properties and optical trapping properties of the 4π tight focusing of radially polarized beams. Based on the Richards-Wolf vector diffraction theory and Rayleigh scattering theory, the radiation forces acting upon a metallic Rayleigh particle are calculated for the case where a 4π high numerical aperature (NA) focusing system under the illumination of two counter-propagating radially polarized beams is applied. Compared with the conventional single-lens system, the main advantage of the 4π focusing system is that it can greatly improve the axial trapping efficiency of nanoparticles. The extremely sharp and almost spherical focal spot of the 4π focusing system provides a large gradient force in both the transverse and axial directions. Meanwhile, the scattering/absorption force near the focus can be strongly suppressed for the reversed-phase superimposition of two counter-propagating beams in this 4π focusing system and consequently the axial force is represented only by the axial gradient force at the focal plane. In addition, the influence of the off-focus distance and the off-axis distance of a trapping particle on the trap stiffness is also investigated in detail.