Random Electromagnetic Beam Focusing And Transmission Of A Large Numerical Aperture

Laser beams focusing and propagating is a subject of considerable importance. An extremely small focal spot of subwavelength size, obtained by using the high-numerical aperture objective, can be used in applications of micromachining and data storage etc. On the other hand, the research of the production and properties of partially coherent vortex beams will be of great significance in practical applications. In this dissertation, the high-numerical aperture focusing of laser beam and the experiment of partially coherent vortex beams have been discussed in great detail. The main research topics are listed as follows:1.Based on the vectorial Debye diffraction theory, the field distribution expressions in the focal region when the partially polarized beam and the partially coherent partially polarized beam are focused by a high-numerical aperture objective are derived. The influence of the source polarization, source coherent length and the numerical aperture on the intensity distribution and the degree of coherence are investigated. It is found that the distribution of intensity and the degree of coherence is related to the degree of polarization of incident beam and the numerical aperture of lens.2.Based on Richards-Wolf vectorial diffraction integral, the expressions for the electric field, the group velocity and the total angular momentum of the focused femtosecond elliptically polarized vortex light pulses are derived. It is shown that near the focus the femtosecond elliptically polarized vortex light pulse can travel at various speeds, that is, slower or faster than light speed in vacuum, depending on the numerical aperture of the focusing objective system. Moreover, the influence of the numerical aperture of the focusing objective and the time duration of the elliptically polarized vortex light pulse on the total angular momentum distribution in the focused field is also studied.3.The intensity distribution of partially coherent vortex beam is investigated. Various degree of coherent and topological charge of partially coherent vortex beam can be obtained. The results show that the intensity distribution of vortex beam will be changing with the varying of coherent characteristics. It is also shown that, the intensity of the vortex beam center can be changed with the decreasing of the degree of coherence of incident beam and the size of the hollow core of vortex beam has close relationship with the topological charge.