Research On The Mechanism And Method Of Bessel Beams

As an exact solution of the free-space scalar wave equation, the zero-order Bessel beams were firstly introduced by Durnin in 1987. The transverse intensity profile of Bessel beams consist of a group of concentric rings with a central bright spot, and it will remain approximately unchanged for a long distance even disturbed by nontransparent obstacles. Over the past few decades, Bessel beams have been extensively studied due to their non-diffracting behavior and their self-healing property. This uique properties make them useful in many fields, such as optical alignment and imaging, particle mciromanipulation, laser microfabrication and nonlinear optics. Nevertheless, present methods and techniques still have many drawbacks. Hence, further researches are still required and have attracted much interest.In this thesis, the mechanism and method of Bessel beams were studied systematically. A series of novel theories and methods of Bessel beams were proposed. The chief content of this dissertation includes the following sections.Based on the diffraction integrated theory, the formation mechanism of Bessel beams was investigated, especially the propagation properties of Bessel beams generated by annular slit and axicon were investigated theoretically and experimentally. Besides, Several practical considerations associated with the formation of Bessel beams based on the employment of axicons were discussed. This includes oblique angle of the input field, imperfections of the axicon tip shape, lateral misalignments with respect to the propagating field, and the ellipticity of the input field. It is shown that the above factors may have a significant impact on the on-axis intensity distribution and the shape of the resulting field, thus the aspects needs to be account for when using Bessel beams in practical applications.The Bessel beam generated by a single axicon begins at the axicon apex and appears to have relatively short work distance, which restricts some fundamental applications. To this end, two novel type multi-axicon systems were proposed to generate Bessel beams with long working distance. Moreover, the systems presented here featuring high compactness and flexibility.The basic principles of interaction between laser and metal materials as well as picosecond pulse ablation mechanism based on one-dimension double-temperature equation were analyzed. In addition, simple analytical formulas were derived which allow to predict the required laser pulse energy and the ablation widths. Theoretical and experimental results concerning the use of axicons for picosecond laser-ablation were carried out.Within the framework of the finite-difference time-domain (FDTD) technique, three methods of Bessel beams generated by photonic nanojet (PNJ) were proposed. Firstly, a liquid-immersed core-shell dielectric microsphere, which can generate an ultralong PNJ while retaining high intensity and large working distance was numerically investigated. Secondly, elongated PNJs produced by microaxicon were demonstrated. The results show that optical focusing by microaxicons produces PNJs of ultralong extent as compared to that formed by micorsphere of the same cross section. Then the properties of PNJs emerging from spheroidal particles illuminated by a focused Gaussian beam were studied. The simulation results show that it is possible to control the PNJ characteristics by varying the ellipticity of the spheroidal particle.