Studies On The Phase Vortex Array And Its Evolutions In The Extremely Deep Fresnel Diffraction Region Generated By Asymmetrical Nanoholes Metal Film

Vortex field is a special kind of light field with a particular phase factor exp(il). Theintensity in the center of the vortex is zero and the phase is undefined, it is also called phasesingularities. The phase of the singularities has a helical variation along with the increase ofazimuthal angle, the total phase variation depends on the topological charge. The topologicalcharge of the light field with phase factor exp(il)is l, and the orbital angular momentum ofthe photon in the fields is l. Due to the special phase distribution and the orbital angularmomentum, the optical vortices has been extensively used for atomic optics, material science,quantum optics, optical micromanipulation, remote sensing, informationtransmission,biomedicine, and so on. Recently, the studies on the realization methods and theevolution of optical vortices have been the research focuses.The method of the multi-beam interference with multipoint for the optical vortexrealization has been widely used due to easy operation and adjusting. Generally, this methodcan be processed as forming pattern at the Fourier plane of the pinhole screen usingFraunhofer diffraction, and the pinholes are mostly at micron dimension or more. The fields indeep Fresnel diffraction region conform to neither Fresnel diffraction nor Fraunhoferdiffraction. Whether the vortices can form in the deep Fresnel diffraction region or not, andthe characteristic of the vortices in this case have rarely been considered in the literature.In this paper, we report a study on a heptad vortex array structure and its evolution indeep Fresnel diffraction region generated by an asymmetrical nanohole screen illuminatedwith linearly polarized light. A Mach-Zehnder type interferometer with a microscopicobjective is set up to image the fields at different distances pass the object plane. A CCDrecords the intensity and the interference patterns of the fields and reference beam. Theintensity and the phase distribution patterns of the diffraction fields are reconstructed usingFourier transform method. From the intensity and the phase distribution patterns, we find anarray of seven vortex structures appears in the central area of the patterns. The patterns of thevortex structures show evolution properties by changing the diffraction distance. The phase and the intensity distributions are corresponding to each other. The vortex in the center of thearray varies uniformly around the vortex core. Using the Kirchhoff diffraction theory, wecalculate the diffraction fields of the nanohole screen in the deep Fresnel diffraction region.Compared with the experiment result, they are different at near distance of the diffractionfields, and they agree with each other. Based on the theory of the light propagation when thelight illuminate on the pinholes, we calculate the diffraction field for only one pinhole atdifferent distances. We analyze the reason of the differences between the calculations and theexperimental results. Since we are not familiar with the surface plasmon polaritons, we canonly conjecture that the surface plasmon polaritons has influence on the intensity and phase atnear distance. With the increasing of the diffraction distances, the surface plasmonpolaritons perpendicular to the sample surfac rapidly reduce, and the influences on theintensity and phase also gradually decrease. This paper is divided into five chapters:Chapter1: First, we simply introduce the development of optical vortices. Then, wesummarize the current state and the purpose of research on optical vortices.Chapter2: we describe the basic mathematical model and characters of optical vortex.Then, we introduce the common methods for generating the phase vortex and their advantageand disadvantage in practical applications.Chapter3: this chapter, we study the phase vortices produced by an asymmetricalnanohole screen in experiment. We describe the principle and process of our experiment.The method and results for the experiment are given in details.Chapter4: We calculate the intensity and phase distributions of the fields in deep Fresneldiffraction region generated by the asymmetrical nanohole screen. In addition, we alsocalculate the field in deep Fresnel diffraction region generated by the one nanohole. Finally,we analyze the formation and evolution of the phase vortex array with the increasingdiffraction distance.Chapter5: We summarize the mian achievements of this paper and expound the matterand shortage in the entire research process. Then, we describe the main future work.