【作者】 周哲海；

【导师】 金国藩；

【作者基本信息】 清华大学 ， 光学工程， 2010， 博士

【摘要】 轴对称偏振光束是一种偏振态空间变化具有轴对称特性的偏振光束,对称轴为光束的传播轴。因其独特的光学特性,该类型光束在光学成像、光学捕获、材料加工及表面等离子体激发等方面具有巨大的潜在应用价值。论文针对目前的研究现状和不足,对轴对称偏振光束的生成方法、光学特性及潜在应用进行了研究。基于亚波长金属光栅的偏振分束特性,提出了利用分块亚波长金属光栅在可见光波段内生成径向偏振光及高偏振级次偏振光束的方法。基于傅里叶模态方法优化设计了亚波长金属光栅的结构,并由清华富士康纳米科技研究中心利用电子束直写技术和反应离子刻蚀技术制作了偏振级次分别为1、2、3和4的偏振光束转换器件。分析了转换光束的相位分布,基于Stokes方法测试了器件转换光束的强度及偏振分布,获得的径向偏振光的偏振纯度达到94.8%,偏振级次P=2、3、4的偏振光束的偏振纯度分别为90.4%、89.3%和84.7%,器件可应用于较宽波段,具有消色差特性。基于矢量衍射理论,推导了轴对称偏振光束在自由空间聚焦场的数学表达式,数值分析了不同类型轴对称偏振光束聚焦场的强度、相位及偏振分布。研究表明轴对称偏振光束高数值孔径的聚焦场具有很强的轴向分量,在焦平面附近可产生多个超分辨聚焦光斑,焦斑数量由光束偏振级次P决定,为2×(P 1);聚焦光场依然保持轴对称偏振分布。基于共焦显微镜搭建了光学系统,对径向和切向偏振光的高数值孔径聚焦场进行了初步测试,实验结果与理论分析吻合。研究了利用衍射光学元件实现轴对称偏振光束聚焦整形的基本方法,并针对某些特定需要,设计实现了平顶聚焦场、超小光斑及长焦深聚焦场及光链分布聚焦场,同时研究了径向偏振光实现衍射超分辨聚焦的衍射光学元件的优化设计方法。初步研究了两种利用轴对称偏振光束进行光学捕获的方法,即利用高数值孔径聚焦的轴对称偏振光束在焦平面附近捕获多个粒子,以及利用高数值孔径聚焦的轴对称偏振光束激发表面等离子体波在金属表面捕获多个粒子。介绍了上述两种方法的基本原理,数值分析了方法的可行性。更多还原

【Abstract】 Axially Symmetric Polarized Beams (ASPBs) are space variant polarized beams with axial symmetry, whose symmetry axis is the propagation axis of the light beam. Due to their unique optical properties, such beams have many potential applications in imaging, optical trapping, material processing, surface plasmon excitation, and so on. In the dissertation, the generation methods, optical properties and potential applications of ASPBs have been studied on the basis of the analysis of state of the art.Based on the polarizing beam splitting properties of subwavelength metallic gratings, the generation method of radially polarized beams and high order polarized beams in the visible range using segmented subwavelength metallic gratings (SSMGs) is proposed. The structure parameters of SSMGs are optimized using Fourier Modal Method (FMM), and the polarization transformed devices of space variant polarized beams with the orders 1, 2, 3, and 4 are fabricated by Tsinghua Foxconn Nanotechnology Research Center. The phase distribution of transmitted beams is analyzed, and the intensity and polarization distribution of transmitted beams are also measured based on Stokes measurement method. The polarization purity of the generated radially polarized beam is 94.8%, and the numbers for transformed P=2, 3, 4 space variant polarized beams are 90.4%, 89.3%, 81.7% respectively. The SSMG devices can operate in a wide wavelength range, and show a broadband achromatic property in visible range.The mathematical expressions of focused fields of ASPBs in free space have been derived based on vectorial diffractive theory. The amplitude, intensity, phase and polarization distributions of focused field of different types of ASPBs have been numerically calculated, which show the high NA focusing fields of higher polarization order ASPBs have strong axial components and present multiple superresolution spots near focus. The number of focal spots is related to the polarization order P as 2×(P 1). The focused beams are still axially symmetric, but present polarization singularities and phase singularities in some spatial positions. An optical measurement system is built based on a confocal microscope, and the high NA focused field intensities of radially polarized beam and azimuthally polarized beams are measured, and the measurement results verify the theoretical analysis.3D focus shaping using diffractive optical elements (DOEs) is studied, and DOEs for some special field distributions, such as flat top, optical chain and ultra small focal spots with long depth of focus, have been designed. Meanwhile, an optimum design method for diffractive superresolution elements for radially polarized beam is also studied.Two methods to implement optical trapping using ASPBs are discussed. Multiple ultra small focusing spots of high NA focused ASPBs formed near the focal plane can trap multiple micro particles simultaneously, and surface plasmon interfering fields excited by tightly focused ASPBs can trap multiple particles near the metal film as well. The basic theories for both methods are presented, and some simulations are carried out to verify the feasibility of the methods.更多还原