轴向位置可调的双中空环形光斑

Double doughnut-shaped focal spots with controllable position in axial direction

为了产生轴向双焦点中空环形光斑,基于矢量衍射积分得出的环带半径公式,设计产生了呈环带分布的轴向双焦点的螺旋相位,并研究了这种螺旋相位在高数值孔径物镜聚焦区域的光斑特性。首先,给出了线偏振以及圆偏振的涡旋光束在高数值孔径物镜聚焦条件下的积分表达式。然后,利用此积分表达式数值模拟了线偏振光与圆偏振光在不同轴向偏移距离及螺旋拓扑荷值时的聚焦光场分布。最后,将轴向双焦点螺旋相位加载到纯相位空间光调制器上,分别对圆偏振光与线偏振光入射进行实验研究。线偏振光入射时,实验产生了拓扑荷为1且轴向距离为±10μm、±15μm的双聚焦环形光斑;圆偏振光入射时,产生了轴向距离为±20μm且拓扑荷为1到4时的双聚焦环形光斑。数值模拟与实验结果表明:圆偏振光与线偏振光经此螺旋相位调制后,在紧聚焦区域可产生轴向距离与暗斑大小可调的中空环形双焦点;圆偏振光较线偏振光产生的空心光斑光强分布更均匀,呈圆对称分布。此轴向双焦点螺旋相位有望在光学微操控、双光束超分辨纳米光刻以及STED显微成像方面获得一定的应用。

In order to generate double doughnut-shaped focal spots at adjustable positions along the axial direction,a vortex phase zone plate based on a formula of annular radius derived from vector diffraction integral was designed.The focusing properties of the modulated vortex phase zone plate were further investigated in a tightly focused system.First,integral formulas of linearly and circularly polarized vortex beams were calculated under high NA focusing conditions.Then,the intensity distributions of linearly and circularly polarized vortex beams in a high NA focusing system were simulated by integral formulas with various axial shifting distances and topological charges.Finally,the corresponding experimental results of linearly and cir-cularly polarized light were also given,utilizing a spatial light modulator loaded on double doughnut-shaped phase patterns.The double doughnut-shaped focal spots with a topological charge of 1 and axial distances of±10μm and±15μm were produced when the incident light was linearly polarized.As well as the double doughnut-shaped focal spots with axial distances of±20μm,topological charges of 1−4 were also produced when the incident light was circularly polarized.The simulated and experimental results demonstrated that two doughnut-shape focal spots with controllable axial shifting distances and dark spot sizes could be pro-duced in the tight focusing region of a high NA objective when it is modulated by the vortex phase zone plate.This kind of vortex phase zone plate could be applied in the field of optical micromanipulation,two-beam super-resolution nanolithography,and Stimulated-Emission-Depletion(STED)fluorescence micro-scopy.