基于空间光调制器的轴向动态三维达曼阵列

It is well-known that, in traditional point-by-point laser scanning techniques, the single focus spot generated by focusing a laser beam via an objective is mechanically scanning to obtain micro-/nano-scale structure information or induce local laser-material interaction in a small voxel. Because of these properties, the traditional point-by-point scanning techniques inherently possess low efficiency, weak stability and poor robustness, especially for those high numerical-aperture (NA) oil-immersion systems. Here, we propose a scheme to modulate the axial position of the focus spot by introducing a quadratic phase into the pupil plane of the focusing objective based on a spatial light modulator (SLM). Then, by changing the defocus level of the quadratic phase, one can obtain the focus spot scanning along the axial direction. By combining the technique of 3D Dammann gratings proposed by us [Appl. Opt. 51, 1619 (2012)], one can realize a 3D Dammann array of the focus spots scanning along the axial direaction, which will drastically improve the laser processing efficiency. Moreover, the scanning manner is realized by continuously changing the defocus level of the quadratic phase via a programmable SLM, and thus the system should be much more stable and robust because there is no mechanical scanning any more. Therefore, the dynamic 3D Dammann array based on a programmable SLM should be of high interest for its potentials in various applications, such as confocal laser scanning microscope, multifocal fluorescent microscope, stimulated emission depletion microscope, optical trapping and direct laser writing, etc.

传统激光扫描技术是利用物镜的单个聚焦光斑机械式逐点扫描获得物质的微观结构信息或产生激光与物质的局域作用，因而系统效率低、稳定性和可靠性差，尤其对高数值孔径油浸聚焦系统更是如此。本项目拟通过空间光调制器（SLM）在聚焦物镜的入射光瞳中引入二次球面相位项，从而改变物镜聚焦光斑的轴向位置。通过连续调节二次球面相位的离焦水平，即可实现聚焦光斑的轴向扫描。结合我们提出的三维达曼光栅技术[Appl. Opt.51,1619(2012)]，我们将这种轴向扫描的单点聚焦光斑拓展为三维空间分布的焦斑阵列，从而在传统的激光扫描技术中实现轴向扫描的三维达曼阵列,极大提高系统的扫描效率。同时，利用SLM的可编程性实时调节二次相位项来实现达曼阵列整体轴向动态扫描，避免了传统的机械式扫描，从而极大地提升系统的稳定性和可靠性。因而，这种基于SLM的轴向动态三维达曼阵列在各种激光扫描成像或处理技术中有广泛的应用前景。

物镜的聚焦光场是现代光学诸多技术如激光直写、激光光镊、激光共焦/荧光显微成像等技术的核心和基础。传统聚焦光场往往只能产生单个聚焦光斑，因而激光作用效率低。本项目提出利用达曼编码技术实现达曼波带片可实现轴向多焦点，进一步结合二维达曼光栅技术，可实现焦斑三维空间排布，因而可大大提高激光作用效率。更进一步，本项目将球面光瞳函数引入到三维达曼阵列系统，实现了无机械轴向扫描的三维达曼阵列。实验中，我们实现了NA0.1下的轴向无机械扫描5×5×5的三维达曼阵列，轴向扫描范围达-1000～1000微米。本项目还通过非分离达曼编码技术，设计并加工了5×5非分离达曼光栅，将二维达曼光栅的效率提升了约10%。此外，我们还提出并实现了连续相位达曼波带片，大大减弱空间光调制器实现达曼波带片时由于像素化带来了的问题。此外，本项目还通过角向编码技术可进一步提高达曼阵列的复杂度，从而使得复杂达曼光阱和三维微加工技术成为可能。我们实现了NA0.1聚焦条件下的轴向5焦面均为由三个焦斑组成的三角形的三维焦斑阵列。我们还初步验证了这种角向编码技术产生的由4个焦斑组成的“Y”型光阱阵列的并行捕获能力。通过达曼编码技术，我们实现了petal-like光斑阵列，并通过变化两个达曼涡旋光栅横向相对位移，实现了petal-like光斑角向的可控旋转。另外，我们还通过达曼编码光栅结合角锥相位，提出并实现了完美涡旋的二维阵列，这种环形光斑半径与携带拓扑荷无关的完美涡旋阵列可在轨道角动量复合光通信和光学操控等方面具有重要的应用前景。达曼波带片产生的轴向多焦点主要问题是由于轴向间隔与孔径内的径向周期数成正比，因而很难产生小轴向间隔的焦斑阵列。本项目基于圆环达曼光栅技术，提出了一种单物镜4Pi聚焦光学结构，可实现贝塞尔光束的驻波场。该技术可填补达曼波带片技术空白，实现轴向小间隔的多焦斑阵列。