High efficiency generation of tunable ellipse perfect vector beams
Ellipse perfect vector beams in focal plane, generated by superposition of two orthogonally polarized (left and right handed circular) component ellipse perfect vortex beams with opposite phase topological charges.
The regulating of local polarization state of an optical beam has become an interesting and hot topic in optical complex beam shaping. Perfect vector beams (PVBs) with inhomogeneous polarization distribution and tunable beam dimensions have shown great potential in a variety of applications including optical micro-particle manipulation, super-resolution imaging and light-matter interactions. Popular vector beam generation systems are based on the employment of spatial light modulator (SLM) loaded with a designed computer-generated hologram (CGH), but the generated vector beams are usually limited to fixed mode of ring-shaped structures, and the systems are also facing low efficiency of light utilization.
The groups led by Dr. Chenliang Chang from School of Physics and Technology, Nanjing Normal University, and Prof. Jianping Ding from School of Physics, Nanjing University jointly carried out a research of generating ellipse shaped perfect vector beams in a high efficient way. More diverse modes of PVBs are extended from ring to different ellipse structures, based on the modulation of specifically designed hologram calculated by using an advanced holographic beam shaping algorithm. The related work is published in Photonics Research, Volume 6, Issue 12, 2018 (Lin Li, et al., High efficiency generation of tunable ellipse perfect vector beams).
The key to generate different ellipse structured PVBs is the algorithm of the phase-only hologram. The algorithm is a combination of inverse diffraction calculation and iterative scheme. The complex field of the hologram plane is obtained by accumulating discrete points of ellipse curve. After continuous iterations, a phase-only profile is finished by calculation under given constraint conditions. The optical implementation is based on the superposition of two orthogonal polarized component beams via an optical filtering system. Moreover, simultaneous generation of multiple ellipse structured PVBs in both of two-dimensional and three-dimensional focal regions are also achieved.
Dr. Chenliang Chang and Prof. Jianping Ding both believe that this work has important theoretical and practical significance for generating diversely extended modes of PVBs. They also believe that this work will provide an effective solution to increase the light efficiency in vector beam generation system, by concentrating on the improvement and optimization of hologram algorithm.
The further work will focus on the PVB generation of more diverse types such as square, quatrefoil or Archimedean spirals based on the proposed algorithm.
可调谐椭圆形完美矢量光束的高效率整形
焦平面上的椭圆完美矢量光束整形，通过两个具有正交偏振（左旋和右旋）及相反相位拓扑荷的椭圆完美涡旋基矢光束的叠加来实现。
在复杂光束整形中，对光束局域偏振态的调控已经成为了当前的一个研究热点。其中完美矢量光束由于其具有空间非均匀分布的偏振态以及可调节的光束尺寸使得其在光学微操纵、超分辨成像及光与材料的相互作用等领域具有非常大的应用潜力。目前比较典型的矢量光场生成系统主要基于加载了计算全息图的空间光调制器来实现，但是该方法生成的矢量光场往往局限于固定的圆环形结构，同时该类系统光量能利用率也较低。
为了解决上述问题，由南京师范大学物理科学与技术学院常琛亮博士和南京大学物理学院丁剑平教授共同领导的研究团队开展了高效生成椭圆完美矢量光场的研究。该研究主要通过利用改进的全息光束整形算法来设计和计算独特的计算全息图，产生从圆环到不同椭圆结构模式的完美矢量光束。相关研究结果发表于Photonics Research 2018年第6卷第12期上(Lin Li, et al., High efficiency generation of tunable ellipse perfect vector beams)。
生成不同椭圆结构完美矢量光束的关键在于采用了纯相位全息图的一种新型计算方法。该新型算法结合了逆衍射计算和迭代优化的思想。首先通过不断累积椭圆曲线上的离散点来获得全息面的复振幅分布，然后在给定的约束条件下，通过不断迭代，计算出最终的纯相位型全息图。在实验，将经过光学滤波系统后的偏振分量正交的两束光进行叠加，实现了椭圆形矢量光束的合成。此外，利用该方法还能够在二维及三维焦场区域中同时生成多个椭圆完美矢量光束。
常琛亮博士及丁剑平教授都相信该工作对于生成更加多样化模式的完美矢量光场具有重要的理论和实际意义。同时他们还认为该工作通过对全息图算法的改进优化，为提高新型矢量光场的生成效率和光能量利用率提供了有效的解决办法和思路。
该研究工作下一步的目标是将该方法进一步推广到能够生成方形、四叶形或阿基米德螺旋线等更多不同模式和结构的复杂完美矢量光场。