Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere

First row: A vector vortex beam that can be decomposed into a vector polarization and a vortex phase. Second row: The vector polarization and the vortex phase can be generated by a q-plate and a spiral phase plate, respectively. A compact device can be constructed by integrating the q-plate structure into the spiral phase plate.

Vector vortex beams, possessing both vector polarization and helical phase in beam section, have important implications in laser machining, particle manipulation, and quantum information. Compared with vector beam or vortex beam, the vector vortex beam provides more degrees of freedom in light-matter interaction. A variety of tools, e.g. spatial light modulator, liquid-crystal-based polarization converter, and laser resonator, have been employed to generate vector vortex beams. However, the generated polarization states are usually limited to two special cases: azimuthal polarization and radial polarization. In addition, these methods usually face challenges of low damage threshold, lower conversion efficiency, and enormous size. Therefore, to generate arbitrary vector vortex beams, a flexible generation method with high efficiency and compact structure should be taken into account.

In order to overcome the above problems, spin photonics group led by Prof. Hailu Luo from School of Physics and Electronics, Hunan University, carried out the research of generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere by combining a dielectric q-plate and a spiral phase plate. Related research results are published in Photonics Research, Volume 5, No. 1, 2017 (Z. Liu, et al., Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere).

Vector vortex beams can be mapped on the hybrid-order Poincaré sphere, where the evolution of polarization and phase can be clearly represented. Based on this geometrical representation, a vector vortex beam can be decomposed into a vector beam and a vortex. Therefore their generation can be realized by using a q-plate and a spiral phase plate, respectively. By controlling the polarization state of input beam, any desired vector vortex beam on the corresponding hybrid-order Poincaré sphere can be obtained.

Note that the q-plate and the spiral phase plate are both fabricated on silica glasses, suggesting the potential possibility to integrate the two structures on a single plate. Prof. Hailu Luo believes that multiple optical elements with different functions into one glass plate can be integrated. This technique may have potential applications in future photonics and optoelectronics. Further work is to realize the miniaturization of the system by integrating different structures, and enhance the capacity in the manipulation of polarization and phase of light.

产生混合阶庞加莱球上的任意矢量涡旋光束

图片说明：一个矢量涡旋光束可以分解成一个矢量偏振和一个涡旋相位（如第一行所示）。矢量偏振可以用一块电介质q片产生而涡旋相位可以通过螺旋相位片来实现（对应于第二行）。将q片结构集成到螺旋相位片上，即可形成产生矢量涡旋光束的集成器件。

矢量涡旋光束是指在光束横截面上同时具有非均匀偏振态和螺旋相位结构的新型激光光束，它在激光加工、粒子操控、量子信息等方面有着重要的应用前景。与矢量光束或者涡旋光束相比，矢量涡旋光束在光与物质相互作用中具有更多的自由度。

目前，利用空间光调制器、基于液晶的偏振转换器、激光谐振腔等都已能够产生矢量涡旋光束。然而，这些方法所产生的光束通常只限于特定的偏振态，且具有低损伤阈值、低转换效率和大结构尺寸等缺点。因此，为了使矢量涡旋光束得到更加广泛的应用，需要找到一种高效率、更灵活、结构紧凑的产生方法。

为克服上述方法存在的问题，湖南大学物理与电子学院罗海陆教授领导的自旋光子学课题组采用电介质q片和螺旋相位片的组合来产生混合阶庞加莱球上任意点的矢量涡旋光束，相关研究成果发表在Photonics Research 2017年第5卷第1期上（Z. Liu, et al., Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere）。

混合阶庞加莱球用于描述任意矢量涡旋光束，可以让其偏振与相位演化的物理过程变得更加直观。基于混合阶庞加莱球理论，任意的矢量涡旋光束可以分解成一个矢量光束和一个涡旋相因子。其中，矢量光束可以用一块电介质q片产生而涡旋相因子可以通过螺旋相位片来实现。通过控制入射光的偏振态，即可得到混合阶庞加莱球上相应点的矢量涡旋光束。

这一方案的一个重要特点是q片的结构与螺旋相位片均可制作在二氧化硅玻璃基底上，因此完全可以将两个结构集成在单个玻片上。该课题组罗海陆教授认为，这一方案可以实现在一块玻璃片上集成多个具有不同功能的光学元件，在未来的光子学与光电子学中将会有重要的应用。后续工作主要是实现系统的集成化，进一步提升操控光的偏振与相位的能力。