基于几何相位超表面的Ince-Gaussian矢量涡旋光场聚焦

Focusing of Ince-Gaussian vector vortex optical field based on geometric phase metasurface

为了实现电介质超表面的聚焦功能和对光场相位的调控,采用几何相位调制原理设计微元结构及空间分布,以SiO_(2)为基底、亚波长TiO_(2)椭圆柱的六边形晶胞为基本结构,设计了一种相位突变呈抛物线梯度分布的聚焦超表面,适用于480nm~580nm波段。基于此结构进行了理论分析和实验验证,发现该结构对线偏振光聚焦,其归一化后的半峰全宽约为428nm,而对矢量光聚焦约为258nm,获得了更出色的聚焦效果。研究了3阶和4阶Ince-Gaussian矢量光场通过该超表面后的聚焦特性,得到了聚焦场能保持入射矢量光场的基本空间结构,但中心结构信息会有损失的结果,即Ince-Gaussian矢量涡旋光场由于涡旋相位的存在,聚焦后会呈现破缺的空间结构。结果表明,超表面结构和入射光场矢量结构之间的匹配程度是影响聚焦特性的重要因素。该研究为理解复杂矢量光场的超表面聚焦机理提供了参考。

In order to realize the focusing function of the dielectric metasurface and the adjustment of the phase of the light field,the geometric phase modulation principle was used to design the micro-element structure and spatial distribution.Using SiO_(2) as the substrate and the hexagonal unit cell of sub-wavelength TiO_(2) elliptical cylinder as the basic structure,a metalens with a parabolic gradient distribution of phase mutation was designed,which was suitable for the wavelength range of 480nm to 580nm.Based on this structure,theoretical analysis and experimental verification were carried out.It is found that for the linearly polarized light with this structure,the normalized full width at half maximum of the focus is about 428nm,and the full width at half maximum obtained by focusing on vector light is about 258nm,which is better than that of the linearly polarized light.The focusing characteristics of the 3rd-order and 4th-order Ince-Gaussian vector light field after passing through the metasurface were studied,and the basic spatial structure of the focusing field can maintain the incident vector light field,but the center structure information will be lost.That is,the Ince-Gaussian vector vortex light field will show a broken spatial structure after focusing due to the existence of the vortex phase.The results show that the matching degree between the metasurface structure and the incident light field vector structure is an important factor affecting the focusing characteristics.This research provides a reference for understanding the metasurface focusing mechanism of complex vector light fields.