平板衍射透镜在超快激光切割中的应用研究

Nearsightedness is one of the most important diseases that do harm to the health of teenagers all over the world. Based on the traditional objective lenses, the surgery of cornea cutting by ultrafast laser is the most efficient approach to treat nearsightedness. However, constrained by Rayleigh diffraction limit, the traditional objective lenses can only focus light into a spot with the size of 0.61λ/NA, where λ is the operating wavelength and NA is the numerical aperture of lens, which is difficult to be suppressed further for decreasing the damage of issue in cornea. Moreover, most of commercial objective lenses with high NA have the working distances smaller than 400 microns, which implies that they cannot be used to cut the cornea with its thickness of ~500 microns. Based on the previous investigation on optical nanofocusing, the applicant plans to develop a planar diffractive lens with super-long focal length of ~1mm, a NA of 0.9 and the focusing efficiency of ~40% for realizing a beyond-diffraction-limit spot of ~0.43λ. To realize it, an efficient optimization algorithm will be built in combination with precise nano-/micro-fabrication technologies so that such a subwavelength focal spot could be achieved at the focal plane, where constructive interferences happen by modulating thee phase of incident light. To verify its optical properties, the focal spot of the fabricated lens under the illumination of a nano-second laser will be used to cut a PMMA film on substrate, leaving an etched line. The line width will be compared with the resulting width achieved by using traditional objective lenses, providing the initial fundamental researches and verifying the feasibility of applying such a planar diffractive lens into the laser surgery in cornea cutting. Such a planar diffractive lens could also be used in a wide spectrum across far-field nano-lithography, super-resolution nano-imaging and high-density data storage.

近视是世界上危害青少年健康的重要疾病之一。基于传统透镜聚焦的激光角膜切割手术是目前最重要、最有效的治疗手段。但，传统透镜由于光的衍射极限（即0.61λ/NA），无法缩小切割光斑以减少身体损伤;且大多数高数值（NA≥0.9）物镜的工作距离都小于400微米,不能用于切割500微米厚的眼角膜。申请人基于前期的光聚焦研究基础，拟开发一种具有超长焦距（1mm）、高数值孔径（NA=0.9）和高效率（约40%）的平板衍射透镜以实现光的超衍射极限聚焦，其光斑尺寸约0.43λ。申请人拟建立有效的优化算法和精细的微纳加工技术，用以调制光的相位，来实现光的相长干涉，使得透镜在焦平面处实现亚波长的聚焦效果。在纳秒级超快激光的照明下，该透镜形成的焦斑去刻蚀有机薄膜（如PMMA），然后测量薄膜上被刻蚀的线宽，并与传统透镜得到的线宽进行对比分析,为该透镜在激光角膜切割手术中的最终应用做前期的基础研究和技术论证。该技术还能用于远场纳米光刻、超分辨显微成像、以及高密度光存储等领域。

近视是世界上危害青少年健康的重要疾病之一。基于传统透镜聚焦的激光角膜切割手术是目前最重要、最有效的治疗手段。但，传统透镜由于光的衍射极限（即0.61λ/NA），无法缩小切割光斑以减少身体损伤;且大多数高数值（NA≥0.9）物镜的工作距离都小于400微米,不能用于切割500微米厚的眼角膜。本项目组研制一种具有超长焦距、高数值孔径和高效率的平板衍射透镜以实现光的超衍射极限聚焦。为实现该目标，项目组研究了平板衍射透镜实现超衍射极限的聚焦机制、结构设计和优化、加工制备和光学性能测试、超快激光切割等内容，实验上制备了具有焦距1mm、数值孔径NA=0.9、焦斑尺寸0.45λ、直径为4mm的平板衍射透镜，并在有机材料PMMA薄膜上进行激光切割实验，得到430nm的切割线宽，是传统物镜切割线宽的1/4。如果将该平板衍射透镜用于视力矫正手术中，本项目的研究成果在理论和实验上证明了其能减小患者的手术创伤面积，因此该技术在视力矫正手术中具有潜在的应用价值。另外，本项目研制的平板衍射透镜具有高数值孔径、长焦距、亚衍射极限聚焦，能用在光学超分辨成像领域，解决传统物镜焦距受限的问题，对任意厚度的样品进行高分辨率成像。本项目的研究成果已经用于提升激光角分辨光电子能谱仪的空间分辨率，增强微小光学成像系统的空间分辨能力，而微结构设计的相关技术也已用于印刷行业中微凹版辊的设计和制作，增强印刷品的三维显示效果。基于本项目的工作，项目组成员发表18篇SCI高影响力论文，包括Nature Communications, Light: Science & Applications, ACS Nano, Nano Letters, PNAS, Laser & Photonics Reviews, Advanced Optical Materials等，已授权发明专利2项，正申请专利2项。