X射线数字单焦点螺旋波带片聚焦及拓扑荷演化特性

X-ray vortex field represents the frontier of synchrotron radiation micro-nano probe and microscopy technology. In the previous work, single-focus spiral zone plate was proposed, and optical vortex transmutation rule was verified for the first time. However, nanometer-scale x-ray focusing and topological charge evolution characteristics are still two fundamental problems which remain to be solved. In this project, the x-ray digital single-focus spiral zone plate is proposed. The key idea is to introduce a digital prolate spheroidal window, and to consider the constraint conditions of the finely focused electron beam with a Gaussian intensity profile, and to suppress higher-order diffractions by in-depth learning. The physical mechanism of converting phase gradient into energy information with larger energy ratio of the main lobe is studied. Under the constraint conditions of nanofabrication, the dependence of the spectrum characteristic parameters in the generated output light field is investigated by in-depth learning, and the nonlinear coupling mapping relation is established, leading to appropriate distribution of the nanostructures for the purpose of higher-order diffraction suppression. The x-ray digital single-focus spiral zone plate with resolution better than 50nm is fabricated. The point spread function is reconstructed by the photoresist exposure method in synchrotron radiation beamline, and CCD is used to investigate topological charge evolution characteristics in free space. We will explore the influence of incident angle, incident wavelength, numerical aperture, layout design of nano-structure, absorber thickness and steepness and the finely focused electron beam with a Gaussian intensity profile on the manipulation of x-ray vortex focusing and topological charge evolution characteristics. Our results will provide the theoretical basis, technical reserves and basic experimental data for a new type of synchrotron radiation nano-probe, which fulfill the functions of focusing, Hilbert transform and higher-order diffraction suppression simultaneously.

X射线涡旋光场是同步辐射微纳探针和显微技术研究前沿。前期研究中，提出单焦点螺旋波带片，首次实验验证涡旋嬗变规则。但是，纳米尺度X射线涡旋聚焦和拓扑荷演化调控仍是两个待研究问题。本项目提出X射线数字单焦点螺旋波带片，基本思路是引入数字扁平窗函数，考虑高斯圆形纳米束斑制造约束条件，深度学习高次谐波抑制。研究将相位梯度转换为大主瓣能量光强信息的物理机制。制造约束条件下深度学习窗函数频谱特征参数对输出光场调控规律，建立非线性耦合映射关系，求解抑制高次谐波的纳米结构合理布局。集成制造分辨率优于50nm的元件，在同步辐射装置上光刻胶曝光法重构点扩散函数，CCD测试拓扑荷演化特性。探索入射角、入射波长、数值孔径、纳米结构布局、吸收体厚度及陡直度、高斯圆形纳米束斑等参数对X射线涡旋聚焦和拓扑荷演化调控规律。为同时实现聚焦、希尔伯特变换和高次谐波抑制的新型同步辐射纳探针提供理论依据、技术储备和基础实验数据。

X射线涡旋光场是同步辐射微纳探针和显微技术研究前沿。近年来，得益于微纳制造技术，尤其是集成电路平面工艺技术的快速进展,以 X 射线螺旋波带片为代表的腔外生成法逐渐成为X 射线涡旋光束生成的主流手段，有高精度、轻重量、紧凑、大设计自由度和校准容易等优点, 不必借助于4f系统即可同时完成聚焦和希尔伯特变换功能。但是，其存在的问题是：1）固有的高次谐波导致多级聚焦点、多衍射级次亮环等背景噪声；2）对高高宽比纳米结构制造提出苛刻要求，极大程度上挑战微电子制造技术的极限。为了克服这些缺点，本项目提出X射线数字单焦点螺旋波带片，对纳米尺度X射线涡旋聚焦和拓扑荷演化调控仍是两个问题进行了研究。1）提出了将2D Chirp-z变换（CZT）引入X 射线数字单焦点螺旋波带片的理论设计方法，建立了基于严格电磁场理论的X 射线数字单焦点螺旋波带片物理模型。2）基于深度学习算法，建立了焦平面衍射光场与元件透射函数参数之间的卷积神经网络模型。3）建立了结合高分辨率电子束光刻、接近式光学光刻和精细电镀等技术相结合的X 射线数字单焦点螺旋波带片加法集成制造方法，集成制造的特征尺寸达50nm。4）在同步辐射装置上对所研制的X 射线数字单焦点螺旋波带片进行了测试，观测到了一个明亮的环形涡旋光束斑衍射图案，随着拓扑荷数值（共四种）的增大，X射线涡旋光束的束腰半径相应增大，验证了X射线数字单焦点螺旋波带片的优越衍射模式。项目取得了完全自主知识产权的研究成果，为同时实现聚焦、希尔伯特变换和高次谐波抑制的新型同步辐射纳探针提供了理论依据、技术储备和基础实验数据。