高分辨扫描相干衍射成像的关键技术研究

扫描相干衍射成像（ptychography）是一种能同时实现高空间分辨率和大视场范围的定量相位成像技术，其系统结构简单而且算法收敛稳定，在生物学、纳米技术与材料科学中具有广泛的应用前景。Ptychography相位恢复问题的本质是通过数学模型对其衍射强度的形成过程进行非线性拟合，然而真实成像系统中包含的各种噪声和不确定性都会造成不可忽略的拟合误差。虽然不用再考虑物镜的影响，但是照明光的相干性、样品扫描系统的精度、样品的多重散射、装置的稳定性以及检测器的工艺限制（包括信噪比、像元深度、像素尺寸、点扩散函数和像素响应的非均匀性）等都会影响重构样品的空间分辨率、衬度和相位分布的定量测量精度，严重的时候甚至会导致算法不收敛。为此学者们相继提出了一系列改进方案来处理这些不理想因素，并取得了部分进展；而为了进一步提高ptychography的成像质量并扩大其应用范围，还需要对这些关键技术问题进行更加深入的探索，尤其是在电子显微镜上实现并展示该方法的全部潜力仍面临着很多严峻的挑战。

本论文对ptychography数据采集和相位重构的关键环节进行了详细的研究，提出了多种优化方案来提升重构的空间分辨率，以及解决扫描位置的不确定性和照明光的低相干问题。相关研究有助于推动低剂量高分辨电子显微技术的发展，为剂量敏感型材料的研究提供强有力的表征工具。论文的具体贡献可以归纳为以下四个方面：

（1）针对近场ptychography，提出了一种亚像素细分ePIE算法来克服检测器像素尺寸对成像分辨率的限制，使用普通相机就能实现亚微米的空间分辨率。考虑到系统结构和检测器性能的影响，特搭建了四种不同配置的可见光实验系统进行实验研究：包括平面波和发散光照明系统，并配备两种规格差异较大的检测器。实验结果显示，在检测器的像素尺寸分别为6.5 μm和2.4 μm的时候，成像分辨率可达到1.1 μm和0.78 μm，分别提高了大约5.9倍和3.08倍。

（2）提出了一种不用已知扫描位置的ptychography重构算法。数值模拟和光学实验研究表明该算法能够在初始照明探针粗略猜测、物体随机猜测并且初始位置完全置为零的情况下，仅根据衍射强度数据就能精确地重构出物体的透过率函数，照明探针的波前分布以及所有的扫描位置。

（3）对实验系统结构和光源相干性的依赖关系进行了详细的实验研究。鉴于光源的退相干是影响低剂量电子显微的高分辨定量相位成像能力的关键性因素，特详细地研究了部分时间相干性和空间相干性对近场和远场ptychography的影响。研究结果表明近场相较于远场对部分相干光有更强的鲁棒性，并且远场成像对空间相干性的退化比时间相干性的退化更敏感度。而且如果使用一个由相干性较好的数据重构的探针作为已知照明，将可以显著地提高重构质量并有助于避免算法陷入局部极小值。

（4）提出了一种适用于任意ptychography系统结构的退相干解决方法。该方法不需要已知照明光的相干性特征（空间相干长度或者光谱分布），可有效地降低光源的低空间和时间相干性对重构的不良影响。该方法可集成到所有主流的ptychography基础重构算法中，且对一系列不同实验条件下获得的可见光和电镜实验数据的重构均表明该方法能够显著地提高成像质量和重构速度。另外，该方法也能进一步地优化现有的混合模态分解方法和数值单色化方法，促使其更快且更稳定的收敛。

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