实空间重叠还不够：纳米束迭代叠层衍射成像中的歧义问题
Real-space overlap is not enough: ambiguity in nanobeam iterative ptychography
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中文总结 AI 辅助
研究发现纳米束迭代叠层衍射成像在小会聚角下会因布拉格盘无干涉出现重建歧义，实空间重叠无法消除，需从多起点重复重建诊断。
中文摘要 AI 辅助
高分辨率迭代叠层衍射成像通常依赖于对准良好、会聚角大的电子探针。本文探究是否可改用小会聚角开展该成像，从而降低对探针校正器的需求，实现低加速电压实验或采用退磁物镜（如洛伦兹模式）下的实验。通过实验与模拟表明：当会聚角小到衍射盘不再重叠时，重建结果会出现歧义，这对结果的可靠解释构成重大挑战。该挑战源于记录的衍射图样强度中布拉格盘间缺乏干涉，导致各反射无相位信息。用此类数据重建会产生简并物体：晶格的刚性平移及物体的对比度反转，都会使实验数据与叠层衍射前向模型间的误差相同。增大探针位置间的实空间重叠无法消除该简并。非晶衬底可通过为布拉格光束提供盘间间隙中的支撑，补充缺失的相位关系，但这种相位确定是脆弱的，仅在前向模型与实验匹配的区域有效。在固定剂量下，将物体约束为纯相位物体，或在前向模型中引入热运动，单独一项就足以使解非唯一，这凸显了为何在重叠阈值以下的实验重建，即便有充足剂量和实空间冗余，仍存在歧义。最棘手的是，晶格间距和取向总能被正确恢复，因此非唯一重建看似可信，仅能通过从不同起点重复重建来诊断。
英文摘要
High-resolution iterative ptychography typically relies on a well-aligned, high-convergence-angle electron probe. Here we explore whether it can instead be performed at small convergence angles, relaxing the need for probe correctors and enabling experiments at low accelerating voltages or with a de-excited objective lens, as in Lorentz mode. Through experiments and simulations, we show that once the convergence angle is small enough that no diffracted disks overlap, the resulting reconstruction is ambiguous, posing a significant challenge for robust interpretation of results. This challenge arises because of the lack of interference between Bragg disks in the recorded diffraction pattern intensity, leading to no phase information for each reflection. Reconstructions with these data lead to degenerate objects in which rigid translations of the lattice and reversals of contrast of the object produce the same error between experimental data and the ptychography forward model. Increasing the real-space overlap between probe positions does not lift this degeneracy. An amorphous substrate can supply the missing phase relationships by giving the Bragg beams support in the gaps between disks. This phasing is fragile, however, and survives only where the forward model matches the experiment. At fixed dose, either constraining the object to be a pure phase object or introducing thermal motion into the forward model is enough on its own to make the solution non-unique, highlighting why our experimental reconstructions below the overlap threshold are ambiguous despite ample dose and real-space redundancy. Most troublingly, the lattice spacing and orientation are always recovered correctly, so a non-unique reconstruction looks convincing and can be diagnosed only by repeating the reconstruction from different starting points.
发表机构
- Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）
- Diamond Light Source（钻石光源）
- Delft University of Technology（代尔夫特理工大学）
- Stanford University（斯坦福大学）
机构由 AI 辅助整理，请以论文原文为准。
