Dejkameh Atoosa, Nebling Ricarda, Locans Uldis, Kim Hyun-Su, Mochi Iacopo, Ekinci Yasin
ETH Zürich, Rämistrasse 101, Zürich, 8092, Switzerland; Paul Scherrer Institute (PSI), Forschungsstrasse 111, Villigen, 5232, Switzerland.
ETH Zürich, Rämistrasse 101, Zürich, 8092, Switzerland; Paul Scherrer Institute (PSI), Forschungsstrasse 111, Villigen, 5232, Switzerland.
Ultramicroscopy. 2024 Apr;258:113912. doi: 10.1016/j.ultramic.2023.113912. Epub 2023 Dec 29.
Coherent diffraction imaging (CDI) and its scanning version, ptychography, are lensless imaging approaches used to iteratively retrieve a sample's complex scattering amplitude from its measured diffraction patterns. These imaging methods are most useful in extreme ultraviolet (EUV) and X-ray regions of the electromagnetic spectrum, where efficient imaging optics are difficult to manufacture. CDI relies on high signal-to-noise ratio diffraction data to recover the phase, but increasing the flux can cause saturation effects on the detector. A conventional solution to this problem is to place a beam stop in front of the detector. The pixel masking method is a common solution to the problem of missing frequencies due to a beam stop. This paper describes the information redundancy in the recorded data set and expands on how the reconstruction algorithm can exploit this redundancy to estimate the missing frequencies. Thereafter, we modify the size of the beam stop in experimental and simulation data to assess the impact of the missing frequencies, investigate the extent to which the lost portion of the diffraction spectrum can be recovered, and quantify the effect of the beam stop on the image quality. The experimental findings and simulations conducted for EUV imaging demonstrate that when using a beam stop, the numerical aperture of the condenser is a crucial factor in the recovery of lost frequencies. Our thorough investigation of the reconstructed images provides information on the overall quality of reconstruction and highlights the vulnerable frequencies if the beam stop size is larger than the extent of the illumination NA. The outcome of this study can be applied to other sources of frequency loss, and it will contribute to the improvement of experiments and reconstruction algorithms in CDI.
相干衍射成像（CDI）及其扫描版本叠层成像术是无透镜成像方法，用于从测量的衍射图案中迭代恢复样品的复散射振幅。这些成像方法在电磁频谱的极紫外（EUV）和X射线区域最为有用，因为在这些区域高效成像光学器件难以制造。CDI依靠高信噪比的衍射数据来恢复相位，但增加通量会导致探测器出现饱和效应。解决这个问题的传统方法是在探测器前放置一个光束阻挡器。像素掩膜方法是解决因光束阻挡器导致频率缺失问题的常用方法。本文描述了记录数据集中的信息冗余，并详细阐述了重建算法如何利用这种冗余来估计缺失的频率。此后，我们在实验数据和模拟数据中修改光束阻挡器的尺寸，以评估缺失频率的影响，研究衍射光谱丢失部分能够恢复的程度，并量化光束阻挡器对图像质量的影响。针对EUV成像进行的实验结果和模拟表明，使用光束阻挡器时，聚光镜的数值孔径是恢复丢失频率的关键因素。我们对重建图像的深入研究提供了关于重建整体质量的信息，并突出了如果光束阻挡器尺寸大于照明数值孔径范围时的易损频率。这项研究的结果可应用于其他频率损失源，并将有助于改进CDI中的实验和重建算法。
