Morgan Andrew J, Quiney Harry M, Bajt Saša, Chapman Henry N
ARC Centre of Excellence in Advanced Molecular Imaging, School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia.
CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
J Appl Crystallogr. 2020 May 29;53(Pt 3):760-780. doi: 10.1107/S1600576720005567. eCollection 2020 Jun 1.
A method is presented for the measurement of the phase gradient of a wavefront by tracking the relative motion of speckles in projection holograms as a sample is scanned across the wavefront. By removing the need to obtain an undistorted reference image of the sample, this method is suitable for the metrology of highly divergent wavefields. Such wavefields allow for large magnification factors that, according to current imaging capabilities, will allow for nanoradian angular sensitivity and nanoscale sample projection imaging. Both the reconstruction algorithm and the imaging geometry are nearly identical to that of ptychography, except that the sample is placed downstream of the beam focus and that no coherent propagation is explicitly accounted for. Like other X-ray speckle tracking methods, it is robust to low-coherence X-ray sources, making it suitable for laboratory-based X-ray sources. Likewise, it is robust to errors in the registered sample positions, making it suitable for X-ray free-electron laser facilities, where beam-pointing fluctuations can be problematic for wavefront metrology. A modified form of the speckle tracking approximation is also presented, based on a second-order local expansion of the Fresnel integral. This result extends the validity of the speckle tracking approximation and may be useful for similar approaches in the field.
本文提出了一种通过跟踪投影全息图中散斑的相对运动来测量波前相位梯度的方法，该方法是在样品扫描穿过波前时进行的。通过无需获取样品的无畸变参考图像，该方法适用于高度发散波场的计量。这种波场允许大的放大倍数，根据当前的成像能力，将实现纳弧度角灵敏度和纳米级样品投影成像。重建算法和成像几何结构与叠层成像术几乎相同，只是样品放置在光束焦点的下游，并且没有明确考虑相干传播。与其他X射线散斑跟踪方法一样，它对低相干X射线源具有鲁棒性，适用于基于实验室的X射线源。同样，它对注册样品位置的误差具有鲁棒性，适用于X射线自由电子激光设施，在该设施中，光束指向波动可能会给波前计量带来问题。还基于菲涅耳积分的二阶局部展开提出了散斑跟踪近似的一种修正形式。这一结果扩展了散斑跟踪近似的有效性，可能对该领域的类似方法有用。
