The Influence Of X-ray Divergence On The Image Quality Of Multi-slice Scanning Coherent Diffraction

As a newly emerging lensless microscopy,coherent diffractive imaging(CDI)has been successfully applied in many 2D and 3D high-resolution imaging studies in materials,biology,medicine and other fields.In the traditional plane wave CDI,only an isolated sample can be imaged and its size must be smaller than the spot size,which seriously limits the field of view of imaging.The traditional CDI has many other defects,such as slow convergence,stagnation,non-convergence or non-uniqueness of the reconstruction result.To overcome these shortcomings,a new type of CDI method,PCDI(ptychographic CDI,or ptychography)was proposed by Rodenberg et al.In principle,an arbitrarily large 2D sample(perpendicular to the optical axis)can be imaged by PCDI.This method uses a local probe to scan across the sample,and the neighboring scanned areas are partially overlapped.Then a detector records the far field diffraction pattern of each illuminated spot for subsequent phase retrieval calculation and image reconstruction.Due to the overlapped samplings,a dataset of the recorded diffraction patterns has a lot of correlative redundant information.The redundant information can not only greatly improve the convergence of the algorithm,but also recover both the sample transmission function and the probe function simultaneously.In addition,it can provide many aspects of sample physical and structural information as well as the highresolution and quantitative image.With the development of imaging technology,the simple 2D imaging cannot meet the demand for the sample information any more.A number of 3D imaging methods based on PCDI are proposed and used.In traditional 3D PCDI,a sample needs to be rotated many times to record adequately many diffraction patterns at different projection angles.It means that hundreds of PCDI measurements need to be performed,which leads to a huge amount of experimental work and data,definitely reducing the experiment efficiency.In recent years,a novel multi-slice 3D ptychography imaging method has been proposed,in which the third dimension information of a sample structure can be obtained from a single 2D ptychography dataset.The core part of multi-slice PCDI is that the sample is separated into a number of slices along the direction of the incident light in reconstruction and assuming the propagation of the wavefront between neighbored slices as the near-field freespace propagation.By iteratively inverting the collected far-field diffraction patterns,we can reconstruct the sample transmission functions and incident probe functions of respective slices,then the complex refraction index information of the sample 3D structure can be retrieved.To improve the convergence and reconstruction quality of multi-slice PCDI,we firstly analyzed the influence of relaxation coefficients and the overlapping degree on imaging quality.It was found that a bigger relaxation parameter ? of incident wave(closer to the sample function relaxation parameter ?)or a higher degree of overlapping leads to a higher imaging quality.Based on these,we chose optimal parameters to systematically study the effect of the illumination divergence degree on the reconstruction results of the multi-slice method by simulative computations.The results indicated that the higher the divergence degree of the incident light,the faster the convergence of iterative reconstruction of multi-slice PCDI,and the higher the reconstructed image quality.On the contrary,a lower divergence degree of the incident light would degrade the quality of 3D imaging.We also investigated the extreme situation of a low divergence,i.e.,a parallel incident illumination for multislice method.It was found that although we can still get an outline of the sample image,its quality is dramatically worse than that produced by a high-divergence light,so that the reconstruction under parallel light can be considered as unconvergable.Therefore,a parallel light is not appropriate in multi-slice PCDI.The above conclusions provide important guidelines for the choice and setup of focusing optics in multi-slice 3D imaging experiments.For example,if a zone-plate of smaller focal length or an X-ray beam of lower energy is chose,a better multi-slice image can be hopefully obtained.