Serial coherent diffraction imaging of dynamic samples based on inter-frame constraint

We proposed a novel approach to coherent imaging of dynamic samples. The inter-frame similarity of the sample's local structures is found to be a powerful constraint in phasing a sequence of diffraction patterns. We devised a new image reconstruction algorithm that exploits this inter-frame constraint enabled by an adaptive similar region determination approach. We demonstrated the feasibility of this technique in visible light experiments with various real samples, achieving reconstructions of good quality within a few hundred iterations. With a setup as simple as conventional coherent diffraction imaging but with much-improved convergence and robustness to missing data and noise, our method is expected to enrich X-ray imaging techniques and electron microscopy, offering a new tool for dynamics studies.

A high-performance reconstruction method for partially coherent ptychography

Ptychography is now integrated as a tool in mainstream microscopy allowing quantitative and high-resolution imaging capabilities over a wide field of view. However, its ultimate performance is inevitably limited by the available coherent flux when implemented using electrons or laboratory X-ray sources. We present a universal reconstruction algorithm with high tolerance to low coherence for both far-field and near-field ptychography. The approach is practical for partial temporal and spatial coherence and requires no prior knowledge of the source properties. Our initial visible-light and electron data show that the method can dramatically improve the reconstruction quality and accelerate the convergence rate of the reconstruction. The approach also integrates well into existing ptychographic engines. It can also improve mixed-state and numerical monochromatisation methods, requiring a smaller number of coherent modes or lower dimensionality of Krylov subspace while providing more stable and faster convergence. We propose that this approach could have significant impact on ptychography of weakly scattering samples.

Broadband ptychographic imaging of biological samples using a deconvolution algorithm

Ptychography is an attractive advance of coherent diffraction imaging (CDI), which can provide high lateral resolution and wide field of view. The theoretical resolution of ptychography is dose-limited, therefore making ptychography workable with a broadband source will be highly beneficial. However, broad spectra of light source conflict with the high coherence assumption in CDI that the current reconstruction algorithm were built upon. In this paper, we demonstrated that incorporation of a blind deconvolution in the reconstruction algorithm can improve the image quality of ptychography with broadband source. This broadband reconstruction algorithm can obtain high-quality amplitude and phase images of complex-valued samples requiring no knowledge of the illumination spectrum. Optical experiments using biological samples demonstrate the effectiveness of our method. The significant improvement in low coherence tolerance by our approach can pave the way for implementing ultrafast imaging with femtosecond or attosecond lasers or high-flux ptychographic imaging with laboratory EUV or X-ray sources.

Scanning phase imaging without accurate positioning system

Ptychography, a high-resolution phase imaging technique using precise in-plane translation information, has been widely applied in modern synchrotron radiation sources across the globe. A key requirement for successful ptychographic reconstruction is the precise knowledge of the scanning positions, which are typically obtained by a physical interferometric positioning system. Whereas high-throughput positioning poses a challenge in engineering, especially in nano or even smaller scale. In this work, we propose a novel scanning imaging framework that does not require any prior position information from the positioning system. Specifically, our scheme utilizes the wavefront modulation mechanism to reconstruct the object functions at each scan position and the shared illumination function, simultaneously. The scanning trajectory information is extracted by our subpixel image registration algorithm from the overlap region of reconstructed object functions. Then, a completed object function can be obtained by assembling each part of the reconstructed sample functions. High-quality imaging of biological sample and position recovery with sub-pixel accuracy are demonstrated in proof-of-concept experiment. Based on current results, we find it may have great potential applications in high-resolution and high throughput phase imaging.

An Integrated Constrained Gradient Descent (iCGD) Protocol to Correct Scan-Positional Errors for Electron Ptychography with High Accuracy and Precision

Correcting scan-positional errors is critical in achieving electron ptychography with both high resolution and high precision. This is a demanding and challenging task due to the sheer number of parameters that need to be optimized. For atomic-resolution ptychographic reconstructions, we found classical refining methods for scan positions not satisfactory due to the inherent entanglement between the object and scan positions, which can produce systematic errors in the results. Here, we propose a new protocol consisting of a series of constrained gradient descent (CGD) methods to achieve better recovery of scan positions. The central idea of these CGD methods is to utilize a priori knowledge about the nature of STEM experiments and add necessary constraints to isolate different types of scan positional errors during the iterative reconstruction process. Each constraint will be introduced with the help of simulated 4D-STEM datasets with known positional errors. Then the integrated constrained gradient decent (iCGD) protocol will be demonstrated using an experimental 4D-STEM dataset of the 1H-MoS2 monolayer. We will show that the iCGD protocol can effectively address the errors of scan positions across the spectrum and help to achieve electron ptychography with high accuracy and precision.