New algorithm enhances ptychographic image resolution

Ptychographic X-ray imaging is used to characterize the structure and properties of matter and materials. While the method has been around for 50 years or so, wide use was at first hampered by the experimental process being slow and the computational processing of the data to produce a reconstructed image being expensive. But in recent years, advances in detectors and X-ray microscopes at light sources such as Lawrence Berkeley National Laboratory's Advanced Light Source have made it possible to measure a ptychographic data set in seconds. As a result, today ptychography is used in a range of scientific domains, including condensed-matter physics, cell biology and electronics.

Reconstructing ptychographic data sets can be a data-intensive challenge, as it involves solving a difficult phase-retrieval problem, calibrating optical elements and dealing with experimental outliers and 'noise'. To address this challenge, Berkeley Lab scientists developed SHARP (scalable heterogeneous adaptive real-time ptychography), which enables the reconstruction of millions of phases of ptychographic image data per second. Since being introduced in 2016, SHARP has had notable successes in the analysis of magnetic thin films, magnetozomes and three-dimensional battery materials.

Now members of the SHARP collaboration have developed a model that further enhances SHARP's reconstruction capabilities. The new algorithm, GDP-ADMM (gradient decomposition of the probe/alternating direction method of multipliers), is described in a recent article in Acta Crystallographica Section A [Chang et al. (2018) Acta Cryst. A74, 157-169] and was featured on the cover of the May issue. GDP-ADMM takes advantage of state-of-the-art mathematical aspects of phase retrieval, background-noise optimization and detector 'denoising', allowing SHARP to handle more light than before, enabling faster data acquisition and higher time resolution, and ultimately more scientific discoveries.

'The goal was to offer the ability to quickly discover interesting nanoparticles at full resolution by enabling rapid feedback from the microscopists at the beamlines,' said Stefano Marchesini, a staff scientist at Berkeley Lab and a co-author of the article. 'Even when the next-generation coherent light sources come online, we may be able to extend the X-ray energies that can be used in ptychography by using this model.'