Ptychography over all wavelengths

Ptychography has emerged as a transformative coherent imaging technique for both fundamental and applied sciences. Its applications in optical microscopy, however, often fall short for its relatively low imaging throughput and limited resolution. In this talk, Dr Zheng will discuss a coded ptychography technique that achieves an imaging throughput order of magnitude greater than previous demonstrations. In this platform, we translate the samples across the disorder-engineered surfaces for lensless diffraction data acquisition. The engineered surface can be made by smearing a monolayer of blood on top of the image sensor. The entire system can be built using a modified Blu-ray player, where the 405 nm laser from the optical pickup head can be used as a coherent light source for sample illumination. By tracking the phase wraps of the recovered images, the group reports the direct observation of bacterial growth in a 15-s interval at a 120 mm^2 area, with a phase sensitivity like that obtained from interferometric measurements. They also characterize cell growth via longitudinal dry mass measurement and perform rapid bacterial detection at low concentrations. For drug-screening application, they demonstrate antibiotic susceptibility testing and perform single-cell analysis of antibiotic-induced filamentation. The combination of high phase sensitivity, high spatiotemporal resolution, and ultra-large field of view is unique among existing microscopy techniques. Dr Zheng will also discuss several extensions of the coded ptychography technique, including optofluidic ptychography, synthetic aperture ptychography, and depth-multiplexed on-chip imaging, thereby expanding its applicability in different fields.

Dr Guoan Zheng, University of Connecticut, USA

Dr Guoan Zheng, University of Connecticut, USA

Dr Guoan Zheng received his BS degree in Electrical Engineering from Zhejiang University in 2007, and his MS and PhD degrees in Electrical Engineering from Caltech in 2008 and 2013, respectively. After completing his PhD, he joined the University of Connecticut (UConn) as an assistant professor, and is now the United Technologies Corporation (UTC) Associate Professor in the areas of Biomedical Engineering and Electrical Engineering. Dr Zheng's expertise includes microscopy, ptychography, optical engineering, biophotonics, computational optics, and lab-on-a-chip devices. He has published more than 100 journal papers and has an h-index of 50. His current research efforts focus on the development of novel optical imaging tools to tackle measurement problems in biology and medicine. 

Ptychography is a computational method of microscopic imaging that can generate high spatial resolution complex phase information of an object by processing coherent diffraction patterns scattered from the object. With the development of Ptychography, it has been extensively used to study materials in combination with other imaging techniques, which can give multiple contrast mechanisms of the measured sample – for example, in combination with X-ray fluorescence (XRF) microscopy. While Ptychography can achieve diffraction-limited spatial resolution, the used X-ray probe profile information usually limits the resolution of XRF images. In this talk, Dr Wu will present a machine learning based approach utilizing multimodal measurement (ie the combination of Ptychography and XRF microscopy) to enhance the resolution of the corresponding XRF microscopy by deconvolving the X-ray probe from the XRF signal. The enhanced spatial resolution was observed for both simulated and experimental XRF data, showing superior performance over the state-of-the-art scanning XRF method with different nano-sized X-ray probes. Enhanced spatial resolutions were also observed for the accompanying 3D XRF tomography reconstructions.

Dr Longlong Wu, Brookhaven National Laboratory, USA

Dr Longlong Wu, Brookhaven National Laboratory, USA

Dr Longlong Wu is an assistant research scientist at the Condensed Matter Physics and Materials Science Department of Brookhaven National Laboratory (BNL). During his PhD, he mainly focused on using surface X-ray scattering techniques to study the structure and dynamic phenomena of soft materials at surfaces. After joining the BNL in July 2019, his research interest focuses on using coherent X-ray diffraction techniques to study the dynamics of correlated materials with X-rays from synchrotron and X-ray Free-Electron Lasers. He also works on the project of the development of Machine Learning phasing algorithms and Bragg ptychography.

Due to the optically transparent nature of many biological specimens, high-resolution microscopic bio-imaging often requires the use of dyes or fluorescent labels to produce sufficient contrast that are not ideal for many in vivo experiments. By measuring the optical phase delay introduced by semi-transparent tissue, quantitative phase imaging methods like ptychography offer an effective label-free imaging platform. Many transparent specimens also exhibit alternative endogenous optical contrast mechanisms, including anisotropic properties such as material birefringence and orientation. Here, Dr Horstmeyer describes several relatively straightforward extensions of ptychography that allow us to measure this phase-sensitive anisotropic information in 3D. The only hardware modifications required to a standard microscope include two polarization-sensitive optical elements and an LED array for specimen illumination. Dr Horstmeyer then extends the standard Fourier ptychography model and inverse solver to account for the vectorial nature of light, the sample, and the microscope itself, which allows us to computationally remove polarization-dependent imaging system aberrations. He demonstrates and validate large-area, high-resolution volumetric reconstructions of specimen refractive index, birefringence, and orientation for muscle fibre sections and diseased heart tissue, the latter of which carries important information for detecting cardiac amyloidosis.

Dr Roarke Horstmeyer, Duke University, USA

Dr Roarke Horstmeyer, Duke University, USA

Roarke Horstmeyer is an assistant professor of Biomedical Engineering, Electrical and Computer Engineering, and Physics at Duke University. He develops microscopes, cameras and computer algorithms for a wide range of applications, from forming 3D reconstructions of organisms to detecting blood flow and brain activity deep within tissue. Before joining Duke in 2018, Dr Horstmeyer was a visiting professor at the University of Erlangen in Germany and an Einstein International Postdoctoral Fellow at Charité Medical School in Berlin. Prior to his time in Germany, Dr Horstmeyer earned a PhD from Caltech’s EE department (2016), an MS from the MIT Media Lab (2011), and bachelor’s degrees in Physics and Japanese from Duke in 2006.

Regardless of wavelength, ptychography is conventionally configured as a far-field method, where dark-field signal is processed to realise super-resolution beyond the aperture of the probe-forming optics. Operating in this high-resolution, lens-free imaging mode, it has proven more-or-less unbeatable. However, ptychography has advantages beyond resolution gains, particularly as a high-accuracy, large field-of-view and experimentally undemanding form of quantitative phase imaging (QPI). Applications here range from unstained imaging of live cells at optical wavelengths to imaging electric and magnetic fields in the electron microscope. For these applications, ptychography in a near-field configuration is appealing because the data requirements are radically reduced: the experiment is easier and quicker, and the reconstruction process quicker and less memory-intensive than far-field ptychography. In this talk Dr Maiden will present his group's work on this near-field form of ptychography and multi-slice ptychography, spanning a huge range of wavelengths from the optical, through X-ray to the electron regimes.

Dr Andrew Maiden, University of Sheffield, UK

Dr Andrew Maiden, University of Sheffield, UK

Dr Maiden studied Electronic and Electrical Engineering at the Universities of Birmingham and Melbourne, before moving to Durham University to complete his PhD on the use of computer-generated holograms for lithography. He is now a Senior Lecturer in Computational Imaging at the University of Sheffield and a Visiting Researcher at the Diamond Light Source synchrotron. For the past decade, Dr Maiden has worked together with Professor John Rodenburg on novel experimental implementations and algorithms for ptychography. His current projects are exploring new varieties of 3D and multislice ptychography for X-ray imaging of large volumes, near-field electron ptychography for imaging electric and magnetic fields, and distributed ptychographic algorithms for efficient processing of big ptychographic data sets.

To date, scanning coherent X-ray microscopy (X-ray ptychography) has been largely limited to X-ray energies below 20 keV, mainly due to the available coherent photon flux at current third generation synchrotron-radiation sources and the capabilities of existing detectors. The first problem of low coherent flux will be solved by future diffraction-limited synchrotron radiation sources that will allow coherent imaging experiments at these higher X-ray energies, and the second problem could be solved by a new generation of high-energy pixel detectors. 

In this talk, an introduction to the Hard X-ray Micro/Nano-Probe Beamline P06 at PETRA III (DESY) will be given, and various ptychographic imaging results obtained in the hard X-ray regime will be summarized. In a recent test experiment, an Eiger2 CdTe photon counting detector was integrated into the instrument to extend the energy range accessible for X-ray ptychography beyond 20 keV. Since the penetration depth is large at these high X-ray energies, initial measurements were performed on thicker industrial catalyst samples that produce a significant phase shift of multiple π in X-ray transmission. To avoid strong phase wrapping effects, a so-called refractive-index update is used, which directly refines the real and imaginary part of the refractive index instead of amplitude and phase of the object transmission function. The performance of the CdTe detector is discussed and some preliminary ptychographic imaging results of an 80μm-thick Pt/Rh-wire measured at an X-ray photon energy of E = 35 keV are summarized. 

Dr Andreas Schropp, Deutsches Elektronen-Synchrotron DESY, Germany

Dr Andreas Schropp, Deutsches Elektronen-Synchrotron DESY, Germany

Dr Schropp studied physics at RWTH Aachen before obtaining his PhD on coherent X-ray diffraction imaging at the Universität Hamburg. By combining this technique with scanning microscopy using a nanofocused X-ray beam, the first ptychographic images were obtained during a postdoctoral stay at TU Dresden. Ptychography enabled a comprehensive characterization of nanofocusing X-ray optics at synchrotron radiation sources and XFELs, which contributed significantly to the further improvement of modern diffraction-limited X-ray optics. After an extended stay of three years at the SLAC National Accelerator Laboratory (Stanford University) as a visiting scientist to investigate X-ray imaging capabilities at XFELs with high temporal and spatial resolution, he is now working enthusiastically in the field of high-resolution X-ray microscopy as a research associate in the 'X-ray Nanoscience and X-ray Optics' group at DESY. 

High-harmonic generation (HHG) driven by ultrafast lasers is an effective way to produce broadband coherent extreme ultraviolet (EUV) radiation with a table-top setup. Such sources open up the prospect of spectroscopic nanoscale imaging of integrated circuits and biological structures. Given the limitations of high-resolution optical components for EUV radiation, lensless imaging concepts such as ptychography are essential for efficient HHG-based imaging. 

To use the full bandwidth of HHG sources effectively, ptychography is ideally performed with multiple wavelengths in parallel. As perhaps the main requirement for robust image reconstruction is diversity, an important question is how to control the HHG source properties to optimize diversity, among scan positions as well as between wavelengths. A unique property of ptychography is its ability to retrieve the probe field and object response separately, making ptychography a powerful wavefront sensing technique. This feature enables a detailed investigation of the properties of HHG beams, which can be used to study the physics of HHG. 

Dr Witte will present the group's recent work on multi-wavelength HHG-based ptychography, and its applications in EUV imaging and wavefront sensing. Specifically, he will discuss strategies towards efficient spectroscopic imaging, and how the group used ptychographic wavefront sensing to characterize complex HHG beams, and study how properties such as wavefronts and orbital angular momentum transfer from the fundamental beam to the harmonics. 

Dr Stefan Witte, Advanced Research Center for Nanolithography, The Netherlands

Dr Stefan Witte, Advanced Research Center for Nanolithography, The Netherlands

Stefan Witte received his PhD in 2007 from the Vrije Universiteit Amsterdam, for research on intense ultrafast laser development and precision spectroscopy. He did postdoctoral work at on nonlinear microscopy and biomedical imaging (Vrije Universiteit) and on ultrafast electron dynamics and lensless imaging with high-harmonic sources (JILA, University of Colorado). 

Since 2014 he has worked at the Advanced Research Center for Nanolithography (ARCNL), where he leads the EUV Generation and Imaging group at ARCNL and is the head of the Metrology Department. He is also an associate professor at the Vrije Universiteit Amsterdam. He was awarded an ERC Starting Grant in 2014, and an ERC Consolidator Grant in 2019. His present research interests include coherent diffractive imaging with visible and EUV radiation, high-harmonic generation and its applications, photo-acoustic imaging, and laser development.

Electron microscopes use electrons with wavelengths of a few picometers, and are potentially capable of imaging individual atoms in solids at a resolution ultimately set by the intrinsic size of an atom. Even with the rapid advances in aberration-corrector technology, both residual aberrations in the electron lenses and multiple scattering of the incident beam inside the sample, the best resolution possible was an order of magnitude worse than this limit. However, with recent advances in detector technology and ptychographic algorithms to unscramble multiple scattering, the resolution of the electron microscope is now limited only by the dose to the sample, and thermal vibrations of the atoms themselves. At high doses, these approaches have allowed us to image the detailed vibrational envelopes of individual atom columns as well as locating interstitial dopant atoms that would be hidden by channelling of the probe with conventional imaging modes. Even the location of all atoms in thin amorphous films now seems within reach. However, as the dose is reduced, so is our ability to robustly reconstruct the object. Challenges in characterizing and predicting performance will be discussed.

Professor David Muller, Cornell University, USA

Professor David Muller, Cornell University, USA

Biography not available