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1. Emittance minimization for aberration correction I: Aberration correction of an electron microscope without knowing the aberration coefficients

   https://doi.org/10.1016/j.ultramic.2025.114137  

   Ma, Desheng; Zeltmann, Steven E; Zhang, Chenyu; Baraissov, Zhaslan; Shao, Yu-Tsun; Duncan, Cameron; Maxson, Jared; Edelen, Auralee; Muller, David A (July 2025, Ultramicroscopy)

   Precise alignment of the electron beam is critical for successful application of scanning transmission electron microscopes (STEM) to understanding materials at atomic level. Despite the success of aberration correctors, aberration correction is still a complex process. Here we approach aberration correction from the perspective of accelerator physics and show it is equivalent to minimizing the emittance growth of the beam, the span of the phase space distribution of the probe. We train a deep learning model to predict emittance growth from experimentally accessible Ronchigrams. Both simulation and experimental results show the model can capture the emittance variation with aberration coefficients accurately. We further demonstrate the model can act as a fast-executing function for the global optimization of the lens parameters. Our approach enables new ways to quickly quantify and automate aberration correction that takes advantage of the rapid measurements possible with high-speed electron cameras. In part II of the paper, we demonstrate how the emittance metric enables rapid online tuning of the aberration corrector using Bayesian optimization. 

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2. Emittance minimization for aberration correction II: Physics-informed Bayesian optimization of an electron microscope

   https://doi.org/10.1016/j.ultramic.2025.114138  

   Ma, Desheng; Zeltmann, Steven E; Zhang, Chenyu; Baraissov, Zhaslan; Shao, Yu-Tsun; Duncan, Cameron; Maxson, Jared; Edelen, Auralee; Muller, David A (July 2025, Ultramicroscopy)

   Aberration-corrected Scanning Transmission Electron Microscopy (STEM) has become an essential tool in understanding materials at the atomic scale. However, tuning the aberration corrector to produce a sub-Ångström probe is a complex and time-costly procedure, largely due to the difficulty of precisely measuring the optical state of the system. When measurements are both costly and noisy, Bayesian methods provide rapid and efficient optimization. To this end, we develop a Bayesian approach to fully automate the process by minimizing a new quality metric, beam emittance, which is shown to be equivalent to performing aberration correction. In part I, we derived several important properties of the beam emittance metric and trained a deep neural network to predict beam emittance growth from a single Ronchigram. Here we use this as the black box function for Bayesian optimization and demonstrate automated tuning of simulated and real electron microscopes. We explore different surrogate functions for the Bayesian optimizer and implement a deep neural network kernel to effectively learn the interactions between different control channels without the need to explicitly measure a full set of aberration coefficients. Both simulation and experimental results show the proposed method outperforms conventional approaches by achieving a better optical state with a higher convergence rate. 

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3. Adaptive aberration correction using an electrowetting array

   https://doi.org/10.1063/5.0133473  

   Zohrabi, Mo; Lim, Wei Yang; Gilinsky, Samuel; Bright, Victor M.; Gopinath, Juliet T. (February 2023, Applied Physics Letters)

   We demonstrate a method that permits wavefront aberration correction using an array of electrowetting prisms. A fixed high fill factor microlens array followed by a lower fill factor adaptive electrowetting prism array is used to correct wavefront aberration. The design and simulation of such aberration correction mechanism is described. Our results show significant improvement to the Strehl ratio by using our aberration correction scheme which results in diffraction limited performance. Compactness and effectiveness of our design can be implemented in many applications that require aberration correction, such as microscopy and consumer electronics. 

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4. Aberration compensation for enhanced holographic particle characterization

   https://doi.org/10.1364/OE.494593  

   Snyder, Kaitlynn; Grier, David G (January 2023, Optics Express)

   Holographic particle characterization treats holographic microscopy of colloidal particles as an inverse problem whose solution yields the diameter, refractive index and three-dimensional position of each particle in the field of view, all with exquisite precision. This rich source of information on the composition and dynamics of colloidal dispersions has created new opportunities for fundamental research in soft-matter physics, statistical physics and physical chemistry, and has been adopted for product development, quality assurance and process control in industrial applications. Aberrations introduced by real-world imaging conditions, however, can degrade performance by causing systematic and correlated errors in the estimated parameters. We identify a previously overlooked source of spherical aberration as a significant source of these errors. Modeling aberration-induced distortions with an operator-based formalism identifies a spatially varying phase factor that approximately compensates for spherical aberration in recorded holograms. Measurements on model colloidal dispersions demonstrate that phase-only aberration compensation greatly improves the accuracy of holographic particle characterization without significantly affecting measurement speed for high-throughput applications. 

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5. Achieving sub-0.5-angstrom–resolution ptychography in an uncorrected electron microscope

   https://doi.org/10.1126/science.adl2029  

   Nguyen, Kayla X.; Jiang, Yi; Lee, Chia-Hao; Kharel, Priti; Zhang, Yue; van der Zande, Arend M.; Huang, Pinshane Y. (February 2024, Science)

   Subangstrom resolution has long been limited to aberration-corrected electron microscopy, where it is a powerful tool for understanding the atomic structure and properties of matter. Here, we demonstrate electron ptychography in an uncorrected scanning transmission electron microscope (STEM) with deep subangstrom spatial resolution down to 0.44 angstroms, exceeding the conventional resolution of aberration-corrected tools and rivaling their highest ptychographic resolutions​. Our approach, which we demonstrate on twisted two-dimensional materials in a widely available commercial microscope, far surpasses prior ptychographic resolutions (1 to 5 angstroms) of uncorrected STEMs. We further show how geometric aberrations can create optimized, structured beams for dose-efficient electron ptychography. Our results demonstrate that expensive aberration correctors are no longer required for deep subangstrom resolution. 

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