Phase offset method of ptychographic contrast reversal correction

• Published in: Ultramicroscopy 2024-04, Vol.258, p.113922-113922, Article 113922
• Main Authors: Hofer, Christoph, Gao, Chuang, Chennit, Tamazouzt, Yuan, Biao, Pennycook, Timothy J.
• Format: Article
• Language: English
• Subjects: 4D STEM; Electron ptychography; Phase wrap
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/j.ultramic.2024.113922

The contrast transfer function of direct ptychography methods such as the single side band (SSB) method are single signed, yet these methods still sometimes exhibit contrast reversals, most often where the projected potentials are strong. In thicker samples central focusing often provides the best ptychographic contrast as this leads to defocus variations within the sample canceling out. However focusing away from the entrance surface is often undesirable as this degrades the annular dark field (ADF) signal. Here we discuss how phase wrap asymptotes in the frequency response of SSB ptychography give rise to contrast reversals, without the need for dynamical scattering, and how these can be counteracted by manipulating the phases such that the asymptotes are either shifted to higher frequencies or damped via amplitude modulation. This is what enables post collection defocus correction of contrast reversals. However, the phase offset method of counteracting contrast reversals we introduce here is generally found to be superior to post collection application of defocus, with greater reliability and generally stronger contrast. Importantly, the phase offset method also works for thin and thick samples where central focusing does not. Finally, the independence of the method from focus is useful for optical sectioning involving ptychography, improving interpretability by better disentangling the effects of strong potentials and focus. •Contrast reversals in ptychographic phase images can appear with sufficiently strong potentials.•These can occur even with strong single atom potentials due to phase wraps in reciprocal space.•While defocus can often correct these reversals, it is often not desired as it can reduce contrast.•A new method to overcome contrast reversals in ptychographic images by applying a phase offset is presented.•The new method helps improve contrast compared to post collection defocus correction.