Phase offset method of ptychographic contrast reversal correction.

Area of Science:

• Electron microscopy
• Materials science
• Imaging techniques

Background:

• Direct ptychography methods, like single side band (SSB), often exhibit single-signed contrast transfer functions.
• Contrast reversals can occur in SSB ptychography, particularly with strong projected potentials or in thicker samples.
• Central focusing is often used to mitigate contrast issues in thicker samples, but it can degrade annular dark field (ADF) signals.

Purpose of the Study:

• To investigate the cause of contrast reversals in SSB ptychography.
• To introduce and evaluate a novel phase offset method for counteracting contrast reversals.
• To demonstrate the advantages of the phase offset method over post-collection defocus correction.

Main Methods:

• Analysis of phase wrap asymptotes in the frequency response of SSB ptychography.
• Development of a phase manipulation technique to shift or damp these asymptotes.
• Comparison of the proposed phase offset method with post-collection defocus correction.

Main Results:

• Phase wrap asymptotes in SSB ptychography's frequency response are identified as the cause of contrast reversals, independent of dynamical scattering.
• The introduced phase offset method effectively counteracts contrast reversals by manipulating phase asymptotes.
• The phase offset method demonstrates superior reliability and contrast compared to post-collection defocus correction.
• This method is effective for both thin and thick samples, unlike central focusing.

Conclusions:

• Contrast reversals in SSB ptychography are attributable to phase wrap asymptotes and can be corrected by phase manipulation.
• The novel phase offset method offers a superior and more reliable approach to correcting contrast reversals.
• The focus-independent nature of the phase offset method enhances its utility for optical sectioning and sample analysis.