Contrast transfer and noise considerations in focused-probe electron ptychography

Area of Science:

• Materials Science
• Electron Microscopy
• Imaging Physics

Background:

• Electron ptychography is a 4D STEM phase-contrast imaging technique.
• It is crucial for imaging light-element and beam-sensitive materials.
• Electron dose is a key metric, but contrast transfer properties also significantly impact image quality.

Purpose of the Study:

• To explore and characterize the contrast transfer properties of electron ptychography.
• To investigate the influence of different ptychographic methods (SSB and ePIE) on contrast transfer.
• To determine optimal conditions for high-contrast imaging while managing electron dose.

Main Methods:

• Experimental demonstration of focused-probe, non-iterative electron ptychography using the single-side-band (SSB) method.
• Analysis of the phase-contrast transfer function (PCTF) for both SSB and extended ptychographic iterative engine (ePIE) algorithms.
• Investigation of normalisation techniques to optimize the transfer window relative to noise levels.

Main Results:

• The SSB method exhibits band-pass contrast transfer, imposing limitations on probe convergence angles for high-contrast imaging.
• The ePIE algorithm provides a broader PCTF compared to SSB.
• Band-pass filtering is necessary for ePIE to mitigate artefacts when transferring both high and low spatial frequencies.
• Normalisation increases the transfer window without amplifying noise.
• Avoiding deconvolution steps in algorithms can enhance dose efficiency.

Conclusions:

• Understanding and optimizing the phase-contrast transfer function is critical for effective electron ptychography.
• The choice of algorithm (SSB vs. ePIE) and processing steps significantly affects imaging performance and dose efficiency.
• Further improvements in dose efficiency can be achieved by carefully selecting reconstruction algorithms.