LAUSR

Defocused probe ptychography aims to determine the amplitude and phase shift that a sample object imparts upon an impinging electron beam, using an algorithm that converges modelled and experimentally determined diffraction patterns arising from illuminating the sample in small overlapping regions. One of the most successful and widely used algorithms for this technique is the extended ptychographical iterative engine (ePIE) [1]. This algorithm treats the sample as a simple multiplicative phase object, and so cannot deal with multiple scattering. However, it has been that noted in some situations in electron microscopy, where extinction distances are similar to typical sample thicknesses, some experimental configurations still give useful reconstructions [2]. Within the currently dominant xray and optical applications of ptychography, the situation has been treated more comprehensively, and it has been found that propagation within the sample is only adequately considered when the sample thickness t ≤ 5.2 r2 /  ts, where  is the wavelength [3]. The validity of this useful relationship appears to have hitherto not been experimentally investigated in electron microscopy.