An autofocusing strategy based on dichotomy for fast correcting the axial distance error of ptychography

Abstract

In ptychography, the precise axial distance between the sample and the detector is of vital importance to retrieve the tested object with high quality, but the axial distance error cannot be entirely eliminated in the practical experiments. An autofocusing strategy based on dichotomy is proposed to rapidly correct the axial distance error. In this strategy, the total variation function that is valid for both amplitude- and phase-modulating specimens is applied to determine whether the current distance is smaller or larger than the true distance. Further, the dichotomy is used to exponentially shrink the step-size of the correction by iterative computations, which means that for the early iterations large modified step-size is used and for later iterations only small step-size is considered. Therefore, the dichotomy is beneficial to quickly confirm the accurate axial distance with few computations. Simulations and experiments are conducted to demonstrate that our proposed strategy, based on the extended ptychographic iterative engine (ePIE), is capable of calibrating the sample–detector distance with high accuracy and efficiency.