Extending the depth of field for ptychography using complex-valued wavelets

Ptychography is a scanning variation of the coherent diffractive imaging method for providing high-resolution quantitative images from specimen with extended dimensions. Its capability of achieving diffraction-limited spatial resolution can be compromised by the sample thickness, which is generally required to be thinner than the depth of field of the imaging system. In this Letter, we present a method to extend the depth of field for ptychography by numerically generating the focus stack from reconstructions with propagated illumination wavefronts and combining the in-focus features to a single sharp image using an algorithm based on the complex-valued discrete wavelet transform. This approach does not require repeated measurements by translating the sample along the optical axis as in the conventional focus stacking method, and offers a computation-efficient alternative to obtain high-resolution images with extended depth of fields, complementary to the multi-slice ptychography.