Ptychographie

Ptychography is a computational imaging technique used in X-ray, electron, and visible-light microscopy. It reconstructs the complex transmission function of a specimen from multiple diffraction measurements obtained while a coherent illumination is scanned across the sample with substantial overlap between adjacent positions.

During a ptychographic scan, the sample is illuminated by a coherent probe that is translated to many overlapping positions. For each position, a far-field diffraction pattern is recorded. The overlap between adjacent positions provides redundancy that enables simultaneous recovery of the sample's complex transmission function and the illumination probe.

Reconstruction is performed with iterative phase retrieval algorithms, such as the Ptychographic Iterative Engine (PIE) and its variants (ePIE). These algorithms alternately update estimates of the object and the probe to minimize mismatch between measured and calculated diffraction patterns. The method yields both amplitude and phase information, enabling quantitative imaging and high spatial resolution beyond the diffraction-limited resolution of conventional lenses.

In X-ray ptychography, coherence from synchrotron or laboratory sources is required; in electron ptychography, a scanning transmission electron microscope is used; imaging with visible light has also been demonstrated. Advantages include improved spatial resolution, robustness to partial coherence, and the ability to characterize complex samples without relying on traditional imaging optics. The redundancy from overlaps enhances dose efficiency and tolerance to certain experimental imperfections.

Limitations include long acquisition times due to scanning, sensitivity to sample drift and mechanical instabilities, and substantial computational demands. History traces the development of ptychography to the early 2000s as an extension of coherent diffractive imaging, with rapid adoption in X-ray and electron microscopy and ongoing algorithmic refinements.