Ptychography

Collection of a ptychographic imaging data set in the simplest single-aperture configuration.
Collection of a ptychographic imaging data set in the simplest single-aperture configuration. (a) Coherent illumination incident from the left is locally confined onto an area of the specimen. A detector downstream of the specimen records an interference pattern. (b) The specimen is shifted (in this case, upwards) and a second pattern is recorded. Note that regions of illumination must overlap with one another to facilitate the ptychographic shift-invariance constraint. (c) A whole ptychographic data set uses many overlapping regions of illumination. (d) The entire data set is four-dimensional: for each 2D illumination position (x, y), there is a 2D diffraction pattern (kx, ky).

Ptychography (/t(ʌ)ɪˈkogræfi/ t(a)i-KO-graf-ee)[citation needed] is a computational method of microscopic imaging.[1] It generates images by processing many coherent interference patterns that have been scattered from an object of interest. Its defining characteristic is translational invariance, which means that the interference patterns are generated by one constant function (e.g. a field of illumination or an aperture stop) moving laterally by a known amount with respect to another constant function (the specimen itself or a wave field). The interference patterns occur some distance away from these two components, so that the scattered waves spread out and "fold" (Ancient Greek: πτύξ is 'fold'[2]) into one another as shown in the figure.

Ptychography can be used with visible light, X-rays, extreme ultraviolet (EUV) or electrons. Unlike conventional lens imaging, ptychography is unaffected by lens-induced aberrations or diffraction effects caused by limited numerical aperture[citation needed]. This is particularly important for atomic-scale wavelength imaging, where it is difficult and expensive to make good-quality lenses with high numerical aperture. Another important advantage of the technique is that it allows transparent objects to be seen very clearly. This is because it is sensitive to the phase of the radiation that has passed through a specimen, and so it does not rely on the object absorbing radiation. In the case of visible-light biological microscopy, this means that cells do not need to be stained or labelled to create contrast.

  1. ^ Rodenburg J, Maiden A (2019). "Ptychography". In Hawkes PW, Spence JC (eds.). Springer Handbook of Microscopy (PDF). Springer Handbooks. Springer International Publishing. pp. 819–904. doi:10.1007/978-3-030-00069-1_17. ISBN 978-3-030-00068-4.
  2. ^ Hegerl R, Hoppe W (1970). "Dynamische Theorie der Kristallstrukturanalyse durch Elektronenbeugung im inhomogenen Primärstrahlwellenfeld". Berichte der Bunsengesellschaft für physikalische Chemie (in German). 74 (11): 1148–1154. doi:10.1002/bbpc.19700741112.