Ptychography

Ptychography is a name given to a technique invented by W. Hoppe.Hoppe, W. (1969). Acta Cryst. A25, 495-501] that aims to solve the diffraction-pattern phase problem by interfering adjacent Bragg reflections coherently and thus determining their relative phase. In the original formulation, Hoppe envisaged that such interference could be effected by placing a very narrow aperture in the plane of the specimen so that each reciprocal-lattice point would be spread out and thus overlap with one another. The name ptychography, from the Greek for "fold", derives from this optical configuration; each reciprocal lattice point is convolved with some function, and thus made to interfere with its neighbors. In fact, measuring only the intensities of interfering adjacent diffracted beams still leads to an ambiguity of two possible complex conjugates for each underlying complex diffraction amplitude. The original formulation of ptychography is equivalent to the well known theorem that for a finite specimen (that is one delineated by a narrow aperture, sometimes known as a "finite support"), the one dimensional phase problem is soluble to within an ambiguity of 2N, where N is the number of Fourier components that make up the specimen. [Rodenburg, 1989] However, such ambiguities may be resolved by changing the phase, profile or position of the illuminating beam in some way. The fact that not only the intensities of the diffracted beams but also the intensities lying midway between the beams, where the convolved Bragg beams interfere, is an alternative statement of the Nyquist–Shannon sampling theorem for components of diffracted intensity. These components general have twice the frequency (in reciprocal space) of their underlying complex amplitudes.

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References

cited from T. Plamann and J. M. Rodenburg Electron Ptychography. II. Theory of Three-Dimensional Propagation Effects Acta Cryst. (1998). A54, 61-73 [ doi:10.1107/S0108767397010507 ]