X-ray holography at atomic resolution

Dear Bill,

I agree with your recent comments on the work by Tegze et al. and Gog et al. on X-ray holography at atomic resolution. Indeed, readers of the many reviews of the recent work on X-ray holography could be forgiven for believing that Gabor's 1948 Nobel-prize winning proposal was for X-ray holographic imaging of atoms, and that this has at last been achieved. This impression is grossly misleading because Gabor did not propose X-ray holography, and the new experimental results are diffraction patterns and not holograms. Gabor proposed electron holography, not X-ray holography, for the purpose of eliminating electron microscope aberrations. Since the first implementation of Gabor's proposal by Haine and Mulvey in 1952 this field has made steady progress, demonstrating both spectacular images of superconducting vortices in motion, and atomic resolution images of defects in crystals.

For the more limited case of diffraction experiments involving periodic structures which recent work describes, the use of the scattering from particular types of atoms to form reference beams has its origins in Bragg's two-wavelength X-ray microscope of 1942, which formed optical reconstructions of diopside from X-ray diffraction patterns, using heavy atoms to solve the phase problem. For the more challenging case of an isolated, non-periodic atomic structure, X-ray holography probably started with the remarkable optical reconstruction of the X-ray image (showing four Fresnel fringes) of a needle by El-Sum and Kirkpatrick in 1952. Again, continuous progress in this field of true X-ray holography has been made since then using synchrotrons by researchers such as Jacobsen, Kin and Howells in this country.

The "new" XFH scattering distribution is not a hologram because the reconstructed wavefield is not conjugate to the object - it is not an image as Gabor clearly intended. Because the method assumes crystallographically equivalent emission sites, it is much more similar to the conventional Kossel diffraction patterns analyzed by von Laue in 1935. (Von Laue gave the theory for both XFH and MEXH, which are related by reciprocity, in 1935.) The MEXH technique continues a tradition of work begun by Knowles in 1956, who studied the effect of neutron standing waves in crystals on X-ray emission, and continued by Batterman, Bedzyk, Duncumb, and many others, including the authors, who made the connection with structural analysis [Acta Cryst. 17, p.33 (1964); J. Micros. 130, p.147 (1984)].

Both the recent synchrotron experiments are remarkable experimental achievements, for which the tunability and intensity of the sychrotron provides exciting new possibilities. But computer modelling of diffraction data must be distinguished from Gabor's holography. By creating a spurious impression of novelty and confusing the distinct topics of diffraction and holography, readers may be misled, and deprived of the rich history of these subjects.