Keyhole coherent diffractive imaging

Recent advances in highly brilliant synchrotron sources including soft X-ray free-electron lasers have ushered in many new methods of microscopy. Coherent diffractive imaging (CDI) is one of the most promising ways of determining the nanoscale structures of non-crystalline materials. However, to enable phase determination, the intensity distribution must be sampled at a spacing finer than its Nyquist frequency, which in turn requires the sample to be finite. In other words, there are some limitations in the sample size. Recently, an Australian group led by Professor K. A. Nugent (University of Melbourne) proposed a new method, 'keyhole' CDI, which can reconstruct objects of arbitrary size. In this case, a beam is focused and the object is placed downstream of the focal point so that it is illuminated by a diverging wave. The geometry looks similar to that of in-line holography, but the requirements placed on the source and detector are different. The group attempted imaging by visible light and X-rays, and, using the latter, part of an extended object was imaged with a detector-limited resolution of better than 20 nm. For more information on the present experiments, see the paper, "Keyhole coherent diffractive imaging", B. Abbey et al., Nature Physics, advanced online publication, DOI: 10.1038/nphys896