Heterodyne Near Field Speckles: from laser light to X-rays

A fascinating way of generating speckle patterns is by interfering the weak fields scattered by a disordered sample with the intense trans-illuminating beam. The resulting intensity fluctuations are known as Heterodyne Near Field Speckles. Thanks to the self-referencing layout, the intensity distribution allows direct assessment of the electric fields, thus preserving both amplitude and phase information. Originally observed with visible laser light, during the last years Heterodyne Near Field Speckles have been extended to partially coherent radiation and to X-ray beams. We give in this review a uniform argumentation of Heterodyne Near Field Speckles based on Fourier Optics, valid with both coherent and partially coherent illumination. Emphasis is given to the speckle size, a fundamental property of any speckle field and a basis for earlier and state-of-the-art development of the technique. We review the applications of Heterodyne Near Field Speckles in the fields of particle sizing, velocimetry, coherence measurements, X-ray wavefront sensing and X-ray phase-contrast imaging and tomography. Throughout the discussion, we also emphasize the common aspects shared with many different research areas, such as astronomical observations, holography and TEM imaging, thus evidencing the encompassing nature of the underlying physical principles.