Shadow images for in-line holography in STEM

Shadow images are obtained on the detector plane when the specimen in a STEM instrument is illuminated by a stationary beam formed with a very large or no objective aperture. It has been recognized for some time that the shadow image formed after a thin crystal specimen in the diffraction plane of a STEM instrument with a field emission gun can be treated as an in-line hologram. Since the transmitted and diffracted beams are added to give interference effects, all the information on the relative phase of the scattered beams is recorded in the shadow image. In fact, this is basically equivalent to the original scheme envisioned by Gabor, where he proposed that a reconstruction could be made from a hologram so that the imperfection due to spherical aberration of the objective lens could be corrected and improved resolution could be achieved.