Seminario de Sistemas Complejos, Erik Guehrs

21 Feb 2017 by Jorge Noreña in Noticias, Noticias 2017, Seminarios
Sala Sergio del Campo, Jueves 23 de febrero, 14:30

Título: Three Dimensional Lensless X-ray Holography

Resumen: Soft X-ray Fourier transform holography (FTH) is a lensless imaging technique. In the past, it has been mostly used to image magnetic samples as well as biological specimens with a lateral resolution up to 40 nm. Through the interference of the object’s scattered wave with a known reference wave the relative phase information is encoded as intensity modulation in the hologram. This allows for the obtaining of a 2D real space image of the sample by a simple Fourier transform of the hologram. Usually, FTH with soft X-rays is realized by using a fixed mask-sample design where the sample is rigidly connected to the holographic mask and the reference wave is produced by a small pinhole.

This design doesn’t allow for the recording of a three dimensional tomographic dataset. This is due to the high aspect ratio of the reference hole (typically 1:10) which blocks X-rays already at small rotation angles. We investigate several techniques to combine X-ray holography with tomography to record 3D images. A small gold sphere on a transparent silicon nitride membrane can be used to produce the reference wave instead of a small aperture. This geometry makes it possible to rotate the sample reference structure. A complete dataset of two dimensional projections can be recorded to compute a three dimensional tomogram of the sample.

The approach is successfully established using a diatom test sample where the 3D structure is reconstructed. This constitutes the first X-ray tomogram recorded via FTH. Furthermore, the gold sphere is replaced by extended reference structures (coded platinum dot array and a thin slit milled trough a gold membrane), which also allows to rotate the sample to record a tomographic dataset. Both reference structures are more robust against noise than a single gold sphere. This is advantageous when the photon flux is limited or the sample has to be protected from radiation damage. Both alternative reference structures are successfully used to record tomographic datasets of a test sample. On this basis two 3D tomograms are calculated.