CASUS Institute Seminar, Silvio Fuchs, Friedrich-Schiller-Universität Jena

Optical Coherence Tomography (OCT) is an established method for the non-invasive acquisition of 3D cross-sectional images of biological samples using near infrared radiation. The axial resolution of OCT depends on the coherence length of the light source. For visible or near infrared sources, the axial resolution is in the range of a few micrometers. In XUV coherence tomography (XCT), broadband XUV or SXR radiation is used instead. This allows XCT to benefit from the significant reduction in coherence length and thus to reach axial resolution of a few nanometers. In this spectral range silicon can be penetrated. Therefore, the technique has a high application potential in the investigation of semiconductors.
Recently, a laboratory-based setup for XCT was built using XUV radiation from a laser-driven high harmonic source. In the XCT setup, the broadband reflected spectrum of the sample is directly recorded without a distinct reference wave as it would be the case in conventional infrared OCT. Therefore, the signal only contains the autocorrelation of the sample’s structure. The underlying problem is similar to the well-known phase problem in two-dimensional lensless imaging like Coherent Diffraction Imaging (CDI). However, in XCT the problem is one-dimensional and thus counterintuitively much harder to solve.
We developed a novel three-step one-dimensional phase retrieval algorithm based on known iterative approaches and succeeded in reconstructing the axial structure of samples without any autocorrelation artifacts. The algorithm is surprisingly stable and converges to the right physical solution. The reconstructed phase information can further be used to even extract material and other structural information like roughness or layer thicknesses.