Lab-XCT_Nanometer resolution Optical Coherence Tomography (OCT) using extreme ultraviolet and soft X-rays produced with laboratory laser-driven sources

• Applicant: Professor Dr. Gerhard G. Paulus
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas ; Synthesis and Properties of Functional Materials
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 381425468

• Final Report Year: 2022

Final Report Abstract

The goal of project was to further develop extreme UV (XUV) coherence tomography (XCT) in several directions and to apply it to achieve cross-sectional nano-scale imaging of samples relevant for nanotechnology and biology. An essential part of the project was the collaboration with colleagues from the Military University of Technology, Warsaw, Poland. The research was following two main threads, namely XUV imaging in the so-called silicon transmission window (30 to 100 eV) and in the water window (280 to 530 eV), respectively particular relevant for nanotechnology and biology. The main results of the project are as follows: 1. Based on succeeding to reconstruct amplitude and phase of the XUV radiation reflected from the sample, it became possible to determine the field reflectivity (i.e. amplitude and phase) of structures buried beneath the surface of the sample. It is even possible to determine properties such as thickness and roughness using a model-based approach. 2. The phase reconstruction is applied and helps in the investigation of 2D material (here graphene) embedded in bulk samples. We successfully characterized mono-, bi-, and tri-layers and verified our results by comparing with conventional imaging techniques. 3. We measured the near-edge X-ray absorption fine structure of the aluminum L2/3 absorption edge in α-Al2 O3 and compared our results to synchrotron data. The agreement proved to be almost perfect with the remarkable side aspect that the XCT measurement was considerably faster than the synchrotron measurement due to the broadband scheme of XCT. 4. A correlative water-window XUV / visible fluorescence microscope has been designed and built. To our knowledge, it is the first lab-based instrument of this type that incorporates both modalities in the same vacuum recipient, i.e. allows to switch both modalities without moving the sample. First correlative images proving an XUV resolution below 100 nm were obtained. Besides the above outlined scientific results, a number of technical advances were made. Particularly noteworthy is the development and commissioning of a versatile high-resolution XUV spectrometer. The project has also been instrumental in encouraging two graduate students to found a company for developing and marketing XUV instrumentation and services. In addition, we expect that XCT has a considerable potential for applications in both science and industry.

Publications

• A high resolution extreme ultraviolet spectrometer system optimized for harmonic spectroscopy and XUV beam analysis, Review of Scientific Instruments 90, 023108 (2019)
  M. Wünsche, S. Fuchs, T. Weber, J. Nathanael, J. J. Abel, J. Reinhard, F. Wiesner, U. Hübner, S. J. Skruszewicz, G. G. Paulus, C. Rödel
  
• Laboratory setup for extreme ultraviolet coherence tomography driven by a high-harmonic source, Review of Scientific Instruments 90, 113702 (2019)
  J. Nathanael, M. Wünsche, S. Fuchs, T. Weber, J. J. Abel, J. Reinhard, F. Wiesner, U. Hübner, S. J. Skruszewicz, G. G. Paulus, C. Rödel
  
• Coherence tomography with broad bandwidth extreme ultraviolet and soft X-ray radiation, Applied Physics B 127, 55 (2021)
  S. J. Skruszewicz, S. Fuchs, J. J. Abel, J. Nathanael, J. Reinhard, C. Rödel, F. Wiesner, M. Wünsche, P. Wachulak, A. Bartnik, K. Janulewicz, H. Fiedorowicz, G. G. Paulus
  
• Material-specific imaging of nanolayers using extreme ultraviolet coherence tomography, Optica 8, 230 (2021)
  F. Wiesner, M. Wünsche, J. Reinhard, J. J. Abel, J. Nathanael, S. J. Skruszewicz, C. Rödel, S. Yulin, A. Gawlik, G. Schmidl, U. Hübner, J. Plentz, G. G. Paulus
  
• Absolute EUV reflectivity measurements using a broadband high-harmonic source and an in situ single exposure reference scheme, Optics Express 30, 35671 (2022)
  J. J. Abel, F. Wiesner, J. Nathanael, J. Reinhard, M. Wünsche, G. Schmidl, A. Gawlik, U. Hübner, J. Plentz, C. Rödel, G. G. Paulus, S. Fuchs
  
• Characterization of encapsulated graphene layers using extreme ultraviolet coherence tomography, Optics Express 30, 32267 (2022)
  F. Wiesner, S. Skruszewicz, C. Rödel, J. J. Abel, J. Reinhard, M. Wünsche, J. Nathanael, M. Grünewald, U. Hübner, G. G. Paulus, S. Fuchs