Ινστιτούτο Φυσικής Πλάσματος και Λέιζερ

Hellenic Mediterranean University

XUV Coherent Diffraction Imaging

Summary description

This research focuses on developing a novel table-top system for rapid nanoscale imaging, using coherent XUV radiation generated via high-order harmonic generation (HHG) in gases. Unlike large-scale XFEL facilities, this system operates at the laboratory scale, supports automated in-vacuum wavelength switching, and enables multispectral imaging. The technology is based on femtosecond laser pulses focused into a semi-infinite gas cell (Ar, Kr, Xe), producing harmonics. Wavelength selection is performed by multilayer XUV mirrors, while phase retrieval is achieved using iterative algorithms. The system is recognized for its potential in nanotechnology, high-resolution microscopy, and ultrafast time-resolved imaging.

Keywords: Coherent Diffraction Imaging · High-order harmonic generation · Multispectral imaging · Ultrafast imaging · Coherent XUV radiation · Semi-infinite gas cell

Detailed overview

The generation of coherent XUV radiation with controlled characteristics is critical for modern nanoscale microscopy. Existing technologies (XFEL, synchrotrons) require massive investments.

Our research team has developed a table-top approach: An automated multi-wavelength XUV CDI system based on HHG in a semi-infinite gas cell.

Operating Principle

  • XUV Generation: fs IR pulses are focused near the exit pinhole of a semi-infinite gas cell filled with noble gas. Intense laser-gas interaction produces a harmonic comb in the XUV.

  • Filtering & Wavelength Selection: An Al filter removes the residual IR beam. Multilayer XUV mirrors select the desired spectral region. A motorized platform enables automated in-vacuum wavelength switching.

  • Diffraction Recording & Phase Retrieval: The monochromatic XUV beam illuminates the sample. The far-field diffraction pattern is recorded by an XUV CCD camera. Phase is numerically retrieved using iterative algorithms (HIO, Shrink-Wrap).

  • Additional Capabilities: Real-time spectral monitoring, plasma imaging, high repetition rate operation.

 

Methodology

  • Four-stage experimental setup: Gas cell (stage A) → Differential pumping (stage B) → Filtering & selection chamber (stage C)  → Sample & recording chamber (stage D)

  • XUV Optimization: Beam diameter, focus position, gas comparison (Kr for longer wavelengths, Ar for shorter), chirp control.

  • CDI Reconstruction Algorithms: Random phase start, constraint application, iterative shrinking support, final real-space image.

 

Applications and Key Findings

Applications

  • Microscopy of nanostructured materials (e.g., holes in metal foils, periodic structures)

  • Multispectral imaging for material identification and chemical mapping

  • High-resolution 3D tomography (enabled by MHz repetition rate capability)

  • Time-resolved imaging of ultrafast processes (fs to ps)

  • Potential extension to soft X-rays for imaging

Key Findings

  • The system generates coherent XUV radiation at two distinct wavelengths, with automated switching capability.

  • Image reconstruction from diffraction patterns is achieved with high fidelity, revealing sub-micrometer details.

  • Laser chirp control enables selective suppression of the long electron trajectory, resulting in a low-divergence XUV beam – ideal for high-resolution imaging.

  • At high gas pressures, the spectrum becomes quasi-continuous, offering wavelength tunability.

 

Examples

Schematic of the CDI experimental setup: CAD design and photographs of the different stages,

XUV harmonic spectral images (left) and corresponding plasma formation images (right) measured for various laser pulse durations and Ar gas pressures in the semi-infinite cell.The negative/positive signs of the laser pulse durations correspond to the imposed negative/positive chirp, respectively.

CDI results: SEM image of object, diffraction patterns for two different wavelengths, reconstructed real-space images.

 
Selected Publications

Petrakis, S., Skoulakis, A., Orphanos, Y., Grigoriadis, A., Andrianaki, G., Louloudakis, D., Kortsalidouakis, N., Tsapras, A., Balas, C., Zouridis, D., Benis, E.P., Tatarakis, M., Dimitriou, V., Papadogiannis, N.A. (2022). Coherent XUV Multispectral Diffraction Imaging in the Microscale. Applied Sciences, 12(20), 10592. https://doi.org/10.3390/app122010592

Petrakis, S., Bakarezos, M., Tatarakis, M., Benis, E.P., Papadogiannis, N.A. (2022). Spectral and Divergence Characteristics of Plateau High-Order Harmonics Generated by Femtosecond Chirped Laser Pulses in a Semi-Infinite Gas Cell. Atoms, 10(2), 53. https://doi.org/10.3390/atoms10020053

Petrakis, S., Bakarezos, M., Tatarakis, M., Benis, E.P., Papadogiannis, N.A. (2021). Electron quantum path control in high harmonic generation via chirp variation of strong laser pulses. Scientific Reports, 11, 23882. https://doi.org/10.1038/s41598-021-03424-3