Water Window in the Laboratory
Soft X-ray Imaging with Nanometer Depth Resolution Using High Harmonics
In its NanoMovie Application Laboratory, MBI offers femtosecond (≈10-15 seconds) short soft X-ray pulses produced by High Harmonic Generation (HHG) to users. The HHG radiation extends from 100 to 600 eV, covering the entire “water window”, i.e., the spectral region below the oxygen K-shell absorption edge at about 540 eV (2.3 nm), with exceptionally high photon flux and stability in a user-friendly environment.
Exploiting these unique capabilities in a combined reflectometry and spectroscopy setup developed at MBI, a team of researchers from Thuringia, Saxony and Berlin has taken an important step toward non-destructive nanoscale imaging with soft X-rays. In the work published in Light: Science & Applications, the researchers show for the first time that high harmonics can be used for imaging in the so-called water window (approx. 2.3–4.4 nm wavelength)—a spectral range that is particularly attractive because it enables high spatial resolution and comparatively high penetration depth in conjunction with strong elemental contrast.
At the core of the work is a broadband method called Soft X-ray Coherence Tomography (SXCT), derived from optical coherence tomography (OCT), a standard imaging method used in medicine to obtain depth profiles, e.g., of the retina in the human eye. Using this novel technique with soft X-rays, the team explored how axial depth profiles can be obtained in reflection from solid state heterostructures and buried interfaces, structures which are of critical importance in materials science and microelectronic devices.
Until now, HHG-driven imaging of nanometer scale structures in the water window was considered hardly practical, primarily due to the limited photon yield at higher energies. The proof-of-principle-study on a test structure consisting of aluminum oxide and platinum layers now shows that the combination of a powerful, broadband HHG source (exceeding a photon flux of 10⁶ photons/eV/s on the sample) and bandwidth-efficient coherence tomography can overcome this barrier. In the SXCT approach, depth resolution in axial imaging is achieved by recording the interference signal of the soft X-ray pulses reflected from the different interfaces within a sample. By exploiting the full spectral bandwidth of the HHG source to observe the interference in the wavelength domain, the team reconstructed depth profiles with only 12 nm axial resolution in a non-destructive imaging modality. It was demonstrated that this was possible even for very weakly reflecting samples with a rough surface, thanks to the Fourier-based, lock-in-like noise suppression of the SXCT approach.
The results are obtained in a collaboration between several research organizations with contributions ranging from laser and radiation source development to tomography methodology, sample preparation and validation by electron microscopy. Partners include, among others, Friedrich Schiller University Jena and the Helmholtz Institute Jena, the Max Born Institute and TU Berlin, the Laser Institute at the University of Applied Sciences Mittweida, as well as the Leibniz Institute of Photonic Technology (IPHT) Jena.
Figure:
a Scheme of the experimental approach. The High Harmonic Generation (HHG) source is driven by an intense infrared (IR) laser and provides broadband soft X-ray pulses, which are focused onto the sample. The interference of the soft X-rays reflected by the different interfaces within the sample leads to a modulation of the soft X-ray spectrum, which is recorded using a spectrometer and allows reconstruction of the axial depth profile.
b Sketch of the test sample consisting of aluminum oxide (Al2O3) and platinum (Pt) layers on a zinc oxide (ZnO) substrate.
c SXCT cross-section of the studied sample, interpolated between the reconstructed depth profiles at five positions (indicated by the same colors as in panel b). The SXCT signal clearly resolves the surface of the Al2O3 layer at d ≈ 70 nm and its increasing height along the lateral scan axis (x), the interface between Al2O3 and ZnO subtrate at d ≈ 0 nm, as well as the emergence and thickness of the buried Pt layer for x ≥ 0.8 mm.
Publication:
Soft X-ray imaging with coherence tomography in the water window spectral range using high-harmonic generation
Julius Reinhard, Felix Wiesner, Martin Hennecke, Themistoklis Sidiropoulos, Sophia Kaleta, Julian Späthe, Johann Jakob Abel, Martin Wünsche, Gabriele Schmidl, Jonathan Plentz, Uwe Hübner, Katharina Freiberg, Jonathan Apell, Stephanie Lippmann, Matthias Schnürer, Stefan Eisebitt, Gerhard G Paulus, Silvio Fuchshs
Light: Science & Applications 15, 79 (2026). URL, DOI oder PDF
Contact:
Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
mbi-berlin.de
MBI press release, 31 January 2026