Optical coherence tomography (OCT) is an established method for non-invasive cross-sectional imaging of biological samples using visible and near infrared light. The axial resolution of OCT only depends on the coherence length l_c∝λ_0^2/Δλ_FWHM, with the central wavelength λ_0 and the spectral width Δλ_FWHM of the light source. For OCT, the axial resolution is in the range of a few micrometers.
XUV coherence tomography (XCT) extends OCT into the extreme ultraviolet range. The significant reduction of the coherence length of a broadband XUV source allows nanoscale axial resolution. The usable spectral bandwidth in XCT is mainly limited by absorption edges of the sample under investigation. For example, the so-called silicon transmission window allows cross-sectional imaging of silicon-based samples like semiconductors.
A laboratory-based XCT setup has been implemented by using XUV radiation from a laser-driven high-harmonic source. By averaging harmonic combs generated by different fundamental wavelengths, a quasi-supercontinuous spectrum, which is well-suited for XCT, is generated.
The radiation is focused onto the sample and the reflected radiation is recorded. Interferences due to reflections at structures in different depths result in a modulated spectra that can be used to reconstruct the axial structure of the sample. Experimentally, we achieve an axial resolution of 24 nm.
In the XUV range, focusing with high numerical aperture (NA) is extremely expensive. Therefore, XCT uses low-NA optics, which limits the lateral resolution to the micrometer range. A combination of XCT with coherent diffraction imaging would provide improved lateral resolution. We present first results a proof-of-concept experiment at a synchrotron source.