Nanometer optical coherence tomography using broad-bandwidth XUV and soft x-ray radiation — XCT

Optical coherence tomography (OCT) is a well-established method to retrieve three-dimensional, cross-sectional images of biological samples in a non-invasive way using near-infrared radiation. The axial resolution of OCT is in the order of the coherence length l_c ∝ λ₀²/Δλ_FWHM which depends on the central wavelength λ₀ and the spectral width (FWHM) Δλ_FWHM of a light source. OCT with broadband visible and near-infrared sources typically reaches axial (depth) resolutions in the order of a few micrometers [1]. The new method presented here takes advantage of the fact that the coherence length and therefore the axial resolution of OCT can be significantly reduced if broadband XUV and SXR radiation is used. The broadness of the usable XUV spectrum is limited by absorption edges of the sample materials. For instance, the silicon transmission window (30-99 eV) corresponds to a coherence length and therefore a possible axial resolution of about 12 nm, thus suggesting applications for semiconductor inspection. In the water window at 280-530 eV a coherence length as short as 3 nm can be achieved and highlights possible applications of XCT for life sciences.