Structure probing by holographic imaging at nanometer scale with X-ray lasers (SPHINX)

Abstract

The SPHINX project aims generating femtosecond-exposure X-ray holograms of microscopic samples and of their internal parts with nanometer resolution. The application is based on a new implementation of the phase-contrast holography that overcomes the main limitations encountered in the current systems (most based on absorption-contrast), namely the low energy range, the limited detector granularity and the weak illumination. The combination of polycapillary lenses, large X-Ray CCD arrays with small pixel size and XFEL sources allows splitting the beam, focusing, magnification and phase-contrast imaging in the keV energy range. Due to the wavelength-dependent optical behavior of the samples, the refractive diffraction reduces the diffraction limit together with a fast drop of the characteristic angles, both essential for the resolving power (given the limited X-ray detector pitch), while also eliminating the shadow effect and giving access to full structure probing. The key parameters are defined by the focusing optics, which could be, according to the beam and sample sizes, a polycapillary micro semi-lens or a combination of the former with a parabolic monocapillary. The advantage of “non-perfect” optics (not providing a point-like focus) is their divergence, driven mostly by the single waveguide. This allows sending on the same detector area the diverging object and reference beams (Fresnel configuration), both provided by a single optical element, condition unreachable with a Fresnel lens or a crystal mirror. Moreover, the femtosecond exposure time on X-FELs permits holographic reconstruction of in-vivo cell elements, viruses and nanorobotic devices during ultrafast molecular processes, yet unexplored by imaging techniques.