DiProI_2012

DiProI beamline at FERMI@Elettra The lensless Coherent Diffraction Imaging (CDI) technique has been developed significantly and is gaining time resolved potentials thanks to the advent of coherent and ultrashort pulses delivered by the X-ray free electron lasers (FEL). The shot-to-shot temporal and energy stability of the seeded-FEL pulses at Fermi@Elettra has opened extraordinary opportunities for CDI and in particular for Resonant Coherent Diffraction Imaging (R-CDI  ),   overcoming some of the limitations imposed by the partial longitudinal coherence of the SASE-FELs.In addition, the multiple (linear and circular) polarization of Fermi-FEL pulses is an added value to explore specific contrast mechanisms, relevant to the spin and orbital sensitive electronic transitions.

Coherent Diffraction Imaging proof of principle In March 2012 the performance of the K-B mirror focusing optics, designed to provide a 3x5 µm2 microprobe, reached some modest microprobe of 40×50 µm2 with preserved coherence. This allowed us to perform the first proof-of-principle single-shot CDI and holography imaging of nano-lithographic test objects, fabricated on Si3N4 windows. The intensity of the photons scattered from the objects was monitored on a CCD camera using a detection set-up with a 45° multilayer (for 32.5 nm) mirror.The speckle pattern of the ‘Christmas tree’ object was obtained in a single-shot mode (pulse energy ~20 µJ and wavelength 32.5 nm). Although the object was destroyed by this intense FEL pulse, the information contained in the diffraction pattern was sufficient for reconstruction of the object image by recovering the missing phase information using computational algorithm. Similar successful CDI experiments with other test samples have confirmed that the Fermi@Elettra FEL-1 peak intensity, coherence and pulse duration are sufficient for performing ultrafast coherent diffraction imaging of non-periodic nano-objects, obtaining the necessary structural information before the radiation damage has occurred.

First proof of principle CDI experiment performed at the DiProI beamline with an indirect detection geometry. The image is reconstructed through phase retrieval algorithm.

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The next planned CDI commissioning efforts will be with a particle injector and samples driven out of equilibrium. This requires more stringent focusing and implementing seed external laser pump and autocorrelator to probe dynamic processes. Moving towards shorter wavelengths with Fermi-FEL-2 will increase the number of the reachable atomic resonances and push the photon-limited resolution. These developments are ongoing and expected to be completed in the nearest future.

Magnetic and Resonant CDI

First proof of principle of resonant magnetic scattering performed at the DiProI beamline: the magnetic domain scattering is more intense at the Co M_2,3 resonance.

Absorption at the Co M2,3-edge. Blue dots represent wavelengths at which magnetic scattering patterns have been measured.

The proof-of-principle resonant CDI experiments were performed with Co/Pt multilayer samples, exploring the strong resonant enhancement of magnetic scattering at the Co M2,3-edge. For these experiments we installed and commissioned a direct detection CCD system (X-CAM) provided by our partner from CFEL-DESY. Tuning the Fermi-FEL wavelength λ to the Co M2,3 absorption edge (20.8 nm), the speckle pattern created by the photons scattered from the magnetic domains gains intensity. This ring structure is typical for a labyrinth-type domain organization and contains information about the average domain size and period of the magnetic structure. The extraordinary opportunities of circularly polarized Fermi-FEL pulses are in using single-shot resonant magnetic scattering in holography approach that will allow easier access to magnetic dynamic experiments.

Direct detection CCD system Modular CCD: the main unscattered beam is let through the gap between two chips.