DiProI Research

Coherent Diffraction Imaging (CDI) In Coherent Diffraction Imaging (CDI), when coherent X-rays impinge on the sample, the magnitude of the scattered radiation field is detected as a diffraction pattern; provided that the pattern is sufficiently "oversampled" to recover the radiation phases, it can be mathematically "inverted" to recover an image of the object's charge-density distribution. In this lensless microscopy technique, a phase retrieval algorithm applied to the acquired diffraction pattern replaces the image-forming optics, used in classical X-ray microscopes, making CDI free of the resolution limitations imposed by optics aberrations and efficiency. Using FELs, CDI is approaching the theoretical spatial resolution, determined only by the FEL wavelength, in the nanometer range, the degree of beam coherence and the angle to which the speckle pattern is detected. Thanks to the almost full coherence and extreme brightness of the FEL pulses, CDI has become a key technique for sampling building blocks of matter. Femtosecond single shot CDI experiments not only allow to image the sample before radiation-induced changes occur, but also make femtosecond time resolved experiments possible.

In lensless imaging, the role of the focusing optics between sample and detector is replaced by a phase retrieval algorithm applyed to the acquired diffraction pattern.

Phase retrieval algorithm The microscopy image of the sample is the inverse Fourier transform of the complex electromagnetic field observed at the detector plane. Since the detectors can record only the intensity of the speckles, the phase is lost and has to be recovered through computational trials to reconstruct the specimen. Many kinds of recursive phase retrieval algorithms can be implemented, depending on previous knowledge of the sample, symmetry, boundary conditions. Starting from the acquired diffraction pattern, with arbitrary phases given to each point, as a guess for the electromagnetic field at the detector, a first guess of the sample’s image is obtained through an inverse fast Fourier transform. Real space constraints are applied before implementing a fast Fourier transform, giving a second guess for the electromagnetic field, which phases are kept, while intensities are replaced with the actual data image. After several iterations, the algorithm converges to the two actual complex images, both in the Fourier space and in the real space domain, giving the desired microscopy image of the sample.

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.

Pump & probe: stroboscopic experiments The single-shot CDI speckle pattern formed by the short femtosecond FERMI-FEL pulse is an excellent probe to monitor the evolution of the transient states of the sample. These can be followed, at variable delays after the excitation by a pump photon pulse, with time resolution on scales down to the pulse duration. Even before applying Fourier inversion methods, information on the induced dynamics or changes in the average size and size distribution of the microscopic building blocks is already coded in the intensities and dimensions of CDI patterns. The probe pulse will be provided either by a synchronized Ti:Sapphire user laser, tunable from the infrared to the near ultraviolet, or by a split-delay correlation system, separating the pump pulse from the probe FEL soft X-ray beam, providing a variable delay up to 1 ns. By splitting the pulse and simultaneously hitting an object from two directions, it is possible to explore stereo 3D imaging as well.

Time resolved experiments can be carried out impinging on sets of identical samples with pump & probe pulses with variable delays.

Resonant CDI and magnetic dichroism Resonant CDI at the atomic absorption edges in a single-shot mode, enabled by the full longitudinal coherence and tunability of the seeded FERMI-FEL pulses, adds elemental sensitivity. This selective "chemical" imaging, combined with the FERMI-FEL circular or linear polarization, extends the information to spin and orbital momentum: in non-destructive mode, with attenuated FEL beam, the difference between images obtained with photons of opposite chirality enhances the contrast related to the magnetic domain structure. Magnetic dichroism can be used to image magnetic domains thanks to variations in the response to circular light of different chirality.

Identical samples or integrating mode

Since the full power of focalized FEL pulses will destroy any sample, experiments planning to compare results from different shots need either to be performed using sets of identical samples or in an integrating mode with attenuated beam intensity.

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.