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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.

Faster chiral magnetic order recovery

Studying the ultafast dynamics of chiral spin structures (Skyrmions) stabilized by Dzyaloshinskii-Moriya interaction (DMI) after optical excitation, the observed recovery of the chiral magnetic order is faster than the average collinear domain magnetization.

Kerber et al., Nature Communications, Vol. 11 - 1, pp. 6304 (2020).

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Diffractive fluence mapping

The FEL fluence spatial distribution on the target can be mapped by drilling customized diffraction gratings on the sample membrane, providing real space imaging of the beam spot at the sample plane. This can be used to properly align the sample on the beam focus and to recover the single shot intensity profile of the pulse used in the experiment.

Schneider et al., Nature Communications, Vol. 9 - 1, pp. 214 (2018).

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Multi-color magnetic imaging

Nanoscale magnetic domain networks in Co/Pt heterostructure are spatially resolved through coherent imaging with Fourier-transform holography. Irradiating the holographic sample at the same time with two harmonics of the FEL seed, at resonance with O and Pt respectively, two element specific images are retrieved at the same time.

Willems et al., Structural Dynamics, 4, 014301 (2017).

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Mini-TIMER: four wave mixing with FEL transient gratings

Extreme ultraviolet four wave mixing have been demonstrated at the DiProI beamline producing a transient grating with 70 fs FEL pulses, split into two halves and recombined on the sample with a known delay, and probing it with a 100 fs ultraviolet pulse to produce a fourth signal beam.

Bencivenga et al., Nature 520, 205 (2015).

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Ultrafast demagnetization dynamics

When an ultrashort optical laser pulse excites a magnetic material, it responds with an almost instantaneous reduction of its magnetization, followed by a slower recovery. This dynamics can be followed by time resolved magnetic holography, taking FEL images of samples excited by IR pulses.

von Korff Schmising et al., Physical Review Letters 112, 217203 (2014).

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Holography with customizable reference

Fourier transform holography retrieves microscopic images encoding in the X-ray scattered wave the interference between a known reference and the sample. The presented algorithm allows reconstruction from customizable references that can be designed in order to optimize signal from each particular sample, overcoming the limitations of standard holography geometries.

Martin et al., Nature Communications 5, 4661 (2014).

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Pump-probe with twin-seeded two-color FEL

The time resolved dynamics of matter under extreme non-equilibrium conditions can be studied by pumping the sample with an intense ultrafast X-ray pulse and probing the system response with a second FEL pulse after a known delay.

Allaria et al., Nature Communications 4, 2476 (2013).

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Sorting CDI movie frames

One of the main goals of CDI is to create single shot images of identical objects, captured at different times during an undergoing transformation. These "frames" must be sorted in the right order to obtain the "movie" of the dynamic process.

Yoon et al., Optics Express 22, 8085 (2014).

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Magnetic resonant holography

Cobalt/palladium multilayers present magnetic domains, with opposite polarizations perpendicular to the surface, that can be prepared in a disordered maze state. The electron density of the system is uniform, but the magnetic structure can be investigated by photon scattering, which depends on light polarization.

Müller et al., Synchrotron Radiation News 26, 27–32 (2013).

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Towards jitter-free time-resolved FEL-IR experiments

Synchronization of FELs and external IR lasers opens the possibility to investigate ultrafast electron dynamics. The seeded scheme of FERMI allows to couple FEL and IR laser pulses with an unprecedented time jitter as low as 6 fs on the sample.

Danailov et al., Optics Express 22, 12869 (2014).

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The DiProI experimental chamber can work in many flexible configurations. A newly available setup (developed in close collaboration with the group of Prof. Dr. Christian Gutt, Siegen University) is organized as a reflectometer in which the scattering from the FEL, impinging on the sample rotated at an angle Theta, is acquired on a detector placed at an angle 2Theta. Reflected signal can be acquired both by a CCD detector and a photodiode, allowing both scattering spatial imaging and fast angular reflectometry scans.

Magnetic scattering in reflection.

Magnetic scattering in reflection geometry

Reflectometer set up

Reflectometer setup

Drowing of the reflectometer setup

Reflectometer Main features

Theta ranges from 0° to 55° in
Theta-2Theta configuration

IR pump available at a nearly collinear geometry, 2° from the FEL trajectory

CCD detector to sample distance
adjustable between 50 and 150 mm

5 axis piezoelectric sample stage
X, Y, Z ±15 mm; Θy, Θ±5° ranges

Static electromagnet on sample holder

A special sample environment, able to provide an in plane static magnetic field (up to ±35 mT), has been designed (in collaboration with the group of Prof. Dr. Jan Lüning, Paris IV University) to perform scattering experiments both in T- and L-MOKE geometry. The device has been tested on permalloy film irradiated with left and right circular polarized FEL radiation in reflection geometry. The inversion of the in plane film magnetization, induced by the external static magnetic field,is visible in data shown in the following figure.
Sample electromagnet features

MiniTIMER delay line

The MiniTIMER compact split and delay line can be set up inside the DiProI chamber before the sample region to split each FEL pulse into to branches and recombine them on the sample with tunable incidence angle and delay (in collaboration with the EIS-TIMER beamline). This set up has been used for transient grating, coherent anti-Stokes Raman scattering and time-resolved magnetic scattering experiments.
The typical crossing angle between the two beams is of about 6° and the delay can be scanned in the range of ±500 fs.
MiniTIMER and reflectometer

MiniTIMER compact delay line

MiniTIMER compact delay line

Four wave mixing setup

Four wave mixing setup

Four-quadrant photodiode: measure of pointing and intensity stability

A four-quadrant photodiode is set at the entrance flange of the DiProI vessel. Under moderate FEL intensity conditions (average pulse intensity 10 µJ), it can provide an accurate shot by shot measurement of the FEL intensity and pointing instability. This will ensure a smooth treatment of data in post processing analysis, keeping into account the beam-transport losses.  
As an example, the panel on the right shows a laser-induced fast demagnetization curve on Co/Pd multilayer film, presenting an improved signal to noise ratio after normalization with the four-quadrant photodiode signal instead of the I0 monitor gas detector placed 60 m upstream the photon beam transport.
Magnetic scattering normalization example

Four-quadrant photodiode and signals

Optical laser

Pump-probe experiments can be performed with femtosecond temporal resolution using both FEL and optical laser radiation. The current optical source is the laser used to generate the FERMI seed, part of which is transported trough a 150 m long optical beamline to the end-stations, where the very high pointing stability is achieved by Rayleigh imaging and beam-position active stabilization, guaranteeing an added timing jitter as low as 3 fs RMS (Cinquegrana et al. Physical Review Special Topics - Accelerators and Beams, 17 040702 (2014)), 10 fs for DiProI in particular (Danailov et al. Optics Express, 22 12869 (2014)). A compact pulse compressor, integrated in the beam transport, controls the pulse length in the range 90-250 fs.
Optical laser transport
Three optical breadboards are available for laser beam manipulation, IR beam transport into the chamber and diagnostics. The first one, of size 900x600 mm2, provides different options fo matching the optical pulse parameters to the experiment. An optical attenuator (Atten.), made of half wave plate and two thin film polarizers, is used to adjust the exact pulse energy impinging on the sample in the 1-700 µJ range (measured on an energy meter (En.). A translation stage (Sc.D.) provides a variable pulse delay in the range of ±570 ps. A compact single-shot auto-correlator (SSAC), built from a Fresnel prism, a thin barium borate BBO second harmonic generation crystal and a CCD camera, is installed for input pulse duration diagnostics. The size of the beam at the sample can be adjusted by a lens telescope (T). The light polarization plane and state can be modified according to the user requirements (WP). A green pilot laser (PL), installed on the breadboard, can assist the preparation of the experiment when the seed laser is used by the other beamlines. After all parameters (energy, beam size, divergence and polarization) are set, the IR laser pulse passes on a small insertion breadboard with focusing lens and beam pointing stabilization system, compensating the beam pointing drifts. A compact second and third harmonic generation setup is available as well. The third breadboard is placed at the output port of the chamber allowing for characterization of the transmitted or reflected beams after interacting with the sample.
For time resolved experiments, two optical laser paths have been conceived inside the vacuum vessel, impinging on the sample plane at an angle of 45° (red line) or 15° (orange dashed line), that can be switched without breaking the vacuum by inserting a movable Second Path (SP) mirror. Both the reflected beam for 45° geometry and the transmitted beam through a Si3N4 thin film or membrane for the 15° one can be used to determine the time jitter between the FEL and IR pulses.
Last Updated on Friday, 18 October 2019 15:16