Deliverables
New x-ray monochromators (crystals) have been designed, constructed and tested to provide the highest flux of photons that can be collected and focused onto a sample using femtosecond laser-produced plasma x-ray sources. These developments are crucial for the success of time-resolved x-ray diffraction experiments with a laser-plasma x-ray source as the number of useable x-ray photons is weak at this present stage of the source development. The main challenge was to keep the ultrafast nature of the radiation unperturbed. Extended simulations of potential new bent crystals were done. They have addressed new materials, thin crystals, and new bending shapes. It resulted in the production and the tests of selected crystals like the toroïdal Germanium 100, the toroïdal thin CdTe and the ellipsoidal HOPG as examples.
These crystals have been used within the FAMTO project by different partners to achieve their successful experiments. Some of them are now part of the new ultrafast x-ray workstation developed at LOA. It is now possible for teams outside of the consortium to benefit from the expertise gained in the development of such x-ray crystals by having the possibility to use them in close collaboration with the University of Jena.
Upgraded toroïdal crystals: The 4th order of (100) reflection of germanium crystals and the 400 reflection in Gallium arsenide crystals were selected following extensive simulation of x-ray properties in different materials. Compared to the crystals used so far, a higher reflectivity and a not too wide reflection curve for a good angular resolution was expected. Furthermore, no additional temporal smearing of the x-ray pulse by the absorption depth inside the bent crystal is produced. As x-ray line for pump-probe experiments, Ti Kalpha was chosen.
Two Ge 400 crystals with different bending radii have been produced (300mm and 500mm) and are now in use. Gallium arsenide has almost the same x-ray diffraction properties as well as the same reflection angle (Bragg angle) than Germanium. Two tests of crystal bending of GaAs standard wafer were very successful with respect to the bending of the crystal surface. The surface bending of these crystals is much better than for germanium. One crystal which was bent with a bending radius of 500mm has been produced. This crystal showed reasonable x-ray focusing and is now in use.
Ellipsoïdal crystals: A new generation of x-ray optics has been developed for time-resolved x-ray diffraction experiments with laser-produced plasma x-ray sources, with the goal to increase by orders of magnitude the x-ray flux that can be focused onto a sample compared to the existing technologies. Bent crystal of a Highly Oriented Pyrolytic Graphite (HOPG) crystal foil have been produced with an ellipsoidal shape. The crystal was designed for Ti K radiation at 4.5keV. The former has appeared to be a key point for that purpose and has been produced using a high precision turning machine.
The new crystal has the following properties. The efficiency of the optic, determined by the ratio of photons focused by the crystal to the photons emitted by an x-ray source, is 3x10-3. This is more than 30 times of the efficiency of crystals used so far in ultrafast experiments using laser-produced plasma sources. This is really a new quality with respect to the brightness. A solid angle of 0.43 steradians can be used to collect the x-ray radiation. The smallest focus of a 120µm fluorescence source was measured to be 480 µm. This is not as good as for the perfect crystals like GaAs or silicon, but is still sufficient for a number of experiments. The rocking curve of the graphite foil was measured by using Ti Kalpha. The integrated reflectivity was 5 times higher than for the strongest reflection of quartz at this photon energy. The width of the rocking curve was determined to be 0.1°. This is, to our knowledge, the best value for such a mosaic crystals and explains the lower limit of 480µm spot size. Due to the crystal thickness, a time smearing of the x-ray pulse of not more than 200fs can be achieved. This is very reasonable for many time resolved experiments in the subpicosecond regime.
A test experiments has shown that 10+7 photons can be detected. This result represents an outstanding increase of the x-ray flux by a factor 100 compared to the previous generation of x-ray optics (toroïdal crystals). This will significantly improve the running time-resolved x-ray diffraction experiments.
A pioneered application of femtosecond x-ray diffraction has been realized: the characterization of ultrafast melting of semiconductor (InSb) in a time-scale at which such processes occur (100fs). These experiments were done using the ultrafast laser-produced plasma x-ray source. They have shown that an ultrafast atomic transition has been directly observed and characterized using x-rays for the first time. This has been published in the Journal Nature in 2001.
This scientific result shows to the user community that a new range of time scale is available for the study of the dynamics of structures and for the characterisation of a non-thermal processes. These processes are characterised by directed, rather than diffusive, motion of atoms, and are of great current interest in biology, chemistry and physics. However, measurements of these phenomena were only performed at optical wavelengths, probing the state of the valence electrons rather than the positions of the atoms. Thanks to this result, the existence of non-thermal melting of semiconductors was established unambiguously, answering a long-standing question in solid-state physics. This first structural measurement of a non-thermal process is of interest to physicists, chemists and biochemists. Large dissemination has taken place following the publication of the results with significant Press coverage (scientific and non-scientific).
Achievement of this result was seen -at an international level- as the key demonstration experiment that had to be tackled before any further investigations of more complex ultrafast processes. It has played a major role in the design and construction of femtosecond x-ray workstation in laser Facilities by identifying the remaining bottlenecks to be solved. Following the success of this experiment, extended studies have been launched and recently leaded to major publications by laboratories outside of the consortium (Nature & Science 2004) using the same tool.
The results can be described as the following: the sample was excited by a laser delivering pulses of 800nm and 120fs, and the response was probed by x-ray radiation at 7.13 Angtroms generated by focusing a 23mJ, 800nm and 120fs laser pulse from a second arm of the visible laser on a silicon target. The emitted x-rays were collected by a toroïdally bent quartz (100) crystal and focused on the samples
The first order Bragg reflection of InSb in the (111) orientation (72.3° Bragg angle) was detected by a cooled x-ray CCD camera, with the estimated number of diffracted photons reaching about 500 in one laser shot. Both 111, 100 and asymmetrically cut 100InSb crystals were used to change the probe depth l of the x-ray. The optical probe beam is a continuum produced by focusing 20mJ of the infrared laser pulse into a water cell. The reflected p-polarized signal is collected with a cooled 16-byte CCD camera through a spectrometer.
The diffracted x-ray intensity integrated over the whole rocking curve has been recorded as a function of the delay between the 100fs x-ray probe and the 100fs infrared exciting pulse. At 200mJ/cm² for InSb, the x-ray intensity drops by 20 % in 350fs and then remains constant during few picoseconds. We attribute this drop to the melting of 650Angtroms at the surface of the semiconductor. The time-scale of this phase transition is too short to be produced by the usual thermal response of the lattice. Ultrafast melting is believed to arise from a strong modification of the inter-atomic forces due to laser-induced promotion of a large fraction (10%) of the valence electrons to the conduction band. Following excitation of the electrons, the atoms find themselves far from the new equilibrium positions and immediately begin to move, gaining enough kinetic energy to produce very rapid melting.
As the fluence is decreased, we have observed that the duration of the phase transition is getting longer while the thickness of the melted layer is becoming shorter. At 50mJ/cm², it takes almost 1 picosecond to melt less than 100 Angstroms at the surface. This fluence is still larger than the expected threshold of the transition (15mJ/cm²) and the amplitude of the drop is found to be close to the amplitude of the error bars. The use of asymmetrically-cut InSb and thin films of CdTe has allowed us to increase significantly the amplitude of the drop at a fixed exciting fluence. The thickness of the molten layer and the duration of the phase transition have been fully determined as a function of the exciting fluence. At high exciting fluences (200mJ/cm²), we have found that it takes 350fs to melt 700 Angtroms at the surface of the semiconductor.
We have demonstrated that synchrotron x-ray radiation can be generated by simply focusing a single high intensity laser pulse into a gas jet. For that purpose, a conceptually new strategy has been used from the marriage of laser expertise and synchrotron radiation concepts. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate an ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a pulse of broadband radiation, which has keV energy and lies within a narrow (50mrad) cone angle. An additional property is its ultrafast pulse duration, which is expected to be few 10fs. This has been published in Phys. Rev. Lett. in 2004.
This is the first time that a beam of collimated x-rays can be produced using lasers, pioneering a new road in laser science towards the advent of efficient secondary sources. This will allow a new class of infrastructure to be developed, combining the specific properties of compact ultrafast lasers and energetic accelerator based devices. A striking advantage of this next generation of ultrafast x-ray source is to significantly increase the x-ray flux that can be focused onto a sample. At Laser Facilities, the time resolved x-ray diffraction experiments in the femtosecond time scale are presently done using a the Kalpha radiation. This source is fully divergent and x-ray optics must be used to collect the radiation before focussing. Despite the significant achievements in the development of large bent x-ray crystals, huge x-ray photon losses occur due to limited solid angle collection and x-ray diffraction efficiency. This is the main limitation of the existing ultrafast laser-produced plasma x-ray sources. Here, a collimated beam of x-ray radiation will not be affected by these limitations and will dramatically improve the number of x-ray photons that could probe a sample. The range of experiments in ultrafast x-ray science can then be significantly extended by meeting more closely the user's need. This new generation of X-ray sources from laser systems paves the way towards efficient small scale and costless devices that could complement -in the future- the 4th generation of Large Scale instruments developed in the accelerator community like the X-ray Free Electron Laser.
The development this x-ray source has not reached a stage to be useable by potential users coming from other scientific fields. Characterization and optimisation studies must be first done as well as a first demonstration experiment in ultrafast x-ray science to show the real capabilities of this new generation of radiation.
This collimated x-ray source has been produced as follows. A titanium-doped sapphire laser operating at 10Hz with a wavelength 0 of 820nm in chirped-pulse amplification mode is used. It delivered energies up to 1 J on target in 30fs, with a linear horizontal polarization. The laser beam was focused with an f=18 off-axis parabolic mirror onto the edge of a supersonic helium gas jet (diameter 3 mm). The laser distribution in the focal plane was Gaussian with a waist w0 of 18 Ým containing 50% of the total laser energy. This produces vacuum-focused intensities IL on the order of 3 10+18 Wcm². We have measured the x-ray radiation produced in the plasma using a cooled x-ray CCD camera placed directly on the laser axis without any focusing x-ray optic. The images recorded clearly show the beam of x-rays.
Its brightness was estimated from the pulse duration and the size of the x-ray source. The temporal pulse width is fully determined by the temporal profile of the electron bunch, which is close to that of the laser (30fs). The average brightness is5 10+6ph/s/mm²/mrad²/0:1% BW and the peak spectral brightness is 2 10+22ph/s/mm²/mrad²/0:1% BW. Furthermore, 5 10+6 photons/pulse/0:1% BW are produced and the x rays are perfectly synchronized with the laser system, enabling visible pump/x-ray probe diffraction experiments with insignificant time-jitter.
An ultrafast x-ray workstation has been constructed in a Laser Facility (LOA) to provide a time resolution of 100fs for time resolved x-ray diffraction experiments.
It provides the user community with the required scientific environment to realise application experiments in their field using a femtosecond laser-produced plasma x-ray source. It will be open to the European pool of users through different channels (Access with Research Infrastructures like LaserLab and collaboration between teams within Networks like FLASH).
We will ensure a dynamic and broad dissemination of this workstation. This is motivated, at first, by the significant technological and scientific advances achieved in the FAMTO project. Experimentalists from multidisciplinary fields have repetitively expressed their desire to have access to still better performance particularly in one direction, subpicosecond x-ray time resolution. FAMTO has strongly participated to the development of this next generation of peripheral equipment at Large Scale Laser and Synchrotrons radiation research infrastructures. Second, beam time is in very high demand by European and National users owing to the increased interest in novel applications. It will be limited at the future large scale X-ray Free Electron lasers that are under development, mainly because the European project will not provide ultrafast x-ray radiation before 10 years, while limited number of beam lines will meet the high demand. Promotion of alternative ultrafast x-ray radiation is a must.
The ultrafast x-ray workstation is ready. A first test experiment (in a time-resolved mode) will be realized in house before opening it to the European pool of users.
The ultrafast X-ray station contains the following parts:
- A 10mJ and 50fs laser system running at high repetition rate (1 kHz);
- An x-ray source working at 1 kHz;
- A toroïdal crystal to collect and focus the x-ray beam onto the sample;
- A full x-ray diffractometer to support the sample, equipped with an additional standard x-ray tube to control its orientation.
Two optical layouts share the same sample position. The first optical layout includes Xcalibur conventional X-ray tube, sample and Sapphire-II X-ray area detector. This setup is used for steady state measurements and orientation of the sample. The second optical layout consists of ultrafast x-ray plasma source, sample and Princeton photon-counting area x-ray detector. This layout can be used for visible pump-x-ray probe experiments once the sample has been aligned and tested with the x-ray tube.
The arrangement of all the components of the diffractometer allows us to move the detector arm within limit of a semicircle, which gives us possibility to work in Bragg and Laue geometry without changing anything. It gives unlimited access to crystallographic directions of a sample for the fs x-ray diffraction experiment.
A readout technique for X-ray streak cameras analogous to the well-known time-correlated single photon counting technique for time-resolved spectroscopy on atoms has been developed, tested and implemented at the LLC/MAXLAB D611 x-ray beam line. The time resolution of this x-ray detector has been boosted from few ps to 500fs and the time repetition rate from few Hz to 7 kHz.
The two major bottlenecks (temporal jitter between the light source and the detector and spread in kinetic energy of the secondary photoelectrons) have been addressed and eliminated thanks to the development of this readout technique. We have demonstrated the possibility to completely eliminate trigger jitter and to increase the temporal resolution beyond the single-shot temporal resolution. Single-photon counting is based on the implementation of two distinct elements: Firstly, timing fiducials are used to correct for timing jitter. Secondly, single X-ray photons are imaged on the readout camera and their centers of gravity are accurately determined. Hence the arrival of an X-ray photon can be accurately determined with respect to the timing fiducial.
The development of this technique makes this tool suitable for use at synchrotrons and laser facilities with very cost effective properties. It will be particularly powerful in the synchrotron case for which this fast x-ray detector will provide, at first, a unique opportunity to increase the time resolution from the 100ps time scale to few hundred femtoseconds; and second, a repetition rate of the data acquisition higher than any other comparable facility at this time scale.
Number of considerations have been taken into account in order to ensure the optimum temporal response of the x-ray streak camera. Only some of the mechanisms for temporal smearing are compensated for by the single-photon counting readout presented here. Collective effects due to spreading of the ensemble of secondary electrons such as imperfect focusing and smearing due to the velocity spread of secondary electrons are cancelled. Statistical processes involving single X-ray photons or primary electrons cannot be cancelled. Such an effect is the broadening due to the finite width of the slit. The position where a single photon hits the photocathode will influence at which position the center of gravity of the ensemble is imaged. A mechanism that may ultimately be the limit of the temporal resolution of streak cameras has so far not been an issue. It will take a finite time for the primary electrons with a kinetic energy corresponding to the X-ray photons to be converted to secondary electrons, which then will escape from the cathode in a diffusive process. Since this stage involves statistics of the primary as well of the secondary electrons, this effect will only partially be improved upon by the here described single-photon counting technique, described here.
In order to obtain a sub-ps streak camera capable of averaging at 5-10kHz (tested at 7kHz), the streak camera was built with a CsI photocathode, an acceleration slit which is imaged, a sweep plates in a meander geometry (transmission line), a magnetic lens focusing and a MCP-based back-end suitable for discriminating close-lying pulses at a synchrotron radiation source with high rep-rate. Initial test give a temporal resolution of 5ps. This is limited by the slit width. In order to decrease the pulse duration we have used a smaller slit down to 50-200 micrometers to have sub-ps resolution.
The streak camera has now reached its specifications in high-repetition rate mode 5-10kHz. 800fs. In addition, the single-photon counting has been demonstrated to work at higher repetition rates than 1kHz. The time resolution which is anticipated to be below 500fs has not been possible to demonstrate yet. However, the algorithms have been tested in situ at the beam line looking at a real signal with the ability of finding centers of gravities of single photons with 100fs accuracy.
The feasibility of protein and liquid crystallography at 25ps time scale has been demonstrated at the ESRF beam line. This represents a major step forward as these application experiments shows that the time resolution for the analysis of complex reactions can be boosted by a factor of 4 compared to the duration of the synchrotron x-ray pulse (100ps).
Protein crystallography: Laue diffraction studies of ligand transport in heme proteins such as the myoglobin complex with CO and O2 and similar hemoglobin complexes have been done. For that purpose, we have traditionally used the synchrotron in 1-bunch mode. The reason for that is the high intensity per bunch that this mode offers, typically 2-3 times more photons per pulse which translates into 2-3 times shorter data collection time ANF 2-3 times fewer laser excitations. The latter is important to reduce radiation damage, in particular dehydration caused by heating. The single bunch ode has now been replaced by 4-bunch mode at half the bunch current. On the up side however, we get shorter X-ray pulses, a better focus and a longer lifetime, all in all a better X-ray beam. The x-ray pulse length in the 4-bunch mode is 100-140ps and we have used these pulses in time-slicing experiments. This technique has been described in the Result related to ultrafast x-ray beam lines at synchrotrons. The results are encouraging and we have a least seen the first images with 50ps time resolution that shows the detachment of CO in the mutant L29F of MbCO. The results are shown below. The green pockets correspond to a change in charge density of 0.23e/ Angstroms³ and the purple to - 0.23 e/Angstroms³. The level of photolysis is 25% and the Fourier map is extrapolated to 100 %. The quality of this map and its time resolution of about 50ps, lead us to believe that 25ps time resolution should become possible very soon at the expense of signal to noise.
We can say that the time slicing on proteins has been slowed down due to the low levels of photolysis that could be achieved with the 100fs laser pulses that were used initially. On the basis of these experiments, pulse stretching combined with high-pulse energy from the TOPAS laser will undoubtedly make it possible to reach at least 25ps time resolution in one or two years.
Liquid crystallography: The time slicing technique has also been used to achieve few 10ps time resolution in the study chemical reactions in liquid, and more precisely the structure of the excited states of small molecules such as I2, Br2, HgI2, CH2I2 and C2H4I2 in various solvents. These experiments can, by contrast to protein crystallography, take the full 1000Hz beam from the laser and the X-ray chopper, which makes the data collection fast and precise.
The experiments have shown that not only does X-ray diffraction probe the excited chromophores; it also probes the environment, which can be divided into the first solvation shell (cage) and beyond (bulk liquid). The last part is responsible a long-range oscillations in the Fourier maps of the electron density, which can interpreted in terms of temperature and pressure during the recombination. We have worked out a fairly complete theoretical picture of x-ray diffraction from molecules in action.
Measurements of the change in the radial electron density for I2 in methanol have been obtained. The data were taken with 100ps X-ray pulses and the time delays scanned though the X-ray pulse in steps of 25ps. The signal increases as the laser position moves from -100ps towards 0ps. The maximum amplitude is attained at 50ps where the excited state is recorded with the intensity from the full bunch. The interesting thing is that the shape of the curves changes slightly in going from -50ps to 0ps. At these early time-delays, the excited molecules have not yet thermalised, conditions that are otherwise difficult to attain.
One new and one upgraded beam lines have been constructed at Synchrotrons (LLC/MAXLAB & ESRF) to perform few 10ps and sub-picosecond x-ray experiments.
Beam time is in very high demand by European and National users owing to the increased interest in novel applications with ultrafast x-rays. The new beam line at LLC/MAXLAB will provide increased beam time for ultrafast x-ray experiments. It will be open to the European pool of users through different channels (Access with Research Infrastructures like LaserLab and collaboration between teams within Networks like FLASH). The ESRF beam line is already well known in the field as a pioneered x-ray installation for picosecond experiments. The upgrades achieved within the project will provide significant added value to the users as it will be described below. The beamlines are commissioned and opened to the European pool of users.
LLC/MAXLAB (D611) Beam line. It works with a bending magnet associated with the MAX II pulsed source operating at 100MHz. The duration of the pulses has been measured to be approximately 150ps using a streak camera. A laser providing pulses with durations of 20-30fs has been synchronized to the ring, and the fast x-ray streak camera which yields sub-ps time resolution has been installed. The laser operates at a maximum of 10kHz, which sets the data accumulation repetition rate. When using the single-photon counting streak camera developed in WP4, the repetition rate is reduced to 1kHz.
The characteristics are the following:
- Source: Bending magnet: 4mrad
- Focusing optics: Toroïd.
- Monochromator: Double Crystal monochromator with Si (111) or InSb (111) crystals.
- Energy range: 2.5 - 9 keV.
- Energy resolution: E/DE = 2 10{4}.
- Photon flux on sample: ~ 10{6}ph/s. (on laser repetition rate)
- Spot size on sample: 0.2(v) x 0.2(h)mm².
- Experimental station: The system consists of a short-pulse laser synchronised to the storage ring. A vacuum chamber providing pressures in the 10{-6} mbar range is available for combined laser/synchrotron radiation experiments. A streak camera detector can be used for sub-ps time-resolution or alternatively an avalanche photo diode for the ns range can be used.
ESRF Beam line. Its performance has been boosted in terms of: laser excitation to provide a complete photolysis of samples; optimisation of the sample environment; and time resolution of the x-ray pulse from 100ps to few 10ps.
X-ray Intensity on sample: A new so-called in-vacuum undulator, based on a 17mm magnetic period, has been designed and now gives a 10-20 increase in flux at an x-ray wavelength of 15keV. A second in-vacuum undulator, the U20, which has its first harmonic at 9.0keV has also been constructed. The third harmonic at 27keV will be 6 times more intense than the second harmonic of the U17. The first optical element in the beam line is the monochromator and its cooling system from water cooling has been upgraded to liquid nitrogen cooling. The improvement in flux density on the sample is 4-6 due to the improved flatness of the atomic net-planes. Finally, the toroïdal mirror, placed down stream the monochromator, has been replaced with a single-crystal silicon mirror of unprecedented optical finish. The new mirror produces a round focal spot of 0.100-mm diameter, with an increase of the flux density by 4 times. To summarize the overall gain is 40 on a day to day basis, which will open up new opportunities such as the use of streak cameras and fine-scanning the x-ray pulse.
Improvement of the time resolution: The time-resolution of our pump and probe experiments is given by the convolution of the x-ray pulse length (100ps) and the laser pulse length (125fs). In a single-shot experiment, the x-ray the pulse length is thus determining (and limiting) the time resolution. In more realistic experiments, the x-ray signal is accumulated on a CCD detector, and the stability of the time-stability of the delay between the laser and x-ray sources, has to be included. We have now measured the timing stability to be 3-5ps (rms) and that opens up the possibility to fine-slice the x-ray pulse with the laser pulse. Specifically one collects scan the time-delay in steps of 10-20ps around time zero and try to extract the differential signal.
Laser for sample excitation: The chirped-pulsed amplifier in our femtosecond laser used to excite the sample was replaced by a new diode-pumped system, the Hurricane from Spectra Physics. The energy per pulse has been raised from 0.45mJ to 1mJ. This will provide 2-3 times more laser photons on the sample, which will increase the signal to noise similarly in many chromophore systems. The set-up for time resolved X-ray experiments has then be improved with a new laser, the so-called TOPAS wavelength shifter (OPA), which increased the wavelength range and the available pulse energy.
Searching for OpenAIRE data...
There was an error trying to search data from OpenAIRE
No results available