2008 Advanced Grants for Laserlab researchers

Coherent pulse trains

Professor Anne L’Huillier’s work at the Lund High-Power Laser Facility of the Lund Laser Centre (Sweden) has chiefly been on the generation of high harmonics and attosecond pulses, but with the proposal that got her the Advanced Investigators Grant, she wants to push attosecond physics into a new direction. “The idea is to create controlled sequences of pulses, and to use them to coherently control electronic processes.” The concept of coherent control stems from the chemistry community, says L’Huillier. “The idea of coherent control in chemistry is that you force chemical reactions to go in a certain direction.” In chemistry, this concept has been demonstrated to a certain extent. Instead of chemical reactions, though, L’Huillier would like to control electronic processes. “You have to go to the time and energy scales relevant for these electronic processes, which means higher photon energy and shorter timescales: we need XUV light and attosecond pulses.” Later steps in her project would involve more intense pulses, in order to get into the nonlinear regime. The last step would be to achieve both spatial and temporal resolution of the pulses, such that the pulse trains can be used to study and control electronic processes in more complex systems.

The research group of L’Huillier in Lund was part of the FOSCIL Joint Research Activity (JRA), which ends at the end of this year. Part of the research conducted within FOSCIL will be continued within the ALADIN JRA. L’Huillier says she is very happy to be in Laserlab Europe and the JRA’s. “Apart from the funding that comes with it, the importance of the network lies in collaborations that are stimulated by Laserlab Europe. I think this type of research is strong in Europe because of the networking that exists due to Laserlab Europe and the Marie Curie network. We are performing difficult experiments, and often we do this in collaboration with other groups. That is very useful and efficient.” Over the years her group has cooperated with groups from different Laserlab Europe partners; such as the Polytechnic in Milan, MPQ Garching, and LOA in Palaiseau, as well as with several groups outside Laserlab. One of those collaborations was actually described in the February 2007 issue of Laserlab Forum (p.10/11). About this research L’Huillier says that it can be seen as a first step into the direction that she wants to go using the Advanced Investigators Grant of 2.25 million euros. The grant money will be mainly used to hire people, she says. “It is hard to get funding for postdocs and PhD students here in Sweden.” She is actively seeking candidates at the moment.

Laser Accelerators

Professor Victor Malka from the Laboratoire d’Optique Appliquée (LOA) in Palaiseau near Paris received his Advanced Investigators Grant for a proposal appropriately called PARIS (PARticle accelerators with Intense lasers for Science). In the coming five years he will use the money to develop compact particle accelerators based on the wakefield of laser pulses. Shining a laser into a plasma creates an electric field that can be used to accelerate an electron beam. Using this method, one can produce electric fields that are up to 10,000 times larger than those used in conventional particle accelerators. This means one can create accelerators that are much more compact than accelerators based on other techniques.

Malka explains the mechanism: “The electric field of a laser pulse pushes the electrons away from the path of the beam, separating the electrons from the much heavier ions. This creates a travelling electric field in the wake of the laser pulse with which electrons can be accelerated to relativistic energies.” It is a very promising technique, according to Malka: “The last few years we have been able to improve this approach a lot and therefore the quality of the electron beam we can produce. We are now able to control both the energy and total charge in the electron beam.” According to Malka the money will be used to build a better facility for the project. “For the coming years, we have two main objectives. First we want to improve on the parameters of our beams, that is very fundamental. And second, which I think was quite convincing for the reviewers, we want to work on several important applications.” The electron beam could be coupled into an undulator, a device in which the electrons are forced into a curved trajectory in such a way that high-energy photons are produced. Another application is for cancer treatment: “One can use high-intensity electron pulses to destroy the cancer cells, which can be used to improve the tumours treatment”, says Malka. Finally, there are applications in material science. “Our beams could be used for nondestructive inspection of materials.”

Malka is the coordinator of the new LAPTECH Joint Research Activity, in which about ten labs take part. “In the next few years we want to explore the ‘bubble regime’ in plasma accelerators, which has been done, but not yet with lasers with optimal parameters. Several labs in Europe operate lasers with various different parameters, and together we want to explore this regime. There is not much money involved in this JRA, but that is of lesser importance: it is enough to do the experiments and especially to coordinate the collaboration between the labs involved. It is a well-focused activity on a hot topic. I think this JRA will be a useful instrument to focus on the use of the powerful lasers that are being built in the European laser labs.”

Aiming to control attosecond electronic processes

Prof. Dr. Mauro Nisoli from the Politecnico di Milano was awarded an Advanced Investigators Grant from the European Research Council for 'exceptional established research leaders' last year. In winning this grant of 2.44 million euros, he joins three researchers from LASERLAB-EUROPE who received that same grant in the first round. Nisoli is planning to take attosecond laser physics to the next level.

“In the past few years I have been working on isolated attosecond pulses”, says Nisoli. “In 2006 we were the first to demonstrate a complete temporal characterization of isolated attosecond pulses with durations down to 130 attoseconds.” Since that time he has been involved in a large collaboration with groups from Lund, MPQ Garching and AMOLF Amsterdam. “The experiments have been performed for over a year here in Milan with our attosecond setup. We have studied ultrafast electronic processes in helium atoms and deuterium molecules. The latter was the first example of a molecular attosecond pump-probe experiment.” There are also theorists involved, says Nisoli, due to complexity of the physical processes involved in the attosecond measurements. Nisoli will use the European grant to try and make attosecond pulses with higher energy. “At the moment, attosecond experiments are mainly limited by the energy of the pulses.”

This is due to the fact that the pulses are generated in high-order harmonics generation processes, which are rather inefficient. “The conversion efficiency for those processes is about 10-6 or 10-5, and therefore the isolated attosecond pulses we obtain have too little energy to perform, for example, pump-probe experiments using two attosecond pulses.” This is why Nisoli has so far performed pumpprobe measurements using one attosecond, and one femtosecond pulse. “That still gives you an attosecond resolution, so it is fine for studying certain classes of processes, but there are also a lot of experiments for which you really need two attosecond pulses.” With two highenergy attosecond pulses, one can also study non-linear effects and try to coherently control electronic processes: “One can excite an electron with the first attosecond pulse and then use the second pulse to control it.”

One can use the laser pulses to influence chemical reactions, as is already being done with femtosecond pulses; or to study electron migration in biomolecules involved in photosynthesis. “There are many interesting applications of attosecond laser pulses”, says Nisoli, “one could also think of studying damaging processes in DNA.” The Italian will build a second attosecond beamline in a brand new lab, and plans to study simple molecules once that laser is in operation. Later on, he hopes to move on to more complex molecules. “We will buy a new high-energy femtosecond laser system, and I am currently looking for postdocs and PhD-students. We are planning a really large facility for attosecond science in Milan.”

X-ray Spectroscopy

The Chemical Physics group in the Lund Laser Centre, led by Professor Villy Sundström, focuses on research related to sunlight: they look for new materials for solar cells, and are trying to mimic natural photosynthetic systems in order to produce fuel – such as molecular hydrogen – with sunlight. They also investigate the harmful effect that sunlight has on human skin. Sundström says this research involves studying both the structural and the electronic dynamics of the reactions. The Advanced Investigators Grant will be used to develop a table-top setup in which sub-picosecond x-ray pulses are employed to probe the dynamics of reactions. “The x-rays will be produced by shooting intense femtosecond laser pulses onto a liquid water target. This has as an advantage over solid targets that the shot-to-shot variations are much smaller. In this way we can generate x-rays with energies from approximately two keV to about fifteen keV, the range in which many elements have their core absorptions. We have recently installed a kHz, several mJ laser system that produces a much higher flux of x-rays than the 10 Hz system that we used up to now.”

In order to do spectroscopy, one has of course to be able to determine the energy of the x-ray photons. Since these are generated in all directions, there are too little x-rays to record a spectrum using a conventional crystal monochromator. One would therefore like to measure the energy of single photons coming from the reaction volume. Sundström: “Direct detection of x-rays can be done with CCD cameras but these have a limited energy resolution. We will use the grant to develop a detection system based on ‘transition edge’ detectors using superconducting material. We will cool this detector to just below the critical temperature where the material is superconducting. Now if an x-ray photon is absorbed, the temperature is increased and the material will go into a nonsuperconducting state, and from the change in resistive properties of the material we can deduce the energy of the x-ray photon with a resolution of a few eV. This kind of detector has been utilized in the development of imaging spectrometers for astrophysical observations and in spectrometers for materials analysis applications, but not at all for this kind of spectroscopy. We are very excited to try it.”

The new detection technique will first be applied to simple chemical reactions such as photodissociation, and later to the more complex systems that are studied in Sunström’s group, such as metal-organic complexes for solar cells and multichromophoric complexes for artificial photosynthesis. “Some of the metal-organic molecules we study do not have a good response in the visible part of the spectrum, but we can still study, for example, the transfer of electrons in these complexes with x-ray spectroscopy.” An inner shell electron that is kicked out of an atom scatters on surrounding atoms and causes interference between electron waves, which gives rise to modulated x-ray absorption. The modulation pattern carries information about the nature of surrounding atoms and their distances, and careful analysis of the temporal evolution of the energy spectrum provides information on the conformational dynamics of the molecule. “This type of spectroscopy is sensitive to the local structure of molecules. This is really different from the information you get from x-ray diffraction experiments, where one resolves global structures and you need crystalline samples. For reactions it is often the local structure in the vicinity of a ‘reaction centre’ that is of interest.”