ELI-ALPS is unique – the combination of numerous synchronized ultrashort high intensity laser pulses with cutting-edge secondary sources and diagnostics will create an outstanding environment for fundamental and applied scientific research. These are just a few perceived areas where ELI-ALPS will be outstanding.
Valence and Core electron science
Electronic motion in atomic/molecular systems occurs on the attosecond timescale with nuclear motion being much slower, around the femtosecond to picosecond time scales. Attosecond pulse interactions with matter opens the window for the study of electronic processes where the response of valence electrons can be individually monitored and controlled. These pulses can initiate and probe/steer ultrafast electronic responses in atoms and molecules and as well as trigger, track and control charge oscillation/migration in extended molecules. In addition to the complete control over an electronic process, these ultrafast spectroscopic tools also allow monitoring and control coupled electronic-nuclear motion. Novel spectroscopy techniques related to attosecond pulse - matter interactions include the insitu diffraction of an emitted photoelectron with a wavelength comparable to the interatomic distances within the probed species. The diffraction occurs at scattering centres of the electron emitting molecule and thus, reveals structural information.
The majority of valence electron science experiments require isolated attosecond laser pulses. These pulse will be generated during primary phase of the facility development by using combinations of polarization gating and/or two-colour harmonic generation methods. The second development phase will result in an enhancement of pulse energy and reduced pulse duration of the high repetition rate lasers. This will greatly facilitate and increase the intensity of the isolated attosecond laser pulses that can be generated and significantly extend the range of scientific problems that can be investigated. These isolated attosecond pulses are used as the probe in one or two colour pump-probe experiments with a wide range of pump light, generated by other primary/secondary sources. Many these experiments will use the HR source as the high repetition rate allows for sophisticated coincidence detection methods and in conjunction with low pulse energies, negates space charge issues – an important bottleneck in surface experiments. The intensity of these sources will be limited so pump-probe will be through the envelope of the XUV pulses and the phase of the driving IR field.
Initial valence electronic studies have so far focused on atomic species, particularly Noble gases where the target dynamics have looked at decaying autoionizing or Auger decaying states as well as wave-packet dynamics of coherent superposition of electronic states. The short lifetime of these processes require attosecond probing. Photoelectron streaking was the main experimental technique that determined Auger lifetimes of Kr in the first-ever attosecond pump-probe experiment. Other experiments have addressed coupled electron and nuclear motion in molecular species. In photoelectron streaking, the velocity change of an ejected electron in a co-propagating IR laser field allows the determination of the entry of electron into the laser field with a time-resolution that is a small fraction of the duration of the optical cycle of the IR field (2.7 fs). Using this technique, a very small (21 attosecond) delay between the ejection of Ne 2s and 2p electrons upon ionization by a 100 eV attosecond laser pulses can be detected.
The delay between the emission of 2s and 2p electron in XUV photo-emission of Ne, measured by attosecond streaking
These type of experiments which will use the HR system will greatly benefit from the increased statistics arising from the laser’s high repetition rate and the possibility for detailed studies of correlated systems through coincidence experiments and imaging techniques.
The SYLOS beamline will offer high intensity attosecond pulse with a high repetition rate suitable for the realization of attosecond resolution XUV-pump XUV-probe experiments for both atomic and molecular systems. Traditionally, chemical processes are described using the Born-Oppenheimer approximation that states that electronic states (orbitals, wavefunctions) adiabatically follow the time-evolving structure of a molecule. The breakdown of the Born-Oppenheimer approximation occurs at curve crossings, where electronic and nuclear timescales become comparable to each other. Initial attosciences experiment can be viewed as examples where electron dynamics has been observed in “post Born-Oppenheimer regime”. Upon instantaneous (attosecond) photo-excitation, a regime can be reached where time-dependent electronic motion is enabled without the participation of the nuclei or preceding this participation. Under these conditions the electron motion can control the nuclear motion, setting the state for “charge-directed reactivity” – a novel paradigm in chemistry. The ELI-ALPS facility is ideally suited for studies of charge-directed reactivity as the huge flexibility in the choice of the parameters of the attosecond and primary HR-sources allows the design of experiments that can explore the existing predictions for attosecond to few-femtosecond time-scale electron transfer in extended systems upon attosecond photo-excitation/ionization. Furthermore, the ability to synthesize pulse shapes of almost arbitrary complexity allows the exploration of electron transfer control and thus the outcome of a chemical reaction – a long-standing goal in molecular photochemistry.
The foreseen unique attosecond beam parameters at wavelengths ranging from the XUV to the x-ray spectral region are ideal for novel core electron experiments and the short wavelengths that will be available will be able to access Inner-shell electrons. In the soft x-ray domain, ionization mainly occurs from the removal of electrons from within an atom in contrast to outer-shell peeling at in the IR-VUV wavelengths. This conventional physics has been extensively studied, e.g. at synchrotron installations, but the exceptional combination of different short pulse radiation sources available at ELI-ALPS will enable time-resolved studies of core electron dynamics at highest possible temporal resolution. Furthermore, the expected ultrashort pulse durations and XUV/x-ray intensity levels of ALPS hold for the first time promise for a breakthrough into the non-linear inner-shell processes era. One possible example of inner-shell research is time-resolved photoelectron diffraction and holography. A visible/UV pump pulse from the HR/SYLOS beam-lines can induce molecular dynamics including photo-dissociation, bond rearrangement, photolysis via Norish I,II type reactions, dynamics at conical intersections. An Xray probe pulse from the SYLOS-S2/H1 beam-lines ejects a core electron from the molecule with ~0.01-1 keV kinetic energy. This interacts with neighbouring atoms and produces a diffraction pattern on the imaging detector. This data depends on the photon energy, and, thus, the De Broglie wavelength of the photoelectron and can provide molecular structural and electronic potentials information. In some cases, a holographic reconstruction of the positions of the nuclei is possible and this is illustrated in the figure below published in F. Krasniqi et al., PRA 81 033411 (2010).
4D attosecond imagining
Three-dimensional arrays of nuclei and electrons define the structure and characteristics of all matter. The structure and time evolution of a change in material due to external excitations, e.g., illumination by light, can be recorded in space and time (4D). Electronic and nuclear processes happen on vastly different timescales and the combination of the light sources produced at the ELI-ALPS infrastructure will enable the development of 4D imaging particularly the visualization of ultrafast motion of atoms and electrons with unprecedented resolution in space and time (4D).
ELI-ALPS will advance 4D imaging into the electronic regime by measuring the scattering/diffraction of attosecond X-ray pulse in conjunction with a synchronized laser. Attosecond timing is critical. At the speed of light, 100 attoseconds correspond to a distance of 30 nm which places a hard limit on sample volumes – well above the majority of biologically and/or chemically interesting compounds.
Bond breakage is often triggered by a change in the molecular orbitals of a species. This can occur by light promoting a bonding electron into a repulsive state. ELI-ALPS’ attosecond/few-femtosecond x-ray pulses will enable the imaging of the electronic structure of a photo-excited molecule before photo-induced structural dynamics start. This imaging will reveal the role of orbital changes in the subsequent chemical pathways and will clarify what light pulses could steer chemical reactions by controlling electronic motions and the intermediate shapes of molecular charge densities. In many important cases like dissociation, proton transfer, charge migration, isomerization, and radiationless deactivation of the excited state, e.g. in DNA, conical intersections provide very efficient channels. Research on these processes requires the short wavelengths of the SYLOS driven secondary sources.
These techniques could also be used to explore the origin of refractive index and nonlinear optics; charge transfer in light sensitive materials and electromagnetic field flow in nanostructures – all which have numerous real-life applications in contemporary science and technology.
THz radiation based research
The THz spectral range bridges the gap between microwave and infrared radiation and has great scientific and technological interest for multidisciplinary science. THz technology already has several imagining applications in many industries including semiconductor fabrication, security and cultural heritage conservation.
One of the many unique features of ELI-ALPS will be the highest-intensity pulsed THz radiation which is preciously synchronised to the main laser sources. High-intensity ultrashort THz sources with unprecedented peak electric field strength up to 100 MV/cm and multi-mJ pulse energy will be available in the 0.1–10 THz frequency range. Traditional imaging experiments will also be complemented by brand-new fields of spectroscopic studies as well as manipulation of matter of various size including accelerated particles, molecules and nanostructures. New types of time-resolved techniques include THz pump -THz probe and THz pump-optical probe spectroscopy, where intense THz pulses initiate changes within the sample and further THz/optical pulses detect the resulting changes. One important application is the study of carrier dynamics in semiconductors. The study of structural changes in biological molecules by pulsed THz electron spin resonance (ESR) may be also possible. ELI-ALPS’ various sources, from X-ray to infrared wavelengths, combined with ultrashort pulse duration will enable an unprecedented variety of structural and dynamical studies.
Generation of THz radiation by charge transfer processes in a bacteriorhodopsin molecule. (G. I Groma at al., Proc. Nat. Acad. Sci. 105 (2008) 6888-6893).
New material engineering becomes possible by the Investigation and control of material properties and processes under the influence of extremely high quasi-static (THz) fields. New insight will be gained into the dynamical properties of molecules, clusters, nanostructures and bulk materials by investigating the physical, chemical and biological processes occurring under the influence of strong THz frequency external fields. For example, time-resolved studies of biomolecules in various conformational states will be enabled by combining THz fields with optical pulses. Controlling the pathways of chemical reactions and manipulation of materials may also become possible.
The unique combination of the radiation sources at ELI covering the electromagnetic spectrum from the X-ray to the far infrared make this facility very attractive for research on complex, applied systems.
One of the major application of the ultra-intense laser beams is the generation and subsequent acceleration of particles. These beams are formed by the tight focusing of an ultrashort laser pulse onto a nanometer thick film which is destroyed whilst generating accelerated particles. There are numerous different experimental regimes, each with different energy characteristic and spectra, that arise from different physical processes. Different particles beams can be formed by using different film materials.
The investigation of laser-driven proton acceleration and usage is currently challenging many research laboratories worldwide, with some significant results, in particular for the improved characteristics of laser based particle sources such as compactness, its efficiency its versatility and its tunability. The advantage of lasergenerated particle beams include high current, strong laminarity at the source, short duration, and small source side. Laser-driven proton acceleration using existing multi-hundred-TW table-top laser systems can generate on-target intensities of ~ 1019- 1020 W/cm² which can produce proton energies of ~15-20 MeV, with a typical laser-to-proton energy conversion efficiency of 1-6 % a current in the kA regime and the laminarity at the source 100 times better than conventional accelerators. Despite the technology being in its infancy, laser-driven particle streams offers a highly controlled low cost access to high energetic particles – previously only available at particle accelerators.
There are numerous applications particularly in biological and medical application but there will be two novel application at ELI-ALPS.
Cultural Heritage Diagnostics
The main challenge within the field of cultural heritage is to obtain the greatest amount of information without damaging artefacts. Another challenge is the conservation of artefacts without modifying its aesthetical appearance. Chemical and morphological information on artwork are mainly, obtained using surface spectroscopies or other methods based on particle accelerators and are most effective and sensible in laboratory. The classical techniques of diagnostics and conservation (restoration and consolidation) require, generally, to move the artworks from museum, or archaeological site, into a laboratory, or to make micro sampling. Chemical information on artworks (ceramic, bronzes, metals, pigments) is mainly obtained using surface spectroscopies (such as Photoluminescence, Raman, X-ray photoelectron spectroscopy (XPS), X-Ray-Fluorescence (XRF), Energy Dispersive X-ray Fluorescence (EDX)) in SEM), while morphological information can be gained with SEM. Complete chemistry of material bulks is known using sophisticated techniques of nuclear physics such as Proton Induced X-ray and Gamma Emission (PIXE and PIGE).
Scan made by conventional proton gun (energy ranging from 1 to 3 MeV) using a smaller spot size (order of microns) only allows analysis of a small region of paint and necessitates scanning the paint in hundreds of points to have complete information. This extends the analysis time and the possibility to damage the artefacts e.g., a larger accumulated dose, long time in nonprotective atmosphere. Moreover, the elemental depth profiles obtained by classical PIXE and PIGE have a maximum depth of 2 - 20 microns, which is often the thickness of surface patina, preventing the correct analysis of material bulk. The conventional methods for diagnostic on cultural heritage are not very tunable and adaptable and this limits their use to only a certain range of energies and to areas of a few microns.
Laser driven ion beams for PIXE and PIGE diagnostics can allow obtaining:
Complete chemical analysis on large area of artworks (order of cm2) opposed to smaller investigated surface of conventional accelerator (order of microns).
“Layer by Layer” analysis, obtained by tuning the beam energy from few MeV to tens of MeV. Conventional guns beam energy is in the order of few MeV resulting in sampling only the first micron of materials.
Small damage for artefacts. The short duration and high current of laser-driven beams ensure short duration of measurements and, thus small absorbed dose.
Nondestructive and multi-elemental analysis of trace elements with an excellent detection limit of up to 20 ppb.
Elemental composition of magnetic films. Other methods do not have an enough mass resolution to resolve Mn-Fe-Co-Ni elements.
Lattice location of impurities in single crystalline samples
However, no effort has currently been put to test the efficiency and usage of laser-driven proton acceleration for purposes of cultural heritage. At ELI-ALPS, laser-driven particle sources will be used for cultural heritage applications and results will be compared to those of conventional accelerators.
The enhancement and control of growing techniques for nano and nanostructured materials with well controlled properties e.g., crystallinity, shape, dimensions, band edge, is one of the current key challenges in nanoscience and nanotechnology. The definition of material properties and the setting of exact growth parameters is crucial for obtaining materials suitable for applications. The main problem in the definition of a standard growth protocol is the individuation of parameters to generate the conditions of temperature and pressure required to produce well-defined structures in very short timescales (ps-fs), necessary for the nucleation of particles with dimensions of tens of nm.
Ultra-intense high-energy short laser pulses, obtainable at ELI-ALPS, enable the production of multiMeV proton and ion beams with low divergence, short duration (<1 ps) and high current. These beams are shorter and higher in flux than conventional proton accelerators sources. While laser based proton accelerators for medical applications are under development, little has been done in order to use laser-generated protons for multidisciplinary applications (of industrial interest), such as material science or nanotechnology.
The irradiation of matter with a high-energy laser-generated proton beam opens the possibility of reproducing the temperature and pressure conditions for crystal growth in the very short time required for nanocrystal growth. As an example, the irradiation of a solid target of Al with a proton beam of 20 MeV, 10 protons and a pulse of few ps generates a heating at temperature of about a few eV in 20 ps; at this temperature, the matter is in a plasma state where the atoms can start to nucleate forming nanoparticles. Simulations show that the requested temperature and pressure ranges for growing nanomaterials can easily be achieved using laser-generated protons.
It is known that the proton beam irradiation is a powerful alternative for the synthesis of amorphous nanoparticles in solution: gold nanocrystals and nanorods were achieved through the proton beam irradiation (energy of about 20 MeV) of a solution containing gold ions. The heating of the solution and the very short heating time generated by the laser-driven proton beam irradiation can confine the nucleation time and block the amorphous aggregation. Laser based proton acceleration sources at ALPS contribute to novel nanostructure growth approaches.
High resolution nanometre imaging of biological material (organelles, cells, sub-cellular structures) under functional conditions (in-vitro or living) is a key technology to understand structure-function relationship in biological soft matter which includes cell metabolism, transportation across cell membranes, cell to cell communication. Current state-of-the art high resolution electron or advanced confocal light microscopy is able to image biomaterial with a ~1 nm spatial resolution but requires extensive cryofixation and/or samples staining all of which may alter and contaminate the sample. Soft X-ray microscopy can be used, as a complementary technique, in the water-window spectral range (2.4 - 4.3 nm) and in conjunction with light microscopy in a dual-mode instrument. This allows for in-vitro imaging of unstained but typically cryogenically cooled samples without creating staining artefacts.
Sub-angstrom spatially resolved structural investigation of biological macro-molecules is important in proteomics and pharmaceutical science as the structure-function relationship of proteins and enzymes is fundamental in understanding the biochemical nature of diseases. This is critical in new drug discovery and development by the pharmaceutical industry. Hard X-ray crystallography of proteins has become a powerful tool for structural investigation of macromolecular crystals and recently, coherent x-ray diffraction techniques, in conjunction with oversampling refinement algorithms, have been applied to the structural investigation of non-crystallized macromolecules. These new coherent diffractive imaging technologies are extremely useful for structural investigations of membrane molecules which are hard to crystallize and thus only few membrane protein structures have been determined from protein crystallography data.
Single molecule imaging requires an ultra-brilliant, short pulse X-ray source in order to obtain sufficient structural information from a protein before it disintegrates due the Coulomb explosion induced by the highly ionizing X-ray pulse. Single molecule imaging could advance structural biology by determining structures of medically relevant proteins which do not crystalize.
THz radiation applications
Intense THz sources can combine THz time-domain spectroscopy with imaging techniques. Large samples could be studied using 2D electro-optic sampling, without time-consuming scanning over the sample surface. Multispectral single-shot imaging is an new non-destructive testing tool and other applications including recording the spatial patterns of various chemicals. Intense THz radiation can also be used for security screening and biomedical applications, where the analysis of the chemical composition of the test object is also important as well as visualizing the geometrical shapes of its internal structures. Many materials have a THz spectral fingerprints and spectroscopic analysis in this frequency domain could be a new tool for material characterization.
A femtosecond laser-based X-ray source offering phase-contrast tomography will allow for an easy imaging and detection of tumour growth. The implementation of image-guided radiotherapy based on novel, diagnostic X-ray sources will become possible.
The understanding and control of biological processes can greatly benefit using ultrafast spectroscopy techniques such a pump-probe X-ray absorption spectroscopy, XUV-pump XUV/visible/NIR probe spectroscopy, attosecond pump-probe spectroscopy, Xray microscopy, time-resolved Raman and IR spectroscopy. The advance laser and secondary sources at ELI-ALPS provide optimal features for the implementation of these techniques.
Proton and ion beams provide the highest physical selectivity and millimetre accuracy in radiation dose delivery. The biological effectiveness of heavy charged particles could be 2-4 times higher than photons and electrons. The laser-plasma based particle source is considered by many for improved cancer treatment. There is ongoing research on laser ion acceleration targets, improved operation environment and parameters, reduced size and reduced construction and operational costs in the future. The HF beamline with advanced, high-repetition-rate laser technologies, has the potential for the development of an advanced laser-based compact ion accelerator for medical applications.
Additionally, the mid-infrared spectral region emitted by the MIR OPCPA primary source is well suited to a large number of medical applications. Bio-molecules have specific absorption lines in the 3-15 μm region and so a well-tuned laser can selectively excite and so detect or destroy molecules.