|Prospects of Stimulated X-ray Raman Scattering with Free-Electron Laser Sources
XFELs might open the pathway to transfer non-linear spectroscopic techniques to the x-ray domain, to study electron motion at unprecedented time and length scales. A promising x-ray pump probe technique is based on stimulated electronic x-ray Raman scattering. I will present the first experimental demonstration of stimulated electronic x-ray Raman scattering in a gas sample of neon*. Despite the limited spectral coherence of SASE XFELs, high-resolution spectra can be obtained by statistical methods, opening the path to coherent stimulated x-ray Raman spectroscopy. An extension of these ideas to molecules** and the results of a recent experiment in CO will be discussed. The high-gain regime, involving exponential amplification and strong-field effects will be contrasted to stimulated scattering at moderate x-ray intensities, more appropriate for spectroscopic studies. A critically assessment of the feasibility of nonlinear x-ray spectroscopic techniques and requirements on the stability and pulse parameters of XFEL sources that could enable these new techniques, will be presented.
* C. Weninger et al., Phys. Rev. Lett. 111, 233902 (2013)
** C. Weninger and N. Rohringer, Phys Rev A 88, 053421 (2013)
|Slides FRA01 [3.771 MB]
|Wave-Mixing Experiments with Multi-colour Seeded FEL Pulses
|The extension of wave-mixing experiments in the extreme ultraviolet (EUV) and x-ray spectral range represents one of the major breakthroughs for ultrafast x-ray science. Essential prerequisites to develop such kind of non-linear coherent methods are the strength of the input fields, comparable with the atomic field one, as well as the high temporal coherence and stability of the photon source(s). These characteristics are easily achievable by optical lasers. Seeded free-electron-lasers (FELs) are similar in many respects to conventional lasers, hence calling for the development of wave-mixing methods. At the FERMI seeded FEL facility this ambitious task is tackled by the TIMER project, which includes the realization of a dedicated experimental end-station. The wave-mixing approach will be initially used to study collective atomic dynamics in disordered systems and nanostructures, through transient grating (TG) experiments. However, the wavelength and polarization tunability of FERMI, as well as the possibility to radiate multi-colour seeded FEL pulses, would allow to expand the range of possible scientific applications.
|Slides FRA02 [7.731 MB]
|The Creation of Large-Volume, Gradient-Free Warm Dense Matter with an X-Ray Free-Electron Laser
|We report on an experiment performed using the hard x-ray beamline (X-ray Pump Probe-XPP) at the Stanford Linac Coherent Light Source (LCLS) free electron laser adapted to the study of high-pressure high-energy density states. This warm dense matter regime, which is barely described by present-day theoretical models, is poorly understood due to the difficulty of achieving these conditions in a manner that allows accurate diagnosis. The development of free electron lasers opens a unique opportunity to generate this regime in laboratory allowing one to efficiently and uniformly heat the matter up to 10 eV within less than 100 fs. In this context, we irradiated thin Ag foils with a 9 keV x-ray beam of 60 fs duration and an irradiance approaching 1016 W/cm2. The temporal evolution of the sample was monitored with two time-and-space resolved interferometry diagnostics measuring the phase of an optical laser beam reflected from the front and back of the sample. This measurement had provided crucial information on the heating uniformity and on the achievable temperature. These conclusions have been obtained by means of a precise modelling of this regime of interaction.
|Slides FRA03 [2.181 MB]
|Optimization of High Average Power FEL Beam for EUV Lithography Application
Extreme Ultraviolet Lithography (EUVL) is realized with 100W plasma EUV source at 13.5nm. It is recommended by the EUVL community to evaluate an alternative approach based on high repetition rate FEL, to avoid the power limit of the plasma source. Several papers discuss on the possibility to realize superconducting FEL to generate multiple kW 13.5nm light. We must notice that the present SASE FEL pulse has higher beam fluence than the resist ablation threshold*, and high spatial coherence which results in speckle patterns, and random longitudinal mode beat which leads to high peak powerμspikes. An expanding mirror is installed after the undulator to reduce the beam fluence, external-seeding configuration is employed to reduce the longitudinal mode beat, and total reflection beam homogenizer is used for spatial mode mixing. Pulse repetition rate is more than 3MHz to cancel the speckle patter formation by averaging illumination. This paper discusses on the lowest risk approach to construct a prototype to demonstrate a high average power 13.5nm FEL for the best optimization in EUVL application, including the scaling to 6.7nm wavelength.
*J. Chalupský, L. Juha et.al, “Characteristics of focused soft X-ray free-electron laser beam determined by ablation of organic molecular solids”, OPTICS EXPRESS 15, 6036 (2007)
|Slides FRA04 [1.413 MB]