Structural modification of solids by ultra-short X-ray laser pulses

The motivation behind this Ph.D. project is to determine the structural modification of solids by ultra-short X-ray laser pulses. This Ph.D. project focuses on determining the amorphous carbon (a-C) as a potential coating on the mirrors of the soft X-ray beamline of the European X-ray Free Electron Laser (XFEL) in Hamburg, in particular. Furthermore, chemical vapor deposition (CVD) diamond used in the monochromators for X-ray beamlines of European XFELneeds to be examined. Among high Z materials Nickel (Ni), MoB4C (multi-layer), are studied at 269 eV photon energy. The focus was on testing the behavior of a-C coated mirrors and the CVD diamond monochromators which are the main subject in the performed experiments.XFEL deliver high peak brilliance, high power, femtosecond focused laser pulses. Optical elements in these facilities are of crucial importance as they should distribute the beam with high quality and survive the intense conditions. Hence, understanding the interplay between the X-ray FEL pulses with coatings on the mirrors as well as single crystal monochromators is important. By means of this project it becomes evident that from the fundamental aspect, different mechanisms are involved in the damage process at different time scales. In the early femtosecond (fs) time zone, the photo-ionization is the main mechanism governing the damage process. During this time the material density changes. The system tends to reach its energetically stable potential state (a-C turns into graphite). In the picosecond (ps) time scale, secondary processes initiate. Among those, one can mention Auger, impact ionization, tunnel ionization, carrier diffusion followed by free carriers interaction with the lattice e.g. electron-phonon coupling, etc. The heat diffusion process starts to take place after some 100 ps, which continues until the system returns to room temperature after some μs(7μs). The analysis of the damage process can be divided into three main phases; based on the different time zone named above. The combination of heat diffusion and secondary processes cause a nonlinear increase in the size of the damage spots on the logarithmic axis dependingon the pulse energy. The photo-ionization (non-thermal) damage threshold is determined from experiments performed at different FEL facilities on different photon energies.From heat diffusion simulation via COMSOL (software package based on advanced numerical methods), one can extract the melting energy threshold for each material at different photon energies. To gain a deeper knowledge on the damage process within the scope of this project, several investigations such as Atomic Force Microscopy (AFM), Raman spectroscopy, photoemission spectroscopy, and theoretical simulation via Hybrid XTANT code were conducted based on the subjected samples

Soft x-ray free-electron laser induced damage to inorganic scintillators

An irreversible response of inorganic scintillators to intense soft x-ray laser radiation was investigated at the FLASH (Free-electron LASer in Hamburg) facility. Three ionic crystals, namely, Ce:YAG (cerium-doped yttrium aluminum garnet), PbWO4 (lead tungstate), and ZnO (zinc oxide), were exposed to single 4.6 nm ultra-short laser pulses of variable pulse energy (up to 12 μJ) under normal incidence conditions with tight focus. Damaged areas produced with various levels of pulse fluences, were analyzed on the surface of irradiated samples using differential interference contrast (DIC) and atomic force microscopy (AFM). The effective beam area of 22.2 ± 2.2 μm2 was determined by means of the ablation imprints method with the use of poly(methyl methacrylate) - PMMA. Applied to the three inorganic materials, this procedure gave almost the same values of an effective area. The single-shot damage threshold fluence was determined for each of these inorganic materials. The Ce:YAG sample seems to be the most radiation resistant under the given irradiation conditions, its damage threshold was determined to be as high as 660.8 ± 71.2 mJ/cm2. Contrary to that, the PbWO4 sample exhibited the lowest radiation resistance with a threshold fluence of 62.6 ± 11.9 mJ/cm2. The threshold for ZnO was found to be 167.8 ± 30.8 mJ/cm2. Both interaction and material characteristics responsible for the damage threshold difference are discussed in the article

Investigating the interaction of x-ray free electron laser radiation with grating structure

The interaction of free electron laser pulses with grating structure is investigated using 4.6±0.1 nm radiation at the FLASH facility in Hamburg. For fluences above 63.7±8.7 mJ/cm2, the interaction triggers a damage process starting at the edge of the grating structure as evidenced by optical and atomic force microscopy. Simulations based on solution of the Helmholtz equation demonstrate an enhancement of the electric field intensity distribution at the edge of the grating structure. A procedure is finally deduced to evaluate damage threshold