Mo/Si multilayer-coated amplitude division beam splitters for XUV radiation sources

Amplitude-division beam splitters for XUV radiation sources have been developed and extensively characterized. Mo/Si multilayer coatings were deposited on 50 nm-thick SiN membranes. By changing the multilayer structure (periodicity, number of bilayers, etc.) the intensity of the reflected and transmitted beams were optimized for selected incident radiation parameters (wavelength, incident angle). The developed optical elements were characterized by means of XUV reflectometry and transmission measurements, atomic force microscopy and optical interferometry. Special attention was paid to the spatial homogeneity of the optical response and reflected beam wavefront distortions. Here the results of the characterization are presented and improvements required for advanced applications at XUV free-electron lasers are identified. A flatness as low as 4 nm r.m.s. on 3 × 3 mm beam splitters and 22 nm r.m.s. on 10 × 10 mm beam splitters has been obtained. The high-spatial-frequency surface roughness was about 0.7-1 nm r.m.s. The middle-spatial-frequency roughness was in the range 0.2-0.8 nm r.m.s. The reflection and transmission of the beam splitters were found to be very homogeneous, with a deviation of less than 2% across the full optical element

Study of ultrathin Pt/Co/Pt trilayers modified by nanosecond XUV pulses from laser-driven plasma source

We have studied the structural mechanisms responsible for the magnetic reorientation between in-plane and out-of-plane magnetization in the (25 nm Pt)/(3 and 10 nm Co)/(3 nm Pt) trilayer systems irradiated with nanosecond XUV pulses generated with laser-driven gas-puff target plasma source of a narrow continuous spectrum peaked at wavelength of 11 nm. The thickness of individual layers, their density, chemical composition and irradiation-induced lateral strain were deduced from symmetric and asymmetric X-ray diffraction (XRD) patterns, grazing-incidence X-ray reflectometry (GIXR), grazing incidence X-ray fluorescence (GIXRF), extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) measurements. In the as grown samples we found, that the Pt buffer layers are relaxed and that the layer interfaces are sharp. As a result of a quasi-uniform irradiation of the samples, the XRD, EXAFS, GIXR and GIXRF data reveal the formation of two distinct layers composed of Pt1-xCox alloys with different Co concentrations, dependent on the thickness of the as grown magnetic Co film but with similar ∼1% lateral tensile residual strain. For smaller exposure dose (lower number of accumulated pulses) only partial interdiffusion at the interfaces takes place with the formation of a tri-layer composed of Co-Pt alloy sandwiched between thinned Pt layers, as revealed by TEM. The structural modifications are accompanied by magnetization changes, evidenced by means of magneto-optical microscopy. The difference in magnetic properties of the irradiated samples can be related to their modification in Pt1-xCox alloy composition, as the other parameters (lateral strain and alloy thickness) remain almost unchanged. The out-of-plane magnetization observed for the sample with initially 3 nm Co layer can be due to a significant reduction of demagnetization factor resulting from a lower Co concentration

Contrasting behavior of covalent and molecular carbon allotropes exposed to extreme ultraviolet and soft x-ray free-electron laser radiation

All carbon materials, e.g., amorphous carbon (a-C) coatings and C60 fullerene thin films, play an important role in short-wavelength free-electron laser (FEL) research motivated by FEL optics development and prospective nanotechnology applications. Responses of a-C and C60 layers to the extreme ultraviolet (SPring-8 Compact SASE Source in Japan) and soft x-ray (free-electron laser in Hamburg) free-electron laser radiation are investigated by Raman spectroscopy, differential interference contrast, and atomic force microscopy. A remarkable difference in the behavior of covalent (a-C) and molecular (C60) carbonaceous solids is demonstrated under these irradiation conditions. Low thresholds for ablation of a fullerene crystal (estimated to be around 0.15 eV/atom for C60 vs 0.9 eV/atom for a-C in terms of the absorbed dose) are caused by a low cohesive energy of fullerene crystals. An efficient mechanism of the removal of intact C60 molecules from the irradiated crystal due to Coulomb repulsion of fullerene-cage cation radicals formed by the ionizing radiation is revealed by a detailed modeling

Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single shot ablation threshold

The process of damage accumulation in thin ruthenium films exposed to multiple femtosecond XUV free electron laser FEL pulses below the critical angle of reflectance at the Free electron LASer facility in Hamburg FLASH was experimentally analyzed. The multi shot damage threshold is found to be lower than single shot damage threshold. Detailed analysis of the damage morphology and its dependence on irradiation conditions justifies the assumption that cavitation induced by the FEL pulse is the prime mechanism responsible for multi shot damage in optical coating

Sub-nanometer-scale measurements of the interaction of ultrafast soft x-ray free-electron-laser pulses with matter

Femtosecond pulses from soft-x-ray free-electron lasers (FELs) [1] are ideal for directly probing matter at atomic length scales and timescales of atomic motion. An important component of understanding ultrafast phenomena of light-matter interactions is concerned with the onset of atomic motion which is impeded by the atoms inertia. This delay of structural changes will enable atomic-resolution flash-imaging [2-3] to be performed at upcoming x-ray FELs [4-5] with pulses intense enough to record the x-ray scattering from single molecules [6]. We explored this ultrafast high-intensity regime with the FLASH soft-x-ray FEL [7-8] by measuring the reflectance of nanostructured multilayer mirrors using pulses with fluences far in excess of the mirrors damage threshold. Even though the nanostructures were ultimately completely destroyed, we found that they maintained their integrity and reflectance characteristics during the 25-fs-long pulse, with no evidence for any structural changes during that time over lengths greater than 3 {angstrom}. In the recently built FLASH FEL [7], x-rays are produced from short electron pulses oscillating in a periodic magnet array, called an undulator, by the principle of self-amplification of spontaneous emission [9-10]. The laser quality of the x-ray pulses can be quantified by the peak spectral brilliance of the source, which is 10{sup 28} photons/(s mm2 mrad2 0.1% bandwidth) [8]; this is up to seven orders of magnitude higher than modern third-generation synchrotron sources. For our studies, the machine operated with pulses of 25 fs duration at a wavelength of 32.5 nm and energies up to 21 {micro}J. We focused these pulses to 3 x 10{sup 14} W/cm{sup 2} onto our nanostructured samples, resulting in an the unprecedented heating rate of 5 x 10{sup 18} K/s, while probing the irradiated structures at the nanometer length scale. The x-ray reflectivity of periodic nanometer-scale multilayers [11] is very sensitive to changes in the atomic positions and the refractive indices of the constituent materials, making them an ideal choice to study structural changes induced by ultrashort FEL pulses. The large multilayer reflectivity results from the cooperative interaction of the x-ray field with many layers of precisely fabricated thicknesses, each less than the x-ray wavelength. This Bragg or resonant reflection from the periodic structure is easily disrupted by any imperfection of the layers. The characteristics of the structure, such as periodicity or layer intermixing, can be precisely determined from the measurement of the Bragg reflectivity as a function of incidence angle. These parameters can be easily measured to a small fraction of the probe wavelength, as is well known in x-ray crystallography where average atomic positions of minerals or proteins are found to less than 0.01{angstrom}. Thus, we can explore ultrafast phenomena at length scales less than the wavelength, and investigate the conditions to overcome the effects of radiation damage by using ultrafast pulses

Characterization of megahertz X ray laser beams by multishot desorption imprints in PMMA

Proper diagnostics of intense free electron laser FEL X ray pulses is indisputably important for experimental data analysis as well as for the protection of beamline optical elements. New challenges for beam diagnostic methods are introduced by modern FEL facilities capable of delivering powerful pulses at megahertz MHz repetition rates. In this paper, we report the first characterization of a defocused MHz 13.5 nm beam generated by the free electron laser in Hamburg FLASH using the method of multi pulse desorption imprints in poly methyl methacrylate PMMA . The beam fluence profile is reconstructed in a novel and highly accurate way that takes into account the nonlinear response of material removal to total dose delivered by multiple pulses. The algorithm is applied to experimental data of single shot ablation imprints and multi shot desorption imprints at both low 10 Hz and high 1 MHz repetition rates. Reconstructed response functions show a great agreement with the theoretical desorption response function mode