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- ItemDynamical studies on the generation of periodic surface structures by femtosecond laser pulses(Les Ulis : EDP Sciences, 2013) Rosenfeld, A.; Höhm, S.; Bonse, J.; Krüger, J.The dynamics of the formation of laser-induced periodic surface structures (LIPSS) on fused silica upon irradiation with linearly polarized fs-laser pulses (50 fs pulse duration, 800 nm center wavelength) is studied experimentally using a double pulse experiment with cross polarized pulse sequences and a trans illumination femtosecond time-resolved (0.1 ps - 1 ns) pump-probe diffraction approach. The results in both experiments confirm the importance of the ultrafast energy deposition and the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.
- ItemNonlinear optical mechanism of forming periodical nanostructures in large bandgap dielectrics(Les Ulis : EDP Sciences, 2013) Grunwald, R.; Das, S.K.; Debroy, A.; McGlynn, E.; Messaoudi, H.Nonlinear excitation mechanisms of plasmons and their influence on femtosecond-laser induced sub-wavelength ripple generation on dielectric and semiconducting transparent materials are discussed. The agreement of theoretical and experimental data indicates the relevance of the model.
- ItemPreparation of clay mineral samples for high resolution x-ray imaging(Bristol : Institute of Physics Publishing, 2013) Abbati, G.; Seim, C.; Legall, H.; Stiel, H.; Thomas, N.; Wilhein, T.In the development of optimum ceramic materials for plastic forming, it is of fundamental importance to gain insight into the compositions of the clay minerals. Whereas spectroscopic methods are adequate for determining the elemental composition of a given sample, a knowledge of the spatial composition, together with the shape and size of the particles leads to further, valuable insight. This requires an imaging technique such as high resolution X-ray microscopy. In addition, fluorescence spectroscopy provides a viable element mapping technique. Since the fine particle fraction of the materials has a major effect on physical properties like plasticity, the analysis is focused mainly on the smallest particles. To separate these from the bigger agglomerates, the raw material has to pass through several procedures like centrifugation and filtering. After that, one has to deposit a layer of appropriate thickness on to a suitable substrate. These preparative techniques are described here, starting from the clay mineral raw materials and proceeding through to samples that are ready to analyze. First results using high resolution x-ray imaging are shown.
- ItemPhotoelectron holography in strong optical and dc electric fields(Bristol : Institute of Physics Publishing, 2014) Stodolna, A.; Huismans, Y.; Rouzée, A.; Lépine, F.; Vrakking, M.J.J.The application of velocity map imaging for the detection of photoelectrons resulting from atomic or molecular ionization allows the observation of interferometric, and in some cases holographic structures that contain detailed information on the target from which the photoelecrons are extracted. In this contribution we present three recent examples of the use of photoelectron velocity map imaging in experiments where atoms are exposed to strong optical and dc electric fields. We discuss (i) observations of the nodal structure of Stark states of hydrogen measured in a dc electric field, (ii) mid-infrared strong-field ionization of metastable Xe atoms and (iii) the reconstruction of helium electronic wavepackets in an attosecond pump-probe experiment. In each case, the interference between direct and indirect electron pathways, reminiscent of the reference and signal waves in holography, is seen to play an important role.
- ItemExperimental strategies for optical pump - Soft x-ray probe experiments at the LCLS(Bristol : Institute of Physics Publishing, 2014) McFarland, B.K.; Berrah, N.; Bostedt, C.; Bozek, J.; Bucksbaum, P.H.; Castagna, J.C.; Coffee, R.N.; Cryan, J.P.; Fang, L.; Farrell, J.P.; Feifel, R.; Gaffney, K.J.; Glownia, J.M.; Martinez, T.J.; Miyabe, S.; Mucke, M.; Murphy, B.; Natan, A.; Osipov, T.; Petrovic, V.S.; Schorb, S.; Schultz, T.; Spector, L.S.; Swiggers, M.; Tarantelli, F.; Tenney, I.; Wang, S.; White, J.L.; White, W.; Gühr, M.Free electron laser (FEL) based x-ray sources show great promise for use in ultrafast molecular studies due to the short pulse durations and site/element sensitivity in this spectral range. However, the self amplified spontaneous emission (SASE) process mostly used in FELs is intrinsically noisy resulting in highly fluctuating beam parameters. Additionally timing synchronization of optical and FEL sources adds delay jitter in pump-probe experiments. We show how we mitigate the effects of source noise for the case of ultrafast molecular spectroscopy of the nucleobase thymine. Using binning and resorting techniques allows us to increase time and spectral resolution. In addition, choosing observables independent of noisy beam parameters enhances the signal fidelity.
- ItemA compact laboratory transmission X-ray microscope for the water window(Bristol : Institute of Physics Publishing, 2013) Legall, H.; Stiel, H.; Blobel, G.; Seim, C.; Baumann, J.; Yulin, S.; Esser, D.; Hoefer, M.; Wiesemann, U.; Wirtz, M.; Schneider, G.; Rehbein, S.; Hertz, H.M.In the water window (2.2-4.4 nm) the attenuation of radiation in water is significantly smaller than in organic material. Therefore, intact biological specimen (e.g. cells) can be investigated in their natural environment. In order to make this technique accessible to users in a laboratory environment a Full-Field Laboratory Transmission X-ray Microscope (L-TXM) has been developed. The L-TXM is operated with a nitrogen laser plasma source employing an InnoSlab high power laser system for plasma generation. For microscopy the Ly α emission of highly ionized nitrogen at 2.48 nm is used. A laser plasma brightness of 5 × 1011 photons/(s × sr × μm2 in line at 2.48 nm) at a laser power of 70 W is demonstrated. In combination with a state-of-the-art Cr/V multilayer condenser mirror the sample is illuminated with 106 photons/(μm2 × s). Using objective zone plates 35-40 nm lines can be resolved with exposure times < 60 s. The exposure time can be further reduced to 20 s by the use of new multilayer condenser optics and operating the laser at its full power of 130 W. These exposure times enable cryo tomography in a laboratory environment.
- ItemSetup of an 8 keV laboratory transmission x-ray microscope(Bristol : Institute of Physics Publishing, 2014) Baumbach, S.; Kanngießer, B.; Malzer, W.; Stiel, H.; Bjeoumikhova, S.; Wilhein, T.This article presents a concept and the first results for the setup of an 8keV laboratory transmission x-ray microscope with a polycapillary optic as condenser at the BliX in Berlin. The incentive of building such a microscope is that the penetration depth for hard x-rays is much higher than in the soft x-ray range, e.g. The water window. Therefore, it is possible to investigate even dense materials such as metal compounds, bones or geological samples. The future aim is to achieve a spatial resolution better than 200 nm.
- ItemFormation dynamics of ultra-short laser induced micro-dots in the bulk of transparent materials(Amsterdam [u.a.] : Elsevier, 2013) Mermillod-Blondin, A.; Ashkenasi, D.; Lemke, A.; Schwagmeier, M.; Rosenfeld, A.In this paper, we study the formation dynamics of ultra-short laser-induced micro dots under the surface of transparent materials. Laser-induced micro dots find their application in direct part marking, to address full life cycle traceability. We first demonstrate the possibility of direct laser part marking into the cladding of an optical fiber. Then, we monitor the laser affected zone with the help of a time-resolved phase contrast microscopy setup in a fused silica substrate. We show that the transient energy relaxation processes affect the host material over a region that exceeds the micro dot size by several micrometers.
- ItemOptimization of the energy deposition in glasses with temporally-shaped femtosecond laser pulses(Amsterdam [u.a.] : Elsevier, 2011) Mauclair, C.; Mishchik, K.; Mermillod-Blondin, A.; Rosenfeld, A.; Hertel, I.V.; Audouard, E.; Stoian, R.Bulk machining of glasses with femtosecond laser pulses enables the fabrication of embedded optical functions. Due to the nonlinear character of the laser-matter interaction, structural modifications can occur within the focal region. To reach a full control of the process, ways of controlling the deposition of the laser energy inside the material have to be unveiled. From static and time-resolved pictures of bulk-excitation of a-SiO2 and borosilicate glass, we show that particular laser temporal shapes such as picosecond sequences can better confine the energy deposition than the femtosecond sequence by reducing the propagation artifacts.
- ItemTransition from ballistic to drift motion in high-field transport in GaAs(Les Ulis : EDP Sciences, 2013) Bowlan, P.; Kuehn, W.; Reimann, K.; Woerner, M.; Elsaesser, T.; Hey, R.; Flytzanis, C.With strong THz pulses, we measure ultrafast transport of electrons, holes, and an electron-hole plasma in GaAs. The transition from ballistic to drift-like transport is strongly influenced by electron-hole scattering.