M. Gensch

Single-shot pulse duration monitor for extreme ultraviolet and X-ray free-electron lasers

The resolution of ultrafast studies performed at extreme ultraviolet and X-ray free-electron lasers is still limited by shot-to-shot variations of the temporal pulse characteristics. Here we show a versatile single-shot temporal diagnostic tool that allows the determination of the extreme ultraviolet pulse duration and the relative arrival time with respect to an external pump-probe laser pulse. This method is based on time-resolved optical probing of the transient reflectivity change due to linear absorption of the extreme ultraviolet pulse within a solid material. In this work, we present measurements performed at the FLASH free-electron laser. We determine the pulse duration at two distinct wavelengths, yielding (184±14) fs at 41.5 nm and (21±19) fs at 5.5 nm. Furthermore, we demonstrate the feasibility to operate the tool as an online diagnostic by using a 20-nm-thin Si3N4 membrane as target. Our results are supported by detailed numerical and analytical investigations.

Analysis of Organic Films and Interfacial Layers by Infrared Spectroscopic Ellipsometry

Organic films and surfaces have been shown to be of increasing technological importance. The exploration of their potential for applications in fields such as nanotechnology or for the development of ‘‘smart materials’’ is the focus of numerous R&D projects. Appropriate characterization methods are mandatory for the design and analysis of devices on the basis of organic films. This focal point article focuses on presenting infrared spectroscopic ellipsometry (IRSE) as an emerging powerful technique for the structural analysis of organic films. IRSE has been successfully applied to the study of inorganic materials and films as, for

Photoinduced Melting of Antiferromagnetic Order in La0.5Sr1.5MnO4 Measured Using Ultrafast Resonant Soft X-Ray Diffraction

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.

Optical excitation of Josephson plasma solitons in a cuprate superconductor

Josephson plasma waves are linear electromagnetic modes that propagate along the planes of cuprate superconductors, sustained by interlayer tunnelling supercurrents. For strong electromagnetic fields, as the supercurrents approach the critical value, the electrodynamics become highly nonlinear. Josephson plasma solitons (JPSs) are breather excitations predicted in this regime, bound vortex-antivortex pairs that propagate coherently without dispersion. We experimentally demonstrate the excitation of a JPS in La1.84Sr0.16CuO4, using intense narrowband radiation from an infrared free-electron laser tuned to the 2-THz Josephson plasma resonance. The JPS becomes observable as it causes a transparency window in the opaque spectral region immediately below the plasma resonance. Optical control of magnetic-flux-carrying solitons may lead to new applications in terahertz-frequency plasmonics, in information storage and transport and in the manipulation of high-Tc superconductivity.

High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter

Ultrashort flashes of THz light with low photon energies of a few meV, but strong electric or magnetic field transients have recently been employed to prepare various fascinating nonequilibrium states in matter. Here we present a new class of sources based on superradiant enhancement of radiation from relativistic electron bunches in a compact electron accelerator that we believe will revolutionize experiments in this field. Our prototype source generates high-field THz pulses at unprecedented quasi-continuous-wave repetition rates up to the MHz regime. We demonstrate parameters that exceed state-of-the-art laser-based sources by more than 2 orders of magnitude. The peak fields and the repetition rates are highly scalable and once fully operational this type of sources will routinely provide 1 MV/cm electric fields and 0.3 T magnetic fields at repetition rates of few 100 kHz. We benchmark the unique properties by performing a resonant coherent THz control experiment with few 10 fs resolution.

Microscale imaging of the preservation state of 5,000-year-old archaeological bones by synchrotron infrared microspectroscopy.

AbstractArchaeological bone materials record characteristic markers of life in prehistoric times (dating, climate, environment, diet, human migration) in their isotopic and chemical composition in addition to palaeontological, archaeozoological, anthropological and palaeogenetic information. Thus, the discovery and conservation of archaeological bone materials is of great importance to get access to this information. However, archaeological materials are altered by different postmortem processes and it appears necessary to estimate if the archaeological information is still reliable or if it has been modified during burial. As archaeological bone materials present a high structural hierarchy at the micro- and nanoscale, changes induced by diagenetic phenomena have to be observed at these scales. One method for revealing post mortem changes of the bone structure and composition at the microscale is synchrotron radiation micro-FTIR imaging (SR micro-FTIR). Thus, thin sections of about 5,000-year-old archaeological bones have been analysed in transmission mode at the IRIS beamline (BESSY II, HZB Berlin) to determine markers of the state of bone preservation at the microscale. The archaeological bone material comes from station 19 of the Neolithic site of the Chalain Lake. By using SR micro-FTIR it was possible to image characteristic bone structures, e.g. osteons (the constitutive histological unit of cortical bone), using the absorption band ratios corresponding to different chemical bone constituents (collagen content and quality, phosphate crystallinity, carbonate content). These data allow us to precisely evaluate the state of preservation of a 5,000-year-old bone at the histological level. FigureChemical mapping of a thin section of the archaeological bone AB_CH19nb1 from the Neolithic station 19 at Chalain Lake

Molecule-solid interfaces studied with infrared ellipsometry: Ultrathin nitrobenzene films

This paper aims to demonstrate the high capability of infrared spectroscopic ellipsometry (IRSE) for the characterization of very thin organic films and the organic to inorganic interfaces. It is shown that the detection limit of IRSE facilitates the investigation of ultrathin nitrobenzene (NB) films with monolayer sensitivity. This accounts for substrates from semiconductors to metals. The process of reoxidation of a NB terminated silicon (001) surface is also reflected in the infrared ellipsometric parameters and evidently proceeds despite the organic layer. As a complementary method, x-ray photoelectron spectroscopy (XPS) measurements were performed.

Ultrathin responsive polyelectrolyte brushes studied by infrared synchrotron mapping ellipsometry

An infrared microfocus spectroscopic mapping ellipsometer was set up at the Berlin electron synchrotron storage ring and used to study ultrathin polyacrylic acid brush films with 3nm thickness. The pH-responsive properties of the brush on a gold-coated glass substrate were investigated. The chemical structure of the brush was resolved with a spatial resolution of 300μm using the synchrotron mapping ellipsometer.

Infrared ellipsometric study on the initial stages of oxide growth on Si(001)

The initial stages of native oxide growth on an etched Si(001) surface have been studied by infrared (IR) spectroscopic ellipsometry. The mid-infrared ellipsometric spectra show characteristic vibrational bands of Si–H stretching vibrations between 2100 and 2200 cm−1, and bands of silicon oxide between 1000 and 1290 cm−1. The most intense band in the range of Si–H stretching vibrations is assigned to a vibration of SiH2 groups, while the most prominent bands from the oxide interface can be assigned to bands originating from different Si–O stretching vibrations. By monitoring these bands during degradation of the passivated surface in atmospheric conditions different steps of the oxide growth at the interface are identified. The frequencies of the Si–O bands differ from those in thicker films. It is concluded that at the very beginning the top Si atoms are oxidized by replacement of the hydrogen bonds allowing the subsequent oxidation of the back bonds.

Fourier Transform Infrared Synchrotron Ellipsometry for Studying the Anisotropy of Small Organic Samples

An experimental setup for polarization-dependent and spectroscopic ellipsometric measurements was developed that utilizes the brilliance of synchrotron infrared radiation at the electron storage ring at BESSY II for investigations of small samples and sample areas. During commissioning of the beamline and the experimental setup, a 1 mm2 piece of a well-characterized polyimide film was studied to show the benefits of Fourier transform infrared (FT-IR) synchrotron ellipsometry. The band shapes are interpreted with respect to the anisotropic distribution of transition dipole moments within the film. In comparison to a globar source, the signal intensity has been improved by more than one order of magnitude for this example.