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- ItemSpatiooral coherence mapping of few-cycle vortex pulses(London : Nature Publishing Group, 2014) Grunwald, R.; Elsaesser, T.; Bock, M.Light carrying an orbital angular momentum (OAM) displays an optical phase front rotating in space and time and a vanishing intensity, a so-called vortex, in the center. Beyond continuous-wave vortex beams, optical pulses with a finite OAM are important for many areas of science and technology, ranging from the selective manipulation and excitation of matter to telecommunications. Generation of vortex pulses with a duration of few optical cycles requires new methods for characterising their coherence properties in space and time. Here we report a novel approach for flexibly shaping and characterising few-cycle vortex pulses of tunable topological charge with two sequentially arranged spatial light modulators. The reconfigurable optical arrangement combines interferometry, wavefront sensing, time-of-flight and nonlinear correlation techniques in a very compact setup, providing complete spatiooral coherence maps at minimum pulse distortions. Sub-7â €...fs pulses carrying different optical angular momenta are generated in single and multichannel geometries and characterised in comparison to zero-order Laguerre-Gaussian beams. To the best of our knowledge, this represents the shortest pulse durations reported for direct vortex shaping and detection with spatial light modulators. This access to space-time coupling effects with sub-femtosecond time resolution opens new prospects for tailored twisted light transients of extremely short duration.
- ItemNear-field dynamics of broad area diode laser at very high pump levels(New York, NY : American Inst. of Physics, 2011) Hempel, M.; Tomm, J.W.; Baeumler, M.; Konstanzer, H.; Mukherjee, J.; Elsaesser, T.Near-field properties of the emission of broad area semiconductor diode lasers under extremely high pumping of up to ∼50 times the threshold are investigated. A transition from a gain to thermally-induced index guiding is shown under operation with single pulses of 300 ns duration. At highest output powers, catastrophic optical damage is observed which is studied in conjunction with the evolution of time-averaged filamentary near-field properties. Dynamics of the process is resolved on a picosecond time scale.
- ItemUltrafast structural and vibrational dynamics of the hydration shell around DNA(Les Ulis : EDP Sciences, 2013) Elsaesser, T.; Szyc, LŁ.; Yang, M.Two-dimensional infrared spectroscopy in the frequency range of OH- and NH stretch excitations serves for a direct mapping of hydration dynamics around DNA. A moderate slowing down of structural dynamics and resonant OH stretch energy transfer is observed in the DNA water shell compared to bulk water.
- ItemUltrafast inter-ionic charge transfer of transition-metal complexes mapped by femtosecond x-ray powder diffraction(Les Ulis : EDP Sciences, 2013) Zamponi, F.; Freyer, B.; Juvé, V.; Stingl, J.; Woerner, M.; Chergui, M.; Elsaesser, T.Transient electron density maps are derived from x-ray diffraction patterns of photoexcited [Fe(bpy)3]2+(PF6 -)2 powder. Upon photoexcitation, the 5T 2 quintet state reveals a charge transfer from the PF 6- ions and from the Fe atoms to neighboring bpy units. The charge transfer from the Fe points to a partial and weak charge-transfer character of this state.
- ItemFemtosecond x-ray diffraction using the rotating crystal method(Les Ulis : EDP Sciences, 2013) Freyer, B.; Stingl, J.; Zamponi, F.; Woerner, M.; Elsaesser, T.We demonstrate the rotating-crystal method in femtosecond x-ray diffraction. Structural dynamics of a photoexcited bismuth crystal is mapped in a pump-probe scheme by measuring intensity changes of many Bragg reflections simultaneously.
- ItemUltrafast two-dimensional THz spectroscopy of graphene(Les Ulis : EDP Sciences, 2013) Bowlan, P.; Martinez Moreno, E.; Reimann, K.; Woerner, M.; Elsaesser, T.With two-dimensional THz spectroscopy the dynamics of low-energy carriers in graphene is determined. Both intra- and interband absorption contribute to the observed ultrafast pump-probe signals.
- ItemUltrafast charge relocation in an ionic crystal probed by femtosecond x-ray powder diffraction(Les Ulis : EDP Sciences, 2013) Woerner, M.; Zamponi, F.; Rothhardt, P.; Stingl, J.; Elsaesser, T.Transient electron density maps of potassium dihydrogen phosphate(KH 2PO4, KDP) are derived from femtosecond x-ray powder diffraction patterns. Upon photoexcitation, the low-frequency TO soft mode is elongated impulsively and modulates the electronic charge distribution on the length scale of interatomic distances, much larger than the vibrational amplitude of the nuclear motion. The results demonstrate a charge transfer from the volumes around the P-atoms to those containing the O - H·· ·O units and a quadrupolar distortion of the K+ charge distribution. This behavior reflects the interplay of nuclear motions and electric polarizations in the ionic crystal lattice.
- ItemUltrafast IR pump-probe and 2D-IR photon echo spectroscopy of adenosine-thymidine base pairs(Les Ulis : EDP Sciences, 2013) Greve, C.; Preketes, N.K.; Costard, R.; Koeppe, B.; Fidder, H.; Nibbering, E.T.J.; Temps, F.; Mukamel, S.; Elsaesser, T.We characterize diagonal and off-diagonal anharmonicities of N-H stretching vibrations in adenosine and thymidine monomers and in A•T-base pairs in chloroform solution, showing the important role of coupling between vibrationally excited N-H stretching states.
- 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.
- ItemFemtosecond X-ray diffraction from nanolayered oxides(Amsterdam : Elsevier, 2010) Von Korff Schmising, C.; Harpoeth, A.; Zhavoronkov, N.; Woerner, M.; Elsaesser, T.; Bargheer, M.; Schmidbauer, M.; Vrejoiu, I.; Hesse, D.; Alexe, M.Femtosecond X-ray scattering offers the opportunity to investigate reversible lattice dynamics with unprecedented accuracy. We show in a prototype experiment how strain propagation modifies the functionality of a ferroelectric system on its intrinsic time scale.