T. Elsaesser

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Berlín, Berlin, Germany

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Publications (505)1346.27 Total impact

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    ABSTRACT: Phosphate vibrations serve as local probes of hydrogen bonding and structural fluctuations of hydration shells around ions. Interactions of H2PO−4 ions and their aqueous environment are stud- ied combining femtosecond 2D infrared spectroscopy, ab-initio calculations, and hybrid quantum- classical molecular dynamics (MD) simulations. Two-dimensional infrared spectra of the symmet- ric (νS(PO−2)) and asymmetric (νAS(PO−2)) PO−2 stretching vibrations display nearly homogeneous lineshapes and pronounced anharmonic couplings between the two modes and with the δ(P-(OH)2) bending modes. The frequency-time correlation function derived from the 2D spectra consists of a predominant 50 fs decay and a weak constant component accounting for a residual inhomogeneous broadening. MD simulations show that the fluctuating electric field of the aqueous environment induces strong fluctuations of the νS(PO−2) and νAS(PO−2) transition frequencies with larger fre- quency excursions for νAS(PO−2). The calculated frequency-time correlation function is in good agreement with the experiment. The ν(PO−2) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO−4 /H2O cluster calculations reveal substantial frequency shifts and mode mixing with increasing hydration. Predicted phosphate- water hydrogen bond (HB) lifetimes have values on the order of 10 ps, substantially longer than water-water HB lifetimes. The ultrafast phosphate-water interactions observed here are in marked contrast to hydration dynamics of phospholipids where a quasi-static inhomogeneous broadening of phosphate vibrations suggests minor structural fluctuations of interfacial water.
    The Journal of Chemical Physics 03/2015; 142(21):212406. DOI:10.1063/1.4914152 · 3.12 Impact Factor
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    ABSTRACT: The infrared emission from 980-nm single-mode high power diode lasers is analyzed in the wavelength range from 0.8 to 7.0 μm. A pronounced short-wavelength infrared (SWIR) emission band with a maximum at 1.3 μm is found to originate from defect states located within the waveguide of the devices. The SWIR intensity is verified to represent a measure of the non-equilibrium carrier concentration in the waveguide, allowing for non-destructive waveguide mapping in spatially resolved detection schemes. The potential of this approach is demonstrated by measuring spatially resolved profiles of SWIR emission and correlating them with mid-wavelength infrared thermal emission along the cavity of devices undergoing repeated catastrophic optical damage. The enhancement of SWIR emission in the damaged parts of the cavity is due to a locally enhanced carrier density in the waveguide and allows for in situ analysis of the damage patterns. Moreover, spatial resolved SWIR measurements are a promising tool for device inspecting even in low-power operation regimes.
    Proceedings of SPIE - The International Society for Optical Engineering 03/2015; 9382:93821G. DOI:10.1117/12.2075859 · 0.20 Impact Factor
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    ABSTRACT: The interaction of intense femtosecond pulses with metals allows for generating ultrashort hard x-rays. In contrast to plasma theories, tunneling from the target into vacuum is introduced as electron generation step, followed by vacuum acceleration in the laser field and re-entrance into the target to generate characteristic x-rays and Bremsstrahlung. For negligible space charge in vacuum, the Kα flux is proportional to the incident intensity and the wavelength squared, suggesting a strong enhancement of the x-ray flux by mid-infrared driving pulses. This prediction is in quantitative agreement with experiments on femtosecond Cu Kα generation.
    Structural Dynamics 03/2015; 2(2):024102. DOI:10.1063/1.4915485
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    ABSTRACT: Third-order optical nonlinearities play a vital role for the generation and characterization of ultrashort optical pulses. One particular characterization method is frequency-resolved optical gating, which can be based on a large variety of third-order nonlinear effects. Any of these variants presupposes an instanta- neous temporal response, as it is expected off resonance. In this paper we show that resonant excitation of the third harmonic gives rise to surprisingly large decay times, which are on the order of the duration of the shortest oscillator pulses generated to date. To this end, we measured interferometric third-harmonic frequency-resolved optical gating traces in TiO2 and SiO2, corroborating polarization decay times up to 6.5 fs in TiO2. This effect is among the fastest effects observed in ultrafast spectroscopy. Numerical solutions of the time-dependent Schrödinger equation are in excellent agreement with experimental observations. Our work (experiments and simulations) corroborates that a noninstantaneous polarization decay may appear in the presence of a 3-photon resonance. In turn, pulse generation and characterization in the ultraviolet may be severely affected by this previously unreported effect.
    Optica 02/2015; 2(2):151 -157. DOI:10.1364/OPTICA.2.000151
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    ABSTRACT: Excitons play a key role for the optoelectronic properties of hybrid systems. We apply near-field scanning optical microscopy (NSOM) with a $100\,\text{-nm}$ spatial resolution to study the photoluminescence of surface excitons (SX) in a $20\,\text{nm}$ thick ZnO film capped with a monolayer of stearic acid molecules. Emission from SX, donor-bound (DX), and - at sample temperatures $T>20\,\text{K}$ - free (FX) excitons is separated in steady-state and time-resolved photoluminescence spectra. The $4\,\text{meV}$ broad smooth envelope of SX emission at $T<10\,\text{K}$ points to an inhomogeneous distribution of SX transition energies and spectral diffusion caused by diffusive SX transport on a $50\,\text{nm}$ scale with a SX diffusion coefficient of $D(T<10 K)=0.30\,\text{cm$^2$/s}$.
    Physical Review B 01/2015; 91(12). DOI:10.1103/PhysRevB.91.121415 · 3.66 Impact Factor
  • Thomas Elsaesser
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    ABSTRACT: Aufgrund der begrenzten Bindungsstärke von Wasserstoffbrücken unterliegen wässrige Systeme ultraschnellen strukturellen Fluktuationen im Zeitbereich zwischen 10–14 und 10–11 s. Schwingungsanregungen von Wasserstoffbrücken zerfallen ebenfalls in diesem Zeitbereich. Die mehrdimensionale Ultrakurzzeit-Infrarotspektroskopie erlaubt eine zeitaufgelöste Beobachtung dieser elementaren Dynamik in Wasser, hydratisierten DNA-Oligomeren und Phospholipiden. Neben Kopplungen zwischen Schwingungsfreiheitsgraden des hydratisierten Systems lässt sich die Wasserdynamik an Grenzflächen und im Volumen direkt aufzeichnen. Strukturelle Fluktuationen im Volumen von H2O treten im Zeitbereich zwischen 5×10–14 und 10–12 s auf, Wasserstoffbrücken werden im Takt von 10–12 s gebrochen und neu geformt. An den Grenzflächen zu DNA und Phospholipiden existieren Hydratisierungsgeometrien, die bei geringen Fluktuationen länger als 10–11 s aufrecht erhalten werden. Ein langlebiges ‘Gedächtnis’ des Wassers existiert jedoch nicht.Aqueous systems display ultrafast structural fluctuations in the time domain between 10–14 and 10–11 s, due to the limited strength of hydrogen bonds. Multidimensional ultrafast infrared spectroscopy allows for a time-resolved observation of such elementary dynamics in bulk water, DNA oligomers and phospholipids. Both the couplings of different vibrational degrees of the hydrated system and bulk water dynamics can be mapped directly. Structural fluctuations of bulk water occur in the time range between 5×10–14 and 10–12 s, the breaking and reformation of hydrogen bonds on a time scale of 10–12 s. At the interface between the water hydration shell and DNA or phospholipids, specific hydration geometries exist for periods of some 10–11 s with minor fluctuations of their structure. However, a long-lived 'memory of water' is absent.
    Chemie in unserer Zeit 12/2014; 49(1). DOI:10.1002/ciuz.201400678 · 0.36 Impact Factor
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    ABSTRACT: Ultrafast structural dynamics in the condensed phase represents a key topic of current physics, chemistry and materials science. Femtosecond hard X-ray pulses are important structure probes that have been applied in time-resolved X-ray absorption and diffraction(1-4). Optical pump/X-ray probe schemes with compact laser-driven table-top sources(5-11) have allowed for tiny changes of diffracted intensity to be measured with X-ray photon statistics, which has set the ultimate sensitivity limit(12-14). To address the strong quest for a higher X-ray flux, here we present the first hard X-ray plasma source driven by intense mid-infrared sub-100-fs pulses at 3.9 mu m. The comparably long optical period allows for accelerating electrons from the Cu target to very high kinetic energies and for generating a characteristic K alpha flux of 10(9) photons per pulse, 25 times more than with our 800 nm driver(9,10). Theoretical simulations account for the experimental results in a wide range of driving fields and predict a further enhancement of X-ray flux.
    Nature Photonics 12/2014; 8(12):927-930. DOI:10.1038/NPHOTON.2014.256 · 29.96 Impact Factor
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    R Grunwald, T Elsaesser, M Bock
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    ABSTRACT: 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 spatio-temporal coherence maps at minimum pulse distortions. Sub-7 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.
    Scientific Reports 11/2014; 4:7148. DOI:10.1038/srep07148 · 5.58 Impact Factor
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    ABSTRACT: Near-field scanning optical microscopy (NSOM) is applied for analyzing GaN-based diode lasers. The measurements are carried out at the front facets of standard devices without any additional preparation. Four different schemes for luminescence and photocurrent detection are applied. The results allow for a detailed analysis of the epitaxial layer sequence, the waveguide mode, the impact of defect absorption, and efficiencies of carrier transfer into the quantum well. Moreover, the effective potential profile as formed by both layer structure and doping profile is imaged. Features being spatially separated by only 30 nm are safely resolved. Our results pave the way towards non-destructive nanoscopic analysis of wide-bandgap optoelectronic devices.
    Semiconductor Science and Technology 09/2014; 29(11):112001. DOI:10.1088/0268-1242/29/11/112001 · 2.21 Impact Factor
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    ABSTRACT: Infrared emission from 980-nm single-mode high power diode lasers is recorded and analyzed in the wavelength range from 0.8 to 8.0 μm. A pronounced short-wavelength infrared (SWIR) emission band with a maximum at 1.3 μm originates from defect states located in the waveguide of the devices. The SWIR intensity is a measure of the non-equilibrium carrier concentration in the waveguide, allowing for a non-destructive waveguide mapping in spatially resolved detection schemes. The potential of this approach is demonstrated by measuring spatially resolved profiles of SWIR emission and correlating them with mid-wavelength infrared (MWIR) thermal emission along the cavity of devices undergoing repeated catastrophic optical damage. The enhancement of SWIR emission in the damaged parts of the cavity is due to a locally enhanced carrier density in the waveguide and allows for an analysis of the spatial damage patterns. The figure shows a side view of a diode laser during catastrophic degradation as recorded by a thermocamera within 5 successive current pulses. The geometry of the device is given in grayscale. The position of the laser chip is indicated by the dotted line. The thermal signatures of the internal degradation of the diode laser are overlaid in color. The bi-directional spread of the damage along the laser cavity is clearly visible.
    Laser & Photonics Review 09/2014; DOI:10.1002/lpor.201400045 · 9.31 Impact Factor
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    ABSTRACT: X-Ray diffraction provides insight into the distribution of electronic charge in crystals. Equilibrium electron distributions have been determined with high spatial resolution by recording and analysing a large number of diffraction peaks under stationary conditions. In contrast, transient electron densities during and after structure-changing processes are mainly unknown. Recently, we have introduced femtosecond X-ray powder diffraction from polycrystalline samples to determine transient electron density maps with a spatial resolution of 0.03 nm and a temporal resolution of 100 fs. In a pump–probe approach with a laser-driven tabletop hard X-ray source, optically induced structure changes are resolved in time by diffracting the hard X-ray probe pulses at different time delays from the excited powder sample and recording up to several tens of reflections simultaneously. Time-dependent changes of the atomic arrangement in the crystal lattice as well as modified electron densities are derived from the diffraction data. As a prototypical field-driven process, we address here quasi-instantaneous changes of electron density in LiBH4, LiH and NaBH4 in response to a non-resonant strong optical field. The light-induced charge relocation in LiBH4 and NaBH4 exhibits an electron transfer from the anion (BH−4) to the respective cation. The distorted geometry of the BH4 tetrahedron in LiBH4 leads to different contributions of the H atoms to electron transfer. LiH displays a charge transfer from Li to H, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This unexpected behavior originates from strong electron correlations in LiH as is evident from a comparison with quasi-particle bandstructures calculated within the Coulomb-hole-plus-screened-exchange (COHSEX) formalism.
    Faraday Discussions 07/2014; 171. DOI:10.1039/C4FD00026A
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    ABSTRACT: Ultrafast vibrational dynamics of [Formula: see text] ions, the key units in boron hydride materials for hydrogen storage, are studied in diluted polar liquid solution and in NaBH4 crystallites by femtosecond infrared spectroscopy. Two-color pump-probe experiments reveal v = 1 lifetimes of 3 ps for the asymmetric [Formula: see text] stretching mode ν3 and of 3.6 ps for the asymmetric bending mode ν4 in the solvent isopropylamine. We provide direct evidence for the [Formula: see text] stretching relaxation pathway via the asymmetric bending mode ν4 by probing the latter after femtosecond excitation of ν3. Pump-probe traces measured in the crystalline phase show signatures of radiative coupling between the densely packed [Formula: see text] oscillators, most clearly manifested in an accelerated subpicosecond depopulation of the v = 1 state of the ν4 mode. The radiative decay is followed by incoherent vibrational relaxation similar to the liquid phase. The excess energy released in the relaxation processes of the [Formula: see text] intramolecular modes is transferred into the environment with thermal pump-probe signals being much more pronounced in the dense solid than in the diluted solution.
    The Journal of Chemical Physics 07/2014; 141(3):034506. DOI:10.1063/1.4889743 · 3.12 Impact Factor
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    ABSTRACT: We report the first observation of terahertz higher harmonics in graphene by mapping the nonlinear response with broadband electrooptic sampling. The nonlinear response in the non-perturbative regime is determined by intra- and interband electron motions.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: We demonstrate experimental evidence for non-instantaneous polarization decay in dielectrics. The few-femtosecond relaxation times agree favorable with solutions of the time-dependent Schrödinger equation and relate to resonances of the quantum mechanical dipole
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: We study the nonlinear terahertz response of LiNbO3 using 2D spectroscopy. Dissecting the nonlinear response into different orders in the electric field shows a strong shift current and higher harmonics of the THz fundamental.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: The terahertz (THz) response of the ferroelectric prototype material lithium niobate (LiNbO3) is studied in the nonperturbative regime of light-matter interaction. Applying two-dimensional THz spectroscopy with few-cycle pulses of an amplitude E≈100 kV/cm and a center frequency of 2 THz, we dissect the overall nonlinear response into different orders in the electric field. The underlying nonlinear current is of interband character and consists of a strong low-frequency shift current (SC) and higher harmonics of the THz fundamental. The SC component originates from the lack of inversion symmetry and the strong interband decoherence for long electron trajectories in k space as shown by theoretical calculations.
    Physical Review Letters 04/2014; 112(14):146602. DOI:10.1103/PhysRevLett.112.146602 · 7.73 Impact Factor
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    ABSTRACT: Internal degradation of 980 nm emitting single-spatial-mode diode lasers during ultrahigh power operation is investigated for continuous wave and pulsed operation (2 µJ, 20 W). In situ analysis of the evolution of the optical nearfields with pico-second time resolution enables the observation of the transition from single- to multi-spatial-mode operation at elevated emission powers. Moreover, internal degradation events and defect propagation are monitored by thermal imaging. The results complete earlier findings obtained at broad-area lasers and allow for the establishment of generalized models covering both classes of edge-emitting devices.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2014; 8965. DOI:10.1117/12.2036082 · 0.20 Impact Factor
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    Rene Costard, Ismael A. Heisler, Thomas Elsaesser
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    ABSTRACT: The properties of biomembranes depend in a decisive way on interactions of phospholipids with hydrating water molecules. To map structural dynamics of a phospholipid–water interface on the length and time scale of molecular motions, we introduce the phospholipid symmetric and asymmetric phosphate stretch vibrations as probes of interfacial hydrogen bonds and electrostatic interactions. The first two-dimensional infrared spectra of such modes and a line shape analysis by density matrix theory reveal two distinct structural dynamics components; the first 300 fs contribution is related to spatial fluctuations of charged phospholipid head groups with additional water contributions at high hydration levels; the second accounts for water–phosphate hydrogen bonds persisting longer than 10 ps. Our results reveal a relatively rigid hydration shell around phosphate groups, a behavior relevant for numerous biomolecular systems.
    Journal of Physical Chemistry Letters 01/2014; 5(3):506–511. DOI:10.1021/jz402493b · 6.69 Impact Factor
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    ABSTRACT: The nonlinear interaction between intense terahertz (THz) pulses and epitaxial multilayer graphene is studied by field-resolved THz pump–probe spectroscopy. THz excitation results in a transient induced absorption with decay times of a few picoseconds, much faster than carrier recombination in single graphene layers. The decay times increase with decreasing temperature and increasing amplitude of the excitation. This behaviour originates from the predominant coupling of electrons to the electromagnetic field via the very strong interband dipole moment while scattering processes with phonons and impurities play a minor role. The nonlinear response at field amplitudes above 1 kV cm−1 is in the carrier-wave Rabi flopping regime with a pronounced coupling of the graphene layers via the radiation field. Theoretical calculations account for the experimental results.
    New Journal of Physics 01/2014; 16(1). DOI:10.1088/1367-2630/16/1/013027 · 3.67 Impact Factor
  • T Elsaesser, M Woerner
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    ABSTRACT: Ultrashort soft and hard x-ray pulses are sensitive probes of structural dynamics on the picometer length and femtosecond time scales of electronic and atomic motions. Recent progress in generating such pulses has initiated new directions of condensed matter research, exploiting a variety of x-ray absorption, scattering, and diffraction methods to probe photoinduced structural dynamics. Atomic motion, changes of local structure and long-range order, as well as correlated electron motion and charge transfer have been resolved in space and time, providing a most direct access to the physical mechanisms and interactions driving reversible and irreversible changes of structure. This perspective combines an overview of recent advances in femtosecond x-ray diffraction with a discussion on ongoing and future developments.
    The Journal of Chemical Physics 01/2014; 140(2):020901. DOI:10.1063/1.4855115 · 3.12 Impact Factor

Publication Stats

9k Citations
1,346.27 Total Impact Points


  • 1994–2015
    • Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
      Berlín, Berlin, Germany
  • 2014
    • Technische Universität Berlin
      • Department of solid state Physics
      Berlín, Berlin, Germany
  • 2009
    • University of Colorado at Boulder
      Boulder, Colorado, United States
  • 1983–2007
    • Technische Universität München
      • • Walter Schottky Institut (WSI)
      • • Faculty of Physics
      München, Bavaria, Germany
  • 2001–2004
    • Paul Drude Institute for Solid State Electronics
      Berlín, Berlin, Germany
  • 2002
    • Fraunhofer Heinrich-Hertz-Institute HHI
      Berlín, Berlin, Germany
  • 1998
    • AT&T Labs
      Austin, Texas, United States
  • 1990
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany
  • 1984–1989
    • Deutsches Herzzentrum München
      München, Bavaria, Germany
  • 1988
    • Georg-August-Universität Göttingen
      • Institute of Inorganic Chemistry
      Göttingen, Lower Saxony, Germany