T. Elsaesser

Technische Universität Berlin, Berlín, Berlin, Germany

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Publications (500)1328.5 Total impact

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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.
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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$/s}$.
    01/2015;
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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 impor-tant 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 sensi-tivity 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 μm. The comparably long optical period allows for accelerating elec-trons from the Cu target to very high kinetic energies and for generating a characteristic Kα 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. The key steps in generating characteristic X-ray pulses in a plasma source are the field-induced extraction of electrons from the metal target (step 1 in Fig. 1a), electron acceleration in vacuum by the very strong laser field (step 2), electron re-entrance into the target and X-ray generation via collisional inner-shell ionization followed by a radiative transition of an outer-shell elec-tron into the unoccupied inner shell (step 3). The strong electric driving field reduces the potential barrier for the tunnelling of elec-trons from states close to the Fermi level into vacuum 15 . After its 'birth', a free electron is moved away from the target in the first half-cycle of the laser field and in the next half-cycle of opposite sign it is accelerated and smashed back into the target with a high kinetic-energy gain 5 . The maximum energy of the re-entering elec-trons is proportional to I peak λ 2 where I peak and λ are the peak inten-sity and the centre wavelength of the driving field, respectively. In steps 1 and 2, electrons move in phase with the optical driver field and undergo a coherent motion on the timescale of the optical cycle. Although this type of electron motion is similar to that during soft X-ray high-harmonic generation in atoms 16 , the sub-sequent generation of hard X-ray photons is not. The latter rep-resents an incoherent collisional process to which many electron trajectories in the target contribute and in which a single electron induces ionization events repeatedly. In our experiments, a high-intensity mid-infrared driver system based on optical parametric chirped pulse amplification 17 (OPCPA, see Methods) is combined with a plasma source for Cu Kα radiation 9,10 . The p-polarized Δt = 80 fs idler pulses at λ = 3.9 µm are focused onto the Cu tape target (D tape = 20 µm thick) with an adjustable angle θ (Fig. 1a). The target is placed in a vacuum chamber and moves with a speed of 5 cm s −1 to provide a fresh target volume for each driving pulse (repetition rate 20 Hz). In this interaction geometry, a standing optical wave is formed at the front target surface by constructive interference between the incom-ing and the partly reflected focused laser beam, which results in a peak intensity of a few 10 16 W cm −2 and a peak electric field of 450 V nm −1 for a pulse energy of W P = 15 mJ (angle of incidence on the target of θ = 59°). The optical spot size on the target was derived from knife-edge measurements in which a sharp metal blade was moved through the beam in the focal plane. Such measurements give a beam diameter of d FWHM = 21 µm (FWHM, full width at half maximum). The generated X-rays were collected in a transmission geometry, that is, in a forwards direction, and detected with a calibrated energy-resolving CdTe detector (see Methods). X-ray spectra generated with mid-infrared pulses of W P = 15 mJ for two different angles of incidence θ are presented in Fig. 1c. They consist of the characteristic X-ray lines of Cu at energies Cu Kα 1,2 ≈ 8.0 keV and Kβ ≈ 8.9 keV and of a broad Bremsstrahlung component that extended above 100 keV (for θ = 59°), which rep-resents a measure for the maximum kinetic energy of the accelerated electrons. As shown in the inset of Fig. 1c, the energy resolution of the detector is not sufficient to separate the Kα 1 and Kα 2 lines but measures the integrated Kα flux (shaded area). At W P = 15 mJ, the total flux into a 4π solid angle has a high value of 10 9 photons per pulse, which corresponds to approximately 8 × 10 7 photons sr
    Nature Photonics 12/2014; 8(12):927-930. · 27.25 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. · 5.08 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. · 2.21 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; · 4.19 Impact Factor
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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.
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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: 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 05/2014; · 7.98 Impact Factor
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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. · 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.
    Photonics West, San Francisco; 02/2014
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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. · 6.69 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. · 3.12 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). · 3.67 Impact Factor
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    ABSTRACT: Femtosecond x-ray powder diffraction maps electron density in response to a strong electric field. In LiH, electron correlations lead to an electron transfer from Li to H while NaBH_4 shows a transfer from BH_4^- to Na^+.
    International Conference on Ultrafast Phenomena; 01/2014
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    ABSTRACT: The response of a LiNbO3 crystal to THz pulses in the nonperturbative regime is studied by two-dimensional spectroscopy. Phase-resolved detection allows for separating the THz bulk photovoltaic effect from other nonlinear contributions.
    International Conference on Ultrafast Phenomena; 01/2014
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    ABSTRACT: The nonlinear dynamics of electrons in multilayer epitaxial graphene is investigated by time-resolved terahertz (THz) spectroscopy in a regime where the interaction of electrons with the external field dominates over scattering processes. The predominantly coherent electron response to the THz field involves both intra- and interband currents, leading to coherently driven interband transitions of carriers and to the generation of higher harmonics of the THz carrier frequency. The overall behavior of the graphene layers is always absorptive, even after generation of an initial electron-hole distribution by femtosecond midinfrared excitation. The results are in agreement with theoretical calculations of the nonperturbative THz response.
    Physical Review B 12/2013; 89(4). · 3.66 Impact Factor
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    ABSTRACT: Excitonic and spin excitations of single semiconductor quantum dots currently attract attention as possible candidates for solid state based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single quantum dot on a femtosecond time scale. Transient reflectivity spectra show pronounced oscillations around the quantum dot exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the exitonic quantum dot polarization and continuum excitations. The results show that although semiconductor quantum dots resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time scales.
    Proc SPIE 12/2013;

Publication Stats

8k Citations
1,328.50 Total Impact Points

Institutions

  • 2014
    • Technische Universität Berlin
      • Department of solid state Physics
      Berlín, Berlin, Germany
  • 1994–2014
    • Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
      Berlín, Berlin, Germany
  • 2013
    • University of Rhode Island
      • Department of Chemistry
      Kingston, RI, United States
  • 2011
    • Humboldt-Universität zu Berlin
      • Department of Physics
      Berlin, Land Berlin, Germany
    • Bundesanstalt für Materialforschung und -prüfung
      Berlín, Berlin, Germany
  • 2010
    • Weierstrass Institute for Applied Analysis and Stochastics
      Berlín, Berlin, Germany
  • 2009
    • University of Colorado at Boulder
      Boulder, Colorado, United States
  • 2002–2009
    • Paul Drude Institute for Solid State Electronics
      Berlín, Berlin, Germany
    • Fraunhofer Heinrich-Hertz-Institute HHI
      Berlín, Berlin, Germany
  • 2008
    • Universität Potsdam
      • Institute of Physics and Astronomy
      Potsdam, Brandenburg, Germany
  • 2007
    • Ludwig-Maximilians-University of Munich
      München, Bavaria, Germany
  • 1983–2007
    • Technische Universität München
      • • Walter Schottky Institut (WSI)
      • • Faculty of Physics
      München, Bavaria, Germany
  • 2006
    • Ioffe Physical Technical Institute
      Sankt-Peterburg, St.-Petersburg, Russia
  • 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