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.

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.

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.

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.

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.

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.

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.

2013, Woerner, M., Kuehn, W., Bowlan, P., Reimann, K., Elsaesser, T.

Recent experimental progress has allowed for the implementation of nonlinear two-dimensional (2D) terahertz (THz) spectroscopy in the ultrafast time domain. We discuss the principles of this technique based on multiple phase-locked electric field transients interacting in a collinear geometry with a solid and the phase-resolved detection of the THz fields after interaction with the sample. To illustrate the potential of this new method, 2D correlation spectra of coupled intersubband-longitudinal optical phonon excitations in a double quantum well system and a study of ultrafast carrier dynamics in graphene are presented.

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.

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.