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Singh V.P.,University of Hamburg | Mathey L.,Hamburg Center for Ultrafast Imaging
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2014

We analyze density-density correlations of expanding clouds of weakly interacting two-dimensional Bose gases below and above the Berezinskii- Kosterlitz-Thouless transition, with particular focus on short-time expansions. During time-of-flight expansion, phase fluctuations of the trapped system translate into density fluctuations, in addition to the density fluctuations that exist in situ. We calculate the correlations of these fluctuations both in real space and in momentum space and derive analytic expressions in momentum space. Below the transition, the correlation functions show an oscillatory behavior, controlled by the scaling exponent of the quasicondensed phase, due to constructive interference. We argue that this can be used to extract the scaling exponent of the quasicondensate experimentally. Above the transition, the interference is rapidly suppressed when the atoms travel an average distance beyond the correlation length. This can be used to distinguish the two phases qualitatively. © 2014 American Physical Society.

Nalbach P.,University of Hamburg | Nalbach P.,Hamburg Center for Ultrafast Imaging
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2014

We investigate the dissipative influence of environmental fluctuations on the dynamics of driven quantum systems at an avoided crossing. We derive two simple approximative equations valid for weak system-bath coupling and compare results for the dissipative Landau-Zener problem with numerically exact results. Very good agreement is found for slow, i.e., adiabatic, driving. Specifically, the minimum in the Landau-Zener probability resulting from the competition of driving and dissipation is well described. For system-bath couplings and temperatures, where this minimum is observed in the adiabatic driving regime, good agreement is also observed for fast driving when the dynamics tends toward nonadiabatic behavior. Otherwise, however, for large temperatures and fast driving our approximation fails even for weak system-bath couplings, for which an undriven system is still accurately described by weak-coupling approximations. © 2014 American Physical Society.

Reichert F.,University of Hamburg | Calmano T.,University of Hamburg | Muller S.,University of Hamburg | Marzahl D.-T.,University of Hamburg | And 3 more authors.
Optics Letters | Year: 2013

We report the fabrication and visible laser operation of Pr,Mg:SrAl12O19 waveguides. Waveguiding structures were created by focusing the radiation of a femtosecond Ti:sapphire laser into bulk material. Guiding losses were determined to be as low as 0.12 dBcm-1 at 632.8 nm. By employing a frequency-doubled optically pumped semiconductor laser, waveguide laser operation was realized at wavelengths of 525.3, 644.0, and 724.9 nm with output powers as high as 36, 1065, and 504 mW, respectively. To the best of our knowledge this is the first demonstration of green laser operation in a Pr3-doped crystalline waveguide laser. © 2013 Optical Society of America.

Rohlsberger R.,German Electron Synchrotron | Rohlsberger R.,Hamburg Center for Ultrafast Imaging
Physical Review Letters | Year: 2014

A new type of spectroscopy for high-resolution studies of spin waves that relies on resonant scattering of hard x rays is introduced. The energy transfer in the scattering process is encoded in the precession of the polarization vector of the scattered photons. Thus, the energy resolution of such a spectroscopy is independent of the bandwidth of the probing radiation. The measured quantity resembles the intermediate scattering function of the magnetic excitations in the sample. At pulsed x-ray sources, especially x-ray lasers, the proposed technique allows us to take single-shot spectra of the magnetic dynamics. The method opens new avenues to study low-energy nonequilibrium magnetic processes in a pump-probe setup. © 2014 American Physical Society.

Takahashi E.J.,RIKEN | Lan P.,RIKEN | Lan P.,Huazhong University of Science and Technology | Mucke O.D.,German Electron Synchrotron | And 3 more authors.
Nature Communications | Year: 2013

High-energy isolated attosecond pulses required for the most intriguing nonlinear attosecond experiments as well as for attosecond-pump/attosecond-probe spectroscopy are still lacking at present. Here we propose and demonstrate a robust generation method of intense isolated attosecond pulses, which enable us to perform a nonlinear attosecond optics experiment. By combining a two-colour field synthesis and an energy-scaling method of high-order harmonic generation, the maximum pulse energy of the isolated attosecond pulse reaches as high as 1.3 mJ. The generated pulse with a duration of 500 as, as characterized by a nonlinear autocorrelation measurement, is the shortest and highest-energy pulse ever with the ability to induce nonlinear phenomena. The peak power of our tabletop light source reaches 2.6GW, which even surpasses that of an extreme-ultraviolet free-electron laser. © 2013 Macmillan Publishers Limited. All rights reserved.

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