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udkm1Dsim toolbox
A Simulation Toolkit for
1D Ultrafast Dynamics
in Condensed Matter


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Research Interests

The functions of solids, molecules and nanostructures are essentially governed by the interplay of electronic excitation and structural changes, i.e. nuclear motion. The elementary steps of electronic and nuclear dynamics occur on the ultrafast timescale (femtoseconds to picoseconds). The electrons set the forces which initiate nuclear motion and this motion in turn influences the electronic response (optical absorption, transmission, reflection and coherent wave-mixing processes).

Based on the understanding and the detailed analysis of simple systems we develop an understanding of more complex coupled electronic and nuclear motion. In solid materials we are interested in studying the ultrafast response associated with phase-transitions and collective phenomena such as ferroelectricity and ferromagnetism. On the other hand we prepare and investigate nanostructured “soft” and molecular materials, in particular polyelectrolyte composites with embedded functional molecular groups or nanoparticles.

We use optical and infrared pump-probe experiments to launch and observe specific dynamics. Ultrafast x-ray diffraction is used in addition to monitor the atomic motion more directly, either at dedicated synchrotron beamlines or at our laser-plasma based femtosecond x-ray diffractometer. As a joint research group of the Helmholtz-Zentrum-Berlin we operate the XPP-KMC3 beamline at BESSY II, which allows for x-ray pump-probe studies and x-ray absorption measurements such as EXAFS.

This unique combination of methods aims at understanding and steering the coupled ultrafast dynamics of nuclei and electrons in complex systems, which may be useful for novel nanodevices.

I) Quasi-monochromatic phonons:

research1

We use sequences of optical light pulses or transducers tailored on the nanoscale to produce trains of ultrashort strain pulses which are essentially quasi-monochromatic phonon wavepackets. They can be detected by ultrafast x-ray diffraction (UXRD), as the phonons produce clear sidebands to a material’s Bragg peak.

Appl. Phys. Lett. 100, 094101 (2012)

II)

research2In full analogy to the x-ray detection of coherent phonons we have established an all-optical broadband spectroscopy which allows for a wavevector-selective detection of hypersound waves by Brillouin scattering. While this technique lacks the material specific response of x-ray diffraction, it is also applicable in non-crystalline materials such as polymers.

arXiv (2012)

III)

research3We investigate the nonlinear propagation of sound by using wavevector-selective excitation of phonon wavepackets and wavevector selective probes. This yields direct access to fundamental interactions in solids which are commonly described as three-phonon processes etc. mediated by the anharmonic potential. We are currently investigating how such nonlinearities in sound propagation are connected to structural phase transitions.

Phys. Rev. B 86, 144306 (2012)

IV)

research4We continue the research on ferroelectric materials such as PbZrTiO3 (PZT), BaTiO3 and BaSrTiO3. Ultrafast x-ray diffraction (UXRD) showed that upon squeezing the polar axis of the ferroelectric PZT the polarization followed with a delay of 500 fs.
By extending the UXRD to an ultrafast reciprocal space mapping we were recently able to follow the in-plane strain propagation which is solely induced by the nanosized mosaic blocks present in the ferroelectric.

Phys. Rev. Lett. 110, 095502 (2013)
Phys. Rev. Lett. 98, 257601 (2007)

V)

research6SrRuO3 and LaSrMnO3 are two ferromagnetic metals with perovskite crystal structure. research5While the electrons and phonons are only weakly coupled to magnons in LSMO, the interaction in SRO is very strong and gives rise to ultrafast magnetostriction. We investigate these systems and aim at a microscopic understanding of the relevant couplings, e.g. the coupling of the rotation of oxygen octahedral to the spin structure.

Phys. Rev. B 78, 060404 (2008)

VI)

research7A very efficient tool for preparing nanolayered structures with functional molecular groups or embedded nanoparticles is spin-assisted layer-by-layer deposition.
These structures are carefully characterized, e.g. by x-ray relflectometry, TEM, AFM, and optical spectroscopy.
research8 We prepare layered systems with molecular hypersound-actuators on the one hand and plasmonic detectors on the other hand.

Langmuir 26, 18499 (2010)

VII)

research9The figure shows how the plasmon resonance of gold nanoparticles changes strongly upon deposition of few nanometers of polyelectrolytes.
In a broadband pump-probe setup we study transmission and reflection changes on the femtosecond timescale to deduce changes of the dielectric function in real time.

Phys. Rev. B 84, 165121 (2011)
Langmuir 28, 4800 (2012)

VIII) Methods for ultrafast x-rays

research10The group permanently works on improving setups and methods for ultrafast x-ray science. We operate the XPP-KMC3 beamline which currently undergoes final reconstruction steps. In a future configuration it is planned to use an ultrafast x-ray Bragg switch for shortening the x-ray pulses down to few picoseconds.
It has been developed at the former EDR beamline and was tested at the ESRF.

Rev. Sci. Instrum. 83, 063303 (2012)
Appl. Phys. Lett. 101, 243106 (2012)