@phdthesis{Bojahr2016,
  author    = {Bojahr, Andre},
  title     = {Hypersound interaction studied by time-resolved inelastic light and x-ray scattering},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-93860},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xxiii, 201},
  year      = {2016},
  abstract  = {This publications-based thesis summarizes my contribution to the scientific field of ultrafast structural dynamics. It consists of 16 publications, about the generation, detection and coupling of coherent gigahertz longitudinal acoustic phonons, also called hypersonic waves. To generate such high frequency phonons, femtosecond near infrared laser pulses were used to heat nanostructures composed of perovskite oxides on an ultrashort timescale. As a consequence the heated regions of such a nanostructure expand and a high frequency acoustic phonon pulse is generated. To detect such coherent acoustic sound pulses I use ultrafast variants of optical Brillouin and x-ray scattering. Here an incident optical or x-ray photon is scattered by the excited sound wave in the sample. The scattered light intensity measures the occupation of the phonon modes. The central part of this work is the investigation of coherent high amplitude phonon wave packets which can behave nonlinearly, quite similar to shallow water waves which show a steepening of wave fronts or solitons well known as tsunamis. Due to the high amplitude of the acoustic wave packets in the solid, the acoustic properties can change significantly in the vicinity of the sound pulse. This may lead to a shape change of the pulse. I have observed by time-resolved Brillouin scattering, that a single cycle hypersound pulse shows a wavefront steepening. I excited hypersound pulses with strain amplitudes until 1\% which I have calibrated by ultrafast x-ray diffraction (UXRD). On the basis of this first experiment we developed the idea of the nonlinear mixing of narrowband phonon wave packets which we call "nonlinear phononics" in analogy with the nonlinear optics, which summarizes a kaleidoscope of surprising optical phenomena showing up at very high electric fields. Such phenomena are for instance Second Harmonic Generation, four-wave-mixing or solitons. But in case of excited coherent phonons the wave packets have usually very broad spectra which make it nearly impossible to look at elementary scattering processes between phonons with certain momentum and energy. For that purpose I tested different techniques to excite narrowband phonon wave packets which mainly consist of phonons with a certain momentum and frequency. To this end epitaxially grown metal films on a dielectric substrate were excited with a train of laser pulses. These excitation pulses drive the metal film to oscillate with the frequency given by their inverse temporal displacement and send a hypersonic wave of this frequency into the substrate. The monochromaticity of these wave packets was proven by ultrafast optical Brillouin and x-ray scattering. Using the excitation of such narrowband phonon wave packets I was able to observe the Second Harmonic Generation (SHG) of coherent phonons as a first example of nonlinear wave mixing of nanometric phonon wave packets.},
  language  = {en}
}
@misc{PaesslerEtteHeydetal.2017,
  author    = {P{\"a}ßler, Ulrich and Ette, Ottmar and Heyd, Thomas and Rapisarda, Cettina and Schwarz, Ingo and Stottmeister, Ulrich and Hoppe, G{\"u}nter},
  title     = {HiN : Alexander von Humboldt im Netz},
  series = {HIN : Alexander von Humboldt im Netz ; international review for Humboldtian studies},
  volume    = {XVIII},
  journal   = {HIN : Alexander von Humboldt im Netz ; international review for Humboldtian studies},
  number    = {35},
  editor    = {Ette, Ottmar and Knobloch, Eberhard},
  publisher = {Universit{\"a}tsverlag Potsdam},
  address   = {Potsdam},
  issn      = {2568-3543},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-405028},
  pages     = {97},
  year      = {2017},
  abstract  = {Inhalt: -Ulrich P{\"a}ßler: Plantae des {\´E}tats-Unis Rediscovered Pages from Alexander von Humboldt's United States Diary -Ottmar Ette: Texto-isla y escritura archipi{\´e}lica: „Isle de Cube. Antilles en g{\´e}n{\´e}ral" de Alexander von Humboldt -Thomas Heyd: Alexander von Humboldt y la unidad de la naturaleza -Cettina Rapisarda: Lava memoriae deodati dolomieu Alexander von Humboldts Gesteinsstudien in Neapel -Ingo Schwarz: Fahndung nach geraubtem Gold und Silber - 1839 -Ulrich Stottmeister: Umweltgedanken zu Alexander von Humboldt -G{\"u}nter Hoppe: Ein Museumsdiebstahl vor 144 Jahren},
  language  = {de}
}
@article{Ette2018,
  author    = {Ette, Ottmar},
  title     = {Texto-isla y escritura archipi{\´e}lica},
  series = {HiN : Alexander von Humboldt im Netz ; International Review for Humboldtian Studies},
  volume    = {XVIII},
  journal   = {HiN : Alexander von Humboldt im Netz ; International Review for Humboldtian Studies},
  number    = {35},
  publisher = {Universit{\"a}tsverlag Potsdam},
  address   = {Potsdam},
  issn      = {1617-5239},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-419270},
  pages     = {13 -- 24},
  year      = {2018},
  abstract  = {Alexander von Humboldts Manuskript „Isle de Cube. Antilles en g{\´e}n{\´e}ral" l{\"a}sst sich als Titel f{\"u}r ein ganzes Text-Archipel begreifen. Es fasziniert durch seine radikal offene Strukturierung und liefert uns eine Vorstellung vom Schreib- wie vielleicht mehr noch vom Denkmodell des Kultur- und Naturforschers. Die politische und soziale Komplexit{\"a}t der karibischen Inselwelt erfassen seine Textminiaturen relational und viellogisch. Sie machen ihn - zusammen mit seinen kubanischen Karten und dem Essai politique sur l'{\^i}le de Cuba - zu einem bedeutenden kubanischen Schriftsteller des 19. Jahrhunderts.},
  language  = {es}
}