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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.
El manuscrito “Isle de Cube. Antilles en général” de Alejandro de Humboldt se puede comprender como el título para todo un archipiélago de textos. Su estructuración radicalmente abierta nos proporciona la idea de un modo de escribir y más aún de pensar de este explorador de cultura y naturaleza. Sus miniaturas textuales registran la complejidad política y social del mundo insular del Caribe de forma relacional y polilógica. En combinación con sus mapas cubanos y el Essai politique sur l’île de Cuba, este manuscrito hace de Humboldt un escritor prominente de la literatura cubana del siglo XIX.
Inhalt:
-Ulrich Päßler: Plantae des États-Unis Rediscovered Pages from Alexander von Humboldt’s United States Diary
-Ottmar Ette: Texto-isla y escritura archipiélica: „Isle de Cube. Antilles en général“ 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
-Günter Hoppe: Ein Museumsdiebstahl vor 144 Jahren