title: Second Harmonic Generation of Nanoscale Phonon Wave Packets.
abstract: Phonons are often regarded as delocalized quasiparticles with certain energy and momentum. The anharmonic interaction of phonons determines macroscopic properties of the solid, such as thermal expansion or thermal conductivity, and a detailed understanding becomes increasingly important for functional nanostructures. Although phonon-phonon scattering processes depicted in simple wave-vector diagrams are the basis of theories describing these macroscopic phenomena, experiments directly accessing these coupling channels are scarce. We synthesize monochromatic acoustic phonon wave packets with only a few cycles to introduce nonlinear phononics as the acoustic counterpart to nonlinear optics. Control of the wave vector, bandwidth, and consequently spatial extent of the phonon wave packets allows us to observe nonlinear phonon interaction, in particular, second harmonic generation, in real time by wave-vector-sensitive Brillouin scattering with x-rays and optical photons.
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Bojahr
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Gohlke
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Leitenberger
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Pudell
authors.affiliation: Helmholtz Zentrum Berlin, Albert-Einstein-Strasse 15, 12489 Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Reinhardt
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: A
authors.initials: A
authors.lastname: von Reppert
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Roessle
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Sander
authors.affiliation: Helmholtz Zentrum Berlin, Albert-Einstein-Strasse 15, 12489 Berlin, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Gaal
authors.affiliation: Institut für Physik and Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.|Helmholtz Zentrum Berlin, Albert-Einstein-Strasse 15, 12489 Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Bargheer