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. 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 ultrafast wave-vector-sensitive
Brillouin scattering with x-rays and optical photons.
We show different ways of preparing quasi-monochromatic phonons by
multipulse-excitation or superlattice-excitation. We carefully
monitor the preparation and decay of these wavepackets and compare the
results to numerical and analytic models.
We also use a combination of ultrafast x-ray diffraction (UXRD) and
ultrafast reciprocal space mapping (URSM) at laser-driven sources and
synchrotrons with time-domain-Brillouin-scattering (TDBS) to observe the
conversion of phonons at lattice-defects  and the nonlinear
interaction with domain-walls .
Most of the results presented are on perovskite oxide nanolayers
composed of SrRuO3, La(SrxMn1-x)O3, Pb(ZrxTi1-x)O3 and SrTiO3, but I
will shortly show results on photoactive polymers and on the
antiferromagnetic rare-earth Dysprosium.
 Phys. Rev. Lett 115, 195502 (2015)
 Opt. Express 21, 21188 (2013)
 Phys. Rev. B 87, 184301 (2013)
 Structural Dynamics 1, 064501 (2014)
 Phys. Rev. Lett. 110, 095502 (2013)
 Phys. Rev. Lett. 114, 047401 (2015)