Couplings of phonons to light and one another studied with LCLS [electronic resource]
Keywords: Physics.

Engineering the thermal properties of materials plays a crucial role in technologies including propulsion, computing, optics, and household appliances. Lattice vibrations, or phonons, are at the heart of our understanding of heat in solids. It is thus beneficial to continue exploring the basic science of phonons not only for the purpose of expanding human knowledge, but for making possible further advances in materials engineering as they pertain to heat. Fundamental to the theory of phonons is the phonon dispersion, the relationship between phonons' momentum and their energy. Momentum resolved inelastic scattering of neutrons and x-rays provides experimental access to this dispersion [1, 2]. An accurate measure of a material's phonon dispersion can allow for accurate prediction of its heat capacity, interatomic-bonding strengths, and sound propagation speed amongst other things. However the phonon dispersion gives us no direct information on how phonons couple to other excitations and/or one another, which is important for macroscopic thermal properties. For instance thermal expansion and finite thermal conductivity are consequences of finite phonon-phonon coupling. Current methods for measuring such interactions are indirect, including measurement of phonon linewidths and measurements of thermal conductivity. Time-resolved measurements can be useful for measuring couplings by observation of energy transfer between excitations with time. Thus a time and momentum resolved measure of phonons would seem an obvious tool for studying their couplings to one another and other excitations. While optical measurements can provide exquisite time resolution, they can resolve only very small momentum shifts. Traditional neutron and x-ray sources have the necessary momentum resolution, but the pulses are too long (~100 picoseconds or more) to resolve the transfer of energy of picosecond timescales, relevant to phonon-phonon coupling. Plasma-based x-ray sources have su cient momentum and time resolution, but the flux is too low to measure weak di↵raction signals from phonons in a reasonable amount of time. The Linac Coherent Light Source (LCLS) is the first instrument to provide all the aspects neces- sary for time and momentum resolved measurement of phonon energy transfer, with short-wavelength (~1 angstrom), short-time (~10s of femtoseconds) pulses at high flux (~10^13 photons/second) [3]. Trigo et al showed that this new tool can be used to measure phonon dispersions in a time-resolved way [4]. Specifically, they showed that a short optical pump can be used to excite temporal coherences for phonons throughout the Brillouin zone that can be observed in the time-resolved x-ray scattering from a delayed LCLS x-ray pulse. The work of this thesis was motivated by the desire to use LCLS to make fundamentally new measurements of phonon coupling. To do this we use variations of the technique of Trigo et al. We first explore further the principles behind the technique, specifically the mechanism by which short- wavelength light couples to high-wavevector phonons. We find that the optical-phonon copuling is of second order in the phonons, exciting correlated pairs of phonons with nearly equal and opposite momentum. Next, we explore a method for measurement of energy transfer in the parametric downconversion of a zone-center phonon in crystalline bismuth. The result is, to our knowledge, the first time and momentum resolved measurement of phonon decay channels. Finally, we show that a shaped optical pump pulse may be used to achieve frequency-selective excitation of the phonons. Taken together, these results suggest tools like LCLS may be used to make measurements of phonon-phonon coupling strengths that were previously inaccessible experimentally.