Quantification of Coherent Acoustic Phonon Dynamics with Ultrafast Electron Scattering

Abstract

The advent of ultrafast experimentation has revealed a great deal of dynamics ranging from charge-carrier relaxation to acoustic wave propagation. In this dissertation, we will discuss the development of ultrafast electron microscopy, the appearance of coherent acoustic phonons, and the theory and applications behind ultrafast electron scattering. The interpretation of UEM results is often difficult due to the complicated phenomena at play. In particular, the simulation of electron scattering theory has many forms ranging from the thickness-averaged kinematical model to the Bloch wave model. The multi-slice dynamical theory based around the Howie-Whelan equations is demonstrated as an effective tool to simulate large-scale distortions and demonstrated sensitivity to tilt, experiencing contrast differences up to a factor of ten with only a few degrees of change and matching prior experimental experiences. A prior study has shown an example of such complicated strain patterns by characterizing a series of transient contrast bands as 35 GHz hypersonic first-order symmetric Lamb waves. Multi-slice simulations of these waves showed peak strains up to 3% in the transverse [110] crystallographic direction. Strain and velocity integrands also showed the energies of these waves to only occupy up to 1% of the original absorbed photoexcitation energy. Convergent beam electron diffraction experimentally verified the presence of high 3.5% strains in the specimen via split higher-order Laue zone line analysis. However, the direction this time was found to be in the in-plane [bar-116] direction, perpendicular to the edge of the specimen. The high strains were hypothesized as extremely high strain states that exist for only short time periods, thus allowing the elastic condition of the stroboscopic experiment to remain true. However, the in-plane directionality of the strain provides an interesting perspective on whether or not bright-field scans of the specimen are truly capturing all dynamics, or if the previous discovery on tilt sensitivity is strong enough to suppress the contrast of other Lamb modes. Furthermore, an acoustic frequency conversion after more than 100 picoseconds from photoexcitation showed a sudden jump to the expected 35 GHz Lamb modes, suggesting the presence of other acoustic waves.