Anharmonic Lattice Dynamics In Germanium Measured With Ultrafast X-Ray Diffraction

The ultrafast, acoustic phonon dynamics in a germanium/silicon layered structure, measuring atomic vibrations with 10-fm resolution, were measured using ultrafast X-ray diffraction. The fluence dependence of the oscillations directly yields the temperature dependent coherence rates resulting from anharmonic lattice dynamics within the germanium film. The observed damping was attributed to the four-body elastic dephasing processes which originate largely from four-body interactions that cause decoherence in the evolution of the mode. Ultrafast X-ray diffraction allows direct measurement of coherent acoustic excitations before their thermalization with the environment