Direct observation of dynamics of thermal expansion using pump-probe high-energy-resolution x-ray diffraction

Time evolution of thermal expansion in the crystal lattice of diamond was studied on the time scale from 100 ps to 18 μs upon heating of the crystal through the entire thickness by a penetrating optical laser pulse of ≈8 ps in duration. Bragg reflectivity curves were measured at different instants of time using pump-probe high-energy-resolution x-ray diffraction with 10−8 strain sensitivity. The observed time-dependent variation of Bragg reflectivity was found to be in agreement with dynamical diffraction calculations for a crystal with propagating strain generated by thermally induced stress. The strain propagated at the speed of sound for longitudinal waves from both crystal surfaces into the bulk.