Interband excitation and carrier relaxation as displacive driving force for coherent phonons

We report on displacive excitation of large amplitude coherent lattice motions in TiO₂ under resonant conditions using sub-6-femtosecond UV pulses. Calculations of non-equilibrium potential energy surfaces reveal a new displacive mechanism for the coherent phonon generation, where a unidirectional displacement of the phonon potential occurs both instantaneously due to carrier excitation and dynamically due to cooling of the hot photo-excited carriers. The carrier cooling dynamics in TiO₂ are faster than the phonon period, resulting in an initially anharmonic lattice motion. This effect manifests itself in an effective phase shift of the coherent phonon oscillation, in good agreement between our experiments and the calculations