Photo-induced dynamics in complex materials probed with femtosecond x-rays and few-cycle optical pulses

This thesis reports on experiments and simulations of ultrafast dynamics in correlated materials. Using some of the shortest pulses of light currently available, the response of correlated materials to prompt excitation is probed on their fundamental timescale. Experiments are performed over a broad spectral range, from IR to the hard X-rays, highlighting physics unseen to date. Photo-induced melting of the Mott state in the 1D organic (BEDT-TTF)-F2TCNQ was measured using frequency resolved IR pump-probe spectroscopy with 10fs resolution. By detecting the change in reflectivity at different wavelengths, the melting of Mott phase was observed, and compared to a many-body quantum model to reveal the formation of a coherent state that rapidly de-phased into a collection of localized excitons. Colossal magneto-resistive manganites were also measured with 10fs resolution, focusing on transitions at visible wavelengths. The photo-induced dynamics of the parent compound, LaMnO3, demonstrate the coupling between the light field, electrons, lattice, spins and orbital degrees of freedom. These results provide a mechanism through-which light can influence the magnetic state of the material. The photo-induced phase transition in the doped manganite Pr0.7Ca0.3MnO3 was investigated by measuring the timescale on which the phase transition occurs. Evidence for the observation of a high-energy quasi-particle called the orbiton is also presented and discussed. Time resolved X-ray absorption spectroscopy was used to understandthenatureofthephoto-inducedphasebymeasuringthechangesintheunoccupied electronic density of states near the Fermi-level. Finally, simulations of phonon-polariton propagation were performed in order to interpret femtosecond hard X-ray diffraction measurements in LiTaO3. The simulations showed that the lattice vibrates at a frequency of 1.2THz, with atomic displacements of order 5m˚