Femtosecond Dynamics of Photoinduced Phase Transitions

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The ultrafast dynamics of the photoinduce phase transitions has not been well understood before starting this project. We studied the dynamic behavior of these phenomena in pico-and femto-second regions, by using time-resolved luminescence, reflectance and Raman spectroscopy. The materials studied are (1)quasi-one-dimensional platinum complexes, (2)nickel complexes, (3)vanadium bronze, (4)divalent ion spin-cross-over complexes, and (5)quantum paraelectrics.Followings are the main achievements.1.In quasi-one-dimensional halogen-bridged platinum complexes, we investigated the self-trapped excitons, which are important as the intermediate states of photoinduced lattice rearrangements, by using femtosecond luminescence spectroscopy. The motion of the wave-packets is captured as a movie for the first time. Importance of the non-radiative transitions has been revealed, based on the halogen-ion dependence of the decay dynamics.2.In Ni-Br complexes, we found a luminescence with a large Stokes … More shift, which shows the importance of electron-lattice interactions in the excited state, although it has been believed that the electron correlation is more important than the electron-phonon interaction in the ground state.3.We applied femtosecond spectroscopy to the spin ladder system, α'-NaV_2O_5 for the first time. Elementary excitations were coherently generated and a new mode was found near the twice of the spin gap energy. The charge disordering process induced by photo-excitation has been observed as a reflectance change and a response faster than thermal effect was found. Photo-induced phase transition or charge disordering is suggested to be responsible for this phenomenon. These investigations opened the doorway to dynamical studies in this material.4.In spin crossover complexes containing divalent Fe ions, we found a new phase that cannot be produced through thermal process. By tuning the excitation photon energy, specific spin states are generated selectively.5.A giant dielectric response was found in SrTiO_3 and KTaO_3, by irradiating with ultraviolet photons. Less