Femtosecond Time-Resolved Spectroscopy of Phonon Dynamics in Organic Molecular and Ferroelectric Crystals

The ultrafast time-resolved technique of impulsive stimulated Raman scattering (ISRS) is utilized to probe and manipulate low frequency phonons related to structural and chemical change in the solid state. The soft modes in the ferroelectric crystals KNbO_3 and BaTiO_3 are impulsively excited by ultrashort laser pulses and their time dependence accurately characterized as a function of temperature. In both crystals, the data show only the presence of the heavily damped soft mode and no relaxational modes of the same symmetry. Quantitative comparisons to other works are made and strong support for the "eight site" model is found. Previously observed anomalous polariton dynamics in the related material LiTaO _3 are also studied and explained through ISRS. The wavevector dependent appearance of a weak relaxational mode is observed and a quantitative explanation of the excess damping rate of the polariton is made. The advantages of ISRS over conventional Raman scattering techniques are illustrated. Novel femtosecond pulse shaping techniques are utilized in conjunction with ISRS in the organic molecular crystal alpha-perylene to demonstrate selective excitation of a desired lattice mode. By resonantly driving a particular phonon with a train of pulses spaced to match its vibrational period, mode selectivity is achieved because other modes rapidly become out of phase with the excitation force. Multiple pulse experiments in LiTaO _3, which allow for a far larger amount of energy to be deposited into lattice modes than alpha-perylene, are also discussed and explained in terms of the differing electronic responses of the two materials to the ultrashort pulses. The experiments have important consequences for selecting and amplifying the vibrational amplitudes of modes believed to be related to chemical and structural change. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.).