Abstract

Generation of coherent optic phonons through impulsive stimulated Raman scattering (ISRS) can occur whenever a sufficiently short optical pulse passes through a Raman active medium. The first experiments demonstrating ISRS were performed with α-perylene molecular crystals¹; subsequently, similar observations were reported for a variety of other materials. One difficulty which can be associated with the impulsive scattering technique is an inherent lack of mode selectivity. In this paper we demonstrate multiple pulse impulsive stimulated Raman scattering, a powerful new technique for achieving mode selectivity in ISRS experiments. By using phase-only spectral filtering techniques, we efficiently generate terahertz repetition-rate bursts of femtosecond pulses,² which we use for repetitive impulsive excitation in our experiments. Individual phonon modes are selected by tuning the pulse repetition rate to the vibrational period. We demonstrate this concept by selectively and individually exciting the 104-cm^™1 vibrational mode and the 80-cm^™1 librational mode in α-perylene molecular crystals. This technique permits direct observation of the individual cycles of the vibrational and librational oscillations, without the mode beating which occurred in previous experiments,¹ and makes possible accurate measurements of the dephasing times.

© 1989 Optical Society of America