The extremely early stage of the ferroelectric B2 soft mode of KDP studied by the impulsive stimulated Raman scattering

It has been reported that the ferroelectric B-2 soft mode of KDP in time-domain was observed directly by the impulsive stimulated Raman scattering with a heterodyne detection method (HD-ISRS). With 80 femtosecond laser pulses, the initial process in the extremely early stage of the B-2 mode excitation observed for the first time in time-domain. The signals are analyzed using the modified Debye model (MDM) which gives phenomenologically the initial process of the Debye model. MDM produces a poor fit to the initial process of the B-2 mode in contrast to the good fit to its relaxation process. This indicates that a new model to describe a whole time dependence of the B-2 mode is needed.