Initial process of the ferroelectric B₂ soft mode of KDP studied by the impulsive stimulated Raman scattering with heterodyne detection

The ferroelectric B 2 soft mode of KDP has been observed directly in time domain by the impulsive stimulated Raman scattering with a heterodyne detection method (HD-ISRS). HD-ISRS system gives the time dependence of the B 2 mode with an excellent signal to noise ratio near the ferroelectric phase transition temperature. With 80 femtosecond laser pulses, the initial process in the very early stage of the B 2 mode excitation has been discovered for the first time. The initial process is described well by a single exponential function with a time constant independent of temperature.