Ultrafast Optical Excitation of a Combined Coherent-Squeezed Phonon field in SrTiO3.

We have simultaneously excited a coherent and a squeezed phonon field in SrTiO3 using femtosecond laser pulses and stimulated Raman scattering. The frequency of the coherent state (a 1.3 THz) is that of the A1g-component of the soft mode responsible for the cubic-tetragonal phase transformation at approximately 110 K. The squeezed field involves a continuum of transverse acoustic phonons dominated by a narrow peak in the density of states at a 6.9 THz.