Ultrafast optical excitation of a combined coherent-squeezed phonon field in SrTiO₃

Abstract

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 (~1.3 THz) is that of the A_1g-component of the soft mode responsible for the cubic-tetragonal phase transformation at ≈ 110 K. The squeezed field involves a continuum of transverse acoustic phonons dominated by a narrow peak in the density of states at ~ 6.9 THz.