  Time-resolved two color pump-probe polarization spectroscopy was performed at
room temperature on SrTiO3 films grown directly on Si with film thickness
varying from 2 nm to 7.8 nm. The E soft mode with a characteristic frequency of
0.2 THz is impulsively generated and measured in these coherently strained
tetragonal phase SrTiO3 thin films. Another over-damped signal observed
indicates the possible relaxational hopping of Ti ion between double potential
wells. The dependence of the coherent phonon signal on pump and probe laser
polarization helps to identify the phonon modes.
