Femtosecond pump-probe study of coherent soft phonon in Pb1−xGexTe ferroelectrics

Under the high-density photoexcitation by ultrashort laser pulses, strong coupling between photogenerated carriers and phonons as well as anharmonic phonon-phonon coupling may cause structural phase transition in ferroelectric materials. In the present paper, we report on the ultrafast dynamics of the soft mode (A1 symmetry) in ferroelectric semiconductor Pb1−xGexTe (x=0.07) under the high-density photoexcitation. With the increase of the pump fluence, the coherent A1 phonon in Pb1−xGexTe exhibits linear increase in the amplitude and the softening of the frequency, the latter of which is explained by carrier-phonon interaction. The damping rate also gradually increases with the pump fluence, which is due to the anharmonic phonon-phonon coupling.