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 image 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.