Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb_2Te_3 superlattices

摘要

Femtosecond dynamics of coherent optical phonons in GeTe/Sb_2Te_3 superlattices (SLs), a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity obtained in as-grown SLs exhibits the coherent A_1 optical modes at 5.10 and 3.78 THz while only the single A, mode at 3.68 THz is observed in annealed SLs. The decay rate of the A_1 mode in annealed SLs is strongly temperature dependent while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A, phonons in amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than the anhar-monic phonon-phonon coupling.