Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices
Femtosecond pulses are used to excite A1g optical phonons in Bi2Te3, Sb2Te3, and Bi2Te3/Sb2Te3 superlattice. Time-resolved reflectivity measurements show both the low-frequency and high-frequency components of A1g phonon modes. By comparing the phonon lifetime, it is found that the scattering rate (inverse of lifetime) in superlattice is significantly higher than those in Bi2Te3 and Sb2Te3. This represents the direct measurement of coherent phonon lifetime reduction in superlattice structures, consistent with the observed reduction in thermal conductivity in superlattices.