Ultrafast dynamics of coherent optical phonons in GeTe/Sb2Te3 superlattices: thermal conductivity and coherent control

Muneaki Hase; Junji Tominaga

doi:10.1117/12.908572

20 February 2012 Ultrafast dynamics of coherent optical phonons in GeTe/Sb₂Te₃ superlattices: thermal conductivity and coherent control

Muneaki Hase, Junji Tominaga

Author Affiliations +

Proceedings Volume 8260, Ultrafast Phenomena and Nanophotonics XVI; 82601G (2012) https://doi.org/10.1117/12.908572
Event: SPIE OPTO, 2012, San Francisco, California, United States

Abstract

We report on the evaluation of lattice thermal conductivity of GeTe/Sb₂Te₃ superlattice (SL) by using a coherent phonon spectroscopy at various lattice temperatures. The time-resolved transient reflectivity obtained in amorphous and crystalline GeTe/Sb₂Te₃ SL films exhibits the coherent A1 optical modes at terahertz (THz) frequencies with picoseconds dephasing time. The relaxation time and frequency of the coherent A1 modes are used to compute the lattice thermal conductivity based on the Debye theory, including scattering by grain boundary and point defect, umklapp process, and phonon resonant scattering. The results indicate that the thermal conductivity in the amorphous SL film is less temperature dependent, due to the dominant phonon-defect scattering, while in the crystalline SL it is temperature dependent because of the main contributions from umklapp and phonon resonant scatterings. We argue the higher thermal conductivity in the GeTe/Sb₂Te₃ SL films than that in the Ge₂Sb₂Te₅ alloy films implies that the phase change in GeTe/Sb₂Te₃ SL is not purely promoted by thermal process, i.e., lattice heating, but rather by nonthermal process, i.e., coherent lattice excitation, because the thermal process generally requires lower thermal conductivity.

Citation Download Citation

Muneaki Hase and Junji Tominaga "Ultrafast dynamics of coherent optical phonons in GeTe/Sb₂Te₃ superlattices: thermal conductivity and coherent control", Proc. SPIE 8260, Ultrafast Phenomena and Nanophotonics XVI, 82601G (20 February 2012); https://doi.org/10.1117/12.908572

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