Abstract
Optical excitation of matter with linearly-polarized femtosecond pulses creates a transient non-equilibrium lattice displacement along a certain direction. Here, the pump and probe pulse polarization dependence of the photo-induced ultrafast lattice dynamics in (GeTe)2/(Sb2Te3)4 interfacial phase change memory material is investigated under obliquely incident conditions. Drastic pump polarization dependence of the coherent phonon amplitude is observed when the probe polarization angle is parallel to the c–axis of the sample, while the pump polarization dependence is negligible when the probe polarization angle is perpendicular to the c–axis. The enhancement of phonon oscillation amplitude due to pump polarization rotation for a specific probe polarization angle is only found in the early time stage (≤2 ps). These results indicate that the origin of the pump and probe polarization dependence is dominantly attributable to the anisotropically-formed photo-excited carriers which cause the directional lattice dynamics.
Highlights
Disks and nonvolatile electrical memories[15]
It was found that anisotropic phonon response is related to a phase transition in cubic iPCM material, which is different from the currently-used hexagonal iPCM7
The hexagonal iPCM material is mainly investigated for data storage applications because of good repeatability of SET-RESET cycles caused by small volume change upon phase change operation in contrast to the cubic iPCM material
Summary
Disks and nonvolatile electrical memories[15]. Among the various kinds of phase change memory, iPCM is one of the most promising candidates for the next-generation of non-volatile electrical memory and has received considerable attention due to its topological insulating nature[16,17], which arises from its multi-layered structure[18,19]. It was found that anisotropic phonon response is related to a phase transition in cubic iPCM material, which is different from the currently-used hexagonal iPCM7. The hexagonal iPCM material is mainly investigated for data storage applications because of good repeatability of SET-RESET cycles caused by small volume change upon phase change operation in contrast to the cubic iPCM material. The hexagonal iPCM is expected to present more significant anisotropy in coherent phonon response than the cubic iPCM material because the hexagonal structure is likely to emphasize the contrast between the basal plane and the stacking axis (c–axis) directional phonon response. Thereby, we investigated the polarization dependence of coherent phonons in hexagonal iPCM material
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