Abstract We have used the ultrafast pump-probe technique known as picosecond ultrasonics to generate and detect coherent acoustic phonon pulses with frequencies reaching 40 GHz in exfoliated crystals of MoSe2 and WSe2 on Si and sapphire substrates. We report picosecond time-resolved reflectivity data from samples ranging from 180 nm to 920 nm in thickness and compare our results to a 1D simulation of strain-induced changes in the optical reflectivity. We find the longitudinal sound velocity along the c-axis (the interlayer direction) to be 2800 m/s ± 40 m/s for MoSe2 and 2510 m/s ± 60 m/s for WSe2. We also report the measured lifetime of longitudinal acoustic phonons approaching 40 GHz to be 0.85 ± 0.2 ns and compare this value with predictions of relaxation damping and 3-phonon models, as well as discuss its relationship to the predicted thermal conductivity of MoSe2.

中文翻译：

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2H-MoSe2 和 2H-WSe2 层间弹性性能的皮秒激光超声测量

摘要 我们已经使用称为皮秒超声波的超快泵浦探测技术在硅和蓝宝石衬底上的 MoSe2 和 WSe2 剥离晶体中产生和检测频率达到 40 GHz 的相干声学声子脉冲。我们报告了厚度从 180 nm 到 920 nm 的样品的皮秒时间分辨反射率数据，并将我们的结果与应变引起的光学反射率变化的一维模拟进行比较。我们发现沿 c 轴（层间方向）的纵向声速对于 MoSe2 为 2800 m/s ± 40 m/s，对于 WSe2 为 2510 m/s ± 60 m/s。我们还报告了接近 40 GHz 的纵向声学声子的测量寿命为 0.85 ± 0.2 ns，并将该值与弛豫阻尼和 3 声子模型的预测进行比较，