We have studied the mechanism of coherent acoustic phonon generation in gold nanofilm induced by ultrafast laser-heating. Under the non-equilibrium condition when the lattice heating time is much longer than the film vibration period, we clearly identified the contribution of electronic thermal stress to drive the lattice motion and successfully measured the electronic Grüneisen parameter γ^e to be 1.6 ± 0.3. We also found that lattice heating via the electron–phonon coupling process lagged behind the coherent lattice motion, which we attributed to the prolonged thermalization process of the laser-excited non-thermal electrons under high pumping conditions. By taking such a process into account, the improved model fit our experimental data much better, and the extracted γ^e of gold was still around 1.6.

中文翻译：

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激光加热金纳米膜中相干声子的产生

我们已经研究了超快激光加热在金纳米膜中产生相干声子的机理。下，当晶格加热时间比膜振动周期要长得多的非平衡条件下，我们清楚地识别电子热应力的贡献，以驱动晶格运动和成功地测量所述电子Grüneisen参数γ ^ë为1.6±0.3。我们还发现，通过电子-声子耦合过程进行的晶格加热落后于相干晶格运动，这归因于在高泵浦条件下激光激发的非热电子的长时间热化过程。通过采用这样的工艺考虑，改进的模型适合我们的实验数据要好得多，并且提取的γ ^Ë 的黄金仍在1.6附近。