Size effects on phonon response of bismuth nanoislands to ultrafast laser excitation
Size effects on phonon response of bismuth nanoislands to ultrafast laser excitation
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DOI：
10.1016/j.jpcs.2019.06.014
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发表时间：
2019-11
影响因子：
4
通讯作者：
Hui Xiong;A. Esmail;H. Elsayed-Ali
中科院分区：
文献类型：
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作者：
Hui Xiong;A. Esmail;H. Elsayed-Ali
利用超快电子衍射研究了超快激光激发下铋纳米岛的晶格动力学。布拉格峰强度的衰减时间取决于衍射级。对于5 nm的平坦纳米岛，（110）衍射级的衰减时间接近A1 g光学声子的衰减时间。当厚度增加到约16 nm时，激发能被耗散到低能量声子中，使得能量传输扩散，从而导致更长的衰减时间。（012）布拉格衍射峰的时间演化示出了两个指数衰减，当纳米岛为5 nm厚并且激光能量密度低于1.5mJ/cm 2时，第二衰减时间为1.40ps。这种二次衰减可能是由于尺寸限制引起的声子瓶颈。
The lattice dynamics of bismuth nanoislands induced by ultrafast laser excitation is studied by ultrafast electron diffraction. The decay times of the Bragg peak intensity depend on the diffraction order. For 5-nm flat nanoislands, the decay time of the (110) diffraction order is close to theA1goptical phonon decay time. When the thickness is increased to ∼16 nm, the excitation energy is dissipated into low energy phonons, making energy transport diffusive, thus leading to a longer decay time. The temporal evolution of the (012) Bragg diffraction peak shows a two exponential decay with a second decay time of ∼40 ps emerging when the nanoislands are 5 nm thick and the laser fluence is lower than 1.5 mJ/cm2. This second decay may be attributed to a phonon bottleneck caused by size restriction.