Relaxation of a nonequilibrium phonon distribution induced by femtosecond laser irradiation
Relaxation of a nonequilibrium phonon distribution induced by femtosecond laser irradiation
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DOI：
10.1103/physrevb.98.144306
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发表时间：
2018-10-18
影响因子：
3.7
通讯作者：
Rethfeld, Baerbel
中科院分区：
文献类型：
--
作者：
Klett, Isabel;Rethfeld, Baerbel
超快激光照射固体会导致材料的电子和声子子系统内部和之间的热力学不平衡。由于电子-电子和声子-声子碰撞，两个子系统在特征热化时间内弛豫到各自新的热力学平衡，这对它们中的每一个都是不同的。此外，它们通过电子-声子耦合来平衡它们的温度。电子非平衡态的弛豫及其对电子-声子耦合的影响是许多研究的主题，并且人们对它有很好的理解，而声子子系统内的非平衡态通常被忽略，非平衡声子对其他弛豫过程的影响也不清楚。我们的计算表明，电子和声子之间的能量转移率的显着差异取决于是否假设声子的非平衡分布。在这里，我们提出了一个模型来研究非平衡声子子系统的弛豫。声子之间的碰撞在玻尔兹曼积分的框架内描述。由此，可以提取依赖于能量的弛豫时间并将其插入弛豫时间方法中。在这个模型的框架内，我们研究了超快激光辐照下声子分布的热化。我们表明，这样的分布的热化时间是几百皮秒的顺序。此外，我们讨论了费米分布的电子和非平衡声子之间的能量转移，并将其与两个子系统中的平衡分布的能量转移进行比较。
Ultrafast laser irradiation of solids leads to a thermodynamic nonequilibrium within and between the electron and phonon subsystems of the material. Due to electron-electron and phonon-phonon collisions, both subsystems relax into respective new thermodynamic equilibria within a characteristic thermalization time, which is different for each one of them. Moreover, they equilibrate their temperatures by electron-phonon coupling. The relaxation of the electronic nonequilibrium and its effect on the electron-phonon coupling was the subject of a number of studies, and it is comparably well understood, while the nonequilibrium within the phononic subsystem is usually neglected and the influence of the nonequilibrium phonons on other relaxation processes is unclear. Our calculations show significant differences in the energy transfer rate between the electrons and the phonons depending on whether a nonequilibrium distribution is assumed for the phonons or not. Here, we present a model to study the relaxation of the nonequilibrium phonon subsystem. Collisions between phonons are described within the frame of Boltzmann integrals. From this, an energy-dependent relaxation time can be extracted and inserted into a relaxation-time approach. Within the frame of this model, we study the thermalization of a phonon distribution induced by ultrafast laser irradiation. We show that the thermalization time of such a distribution is of the order of some hundreds of picoseconds. Moreover, we discuss the energy transfer between Fermi-distributed electrons and nonequilibrium phonons and compare it to the energy transfer for equilibrium distributions in both subsystems.