Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals
Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals
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DOI：
10.1103/physrevb.71.184301
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发表时间：
2005-05-01
影响因子：
3.7
通讯作者：
Kitajima, M
中科院分区：
文献类型：
--
作者：
Hase, M;Ishioka, K;Kitajima, M
飞秒光学泵浦探针技术用于研究过渡金属 Zn 和 Cd 中光激发电子和相干光学声子的动力学随温度和激发能级的变化。时域中的光学响应通过相干 E-2g 声子激发引起的阻尼谐波振荡和电子声子热化引起的亚皮秒瞬态响应的线性组合很好地拟合。电子-声子热化时间随温度单调增加，与热调制场景一致，在高温下，系统可以通过二温模型很好地解释，而在低于大约 50 K 的温度下，需要应用非热电子模型。随着晶格温度的升高，相干E-2g声子的阻尼增加，而快速电子响应和相干E-2g声子的振幅减小。 E-2g 声子阻尼的温度依赖性表明，由于非谐波声子-声子耦合导致的相干光学声子的布居衰变主导了衰变过程。我们提出了一个模型，该模型考虑了假设光致吸收机制所观察到的振幅的温度依赖性，其中信号振幅与准粒子密度的光致变化成正比。 E-2g 声子的振幅遵循快速电子瞬态振幅的温度依赖性的结果表明，在谐振条件下，电子和声子响应都与介电函数的变化成正比。
The femtosecond optical pump-probe technique was used to study dynamics of photoexcited electrons and coherent optical phonons in transition metals Zn and Cd as a function of temperature and excitation level. The optical response in time domain is well fitted by linear combination of a damped harmonic oscillation because of excitation of coherent E-2g phonon and a subpicosecond transient response due to electron-phonon thermalization. The electron-phonon thermalization time monotonically increases with temperature, consistent with the thermomodulation scenario, where at high temperatures the system can be well explained by the two-temperature model, while below approximate to 50 K the nonthermal electron model needs to be applied. As the lattice temperature increases, the damping of the coherent E-2g phonon increases, while the amplitudes of both fast electronic response and the coherent E-2g phonon decrease. The temperature dependence of the damping of the E-2g phonon indicates that population decay of the coherent optical phonon due to anharmonic phonon-phonon coupling dominates the decay process. We present a model that accounts for the observed temperature dependence of the amplitude assuming the photoinduced absorption mechanism, where the signal amplitude is proportional to the photoinduced change in the quasiparticle density. The result that the amplitude of the E-2g phonon follows the temperature dependence of the amplitude of the fast electronic transient indicates that under the resonant condition both electronic and phononic responses are proportional to the change in the dielectric function.