飞秒激光热反射系统测量金属薄膜中的热波

金属材料中的热波现象可以利用包含电子弛豫时间影响的双曲两步模型进行理论分析.通过飞秒激光热反射实验系统对金属薄膜材料进行了测量.利用偏振分光棱镜将飞秒激光分成抽运光和探测光,其中较强的抽运光用于加热金属薄膜而较弱的探测光用于探测薄膜表面反射率随时间的变化,两束光之间的光程差通过步进电机进行精确控制.利用金属薄膜反射率和电子温度的正比例关系就可以得到电子温度随时间的变化规律.实验发现在加热激光脉冲过后的电子温度下降区间会出现另一个较弱的电子温度波峰,并利用相同厚度的两块金属薄膜样品重复测量对实验结果进行了验证.理论上这一现象可以解释为金属薄膜中热波在背面反射的结果,并且实验结果和双曲两步模型给出的热波理论计算结果相符合.根据实验结果计算出热波传递速度约为5×105m/s,对应的电子弛豫时间为60fs.

Measurement of the thermal wave in metal films using femtosecond laser thermoreflectance system

The thermal wave phenomenon induced by the ultrafast laser heating in metal films has been studied by different experimental methods in the past. In our experiment, a femtosecond laser pump-probe system is used to study the ultrafast energy transport between the electrons and phonons. The experiments showed a weak wave peak in the electron temperature curve following the main peak caused by the laser pulse. The reproducibility of the results has been tested for different laser intensities and different film samples. This weak peak in electron temperature could be explained by the reflection of the thermal wave from the rear surface. The experimental results agree with the theoretical curves calculated by the hyperbolic two-step model. The speed of the thermal wave is about 5×10⁵ m/s and the electron relaxation time is about 60 fs.