Measurement of thermal diffusivity of super lattice film by pico second pulse laser with variable wave length.

研制了一套测量10 ~ 1000 nm厚薄膜热扩散率的装置。用脉冲激光加热样品表面。由脉冲加热后辐照表面的温度衰减，导出了样品的热扩散率。为了避免薄膜下基底的影响，只需要表面层的信息。这对应于激光脉冲后温度衰减响应的小于100皮科秒的初始时间区域。脉冲激光源采用钛蓝宝石短脉冲激光器，半宽度为80飞秒。通过测量样品表面反射系数的变化来测量温度响应。测量了Mo膜的热扩散率。Mo膜的热扩散系数与体样品相比显著减小。同时，我们还开发了一个模拟激光闪光法的程序，该程序是基于一维晶格模型的调和势。首先产生一维排列的原子，它们通过谐振势相互作用，然后迫使最左边的原子以固定的周期振荡。在此基础上，利用分子动力学方法计算了系统中所有原子的运动，阐明了热脉冲在系统中的传递过程。在本研究中，假设原子的谐振势是公共的，只改变原子质量的排列。我们研究了两种类型的模型，即，均匀（所有质量相同）和超晶格系统（两种质量周期性重复）。在相同条件下进行了多次模拟运行，结果表明，与均匀系统的情况相比，在重原子周期性分散的超晶格系统中，热脉冲的倾倒急剧减少。

We developed the equipment to measure the thermal diffusivity of thin film of 10 to 1000nm thickness. The surface of sample is heated by pulse laser. From the temperature decay of the irradiated surface after pulse heating, the thermal diffusivity of sample is derived. To avoid the effect of substrate under the thin film, only information of surface layer is required. This corresponds to initial time region less than 100 pico second of temperature decay response after laser pulse. The Ti- Sapphire short pulse laser of 80 femto-second half-width time was used as the pulse laser source. The temperature response was measured by the change of reflection coefficient of surface of sample. The thermal diffusivity of Mo film was measured. Thermal diffusivity of Mo film is significantly small compared with bulk sample. Meanwhile, we developed a simulation program which mimics Laser flash method based on the one dimensional lattice model with harmonic potentials. One dimensionally arrayed atoms, which interacts each other via harmonic potential, were produced and then the most left sided atom was forced to oscillate in a fixed period. After that, the movements of all the atoms were computed by the molecular dynamics method to clarify the heat pulse transfer in this system. In the present study, the harmonic potential of the atoms was assumed to be common and only the arrangements of the masses of atoms were changed. We examined two types of model, i.e., homogeneous (all masses are same) and super lattice systems (two kinds of masses were repeated periodically). Many simulation runs were performed under the same conditions and the results showed that the dumping of the heat pulse was reduced drastically in the super lattice system in which heavy atoms were dispersed periodically compared to the case of the homogeneous system.