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

我们研制了测量厚度为10~1000 nm的薄膜热扩散率的装置。用脉冲激光对样品表面进行加热。根据脉冲加热后被辐照表面的温度衰减，推导出样品的热扩散率。为了避免薄膜下衬底的影响，只需要表层的信息。这对应于激光脉冲后温度衰减响应的初始时间区域小于100皮秒。脉冲光源采用80飞秒半宽时间的钛宝石短脉冲激光器。通过样品表面反射系数的变化来测量样品的温度响应。测量了Mo膜的热扩散率。与块体样品相比，Mo薄膜的热扩散率明显减小。同时，基于一维谐振势晶格模型，开发了模拟激光闪光法的模拟程序。产生一维排列的原子，它们通过谐振势相互作用，然后最左侧的原子被强迫以固定的周期振荡。之后，用分子动力学方法计算了所有原子的运动，以阐明热脉冲在该体系中的传输。在本研究中，假设原子的谐和势是公共的，并且只改变原子质量的排列。我们考察了两种类型的模型，即均匀(所有质量相同)和超晶格系统(两种质量周期性重复)。在相同的条件下进行了多次模拟，结果表明，与均匀体系相比，周期性分散重原子的超晶格体系的热脉冲倾倒大大减少。

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.