Acoustic phonons in nanostructures

利用反射激光可以观察到嵌在玻璃块中的金属小球的振动。稳定光束的波长随拉曼散射中振动频率的不同而上下移动。使用短于十亿分之一秒的激光脉冲可以更直接地观察到振动。为了计算这些振动的频率，需要进行理论工作，考虑金属的弹性响应以及金属球体的形状和环境。使用分子动力学模拟，其中许多(多达100000)点质量的位置和速度是在计算机上随时间计算的。运动的傅里叶变换被用来求出各个振型的频率。具有方向相关(各向异性)声速的材料可以用这种方法模拟。也可以处理非球形的纳米团簇。最后，可以研究周围材料(如玻璃块)的效果。这些研究的结果将使我们对非常小的物体的机械特性有新的认识。当设计非常小的机械设备时，这一知识具有潜在的未来应用。

Vibrations of small metal spheres embedded in a glass block can be observed using reflected laser light. The wavelength of a steady laser beam is shifted up or down according to the frequency of the vibration in Raman scattering. The vibrations can be more directly observed using brief pulses of laser light shorter than one billionth of a second. Theoretical work is needed to calculate the frequencies of these vibrations, taking into consideration the elastic response of the metal as well as the shape and surroundings of the metal sphere. Molecular dynamics simulations, in which the positions and velocities of many (up to 100000) point masses are calculated with time on a computer, are used. The Fourier transform of the motion is used to find the frequencies of individual modes. Materials with direction dependent (anisotropic) speed of sound can be simulated in this way. Nanoclusters with non-spherical shape can also be handled. Finally, the effect of a surrounding material, such as a glass block, can be studied. The results of these investigations will give new insight into the mechanical properties of very small objects. This knowledge has potential future application when very small mechanical devices are designed.