Acoustic phonons in nanostructures

嵌入玻璃块中的小金属球的振动可以用反射的激光观察到。稳定激光束的波长根据拉曼散射中振动的频率向上或向下移动。使用短于十亿分之一秒的激光脉冲可以更直接地观察到这种振动。考虑到金属的弹性响应以及金属球体的形状和周围环境，需要进行理论工作来计算这些振动的频率。分子动力学模拟，其中许多（多达100000）点质量的位置和速度计算在计算机上的时间，被使用。运动的傅里叶变换用于找出各个模态的频率。具有方向相关（各向异性）声速的材料可以用这种方法模拟。非球形纳米团簇也可以处理。最后，可以研究周围材料（如玻璃块）的影响。这些研究的结果将对非常小的物体的力学特性提供新的见解。当设计非常小的机械装置时，这种知识有潜在的未来应用。

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.