School of Physics, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa.
J Acoust Soc Am. 2012 Jun;131(6):4491-9. doi: 10.1121/1.4714353.
Propagation of a short acoustic pulse through a polycrystalline film comprised of large randomly oriented elastically anisotropic grains is analyzed theoretically. For average grain size much larger than the film thickness, a short acoustic pulse launched normally into the film will traverse each grain in a time determined by the acoustic slowness in the direction normal to the film, which will depend on the local grain orientation. A typical measurement averages over a large number of grains resulting in the broadening of the composite output pulse. The resulting pulse shape is characterized by distinct features related to stationary values of the directionally dependent acoustic slowness of the crystalline material. Maxima and minima in the slowness yield discontinuities in the pulse shape, while saddle points yield logarithmic singularities. For cubic and hexagonal crystals, power law singularities result from cones of directions in which the slowness is a maximum or minimum. Numerical results, taking into account Gaussian broadening of the input pulse, are presented for thin film materials commonly encountered in picosecond ultrasonic experiments, such as copper, gold, and aluminum.
通过理论分析研究了由大尺寸各向异性弹性随机取向晶粒组成的多晶薄膜中短声脉冲的传播。对于平均晶粒尺寸远大于薄膜厚度的情况，垂直入射到薄膜中的短声脉冲将在由垂直于薄膜的声速决定的时间内穿过每个晶粒，该声速取决于局部晶粒取向。典型的测量是对大量晶粒进行平均，导致复合输出脉冲展宽。所得到的脉冲形状的特征是与晶体材料方向相关的声速的稳定值有关的明显特征。声速的极大值和极小值产生脉冲形状的不连续，而鞍点产生对数奇点。对于立方和六方晶体，来自于声速为极大值或极小值的方向的锥形产生幂律奇点。考虑到输入脉冲的高斯展宽，给出了皮秒超声实验中常见的薄膜材料（如铜、金和铝）的数值结果。
