Modeling Changes in the Second Harmonic Generation of Ultrasonic Waves Having Wavelengths Beyond the Length Scale of Conventional Molecular Dynamics

The nonlinear ultrasonic (NLU) technique is attracting attention as a nondestructive method for the detection of nanostructures in crystalline materials. In this study, we developed a method to quantify the changes in NLU signals associated with nanostructures formation using molecular dynamics (MD) simulations. To achieve this, we used a nonreflective boundary, which reduces the computational cost to the first power of the wavelength; this is distinct from previous proposals using conventional MD method, for which the computational cost is proportional to the square of the wavelength. The nonreflective boundary eliminates the influence of reflected waves at the detection position by setting a buffer region at the end of the simulated cell opposite from the wave source, and the displacements and velocities of all atoms in this region are periodically reset. Using this method, we succeeded in speeding up the calculation by up to 4000 times while maintaining similar accuracy to that of the conventional MD method. Thus, it is possible to extend the NLU wavelength by approximately four orders of magnitude, which approaches the wavelengths used in actual inspections and, thus, simulate the changes in the NLU signals induced by nanostructures by MD. Consequently, this method will contribute to the development of a robust inspection technique based on scientific principles.