Multiscale simulation of terahertz radiation process in benzimidazole crystal by impulsive stimulated Raman scattering.

Comprehensive dynamics of coupled light wave and molecules in the terahertz wave generation process in an organic molecular crystal solid, 5,6-dichloro-2-methylbenzimidazole (DCMBI), induced by impulsive stimulated Raman scattering has been described by our previously developed multi-scale simulation, Maxwell + polarizable molecular dynamics method, where the propagation of macroscopic electromagnetic fields and microscopic molecular dynamics based on the force field model are numerically solved in the time domain. It has shown the behaviors of the excitation of Raman-active phonon modes by the irradiated pulse and terahertz radiation by molecular motions of infrared-active modes. Simulations of terahertz absorption and Raman spectroscopies of the DCMBI solid have also been performed to verify the applicability of the method to the terahertz optics. The calculated spectra are compared with the experimental measurements, showing good agreement. The detailed motions of the interacting electromagnetic fields and molecules occurred in the terahertz spectroscopies have also been provided, and the analyses have shown that rotational motions of the DCMBI molecules play key roles in the terahertz wave generation.