Inversion of optical constants of natural silk fibers based on FDTD-PSO and scattering experiments

The optical constants of fiber materials are of great value in the study of the mechanism and application of radiation regulation, but the conventional methods for obtaining the optical constants of fiber materials suffer from a series of problems such as compositional differences, chemical residues, and microstructural damages, which make it difficult to obtain the optical constants of the fiber materials and make the final results of the measurements doubtful. In this work, a new intelligent inversion method is developed, based on the first-principles calculations of electromagnetic scattering using Finite Difference Time Domain method and particle swarm optimization algorithm (FDTD-PSO), to obtain the optical constants of fibers through the measurement of the scattered radiation properties of fiber materials and the characterization of their surface microstructures. The feasibility and accuracy of the method are demonstrated by theoretical numerical calculation simulations of different kinds of fibers, and the scale effect and error are analyzed from three aspects, namely, scale parameters, roughness and incident light angle direction. The results show that the dimensions, roughness and incident wavelength of the fiber material are in the sub-wavelength scale interval, which requires high model accuracy and gives the appropriate inversion range interval. Finally, the method is experimentally validated by using the natural silk fibers. This work constructs a complete set of theoretical models and experimental methods to accurately obtain the optical constants of actual fiber materials, which provides a new direction for obtaining the optical constants of fiber materials and a numerical basis for the study of the radiation modulation mechanism of fiber materials.