The pseudospectral time-domain method for light scattering

The pseudo spectral time domain (PSTD) method is one of the most accurate and efficient computational methods to solve Maxwell's equations for light scattering by arbitrary shaped particles. It discretizes both the spatial and time domain, and uses the spectral and finite difference method for the spatial and time derivatives, respectively. The near field is transformed into far field by a surface or volume integral methods to obtain the scattering properties of the particle, and the perfectly matched layer (PML) is used to truncate the spatial domain. A parallel PSTD code has been developed to calculate the light scattering by very large particles, and its performance is compared with that of a code using the the discrete dipole approximation (DDA). Combining considerations of the accuracy and CPU time, we find that the PSTD is a better method for large particle sizes x and large refractive indices m. That is, for each relative small refractive index, there is a critical size parameter above which the PSTD is the method of preference, and this value decreases from 80 to 10 as the refractive index increases from 1.2 to 1.6. Above m=1.6 the PSTD appears to perform better than ADDA for all size parameters larger than 10. The PSTD method is used to simulate lighter scattering by spheres with size parameter as large as 200, and shows high performance for randomly oriented non-spherical particles. The results are compared with results obtained using the analytic Mie theory and T-matrix method for spheres and some rotationally symmetric non-spherical particles.