I have been trying to use fdtd-solutions to compute the absorption and scattering spectra of a 35 nm diameter Au sphere in the presence of an external magnetic field. In order to properly treat the difference in permittivity experienced by right- and left-handed circularly polarized light, I’ve been utilizing a user-defined, 3D-sampled permittivity tensor (diagonal anisotropy), and then creating a permittivity rotation grid attribute (as described: https://kb.lumerical.com/en/materials_anisotropic.html). To generate the circularly polarized incident fields, I use two TFSF sources with identical positions & geometries, but with orthogonal polarizations and a relative phase shift of 90 degrees. However, the results I get from running the simulations seemed incorrect - when running the simulation as described above, I observed an additional spectral feature at ~675 nm, redshifted from the LSPR, which I did not expect, but persisted as I varied parameters such as the simulations time, early-termination cut-off, FDTD simulation domain size, mesh override region, etc…
After varying many different parameters, none of which solved this issue, I’ve finally begun to think that this may be a numerical artifact associated with the utilization of the permittivity rotation grid attribute. In an attempt to confirm this, I’ve run three simulations of the same physical situation (.fsp files attached), but with 1) a single linearly polarized source and no grid attribute, 2) circularly polarized source (as described above) and no grid attribute, 3) circularly polarized source with the permittivity rotation grid attribute enabled. In all cases, the strength of the external magnetic field was set to zero so that the permittivity tensor of the sphere is diagonal and isotropic (this is to ensure that circular and linear polarizations lead to identical absorption and scattering spectra - this was also confirmed using Mie theory). Therefore, I would expect all three methods should produce identical scattering and absorption spectra. Instead, I get:
Judging from these plots, it seems that enabling the grid attribute somehow affects the resulting spectra even though it should have no effect. Is this an effect that is well documented/expected for the types of simulations I am running? Any comments/guidance you could provide would be greatly appreciated in helping me understand if I am somehow setting up the simulations incorrectly, or if this is a shortcoming of the FDTD method.
Please let me know if any clarification of the problem is needed, and thanks very much in advance!
- linear polarization, no grid attribute: 35nm_Au_linear.fsp (364.4 KB)
- circular polarization, no grid attribute: 35nm_Au_m90_diag.fsp (402.6 KB)
- circular polarization, grid attribute enabled: 35nm_Au_m90.fsp (405.2 KB)
File containing Real & Imag. xx, yy, zz permittivity components used to generate the user-defined 3D-sampled material:
Au_anistropic_0T.txt (5.1 KB)