Guilin Sun
Ansys Employee

From your description, most likely it is either due to PML: maybe too close to the nano structures, or fewer PML layers, or strong reflection at larger diffraciton angles, or not small enough the autoshut off min in FDTD "Advanced options". Please increase FDTD span along propagation direction, increase the PML distance and number of layers, and reduce the autoshutoff min to be 1e-6 or 1e-7, and make sure the simulation time is long enough and the simulation is terminated before the progress reaches 99%.