Even that is still too course as you have a 0.46 um source and high index contrast. With any FDTD engine you need a bare minimum of 10 points per wavelength in order for the simulation to provide reliable results. So you are looking at something closer to 0.01 um just to start you analysis. You are attempting to model a very large (in terms of wavelengths) domain which is going to require significant computational resources and time to run a simulation.