Lumerical Question about Computation Time in FDTD study - grating coupler at 4200nm wavelength
I have built a model to run the particle swarm optimization for a basic 2-D grating coupler with a gaussian input in Lumerical. The original model was built to optimize a SOI grating coupler at a wavelength of 1550nm with a scalar gaussian source (the typical case study for these couplers). Running the particle swarm on this model with 4 different variables (grating period, fill factor, source x-position, angle of source input) to maximize the transmission into the waveguide takes <20 mins with nice results at a mesh setting of 3 by default.
I have changed this same model to optimize an air clad, silicon-on-sapphire (SOS) grating coupler at 4275nm wavelength with a gaussian source (thin lens). I added the sapphire material into the library via its Sellmeier formula. With the same mesh settings as the previous model this one takes hours upon hours to finish (I can't say how long as it has not finished).
Any ideas as to why there is such a HUGE difference in computation time? Is it the wavelength being much longer (if I run the SOS model at a wavelength of 1550nm it goes much faster)? Perhaps I need to change the mesh?
Or am I missing something/doing something wrong?
I can provide my file if that would help?