Questions about using mode source to solve the mode on the grating


Now I would like to solve the mode on my dielectric grating under single wavelength illumination. The information I would like to get from my simulation is

  1. The effective refractive index of the grating
  2. The mode on the gratings

I have several questions about using the mode source.

  1. What are the setting rules of the mode source?
    (the mode source dimension, the boundary condition of every direction and the distance between the source and FDTD box)
  2. How to make sure the simulation result is correct?

Please see attached file of my settings.
(The mode source is launched from the left side of the gratings and the surface plasmon mode will propagate along the +x direction.)

20191210PDMS_grating_wl785_mode source.fsp (394.6 KB)

Thank you.

Hi @shiang-yu.huang,

Thank you for your question. If you would like to find the effective index and mode profiles of the grating, it is probably best to use the FDE solver in MODE rather than FDTD. To see how to do this, you can take a look at this webinar or this KX post:

For a periodic structure like this you can also use the EME solver in MODE to determine the transmission for various lengths much faster than FDTD.

If you want to simulate this structure using FDTD, I would recommend extending the waveguide structure beyond where the grating begins, and placing the mode source a few wavelengths away from the grating. The span of your mode source should be large enough that the mode profiles are close to zero at the boundaries of the mode source region. Also, because your structure is uniform in the z direction, I would recommend using a 2D FDTD simulation.

I hope this helps. Let me know if you have any questions.

Best Regards,