When using the eigensolver in MODE or the mode source in FDTD, there are many eigensolver settings that can affect the modes found. For some types of structures, finding the correct modes can be challenging. This page provides a checklist of common settings that should be checked or modified when you have trouble finding the right modes or you get the message "no physical modes were found".
It is often recommended that one starts with metal boundaries when looking for well confined modes. Please see Starting with metal boundary conditions for detailed information. If the right boundaries are not chosen, the right modes may not be found. The size of the simulation region is also important. When using metal boundaries, the boundaries should be far enough from the structure interfaces so that the evanescent tails of the modes do not reach the boundaries and get reflected.
If the mode you are interested in is radiative, you will need to use PML boundaries. Please see Lossy modes and dB/m to k conversion for more information.
Note: The Integrated Mode Source
When finding modes in the Mode Source, the default boundaries are metal. These boundary conditions can be changed as explained here.
Enforcing symmetry in one or two directions can greatly reduce the simulation time and memory; however, one should use these boundaries with care when solving for modes. Depending on the type of boundary used, symmetric or anti symmetric, only TE or only TM results could survive. Make sure you know the symmetry of the fields you are looking for before you apply symmetry boundaries. Please see Symmetry boundary conditions for detailed information on how to determine which symmetry boundary to use.
When using symmetry, sometimes it is necessary to "force symmetry" on the mesh as well. This option can be selected under the "advanced options" tab of the simulation region object.
Index to search near
One can specify the index to search near or a range of indices to search in between for modes of interest. The specified value is not just the real part of the index, rather it is the magnitude of the complex index.
If the mode is lossy, you may want to enter a complex value for this index. Please see Lossy modes and dB/m to k conversion for more information.
For the RF frequency range, if the structure is open (eg. transmission line embedded in free space):
- If the mode is guided, the guess index should be greater than or equal to the background index
- If the mode is radiating, the guess index should be less than or equal to the background index
If the structure is closed (eg. a hollow metallic waveguide):
- For frequencies near the mode's cutoff frequency, the guess index should be much less than the background index
- For frequencies away from the cutoff frequency, the guess index should be less than or equal to the background index
Number of trial modes
The number of modes specified to look for around a refractive index value will affect the modes that the solver finds. If this number is too small, the desired modes may not be found. Often, setting this number to 100 modes will ensure that no physical modes near the specified index have been missed. If more than 100 modes exist, then a larger number should be used if one is interested in higher order modes.