In the "Boundary conditions" tab, you can select the boundary condition to apply for
each solver region boundary.
By default, the metal boundary condition is selected for all boundaries.
The other boundary conditions available are PML, Periodic, Symmetric, Anti-Symmetric and
Metal boundaries are perfect metal boundary conditions, they will totally reflect the
light, with no absorption.
PML are absorbing boundaries.
They simulate an infinite space within a finite computational grid.
Periodic boundaries are for periodic structure.
They allow to model only 1 single period.
Symmetric and anti-symmetric boundaries can be used when the structure and the fields
share the same symmetry.
They allow to reduce the simulation size by a factor or 2 or 4.
Refer to the knowledge base to see how to choose between symmetric and anti-symmetric
Finally, PMC are perfect magnetic boundaries.
Similar to Metal boundaries, they reflect 100% of the light with no absorption.
If there is no radiative loss, you should always start your simulations using Metal
They are faster and more computationally efficient than PML.
This is most easily understood in the following manner: If the fields are zero at the boundary,
which should be the case for bound modes), then the choice of boundary doesn't matter.
In addition to being faster, Metal boundaries allow you to avoid other numerical challenges
that come with using PML boundaries, namely artificial modes that are introduced by the
presence of the PML.
You can use symmetry and anti-symmetry to reduce the memory usage.
Using symmetry boundaries also means the solver will only find modes with the specified symmetry.
Other modes will not be found.
If you use anti-symmetry (here in the x direction), the solver will only find the TE modes.
On the other hand, it will only find the TM modes if we set the boundary condition to
Making the correct choice of symmetry vs anti-symmetry can be confusing, so we recommend starting
without using symmetry.
Once you can find the mode of interest without symmetry, try enabling symmetry.
If you select the correct option, you should find exactly the same mode, although some
of the other modes may disappear from the list if they have different symmetries.