Here we have a silicon-on-insulator taper structure which converts incoming light from
a wide silicon slab to a narrow silicon waveguide.
The solver region has already been set up, and you will notice that the slab mode position
and test points indicated by the green and blue crosses are placed outside of the solver
It is actually fine to place these outside of the solver region as long as the cross
section along z is correct.
With the current slab mode position which is at the location of the green cross, this
will include the cross section of the structure including air, silicon, and the glass substrate.
If you edit the varFDTD solver region and use the “user select” option, you can
verify the material refractive index profile along z.
You can also use the “user select” option to select higher order slab modes, however,
this particular structure only supports one TE mode and one TM mode in the silicon slab.
The other modes found in the mode list have strong fields outside the silicon slab in
the glass substrate region.
Change the polarization selection back to “E mode (TE)”.
You can see from the plot below that the fields of this mode are confined in the region of
the silicon slab.
In the graphical XY view port, you can select and drag the position of the slab mode and
The test points are only used to report the calculated effective material properties,
and don’t have any impact on the simulation results, but if I drag the slab mode position
to a region where the silicon is not present, this will change the cross section used for
calculating the slab mode which will affect the simulation results.
After moving the slab mode position, edit the solver region again, and you can see that
the fundamental TE mode now has fields primarily in the glass region since the silicon slab
is no longer present in the structure cross section, and this slab mode would not allow
you to inject light into the silicon slab.