This video is taken from the FDTD Learning Track on Ansys Innovation Courses.
This is a 2D FDTD simulation region with one unit cell of a grating structure in the simulation
Bloch boundary conditions are set at the x min and x max boundaries of the simulation
region since the structure is periodic and the plane wave that will be added will be
injecting light at a non-normal incident angle.
PML boundaries are used above and below the structure in the y direction to absorb transmitted
and reflected light from the structure.
A frequency domain profile monitor is used to measure the fields over space in the XY
plane, and a linear power monitor is placed near the y max boundary of the simulation
region to measure the reflected fields.
To set up the plane wave source incident on the grating structure from the air above the
structure, start by adding the plane wave source from the Sources menu in the main toolbar.
Click on the edit button or use the E keyboard shortcut to open the source edit window.
In the last unit, we went over the three available plane wave types: Bloch/periodic, BFAST, and
Remember that the Bloch/periodic type is recommended for simulating an infinitely wide plane wave
source for plane waves injected at normal incidence or single frequency simulations
at angled incidence.
If the broadband response for an angled source is required then choose the BFAST planewave
Here we will choose the Bloch/periodic plane wave type.
Set the injection axis to y-axis and the direction to Backward to inject the source towards the
negative y direction.
Set the angle theta to 15 degrees and leave angle phi at 0 degrees to inject at 15 degrees
from normal relative to the XY plane, and set the polarization angle to 90 degrees to
rotate the direction of electric field polarization from the x-direction to the z-direction.
Next, in the Geometry tab, set the x span of the source to 5 microns which is wide enough
to span the entire x span of the simulation region.
I'll set the y position to 2 microns so that the source is injected from the air above
the structure, and the z position and span will not be used since the simulation region
is 2D, which means it will be as though the structure and source are uniform and infinite
in the z dimension.
Next, in the Frequency/Wavelength tab, set the start and stop wavelength both to 0.8
microns which is the wavelength of interest.
Check the beam options tab which shows a plot of the injected angle of the plane wave over
Since we have a single wavelength source injected, there is only one point on this plot, however,
if you were injecting a broadband source at an angle you would see that the angle will
vary over wavelength when using the Bloch/periodic plane wave type.
The BFAST source type can be used to get a constant angle of injection over wavelength.
Now click OK to accept the settings.
Check the CAD view ports to make sure that the orientation of the electric fields is
in the z-direction indicated by the blue arrow.
And the injection direction indicated by the pink arrow is angled as expected.
I can also make sure that the white shaded injection region is not overlapping with the
reflection monitor or intersecting with the grating structure.
You can also run the simulation to confirm that the source is injecting the expected
Right-click on the structure group and disable the grating structure temporarily and run
the simulation to simulate the propagation of the plane wave in free space.
After the simulation has completed, right-click on the profile monitor and visualize the E
result to plot the electric field profile.
By default, the plot shows the magnitude of the electric fields over space so you can
see that the fields have an amplitude of 1 in front of the source and 0 behind the source
since there is no reflection.
Under the vector operation column choose the field component to plot.
Since the source is Ez polarized, the Ex and Ey field components are 0 and the Ez field
component shows the angled phase fronts of the plane wave as expected.
Next, right click on the profile monitor and visualize the P result which is the Poynting
The direction of the Poynting vector shows the direction of power flow.
Select the vector plot type.
Here you can see the arrows in the vector plot showing the angled direction of power
The amplitude of the Poynting vector is indicated by the color scale shown on the right.
Switch back to layout mode by clicking the Layout button and re-enable the structure
group and re-run the simulation.
After the simulation has been run, right-click on the monitor above the source and plot the
T result which gives the fraction of injected power which gets reflected by the structure
at a wavelength of 0.8 microns.
Let's review some points to remember about plane wave source types and boundary conditions.
If simulating a Bloch/periodic type plane wave, always use periodic or Bloch boundary
conditions at the sides of the simulation region in the periodic direction.
If PML or metal boundaries are used at the sides there will be edge effects.
When using the BFAST type plane wave, BFAST boundaries will automatically be used in the
directions of periodicity, overriding any other boundary conditions settings in these
When using the Bloch/periodic or BFAST source types, if the source span that you set doesn't
extend the entire span of the simulation region, the source span will be automatically extended
to fill the full width of the simulation region.
This means that if you want to simulate a diffracting plane wave as though the plane
wave is diffracting through an aperture of a specific size, the "Diffracting" type plane
wave must be used.
Finally, the BFAST method should be used to simulate broadband angled injection where
the light is injected at a fixed angle over all wavelengths.
And for more details about BFAST method, see the links below this video.