We start off with the same simulation file as was shown in the demonstration for the
index monitor which contains a photonic crystal cavity resonator made of a dielectric slab
with a pattern of holes etched into the slab forming the cavity.
Symmetry is used in the simulation region in the x and y directions to reduce the memory
Here we want to add a time monitor to measure the resonant fields of the cavity and look
at the spectrum of the fields to determine the resonant frequency that is excited.
Add a time monitor from the monitors drop down menu of the main toolbar.
Edit the monitor, and set the start time to 75 fs.
This sets the time at which the time monitor starts measuring field data.
By delaying the time, I can avoid measuring the initial source pulse which excites the
resonance of the cavity and only measures the resonant fields of the excited mode.
This means that in the spectrum result that I’ll plot later, it won’t include the
spectrum of the source pulse, only the spectrum of resonant modes.
In the Geometry tab, set the x, y, and z position of the point monitor to 0.1 microns.
The reason I offset the position of the monitor from the center is that sometimes the resonant
mode will have a node at the center meaning that the fields of the mode are 0 at the center,
and offsetting the position where I measure the fields can reduce the chance that the
monitor won’t record any resonant fields.
Run the simulation.
After the simulation has completed, right-click and visualize E which generates a plot of
the electric field amplitude over time.
I can see that the time starts at 100 femtoseconds so I don’t see the signal of the source
pulse and I can only see the decaying resonant fields of the cavity over time.
Next, right-click and visualize spectrum.
Here I can see a sharp peak at around 0.5 microns.
If I zoom in and hover over the data point at the peak, I can see that it is at around
Now that we’ve seen a demonstration of the setup of the time monitor, let’s go over
some setup tips.
We can set the recording time to include only the time window that we are interested in,
like in the example we just saw, we were able to exclude the fields due to the initial source
pulse and only record the resonant fields of the cavity by setting the start time for
We can plot the fields over time to check that the fields have decayed by the end of
the simulation time.
In the previous simulation, we also used it to check that the start time that we set was
late enough that it did not include the source pulse.
To be able to get the spectrum result, which is Fourier Transform of the time domain
data, the time monitor type must be a point monitor.
If you want to get the spectrum at multiple locations, you can use multiple point time
Using point monitors is also more efficient in terms of memory compared to using 2D or