This video is taken from the varFDTD 100 course on Lumerical University.

## Transcript

This unit shows the workflow for a varFDTD simulation.

We start by setting up the 3D device geometry where the light will propagate in the xy plane

of the device, and add the solver region where we can select

the vertical slab mode of the core waveguide.

This slab mode is calculated given the cross section of the device along the z-dimension

at the specified x,y position.

1D finite-difference Eigenmode (or FDE) calculation is performed to find the supported slab mode,

and we can plot the calculated slab mode profile as a function of z in the Effective index

tab of the solver region.

The solver region settings will be discussed in more detail in the next section of the

course.

The slab mode is used in the calculation of the effective 2D materials at each x, y position

of the solver region in order to collapse the 3D problem to a 2D problem, and this calculation

will be discussed in the following unit.

For broadband simulations, we can calculate the slab mode profile as a function of wavelength

and the effective material properties are calculated over the broadband range allowing

the simulation to account for material dispersion and waveguide dispersion.

Next, we can add sources to inject light and monitors to record the fields.

The mode source is a commonly used source, and it's important to select the mode to inject

from the mode source only after the slab mode has been selected in the varFDTD solver region,

since the modes are calculated given the effective 2D materials whose properties depend on the

slab mode.

Running the simulation will trigger the calculation of the all the effective material properties

required to represent the 3D geometry by a 2D set of effective materials, and a 2D FDTD

simulation is performed.

After the simulation has been run, the monitor results will be available.

The field data can be expanded along z by multiplying the result from the 2D simulation

with the vertical slab mode.

The key difference between varFDTD and a traditional 2D FDTD simulation is that varFDTD automatically

determines effective material properties for the 2D simulation that will give the closest

representation of the actual 3D device whereas a 2D FDTD simulation in FDTD Solutions will

simply take the cross section of the physical structure assuming that the structure is uniform

in the third dimension.