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

## Transcript

The 2.5D variational FDTD, also called the varFDTD solver works by collapsing a 3D geometry

to 2D, and runs a 2D FDTD simulation which is much faster and uses less memory compared

to a 3D FDTD simulation.

The method used to reduce the 3D problem to an effective 2D problem will be discussed

in the Solver Physics section of the course.

Because the solver is based on the FDTD method, and many of the settings and workflow for

using the solver are shared with FDTD Solutions, we will not be covering the shared features

in detail in this course, and the FDTD 100 course is a recommended prerequisite.

As mentioned, the advantage of the varFDTD solver is that it allows us to simulate 3D

devices using a 2D FDTD simulation.

This is useful for simulating relatively large devices where the dimensions of the device

are hundreds of times larger than the wavelength, and for prototyping designs where you need

to run many simulations to sweep the parameters of the design since this will greatly speed

up the process compared to running 3D FDTD simulations.

A typical design process is to start by using the varFDTD solver for initial prototyping

to determine a range of design parameters that optimizes the performance of the device,

followed by verification of the results and extracting high accuracy results using 3D

FDTD or EME simulations.

Assumptions used by the varFDTD solver limit the type of devices that can be simulated.

The varFDTD solver is ideal for simulating devices where light travels within one plane

of the device, where there is no vertical coupling between different slab modes, or

polarization conversion from TE polarization to TM or vice versa.

Photonic integrated circuit components which are based on slab waveguide structures are

an example where the varFDTD solver works well since they often support two vertical

modes with different polarizations.

In the following unit, we will go over some examples to show when the varFDTD solver can

and cannot be used.

If you are unsure whether your device meets the assumptions of the method, you can test

a simple version of the device and verify the results by comparing with the results

from a 3D FDTD simulation, or EME simulation which does not require the above assumptions.