# EME Setup Tab — Lesson 2

### Cell Geometry

The Cell Geometry settings define the geometry of the Solver Region as well as the number and geometry of the cells in the propagation direction. In this video, you will learn how the various setup parameters under the Cell Geometry section work.

### Cell Group Definition

The cell group definition table divides the solver region into smaller groups and cells based on the cross-sections of the device and how the modes are solved in each cell. In this video, you will learn how to define the number, span and type of cells, span of cell groups and the number of modes.

### Cell Geometry Demo

In this video, we will use a tapered waveguide example to demonstrate how to set up the cell geometry under the EME setup tab of the EME solver object. We will also learn about modifying custom settings to limit the search for the modes to regions with specific index values. The example file used at the start of the following demonstration can be downloaded here. The resulting simulation file can be downloaded here.

### Periodicity

The periodicity section of the EME setup tab allows you to simulate periodic devices in a very efficient manner. The key point to understand is that the modes in each unit cell are the same. Therefore, it is only necessary to calculate modes in one unit cell. The periodicity is included in the EME simulation during the propagation step.

The periodicity section includes the following settings:

1. NUMBER OF PERIODIC GROUPS: Number of regions of the structure that are periodic.

2. PERIODIC GROUP DEFINITION: Defines the cell regions that have periodicity and the number of periods in each region.

3. CELL GROUP SEQUENCE: Displays the current setup and periodicity. For example, [1,(2,3)^3,4] indicates that there are four cell groups and the second/third cell groups are repeated three times.

Note regarding Cell group periodicity and EME profile monitors

EME field profile monitors should not be used with the Cell group periodicity feature. EME profile monitors will not correctly reconstruct the field profile when Cell group periodicity is used.

### Periodicity Demo

Lumerical EME represents the periodicity of a structure in an algebraic format. In this video, we will demonstrate how to define the cell group sequence to represent a waveguide cavity sandwiched by Bragg mirrors.

The simulation file shown in this video can be downloaded from the Phase-shifted Bragg grating example.