# 1.1 Main concepts

Please read the following two sections carefully as it gives the basic idea of what's going on and the definition of the term used. It is probably necessary for understanding of the rest of this manual.

PSlab works on a photonic structure defined by the user. This can be either a fully three-dimensional or two-dimensional one. In the latter case it can be assumed that the device has transational symmetry in one direction in case of Cartesian coordinates or is axisymmetric for cyllindrical coordinates.

Each structure in PSlab consist of the several layers. Regardless of the coordinate system (either Cartesian or cylindrical) the axis is always defined as the one perpendicular to theese layers. There can be many different layers but there are two important requirements:

1. the layer boundaries are planes paraller to each other and perpendicular to -axis (which is the consequence ot the abovementioned definition) and
2. in each layer the structure is uniform along -direction (see fig.); in case of non-planar constructs they must be approximated with several thin layers.

The PSlab software allows to determine the eigenmodes in the defined structure. This is done either be a searching of the eigenfrequency for an arbitrary wavevector, or by looking for a propagation constant at a fixed frequency1.1. When they are found it is possible to compute the field distibution in the whole structure which can be than saved to a file, ploted with some graphical library, or used in your further calculations.

#### Footnotes

... frequency1.1
In PSlab the frequency is always refered as the normalized angular frequency