The purpose of this section is to describe how diffracting plane wave source works and how it differs from Bloch/periodic plane wave source. Well know double slit experiment is used to demonstrate this.

## Discussion and simulation setup

Diffracting plane wave source is a source in which plane wave propagates through a rectangular aperture. The size of the source defined on the Geometry tab is the dimension of the aperture. Additionally, diffraction pattern will be produced as the plane wave travels through the aperture. This differs from the Bloch/periodic plane wave source type, which always automatically expands across the size of the entire simulation region in order to simulate pure plane wave.
For this reason, diffracting plane wave source should be used only in simulations where the diffraction is a desirable effect. In case of the double slit experiment, the diffracting plane wave source allows us to replace each slit with a plane wave source instead of creating a structure representing the slits.
In this example, the sources are 12um apart and each source/slit has size of 2um. Frequency domain field and power monitor is used to represent the screen that is placed 58um from the slits. |

The following analytical formula can be used to calculate the spacing of the interference maxima on the projection plane:

$$ s=\frac{z \lambda}{d} $$

where:

z is the distance of the projection plane from the slits

d is the distance between the slits

lambda is wavelength

Simple calculation shows that the distance between the maxima at 633nm should be approximately 3.05um.

Tip: Reducing the simulation size To minimize the simulation time, it is generally recommended that you do not include large regions of empty space in a simulation. It would be possible to obtain these same results with a much smaller simulation region, by taking advantage of the far field projection functions. This example uses a large simulation region to keep the analysis as simple as possible, even though it is less computationally efficient. |

## Results

The simulation results shown on the figures below demonstrate the diffracting nature of the sources and their constructive and destructive interference. Moreover, the distance between the maxima is ~3.04um, which is well aligned with the analytical result above.