  Plane wave illumination with Gaussian beams
The present example considers a plane wave illumination with Gaussian beams.
Simulation model considers a cubic volume of free space (cube side = 100 mm), with absorbing exterior boundaries (marked as blue box), a surface for near-to-far field transformation (NTF box, marked as reen box) with each wall placed 5 mm from an absorbing boundary and a surface for free space incident wave excitation (plane wave box, marked as red box) with each wall placed 10 mm from an absorbing boundary. Plane wave illumination with Gaussian beams project in QW-Editor.
Illumination with a plane wave well approximates these physical cases where a spot size of the illuminating wave is larger than the scattering object�s dimensions. There are, however, numerous cases where the actual finite size of the spot must be considered and may be smaller than the object�s dimensions. This happens in measurements and scatterometry techniques.

Solutions to the Maxwell equations in free space that provide finite spots are Gaussian beams. The so-called 3D Gaussian beam propagating along a particular dimension focuses in the other two dimensions around a point called a neck centre, creating a Gaussian-shaped spot of size called a neck diameter; it then de-focuses beyond the neck. The so-called 2D Gaussian beam propagates along one dimension, focuses in the second dimension, and remains invariant along the third.

Both types of Gaussian beams are supported in QuickWave with B3D (3D Gaussian beam) and B2D (2D Gaussian beam) options. In the considered example 3D Gaussian beam is used. NeckX, NeckY, NeckZ are coordinates of the neck centre and N_Dia is neck diameter. For the 2D Gaussian beam one more parameter is defined under Beam 2D. Angle of variation determines the direction in which Gaussian shape is produced.

For the plane wave excitation there are three angles to be chosen:
φ (Phi) - azimuthal angle of the direction of wave propagation,
θ (Theta) - elevation angle of the direction of wave propagation,
Polarisation - polarisation angle of the electric field.

It is advised to refer to Free space incident wave for detailed discussion regarding free space excitation with a plane wave or Gaussian beam.

With the settings of Phi=0 degrees, Theta=0 degrees and Polarisation=0 degrees, the wave propagates along +Z axis with Ex, Hy fields (being the only fields in a plane wave, and the dominant ones in Gaussian beams). Configuration dialogue of a plane wave box excitation (free space incident wave). Distribution of time-maximum envelope of Ex field in the central vertical planes at 15 GHz. Distribution of time-maximum envelope of Ex field around the neck at 15 GHz.
Time dependent Ex field distribution around the neck at 15 GHz.