In OptiFDTD, transverse Gaussian Beam is expressed as
where x0 is the center position and T is being called as half width.
In general, Gaussian Beam radius is the radius at which the field amplitude and
intensity drop to 1/e and 1 / e2, respectively. Gaussian Beam Size or Gaussian
Beam Spot size is the beam diameter which is two times of Beam Radius. Suppose
the beam size is denoted as a, which means
Please use the above formula to set the half width in OptiFDTD for your desired
Gaussian Beam size.