The Beam tab allows you to modify the default Gaussian beam for overlap calculations, as well as create Gaussian beams in the deck which will be accessible from the scripting environment. There are two types of Gaussian beams, and the user can choose between the scalar approximation for the electric field or the fully vectorial beam profile option:
For the Gaussian Beam Source, please see the Gaussian source
Fully vectorial beam
- NA: This is nsin(α) where n is the refractive index of the medium in which the source is found and α is the half angle as shown in the figure below. Please note that the index will not be correctly defined in dispersive media and lenses should only be used in non-dispersive media. The refractive index for the source is determined at X, Y (and Z).
- Distance from focus: The distance d from focus as shown in the figure below. A negative distance indicates a converging beam and a positive distance indicates a diverging beam.
- Fill lens: Checking this box indicates that the lens is illuminated with a plane wave which is clipped at the lens edge. If FILL LENS is checked, then it is possible to set the diameter of the thin lens (LENS DIAMETER) and the beam diameter prior to striking the lens (BEAM DIAMETER), as shown in the figure below. A beam diameter much larger than the lens diameter is equivalent to a filled lens.
- Number of plane waves: This is the number of plane waves used to construct the beam. The beam profile is more accurate as this number increases but the calculation takes longer.
Note : References for the thin lens source
The field profiles generated by the thin lens source are described in the following references. For uniform illumination (filled lens), the field distribution is precisely the same as in the papers. For non-uniform illumination at very high NA (numerical aperture), there are some subtle differences. This is due to a slightly different interpretation of whether the incident beam is a Gaussian in real space or in k-space. This difference is rarely of any practical importance because other factors such as the non-ideal lens properties become important at these very high NA systems
- Define Gaussian beam by : This menu is used to choose to define the scalar beam by the WAIST SIZE AND POSITION or the BEAM SIZE AND DIVERGENCE ANGLE.
If WAIST SIZE AND POSITION is chosen, the options are:
- Waist radius: 1/e field (1/e2 power) radius of the beam for a Gaussian beam, or a half-width half-maximum (HWHM) for the Cauchy/Lorentzian beam.
- Distance from waist: The distance, d, as shown in the figure below. A positive distance corresponds to a diverging beam, and a negative sign corresponds to a converging beam.
If BEAM SIZE AND DIVERGENCE ANGLE is chosen, the options are:
- Beam radius: 1/e field (1/e2 power) radius of the beam for a Gaussian beam, or a half-width half-maximum (HWHM) for the Cauchy/Lorentzian beam.
- Divergence angle: Angle of the radiation spread as measured in the far field, as shown in the figure below. A positive angle corresponds to a diverging beam and a negative angle corresponds to a converging beam.
For both types of beams:
- The polarization angle is defined with respect to the horizontal-axis for normal incidence fields. When the incidence is off-axis, the polarization angle should be 0 for p-polarized light and 90 for s-polarized light.
- The refractive index is the refractive index of the homogenous material in which the Gaussian beam is found.
- The angle theta is the angle between the normal-axis and the direction of propagation.
- The angle phi is the angle between the direction of propagation projected onto the Eigenmode Solver plane and the horizontal-axis.
- Create Beam: this button will add a new Gaussian beam to the deck based on the above specifications.