The total-field scattered-field, or TFSF source injects a plane wave with finite span and
separates the computation region into 2 regions.
Inside the source region both the incident and scattered fields are present, and outside
the source only the scattered fields are present.
The total-field scattered-field source works by subtracting at its boundaries any light
which is directly transmitted through the source region or reflected from a flat substrate
so that only the light that is scattered by a feature contained completely inside the
source region will pass through the boundaries and propagate outside to the scattered field
region.
To get a better idea of how the source works, it's useful to consider what happens in the
simple case when the source is injected in free space with no scattering objects, as
shown in this movie.
The plane wave is injected from one side of the source region and subtracted at the other
side.
Since there is no scattering, the fields are 0 outside of the source boundaries.
When there is a substrate, any light which is directly reflected or transmitted through
the substrate gets subtracted at the boundaries.
The source is able to determine what portion of the fields correspond to directly transmitted
and reflected light by using one edge of the source as a reference edge, illustrated by
the yellow line in the image here.
The refractive index profile of the structure along this edge is measured, and the fields
that would be directly reflected or transmitted by the reference refractive index profile
can be calculated.
This is what gets subtracted at the boundaries during the simulation.
Since the refractive index profile at the reference edge is important for this calculation,
the side edges of the source should always pass through the substrate, and not intersect
with the scattering particle.
Since the total-field scattered-field source injects a plane wave over a finite span, the
default normalization method where the transmission result from monitors is normalized by the
amount of power injected by the source, leads to arbitrary values.
This is because the power injected by the source depends on the size of the plane wave
injected in the total field region of the source.
It is usually more physically meaningful to normalize transmission results by the source
intensity and this is also known as cross section units.
Cross section units are usually used in Mie scattering simulations to get the absorption,
scattering, and extinction cross sections.
There is a discussion about the different possible normalization methods when using
the total-field scattered-field source linked below.
The total-field scattered-field source works with absorbing materials and anisotropic materials,
and it works with multi-layered substrates.
It can be used to simulate standalone scattering objects, or periodic structures in conjunction
with periodic or Bloch periodic boundary conditions.
Here are some examples to illustrate some of the rules for setting up total-field scattered-field
sources.
If using PML boundaries at the sides to simulate a single standalone scatterer, the source
should not extend into the PML.
If simulating a periodic structure, the source can extend through periodic boundaries.
If there is a substrate, the source injection axis should be perpendicular to the substrate.
If there is a substrate the edges of the source should intersect with the substrate.
The source should not intersect with the scattering object.
Some applications which use the total-field scattered-field source are Mie scattering,
and defect detection.
The total-field scattered-field source is useful when you want to be able to separate
the scattered fields from the total fields and perform analysis on the scattered fields
only, such as by getting the scattering cross section, or the angular distribution of scattered
light in the far field.
In the next unit, we'll set up a total-field scattered-field source to measure the scattered
fields from a particle which represents a defect in a structure.