This section describes problems that can occur when using the plane wave source is truncated, either because the span is too small, or when PML boundary conditions are used.
Examples of correct usage
Ideally the plane wave source should be used in the following manner: The source should span the entire simulation. Periodic or Bloch boundary conditions should be used in the directions normal to the propagation. PML should be used to to absorb the transmitted and reflected light.The first two examples illustrate this situation.
 
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Description
Simulate a plane wave propagating through free space at normal incidence.
Simulation Settings
- Periodic BC for Y boundaries. PML for X boundaries.
- Plane wave source extends through simulation boundary.
- No physical structures
Results
- An ideal plane wave propagates forward from the source, and is absorbed by the PML on the right side of the simulation.
- In front of the source, a uniform intensity of 1 is measured at all locations. This is expected for a plane wave.
- Behind source injection plane, zero field is recorded because there is no scattered field.
Recommendations
- This is an appropriate way to setup simulations using plane wave illumination and periodic structures.
- For plane wave illumination of non-periodic structures, consider using the TFSF source.
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Description
Simulate a plane wave incident on a periodic array of cylinders at normal incidence.
Simulation Settings
- Periodic BC for Y boundaries. PML for X boundaries.
- Plane wave extends through simulation boundary
- A cylinder with index 1.4 will cause some scattering.
Results
- In front of the source, a complex intensity pattern is formed due to interference from the sphere.
- Behind source injection plane, there is some scattered field visible due to reflections from the sphere.
Recommendations
- This is an appropriate way to setup simulations using plane wave illumination and periodic structures.
- For plane wave illumination of non-periodic structures surrounded by a uniform material, consider using the TFSF source.
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Truncation by PML boundaries
If PML boundary conditions are used in the direction normal to the wave-vector, some undesired diffraction will occur because of energy absorbed by the PML.
 
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Description
Simulate a plane wave propagating through free space, but with PML on all boundaries.
Simulation Settings
- PML BC on all boundaries.
- Plane wave extends through simulation boundary.
Results
- This simulation does not produce an ideal propagating plane wave because the PML absorbs energy at the simulation boundary, causing diffraction.
- Far from the simulation boundary, the field still approximates a plane wave.
Recommendations
- This is not a recommended configuration, since the PML causes non-physical distortions of the plane wave.
- Consider using a focused beam source if you want a finite sized beam.
- Consider using a TFSF source if you want a plane wave on a non-periodic structure.
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Truncation due to short source span
If the source does not span the entire simulation width, diffraction will occur at the source boundaries. Physically, this setup can be understood as an infinite plane wave passing through an aperture the size of the source. Diffraction occurs as the plane wave passes through the aperture.
 
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Description
Simulating a finite sized plane wave propagating in free space with the planewave source.
Simulation Settings
- PML BC on all boundaries.
- Plane wave source does not extend through simulation boundary.
Results
- This simulation does not result in an ideal propagating plane wave because the source has a finite width. This causes diffraction at the edges of the source.
- Far from the source boundary, the field still approximates a plane wave.
- Care must be taken with this type of simulation, since any analysis may have to compensate for the diffraction near the source boundary.
Recommendations
- This is not a recommended configuration. There are very few situations where this simulation setup is actually required.
- Consider using a focused beam source if you want a finite sized beam.
- Consider using a TFSF source if you want a plane wave on a non-periodic structure.
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See also
Source field profiles and movies
TFSF sources