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PML attenuation vanishes at grazing and excludes evanescent modes
Example
For a propagating angle , and the one-way PML amplitude after accumulated stretch is . It equals at normal incidence and tends to as . For a purely evanescent normal branch with , the factor has modulus , so this imaginary stretch adds no modulus decay.
Facts & Assumptions
Given: The Cartesian stretch [F1] and propagating-mode theorem [F2].
Cartesian complex-stretch PML for a homogeneous Helmholtz mode: and a separated normal mode has factor .
Perfect matching and attenuation for non-grazing propagating modes: for real , the one-way amplitude factor is .
Verification
Given: and the modes in [F1]–[F2].
For a propagating mode, substitute into [F2]. At the factor is ; as , , so it tends to .
For , direct substitution into [F1] gives , whose modulus is . The added stretch contributes only a phase, confirming that the propagating attenuation theorem does not cover evanescent modes.
Depends on
Used by
Nothing in the library uses this result yet.
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Sources
- Steven G. Johnson, Notes on Perfectly Matched Layers (PMLs), §§6, 7.2 (standard reference, not scraped)