I do not see any description about what material/device you are testing. Please visualize the source amplitude. In many cases the local intensity can be larger than the source intensity, due to refraction, diffraction, scattering and interference. even for one material half in air, the intensity can be larger than the source since 1+r=t from the Fresnel coefficient.
In addition, please check if the boundary condition is correct: for plane wave, periodic boundary condition should be used.Â
https://optics.ansys.com/hc/en-us/articles/360034382874-Understanding-field-truncation-issues-with-finite-sized-plane-wave-sources
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