Later please write posts in the dedicated forum: Photonics (ansys.com)
Most likely the monitor is in between the source and the front surface of the metamaterial. Please refer this article: https://optics.ansys.com/hc/en-us/articles/360034915753-Tips-for-accurately-measuring-reflection-in-an-FDTD-simulation
For better intituition, please make sure the reflection monitor is behind the source.
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