Hi Mikhailo , This is a known source of error similar to other errors in other algorithm. Although there are many different suggestions for the improvement, the current algorithm is the only one that is robustic. Most importantly we are interested in the fields not the speed. If you want to correct it, you can do a post correction using the numerical dispersion relation, which is usually unnecessary. Many papers have been published to address this numerical dispersion. You can search "FDTD numerical dispersion" for more details. As far as I can tell, FDTD has been the most popular simulation method in computational electromagnetics for more than two decades with reasons.
I tested with the regular backgound material of n=1 and etch and found that they gave exactly the same result. Basically the "etch" is to use the material of n=1. Please use time monitor to compare the signals.
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