On Tue, 20 Jun 2006, Chad Husko wrote:
Sometimes the simulations return negative Q values. Often the solution is to just move the Harminv monitor or to run the simulation longer. My question though is why this arises and whether it has any physical interpretation. It seems to be just an error, but maybe there's more. Also, a general approach to fixing this problem in the simulations (other than the ones I've suggested above) would be most appreciated. See the example output below.
Remember that there is some error bar in the harminv results. If the Q is so large (decay rate is so small) that the error bar is bigger than the decay rate, this can cause it to report negative Q values. The solution is just to run for a longer time, or alternatively using a narrower-bandwidth source usually helps.
If you do all this and you still see a negative Q, this means you may have exponential growth (-Q = growth lifetime). This may come from a numerical instability, or of course it will happen if you include gain in your system.
You can also get -Q values if you run harminv before your source has turned off -- in that case, you are just looking at growth in the mode because energy is being pumped into it.
Steven _______________________________________________ meep-discuss mailing list meep-discuss@ab-initio.mit.edu http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
