Hi, I am not sure exactly what is wrong here. However, if I were to venture a guess, I would think it has something to do with the size of your PMLs. I would try something like (1.5*lambda) or larger and see if the results are reproduced.

Good luck! On Thu, Apr 12, 2012 at 12:00 PM, Filip Dominec <filip.domi...@gmail.com>wrote: > Dear Meep users, > today I am writing because I observed an unexpected behaviour of my > simple simulation: > A dielectric resonator of spherical shape is defined in center of the > volume, through which a broadband gaussian pulse is sent in the Z > direction. The X, Y faces of the volume are defined to be periodic, > the Z faces are equipped with PML (thickness = lambda/10). The > transmitted and reflected waves at the Z faces are recorded and > finally processed by custom FFT. > > Although for first part of the simulation everything works as > expected, the electric field starts to oscillate in a exponentially > growing _transverse_ mode after a longer run. This limits the spectral > resolution of FFT and ove From MEEP mailing list, I learned that FDTD > simulation may become unstablerall simulation accuracy. > > I prepared a specialized page with illustrative images: > http://fzu.cz/~dominecf/misc/meep/instab_long_run/ > > The oscillations seem to depend on the number of simulation steps, so > with coarser grid and greater time step the simulation starts to > oscillate at a later time. > Changing Courant factor from 0.5 to 0.01 resulted in much longer > simulation only, but oscillations surprisingly occured at the same > time. So it is perhaps a numerical, not electrodynamical problem. > > From MEEP mailing list, I learned that FDTD simulation may become unstable > 1) for non-dispersive real negative epsilon [1], or > 2) for any non-dispersive complex epsilon, where (f * Re(eps) < 0) [2], or > 3) for dispersive epsilon with resonance frequency too high [3]. > However, I am using real epsilon=94. > > I would be very thankful if somebody pointed me out where such > oscillations may come from! > Filip Dominec > ____ > [1] > http://www.mail-archive.com/meep-discuss@ab-initio.mit.edu/msg02450.html > [2] > http://www.mail-archive.com/meep-discuss@ab-initio.mit.edu/msg02455.html > [3] > http://ab-initio.mit.edu/wiki/index.php/Conductivity_in_Meep#Numerical_stability > > _______________________________________________ > meep-discuss mailing list > meep-discuss@ab-initio.mit.edu > http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss > -- Abdullah Al Rashid Graduate Student Theory of Nanophotonics and Photonic Materials Department of Physics University of Toronto Mailing Address: 60 St. George St Toronto, ON M5S 1A7 Canada Telephone: +1-416-978-5207 E-mail: abdullah.alras...@utoronto.ca

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