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

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

Telephone: +1-416-978-5207
E-mail: abdullah.alras...@utoronto.ca
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