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tutorial and unit test for mode sources using DiffractedPlanewave
object
#2107
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Should be able to delete these lines as I commented in #2072. (It's possible you were having problems before that are now fixed by #2114.)
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If it's not working, make sure the MPB/Meep verbosity is > 1 and look at the
NEW KPOINT
andMPB solved for frequency
verbose outputs — you should see that it is setting the parallel component ofk
by the boundary conditions, and varying the perpendicular component ofk
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Looks like there is a potential bug because printing out
NEW KPOINT
shows that its value is clearly incorrect:The y component (i.e., the periodic direction of the monitor monitor) should be 1.333... (i.e., 4/3) to match the
k_point
of theSimulation
object. The reason it is not this value is due to these lines:meep/src/mpb.cpp
Lines 426 to 433 in 405b7e6
In this example,
R = (1,1.5,1)
andkcart = (0,1.33333,0)
which results ink = (0, 2.0, 0)
.The fix seems to be to just replace
k[i] = dot_product(R[i], kcart)
withk[i] = kcart[i]
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No, that code is correct — MPB expects the k point in the reciprocal-lattice basis, so it has to convert from the Cartesian basis of Meep. (It does the same thing with the
DiffractedPlanewave
object.)Instead, something weird is going on. You said the output is:
which means that it is taking the nonzero ky into account, but somehow it is getting the wrong kx — somehow, it is acting as though it is passing
ky==0
to MPB, even though we are clearly printing that it is passing a nonzerok[1]
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I think I have finally gotten to the bottom of this issue. TLDR: the current behavior is a feature and not a bug.
Currently, to launch oblique planewaves using an$\left|k_\perp\right| = (n\omega)^2 - \left|k_\parallel\right|$ is only computed automatically for the case when
EigenModeSource
witheig_band
set to an integer (rather than anDiffractedPlanewave
object), the user must provide its wavevectork
(ameep.Vector3
object) viadirection=mp.NO_DIRECTION
andeig_kpoint=k
. This is because even though internally Meep sets the component ofk
parallel to the line source (in the directions with periodic boundaries) to the components of thek_point
of theSimulation
object, the correct perpendicular componenteig_band
is aDiffractedPlanewave
object.It would be nice to extend this feature to when
eig_band
is an integer but that can be a separate PR since it is not directly related to this PR.There was a problem hiding this comment.
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No. A
DiffractedPlanewave
object only enables the analytical calculation of k. Otherwise, it uses a Newton solver to find the k where ω(k) = specified frequency. This of course should be the same thing (but slower, and with more ambiguity about polarization and more difficulty in choosing a specific diffraction order).There was a problem hiding this comment.
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The wavevector of the oblique planewave in Cartesian coordinates should be:
k = (2.68742, 1.33333, 0.00000)
. However, the Newton solver shows:Note that the$y$ component (parallel direction) of the MPB mode ($x$ component (perpendicular direction) (
1.33333
) is correct but the3
) is not2.68742
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Turns out that there was a bug in the output of MPB's mode wavevector which is fixed by #2253. (The mode itself was correct.)
With the bug fix, the output from
run_mode_source
oftest_diffracted_planewave.py
in this PR is:This means
k = (2.68742,1.33333,0)
which is equivalent to thek_point
of theSimulation
class object as expected.