On Wed, 28 Mar 2007, Steven G. Johnson wrote:
If you want to see the *field* pattern for a given angle, then you need a
narrowband (CW) source. But if you want the *flux* from a given angle, then
you can put in a broadband source and only take that frequency component of
the Fourier transform.
Let me make a caveat to the above statement.
The above assumes that if you input a field at multiple angles, then the
scattered power is the sum of the scattered power from each angle
individually. This is true only if the structure you are scattering from
has translational symmetry in the x direction (parallel to the source
plane)---it can be periodic, but in this case your frequency must be below
the diffraction cutoff.
This was the picture I had in my head, but it occurs to me that this may
not be what you are simulating.
If you have a non-periodic structure in the x direction, or care about
frequencies above the diffraction cutoff, then different input angles can
scatter into the same output angle and there can be interference. In this
case, the power does not simply sum.
In that case, you really must put in a narrowband source (I think, unless
there is some additional trick that is not occurring to me now).
However, you should still put in a narrowband Gaussian, and wait for it to
decay to zero, rather than putting in a CW field. Or, if you really want
to use a CW field, then you should use the flux-in-box function (which is
integrated in space not time), not the flux spectrum (which is integrated
in time and space and is problematic if your fields don't go to zero by
the end of the simulation).
Steven
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