On Wed, 28 Mar 2007, matt wrote:
My trouble with this is that it isn't compatible with the periodic plane wave
excitation:
I can create a planewave with oblique incidence and periodic boundaries, but
only if the source is very narrowband (df is small). A large df doesn't work
because the periodic boundaries are frequency specific.
For a gaussian source, a small df practically makes it a continuous source,
producing the same results.
No, that's not true. A small-df Gaussian is still a Gaussian. It still
goes to zero at both the beginning and end of the simulation, unlike a CW
source. Note that, however, you would want to use stop-when-decayed to
wait until the fields die away, rather than running for a fixed time.
Moreover, a Gaussian is in some sense the optimal shape if you want to
maximize localization in both frequency and time.
However, it's still better to use a short pulse, as described below.
The reason you'd want this is to be able to calculate the scattering
properties for a particular angle of incidence.
Your reasoning is incorrect here.
Assuming you have linear materials, you should get the same results if you
put in a narrow-band Gaussian and look at only one frequency component of
the Fourier transform, or put in a broad-band Gaussian and look at only
one frequency component of the Fourier transform. The latter has the
advantage that it requires a shorter simulation for the fields to die
away.
Morever, if you want the scattering properties as function of both
frequency and angle, then the short pulses have a further advantage. Each
simulation with a short pulse and fixed k gives you a broad spectrum
result, each frequency of which corresponds to a different angle. Then
you repeat the simulation for a range of k's, and at the end you'll have a
2d dataset of transmission/reflection vs. both frequency and angle.
Steven
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