IBM Release 1.93 manual Plane Waves

source is therefore an oscillating dipole. FDTD codes model a dipole as a single-cell source whose E field oscillates in time, just as you’d expect. Unfortunately, only linearly- polarized sources are supported because only a single point source can occupy a cell. This restriction is not essential, and may be removed in a future release.

The other problematical thing about point sources is normalization. One might expect that a point source of strength 1 would produce an E field at the source of 1, but that isn’t how TEMPEST’s point sources are normalized. This and other normalization problems (of which there are a few) will be fixed in a future release. The problem doesn’t affect power ratios and efficiencies, which is what we usually care about.

Figure 2.3 TEMPEST divergence due to	Figure 2.4: Side view of the domain of
source inside PML. This box is 3.2	Figure 2.3.
wavelengths across.

3.5.2. PLANE WAVES

Because of the periodic boundary conditions assumed in the simulation geometry, POEMS has a lot in common with FFT programs, e.g. point N is the same as point 0, and only waves whose spatial frequencies are integral multiples of 1/(N dx) can be used. Since dx

<<λ, and memory is limited, in practice only a few dozen or at most a few hundred plane wave components are necessary to synthesize any desired pupil function. The down side is that the resulting beams are spatially periodic, and their side lobes will leak into adjacent domains.

A plane wave source produces radiation in only one direction, and does not interfere with other field components crossing it. Plane waves don’t work well with fully isolated domains, i.e. those with PMLs on all surfaces. It’s useful to put PML boundaries downstream of the illumination, but when using plane waves, some imaginary absorbing material boundary is often better behind the illumination surface.

34