Watch a pulse meet a dielectric

Current-run teaching example · actual FDTD snapshots · 2D TMz · single mesh

Place a dielectric on the right side of the same open domain. The pulse reaches the interface, produces a reflected part, and propagates differently inside the material. The vacuum reference on the left makes the changed field and probe arrival easier to see.

Teaching mesh: preflight reports resolution advisories for the dielectric at the declared band edge. This movie is qualitative; it does not establish phase-velocity accuracy or reflection/transmission coefficients.

The model

Model detail	Value
Cross-section	80 × 60 mm; uniform Yee 2D TMz with 1 mm cells
Left panel	Vacuum reference
Right panel	Vacuum for `x < 40 mm`, ideal nondispersive dielectric `εr = 4` for `x ≥ 40 mm`
Boundary	16 CPML layers outside the physical domain
Source	Same differentiated Gaussian current pulse in both models at (20, 30) mm
Probe	`Ez` at (55, 30) mm
Recording	640 steps, actual field snapshot every 4 steps

The dielectric extends through the right absorber and the transverse edges; there is no second material interface in the simulated interior. The dashed line is the declared interface on the 40 mm grid plane. The source is a point in this 2D model, so the wavefront is curved; this is not a normally incident infinite plane wave.

Both panels use the same fixed signed Ez / Eref scale. Eref is the largest absolute recorded field outside a 4 mm source neighborhood across both models. The source pixels can saturate, and all unmodified values remain in the data. The green-probe trace and cursor share the movie’s actual simulation time.

Run and explore

Follow the pinned installation and generator download in the boundary example, then:

JAX_PLATFORMS=cpu PYTHONPATH=. python field_movies.py \
  --case dielectric-interface --output field-movies

Try a different permittivity or move the interface. Rerun preflight and the simulation; a higher permittivity shortens the material wavelength and may require a finer mesh. For a quantitative slab spectrum, continue with the slab reflection/transmission tutorial, which uses flux monitors and reference subtraction.

Preflight is part of the result

The manifest preserves the complete preflight output. Five advisories were reported and retained:

Three resolution findings: the dielectric has 12.5 cells per effective wavelength at the declared 12 GHz band edge, below the 15-cell recommendation.
Geometry reaches into CPML because this example models a continuing half-space. Absorber-region fields are excluded from the plotted interior.
The material is perfectly lossless in an open domain. This example does not extract Q, material loss or an absorption fraction.

No advisory was suppressed to make a clean banner. No mesh refinement or record-length convergence claim is made.

Download the actual samples

The NPZ contains every 16th movie frame at full spatial resolution. Its *_frame_times_s, *_frame_steps, *_x_m and *_y_m arrays describe the exact samples. Local regeneration retains every frame. Fields were checked for finite values; displayed coordinates and time steps were checked against Result.snapshot_axes. The solver source is a9054338; the manifest records the full SHA, generator hash and CPU environment.