How do I avoid reflections from the simulation edges?

Date	Category
2023-12-15 21:41:47	Boundary Conditions

You should use tidy3d.PML boundary condition to enclose the simulation domain with layers of a special lossy material designed to absorb incoming waves from all angles with minimal reflection. Tidy3D uses PML boundary conditions by default, but you can also set the boundaries explicitly using the all_sides() method. For example:

# Define PML boundary conditions in all sides.
bspec = tidy3d.BoundarySpec.all_sides(boundary=tidy3d.PML())

# Alternatively, you can apply the boundary at specific directions.
# bspec = tidy3d.BoundarySpec.pml(x=True, y=True)

# Build the simulation.
sim = tidy3d.Simulation(
    center=(0, 0, 0),
    size=(10, 4, 4),
    boundary_spec=bspec,
    grid_spec=tidy3d.GridSpec.auto(min_steps_per_wvl=20, wavelength=1.55),
    structures=[waveguide],
    sources=[mode_source],
    monitors=[mode_monitor],
    run_time=1e-12,
)

In some cases, such as when an angled structure or dispersive materials lie within the PML, use tidy3d.Absorber instead. The absorber functions similarly to PML, absorbing the outgoing radiation to mimic the infinite space. However, the absorber has a slightly higher reflection and requires a bit more computation than PML, but it is numerically much more stable.

See this notebook for more details on setting up boundary conditions.