Use the Matched Boundary Condition to make a boundary transparent to a wave with a known scattered wave direction. Since the Scattered wave direction setting is taken into account, this boundary condition is low reflecting also for a wave propagating in a direction at a large angle to the normal of the boundary. This is in contrast to the Scattering Boundary Condition, where the scattered beam should propagate in a direction that is almost parallel to the boundary normal to be efficiently absorbed. The boundary is also transparent to an incoming wave.

If there is an incident field, a Reference Point subnode can be added by right-clicking the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu. Taking into account the Reference Point subnode, the total electric field, including the incident and scattered waves, can be written as

Here, rref is a reference point determined by the Reference Point subnode, the field E0 is the incident wave that travels in the direction ki,dir in a medium with wave number k, and the field Es is the scattered wave that travels in the direction ks,dir.

Select an Incident field — No incident field (the default), Wave given by E field, Wave given by H field, or Gaussian beam.

Enter the expressions for the components of the Incident electric field amplitude E0 or Incident magnetic field amplitude H0, depending on the Incident field selected.

If the Incident field is set to Gaussian beam, edit the Beam radius w0 (SI unit: m) and the Distance to focal plane p0 (SI unit: m). The default values are ((10*2)*pi)/emw.k0 and 0 m, respectively. Select an Input quantity: Electric field amplitude (the default) or Power. If the Input quantity is Electric field amplitude, enter the component expressions for the Gaussian beam electric field amplitude Eg0 (SI unit: V/m). If the Input quantity is set to Power, enter the Input power (SI unit: W in 2D axisymmetry and 3D and W/m in 2D) and the component expressions for the Gaussian beam nonnormalized electric field amplitude Eg0 (SI unit: V/m). The optical axis for the Gaussian beam is defined by a line including a point which is the average position for the feature selection and a direction specified by the Incident wave direction (see below).

If the Incident field is not set to No incident field, edit the Incident wave direction ki,dir vector components. The default direction is the inward normal to the boundary. For 2D axisymmetry, the direction should be parallel or anti–parallel to the symmetry axis. If no scattered field is expected, select the No scattered field checkbox. This prevents COMSOL from returning spurious solutions that otherwise could appear between boundaries with unconstrained scattered fields. Edit the Scattered wave direction ks,dir vector components. The default direction is the outward normal to the boundary.