Matched Boundary Condition
Use the Matched Boundary Condition to make a boundary transparent for a wave (or waves) with the phase(s) as prescribed in the Wave Vectors settings of The Electromagnetic Waves, Beam Envelopes Interface settings.
Since the Wave Vectors settings are taken into account, this boundary condition is low reflecting also for waves propagating with a large angle to the normal of the boundary, in contrast to the Scattering Boundary Condition, where the scattered beam should propagate almost in parallel to the boundary normal to be efficiently absorbed.
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. The Reference Point subnode redefines the incident field to be expressed as
,
where rref is a reference point determined as the average point from the point selection in the Reference Point subnode.
In 2D axisymmetry, the default subnode Symmetry Axis Reference Point is available. This subnode defines a reference point at the intersection between the symmetry axis and the Scattering boundary condition’s boundary selection.
Matched Boundary Condition
When Wave Vectors is set to bidirectional, select an Input waveFirst wave (the default) or Second wave — that the specified input electric field is associated with.
Select an Incident field to specify whether there is No incident field (the default), the input wave is specified by an Electric field, a Magnetic field, or a Gaussian Beam.
When No incident field is selected, it is assumed that the wave is propagating out from the domain the selected boundaries are adjacent to.
Specify the Incident electric field envelope E0 (SI unit: V/m) or Incident magnetic field envelope H0 (SI unit: A/m), depending on the Incident field selected. Notice that you only specify the envelope factor of the incident electric or magnetic field. The envelope function is internally multiplied by the phase function, as specified in the Wave Vectors settings, to form the complete incident electric or magnetic fields.
If the Incident field is set to Gaussian beam, 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). Also 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)/ewfd.k0 and 0 m, respectively. The optical axis for the Gaussian beam is defined by a line including a reference point on the feature selection with a direction specified by the propagation direction for the Input wave (or k1 for the unidirectional formulation). By default, the reference point is the average position for the feature selection. However, by adding a Reference Point subnode any available point (or the average of several selected points) on the feature selection can be used as the reference point. The focal plane for the Gaussian beam is located the Distance to focal plane p0 from the reference point in the propagation direction for the Input wave (or k1 for the unidirectional formulation).
If the Incident field is set to Gaussian beam and Type of phase specification, in the Wave Vectors section for The Electromagnetic Waves, Beam Envelopes Interface, is set to User defined, edit the Incident wave direction kdir for the vector coordinates. The default direction is in the opposite direction to the boundary normal.
When Wave Vectors is set to bidirectional, if no scattered field is expected, select the No scattered field check box. This prevents COMSOL from returning spurious solutions that otherwise could appear between boundaries with unconstrained scattered fields.
Gaussian Beam Incident at the Brewster Angle: Application Library path Wave_Optics_Module/Optical_Scattering/brewster_interface.
Total Internal Reflection: Application Library path Wave_Optics_Module/Waveguides/total_internal_reflection.