This video is taken from the FDTD Learning Track on Ansys Innovation Courses.
The dipole source is a point source which emits a dipole radiation pattern.
There are 2 types of dipole sources available: electric and magnetic.
The electric dipole is equivalent to an oscillating point charge, whereas the magnetic dipole
is equivalent to a current loop.
This movie here shows the emission from an electric dipole source in the XY plane for
a dipole oriented in the z direction.
The power emitted by dipole sources in a homogeneous environment can be calculated analytically,
and this analytic radiated power is used to normalize the transmission result from monitors.
For more details about the equations, see the related information links below.
The actual power that is emitted by the dipole will dependent on the structures in the environment
surrounding the dipole, and whether other sources are present.
This is because any light reflected back from structures around the source or light from
surrounding dipoles can constructively or destructively interfere with the light emitted
by the dipole.
For example, if a dipole is placed next to a metal wall, using the method of images,
the metal wall can be replaced by a mirrored dipole, so the system is equivalent to injecting
2 dipoles which interfere coherently with each other, modifying the total radiated power.
The ratio of power actually radiated by the source to the power that would be injected
in a homogeneous medium is known as the Purcell factor.
After running a simulation, the dipole source returns results dipolepower, purcell, spectrum,
and time signal.
The dipolepower result gives the analytic power injected by a dipole in a homogeneous
medium as function of frequency.
The Purcell result gives the Purcell factor as a function of frequency.
The spectrum result gives the Fourier transform of the source pulse, and time signal result
gives the time domain amplitude of the source pulse.
On an earlier slide, we mentioned that by default, the power transmission result from
a monitor is normalized by the analytically calculated power that would be injected by
the dipole in a homogeneous environment.
You can re-normalize transmission results by the actual power injected by the dipole
by multiplying the transmission result by the Purcell factor.
The Purcell factor can be obtained from the purcell result from the source, or by placing
a box of monitors around the dipole injection region to measure the net power flowing through
the box of monitors.
However, if the dipole is placed within an absorbing medium, some care needs to be taken
in calculating the Purcell factor since the dipolepower result from the source may not
For some tips on what to do when injecting the dipole in an absorbing medium, see the
link to the Purcell factor page below this video.
A single dipole source will inject a dipole radiation pattern like shown in the image
on the left.
To simulate a point source which emits a uniform isotropic radiation pattern, you can run 3
simulations with orthogonal dipole orientations and then average the fields from the three
This slide shows some applications where dipole sources are used.
Dipole sources can be used to excite modes of cavities and resonators, or represent point
sources such as spontaneous emission in organic LED devices.
They can also be used to represent a fluorophore to simulate fluorescence enhancement since
the decay rate of a fluorophore can be related to the power radiated by a dipole.
Dipoles can also be used to excite modes of periodic lattice for calculating the band
structure of periodic lattice structures.
In the next unit, we'll show how to set up dipole sources to excite a specific mode
of a disk resonator.