Oscillating dipoles act as sources in Maxwell's equation to produce electromagnetic fields. Dipoles are used to simulate point source radiators, such as radiation from a fluorescent molecule.
In MODE, for the 2.5D FDTD solver, the orientation of the dipole source partially depends on whether the polarization of the propagator simulation is set to TE or TM. Depending on the simulation polarization and dipole type, the theta, phi values may be locked.
- DIPOLE TYPE: A pull-down menu in which the point source can be configured as an electric dipole (oscillating point charge) or a magnetic dipole (current loop). The radiation pattern of these dipoles is similar, but not exactly the same.
- AMPLITUDE: The amplitude of the point source. The units of the source depend on the dipole type, as explained in the Units and normalization section.
- BASE AMPLITUDE: This is the amplitude that will generate a radiated CW power of 10 nW/m in 2D simulations and 1 fW in 3D simulations.
- TOTAL AMPLITUDE: This is the amplitude actually used in the simulations, it is the product of the AMPLITUDE and the BASE AMPLITUDE.
- PHASE: The phase of the point source, measured in units of degrees. Only useful for setting relative phase delays between multiple radiation sources.
- THETA: The angle with respect to the z-axis of the dipole vector.
- PHI: Angle with respect to the positive x-axis of the dipole vector.
The geometry tab contains options to change the size and location of the sources. The dipole position and direction are specified in terms of the center position and their orientation through angles theta, phi.
The Frequency/Wavelength tab is shown below. This tab can be accessed through the individual source properties, or the global source properties. Note that the plots on the right-hand side of the window update as the parameters are updated, so that you can easily observe the wavelength (top figure), frequency (middle figure) and temporal (bottom figure) content of the source settings.
At the top-left of the tab, it is possible to chose to either SET FREQUENCY / WAVELENGTH or SET TIME-DOMAIN. In most simulations, the 'SET FREQUENCY / WAVELENGTH ' option is recommended.
If you choose to directly modify the time domain settings, please keep the following points in mind:
- PULSE DURATION: Choose a pulse duration that can accurately span your frequency or wavelength range of interest. However, very short pulses contain many frequency components and therefore disperse quickly. As a result, short pulses require more points per wavelength for accurate simulation.
- PULSE OFFSET: This parameter defines the temporal separation between the start of the simulation and the center of the input pulse. To ensure that the input pulse is not truncated, the pulse offset should be at least 2 times the pulse duration. This will ensure that the frequency distribution around the center frequency of the source is close to symmetrical, and the initial fields are close to zero at the beginning of the simulation.
- SOURCE TYPE: In general, you can choose between ‘standard’ and ‘broadband’ source types. Standard sources consist of a Gaussian pulse at a fixed optical carrier, while the broadband sources consist of a Gaussian pulse with an optical carrier which varies across the pulse envelope. Broadband sources can be used to perform simulations in which wideband frequency data is required – for instance, from 200 to 1000 THz. This type of frequency range cannot be accurately simulated using the standard source type.
Set frequency wavelength
If the SET FREQUENCY / WAVELENGTH option was chosen, this section makes it possible to either set the frequency or the wavelength and choose to either set the center and span or the minimum and maximum frequencies of the source.
For single frequency simulations, simply set both the min and max wavelengths to the same value.
Set time domain
The options in the time domain section are:
- SOURCE TYPE: This setting is used to specify whether the source is a standard source or a broadband source. The standard source consists of an optical carrier with a fixed frequency and a Gaussian envelope. The broadband source, which contains a much wider spectrum, consists of a chirped optical carrier with a Gaussian envelope. When the user uses the script function setsourcesignal, this field will be set as "user input".
- FREQUENCY: The center frequency of the optical carrier.
- PULSELENGTH: The full-width at half-maximum (FWHM) power temporal duration of the pulse.
- OFFSET: The time at which the source reaches its peak amplitude, measured relative to the start of the simulation. An offset of N seconds corresponds to a source which reaches its peak amplitude N seconds after the start of the simulation.
- BANDWIDTH: The FWHM frequency width of the time-domain pulse.
For more information, please visit Changing the source bandwidth
- ELIMINATE DISCONTINUITY: Ensures the function has a continuous derivative (smooth transitions from/to zero) at the start and end of a user-defined source time signal. Enabled by default.
- OPTIMIZE FOR SHORT PULSE: Use the shortest possible source pulse.
- This option is enabled by default in the FDTD solver. It should only be disabled when it is necessary to minimize the power injected by the source that is outside of the source range (eg. convergence problems related to broadband steep angled injection).
- This option is disabled by default in the varFDTD solver, as it improves the algorithm's numerical stability.
- ELIMINATE DC: Eliminates the DC component by forcing signal symmetry
Manual calculation of the source time signal
As explained above, the 'Standard' source type uses a fixed carrier with a Gaussian envelope. The following script code shows how to calculate the source time signal used by the source.
# calculate standard pulse time signal frequency = 300e12; pulselength = 50e-15; offset = 150e-15; t = linspace(0,600e-15, 10000); w_center = frequency*2*pi; delta_t = pulselength/(2*sqrt(log(2))); pulse = sin( -w_center*(t-offset)) * exp( -(t-offset)^2/2/delta_t^2 ); plot(t*1e12,pulse,"t (fs)","source pulse time signal");
There are some small differences between the pulse generated by this code and the actual time signal generated by the 'standard' source pulse setting. If you need very precise control over or knowledge of the source time signal, you should create your own Custom time signal.
The 'broadband' option is generated with a more complex function. The precise function is not provided. To create your own arbitrary source time signals, see the Custom time signal page.
This tab only appears for the dipole source. The tab contains a RECORD LOCAL FIELD checkbox. When checked, the fields around the dipole are saved; this box must be checked in order to use the dipolepower script function.
- DIPOLEPOWER: The power injected into the simulation region by a dipole is returned. The units will be in Watts if cw norm is used and Watts/Hertz2 if no norm is used.
- PURCELL: By utilizing the power measurement, the emission rate enhancement of a spontaneous emitter inside a cavity or resonator, the Purcell factor is returned.
- TIME SIGNAL: Time domain signal of the source pulse.
- SPECTRUM: The fourier transform of time signal.