Total-Field Scattered-Field (TFSF) source - Simulation object

The TFSF source is often used to study scattering from small particles illuminated by a plane wave. Typical uses include:

• Particles in a homogeneous medium (which may be lossy or anisotropic). E.g. Mie scattering
• Non-periodic structures in a multi-layer substrate, which may be lossy or anisotropic.
• Periodic structures in a multi-layer substrate, when used in conjunction with Periodic or Bloch boundary conditions.

The TFSF source separates the computation region into two distinct regions:

• Total Field region - includes the sum of the incident field wave plus the scattered field
• Scattered Field region - includes only the scattered field.

The TFSF source is an advanced source and care must be taken to ensure proper setup and analysis of results. See TFSF tips and best practices for more information. Both regions are visible in the top figure. It is important to note that the physical field is the total field and the separation into an incident and a scattered field requires careful interpretation. For particles in a homogeneous medium, the incident field is a plane wave.  For particles on a substrate or multi-layer stack, the incident field is the field that would exist in the multi-layer in the absence of a particle (or defect). 

General tab

• INJECTION AXIS: Sets the axis along which the radiation propagates.
• DIRECTION: This field specifies the direction in which the radiation propagates. FORWARD corresponds to propagation in a positive direction, while BACKWARD corresponds to propagation in a negative direction.
• AMPLITUDE: The amplitude of the source as explained in the Units and normalization section.
• PHASE: The phase of the point source, measured in units of degrees. Only useful for setting relative phase delays between multiple radiation sources.
• ANGLE THETA: In 3D simulations, this is the angle of propagation, in degrees, with respect to the injection axis of the source. In 2D simulations, it is the angle of propagation, in degrees, rotated about the global Z-axis in a right-hand context, i.e. the angle of propagation in the XY plane.
• ANGLE PHI: In 3D simulations, this is the angle of propagation, in degrees, rotated about the injection axis of the source in a right-hand context. In 2D simulations, this value is not used.
• POLARIZATION ANGLE: The polarization angle defines the orientation of the injected electric field, and is measured with respect to the plane formed by the direction of propagation and the normal to the injection plane. A polarization angle of zero degrees defines P-polarized radiation, regardless of the direction of propagation while a polarization angle of 90 degrees defines S-polarized radiation.
• THETA VS WAVELENGTH PLOT: This plot shows the actual injection angle theta for each source wavelength as used in the simulation.

Geometry tab

The geometry tab contains options to change the size and location of the sources.

Frequency/Wavelength tab

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

Advanced

• 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 algorithms 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");

Results returned

• TIME SIGNAL: Time-domain signal of the source pulse.

• SPECTRUM: The Fourier transform of the time signal.

See also

 TFSF tips and best practices, Energy conservation with a TFSF source, Source movies