Gaussian beam source (DGTD) - Simulation object

A Gaussian source defines a beam of electromagnetic radiation propagating in a specific direction, with the amplitude defined by a Gaussian cross-section of a given width. The beam is modeled using a non-paraxial approximation (vector beam approximation) which assumes that the fields in the direction of propagation are zero and is very similar to scalar approximation of a Gaussian beam but unlike the scalar approximation is an exact solution of Maxwell equations. The beam is injected along a line perpendicular to the propagation direction, and is clipped at the edges of the source.

Gaussian source center of focus

Figures below demonstrate how the center of focus for a Gaussian beam can be adjusted to any desired coordinates in the simulation. This is possible by adjusting the center of focus from "beam options" tab in source properties as is discussed further in this page. Visualization of the Ex field component of the source clearly shows the Gaussian beam profile and how it is focused at the desired focus point. To reproduce the results, download the associated file and open it in In DGTD and FEEM. The focus point position can be simply adjusted from the properties window of the source object.

Injecting to a de-centered focus point  

Injecting to a centered focus point

General tab

• AMPLITUDE: The amplitude of the source.
• PHASE: The phase of the point source, measured in units of degrees. Only useful for setting relative phase delays between multiple radiation sources.
• DIRECTION: This field specifies the direction in which the radiation propagates. FORWARD corresponds to propagation in the positive direction, while BACKWARD corresponds to propagation in the negative direction.
• DIRECTION DEFINITION: Specifies whether to define the injection direction using a reference axis or k-vector components
• INJECTION AXIS: Sets the axis along which the radiation propagates.
  • ANGLE THETA: The angle of propagation, measured in degrees, with respect to the injection axis defined above.
  • ANGLE PHI: The angle of propagation, in degrees, rotated about the injection axis in a right-hand context.
• K-VECTOR
  • KX: k-vector component in x axis direction
  • KY: k-vector component in y axis direction
  • KZ: k-vector component in z axis direction
• 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 direction of propagation while a polarization angle of 90 degrees defines S-polarized radiation.

Geometry tab

Volume, surface, line and point in 3D and 2D:

Surface Type

• DOMAIN:EXTERIOR : Select the target domain. The reference geometry is the common surface(s) shared by the uttermost surface(s) of the selected domain and the simulation region. The selected domain has to have at least a surface that is shared with one of the simulation region surfaces.
• DOMAIN:DOMAIN :  Select the target domains. The reference geometry is the common surface(s) shared by the two selected domains.
• DOMAIN : Select the target domain. The reference geometry is the surfaces of the selected domain.
• SOLID : Select the target solid. The reference geometry is the surfaces that enclose the selected volume if the solid is a 3D shape, or the surface if the solid is a 2D plane.
• SIMULATION REGION : Select one or more simulation region boundaries. The reference geometry is the selected boundaries.
• SOLID:SIMULATION REGION : Select one or more simulation region boundaries and the target solid. The reference geometry is the common surface(s) shared by the simulation region and the target solid.
• MATERIAL:MATERIAL : Select the target materials. The reference geometry is the surface(s) that is shared by the two selected materials. This is only available in some boundary conditions.
• SURFACE : Type the identifier of the partition surface. If the target partition surface is SURFACE 3, type 3. If the target partition surfaces are SURFACE 3 and SURFACE 5, enter 3,5.

Frequency/Wavelength tab

This tab can be accessed through the individual 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 length: 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.

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.

Results returned

• BEAM: Provides information about spatial distribution of electric (E) and magnetic (H) fields injected by the source within the limits of simulation region.