Index monitors records the n and k value as a function of frequency/wavelength in a simulation. In the future, the index monitor will be able to capture the time-evolution of the physical properties profile for nonlinear media.
In FDTD, it is possible to preview the index profile while still in Layout mode (i.e. without running the simulation). Note that the index preview returns a slightly simplified version of what the index monitor provides after running the simulation. Some advanced settings (such as the spatial interpolation setting) will be ignored in the preview.
Notes : Memory and computation time
Index monitors generally have modest memory requirements. To minimize the amount of memory required, use 1D or 2D rather than 3D monitors. Similarly, try to minimize the number of frequency points recorded. By default, only one frequency point is recorded. It is also possible to use spatial downsampling to record less spatial resolution.
Generally, Index monitors don't have a large effect on the simulation time, except when recording a very large amount of data. All the index monitor data is calculated at the beginning of the simulation.
Simulation type: Record the type of simulation data, default setting is ALL
Override global monitor settings:
A toggle to override the global monitor settings. If checked, users can specify the frequency range and number of points at which frequency-domain information will be recorded (using the options described below). If unchecked the options below are set from the global monitor settings.
SAMPLE SPACING: This combo-choice parameter determines how the sample frequency/wavelength will be selected. The three sample options are "uniform", "chebyshev" and "custom".
- USE WAVELENGTH SPACING: By default, data is recorded at certain spaced points with respect to frequency. Selecting this option spaces data at certain spaced points with respect to wavelength.
- USE SOURCE LIMITS: When checked these monitors use the source limits. When unchecked, the frequencies/wavelengths at which to record data can be set using the pull down menus and boxes below them.
- FREQUENCY POINTS: Set to choose the number of frequency points at which to record data.
When sample spacing is selected to be "custom", all the above settings will be disabled and a "custom frequency samples" table will be shown. Properties of the table are:
- ADD: Adds an entry above the selected entry to the "frequency (THz)" table
- REMOVE: Removes the selected entry from the "frequency (THz)" table
- SORT: Sorts the table with frequency from low to high
- set global monitor settings: Access to global properties
Monitor type: The monitor type and orientation, this option will control the availablity of spatial settings below
- X, Y, Z: The center position of the simulation region
- X MIN, X MAX: X min, X max position
- Y MIN, Y MAX: Y min, Y max position
- Z MIN, Z MAX: Z min, Z max position
- X SPAN, Y SPAN, Z SPAN: X, Y, Z span of the simulation region
The DOWN SAMPLE X, Y, Z option is used to set the spatial downsampling performed by the monitor. A down sample value of N corresponds to sampling (recording) the data every Nth grid points. Setting the down sample value to 1 gives the most detailed spatial information (i.e. information at each grid point).
|WARNING: This tab includes options which should only be changed if you are quite familiar with the meshing algorithm and techniques used.|
- SPATIAL INTERPOLATION: In FDTD generally, the electromagnetic field components and material properties are not known at the same point in space. Instead each component of the vectorial electric and magnetic fields are recorded at different locations in the Yee cell. Therefore, the material properties are also known at various locations within the Yee cell. This setting controls how the index monitor calculates the refractive index data. The SPECIFIED POSITION (Default) returns the index data where the monitor is located. This type of data is convenient for many purposes, but it is important to remember that the actual FDTD engine calculates the fields at locations other than the Yee cell origin. The NEAREST MESH CELL option is very similar, but the monitor location is snapped to the nearest mesh cell. The NONE option records the index data at the locations actually used by the FDTD engine.
Note: Spatial interpolation - NONE setting
Disabling the spatial interpolation is a very advanced feature. Only expert users who are very familiar with the FDTD method should consider using this feature. Most standard analysis functions (such as the transmission script function, the data visualizer, etc) assume that the spatial interpolation is enabled. They may not give the most accurate result when used to analyze such monitor data. All analysis must be done manually.
- RECORD DATA WITHIN PML: This option is as described above for time domain monitors.
- RECORD CONFORMAL MESH WHEN POSSIBLE: Enabled by default. The index monitor will record the the effect of the conformal mesh. It is helpful to disable this option when doing absorption calculations. The conformal mesh information can only be recorded when using the 'NEAREST MESH CELL' or 'NONE' spatial interpolation setting, and when the spatial downsampling option is set to 1.
- RECORD SURFACE CONDUCTIVITY: The index monitor will record the surface conductivity which is used in the simulation and return this as the SURFACE CONDUCTIVITY result. This may be used if you want to record the surface conductivity components over space when using SPECIFIED POSITION, or NONE as the spatial interpolation option.
The results that index monitors return depends on the modes the simulation is in.
In ANALYSIS mode:
- INDEX: Refractive index as a function of position and frequency/wavelength.
- INDEX_DETAIL: A matrix dataset with parameters x,y,z,f. This is a result when the "spatial interpolation" is set to "none". It also contains duplicate parameter entries for x,y,z called x_offset, y_offset and z_offset, respectively. These are the positions offset in the Yee cell. To obtain data at the correct spatial locations for index_x, you should use parameters (x_offset,y,z,f), for index_y you should use parameters (x,y_offset,z,f) and for index_z you should use (x,y,z_offset,f). This is useful in the script and can also be done in the visualizer. This type of capability would make absorption calculations like this example easier. For further information about why different positions in the Yee cell are used for each field component, please refer to the FDTD Solver Physics page.
- SURFACE CONDUCTIVITY: This will return the same result as the surface conductivity preview by default. If the RECORD SURFACE CONDUCTIVITY option under the ADVANCED TAB is enabled, this will instead return the surface conductivity used in the simulation which will include the effect of the selected spatial interpolation option.
In LAYOUT mode:
In addition to the "index preview" result, the index monitors have "index" results and (when spatial interpolation is set to None) "index_detail" results. All of these 3 results require meshing in the UI when the user asks for the results and therefore could trigger a big calculation, especially for large 3D index monitors. The difference is:
- "index preview" does not try to keep track of conformal meshing or any yee grid offsets. It is therefore the fastest and useful to ensure that the structure is basically correct.
- "index" and "index_detail" are guaranteed to give exactly the same results that you will get in ANALAYSIS mode, as if the simulation had been run. This can be slower if, for example, you are recording conformal mesh properties.
- INDEX PREVIEW: Refractive index as a function of position and frequency/wavelength before the simulation runs. It is important to note that the preview data is not exactly the same as the data returned after the simulation runs. Some advanced settings, such as spatial interpolation options and conformal meshing are ignored when calculating the index preview.
- INDEX: Refractive index as a function of position and frequency/wavelength. Same as in ANALYSIS mode.
- INDEX_DETAIL: Refractive index dataset including the positions offset parameters. Same as in ANALYSIS mode.
- SURFACE CONDUCTIVITY PREVIEW: Surface conductivity as a function of position and frequency/wavelength before the simulation runs. This is useful tool to make sure the graphene object is positioned as desired.