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This page contains 2 independent sections. The simulation can be set up from a new 2D simulation, starting at the Set up Model section. Otherwise the associated files (which you can find at the locations given on the first page of the tutorial) can be used to start at the second section. There is also a script file that builds the simulation entirely using script commands.

 

 

In FDTD, there are usually 4 steps in order to completely setup the project file:

1.add the physical object (geometry and material)

2.add simulation region, setup proper boundary conditions, add override "MESH"icon_sims_override if necessary

3.add source and setup frequency (or wavelength) range

4.add monitors and analysis groups, and then setup frequency points to be recorded

Finally save the simulation file, run, and analyze the results.

 

In this example, we will compute the scattering cross sections using analysis group in the Object Library. Once we got peak wavelength, we will re-run the simulation with the modified mesh and an added profile monitor to demonstrate field intensity at this wavelength.

Set up model

Open a blank simulation file. For instructions see the FDTD Solutions GUI page in the Introduction to the Getting Started Examples. Since the nanowire has dimension in nanometers, we first modify the length unit from default "microns" to "nanometers" by clicking  Settings--> Length Unit as follows

hmfile_hash_272fa2dc

A full simulation file may need to set or modify many settings. However for a short instruction of this getting starting example, we will only show how to set the parameters related to the example and the rest of the parameters will be remained at their default settings.

Press the arrow on the STRUCTURES button icon_structures and select a CIRCLE from the pull-down menu. Set the properties of the circle according to the following table.

tab

property

value


name

nanowire

Geometry

x (nm)

0


y (nm)

0


z (nm)

0


z span (nm)

600


radius (nm)

25

Material

material

Ag (Silver) - Palik (0-2um)

Set the object in the coordinate origin is a good practice, which can make many settings easier.

 

Press the SIMULATION button icon_simulation to add a simulation region.  Note that if your button does not look like the button to the left, you will need to press on the arrow to get the simulation region. Set the properties according to the following table.

tab

property

value

General

dimension

2D


simulation time (fs)

200

Geometry

x (nm)

0


y (nm)

0


z (nm)

0


x span (nm)

800


y span (nm)

800

 

Note that since we are using a 2D simulation region, properties such as "z span" are irrelevant. However, 2D simulation is a section of 3D, so all the objects should keep the same z. The default value ( z = 0) can be used.

Press the arrow on the SIMULATION button icon_simulation and select the MESH region icon_sims_override from the pull-down menu. Set the properties according to the following table.

tab

property

value

General

dx (nm)

1


dy (nm)

1

Geometry

x (nm)

0


y (nm)

0


x span (nm)

110


y span (nm)

110

 

Press the arrow on the SOURCES button icon_sources and select the Total-field scatter-field (TFSF) source from the pull-down menu. Set the properties according to the following table:

tab

property

value

General

polarization angle

0

Geometry

x (nm)

0


y (nm)

0


z (nm)

0


x span (nm)

100


y span (nm)

100

Frequency/Wavelength

Wavelength start (nm)

300


Wavelength stop (nm)

400

 

Please refer TFSF page for more information.

Press the arrow on the ANALYSIS button icon_analysis and select OPTICAL POWER from the pull-down menu. This will open the object library window.

Find the CROSS SECTION analysis group and click "inset",  then click the edit button icon_editand set the properties of this group according to the following table.

tab

property

value


name

scat

Setup à Variables

x (nm)

0


y (nm)

0


z (nm)

0


x span (nm)

110


y span (nm)

110


z span (nm)

110*

* The Z span value is not important, since this is a 2D simulation in the XY plane.

 

Users are encouraged to read the analysis script to get some knowledge of scripting. More script description can be found in the Script section.

Make a copy of "scat" with the COPY button icon_duplicate and set the properties of this group according to the following table.

tab

property

value


name

total

Setup à Variables

x (nm)

0


y (nm)

0


x span (nm)

90


y span (nm)

90

 

Press the arrow on the MONITORS button icon_monitors and select GLOBAL PROPERTIES from the pull-down menu. The settings here will apply for all the Frequency-domain monitors such as inside the analysis groups.  Set the properties according to the following table.

tab

property

value

Frequency Power/Profile

frequency points

100

 

Press the arrow on the MONITORS button and select the FIELD TIME monitor from the pull-down menu. Set the monitor properties according to the following table. The location of this TIME monitor is to record the time response in the total field region.  

tab

property

value


name

time

Geometry

x (nm)

28


y (nm)

26

 

The setup is finished. Click "File" on the top left and choose "Save as" to save the file with a file name in a file folder.

Run simulation, plot cross sections.

Press on the CHECK button icon_check to open the MATERIAL EXPLORER. In "Material" the preset "Ag (Silver) - Palik (0-2um)" is automatically chosen. To obtain a plot of the refractive index as a function of wavelength, press the FIT AND PLOT button. The fitting is reasonably good so no further modification is needed.

Press the arrow next to the CHECK button and select the "Check simulation and memory requirements" buttonicon_checkmem to view the simulation and memory report. Make sure that the usable memory in the computer is enough for the simulation.

Next, check if the mesh is fine enough using the VIEW SIMULATION MESH button icon_viewmesh and ZOOM button in the toolbar. For more information about how to use these tools, see the Layout editor section of the reference guide.

Press the Resources button icon_resources and check the number of processes (number of cores) for the local machine.  Since this example only needs small amount of memory, the number of processes can be set 1,2, or 4.  please note that due to scaling of parallel computing it is not necessary true that the larger the number of processes are the faster the simulation would be. Then, press the "Run Tests" button to make sure the simulation engine is configured correctly.  The first time you run this test, it may fail and ask you to register your username and password for your operating system account. If it does, fill in the appropriate text fields, press "Register", then "OK", and re-run the tests.

Run the simulation by pressing the RUN button icon_run_parallel.  

Once the simulation finishes running, all the monitors and analysis groups in the object tree will be populated with data. The Results View window will show all the results and their corresponding dimensions/values for the selected object. Plot the time domain data by right-clicking on the time monitor and selecting Visualize -> E. Please note that if you are setting up your own project file, at this step the "Profile" monitor has not been added yet.

gs_nanowire_results_zoom40 gs_nanowire_time_visualize_zoom34

 

You can then select which components of the E field data you want to plot in the Visualizer. The screenshot below shows how to plot the real part of the x component of the electric field.

 

gs_nanowire_time_visualize2_zoom40

 

To plot the cross section results, right-click on the "scat" and "total" analysis groups, and select "Run analysis". This will run the analysis scripts in the analysis groups to calculate the cross section results.

Right-click again on the "scat analysis group", and you will see an option to Visualize->sigma, which will plot the scattering cross section as a function of frequency in the Visualizer. To plot the result as a function of wavelength, simply change the "Parameter" option (near the bottom of the Visualizer) from "f" to "wavelength". You can also change the "Units" to nanometers on the right side of the Parameters section (choose 1e-9) .

Without closing the Visualizer, go back to the object tree and right-click on the "total' analysis group, and select add to visualize1->sigma, which will plot the 2 cross section results in the same Visualizer.

You can also select the two analysis groups at the same time and then Visualize "sigma" since in this example both groups output their result with the same variable name.

Note that the absorption cross section is actually negative from the result returned by "total". We want the power flowing into the box, rather than out of the box calculated in the group. You can choose -Re (a negative sign)  in the Visualizer under "Scalar operation" to make it positive, which is physical.

 

gs_nanowire_cross_section_visualize_zoom27

Compare with theoretical results

Even though we can plot everything we need with the Visualizer, comparisons with theoretical calculations will have to be done through the scripting environment.

Open the script file editor (for instructions on how to do this see the Introduction section of Script).

Get the nanowire_plotcs.lsf script file and the cvs file from the first page of this application example and save them in the same folder of the fsp file. The cvs file contains data directly from Mie theory.

Then use the OPEN SCRIPT button icon_groups_container in "Script File Editor" to browse to and open the nanowire_plotcs.lsf script file.

Run the script file using the RUN SCRIPT button icon_run_script which is found on the Script File Editor window This will create the two plots of the cross sections seen in the discussion and results section.

Plot near field data

To plot field profile with high resolution and higher accuracy, the mesh needs to be finer and a profile monitor is to be added.

Switch the simulation back into layout mode by the SWITCH button icon_switch.

Edit the "mesh". Set dx = dy = 0.5nm.

Edit FDTD-->Mesh settings, in "mesh refinement" make sure the "conformal variant 1" is chosen.

Press the arrow on the MONITORS button and select the FREQUENCY-DOMAIN FIELD PROFILE monitor from the pull-down menu. Set the monitor properties according to the following table.

 

tab

property

value


name

profile

General

override global monitor settings

check


use source limits

uncheck


frequency points

1


wavelength center (nm)wavelength center (nm)

(click the down arrow on the right of "frequency" box)

345

Geometry

monitor type

2D Z-normal


x (nm)

0


y (nm)

0


z (nm)

0


x span (nm)

90


y span (nm)

90

 

If "override global monitor settings " is not checked, this newly added profile monitor will inherent 100 frequency points set in GLOBAL PROPERTIES.

 

Save the file. Run the simulation again by pressing on the RUN button. To save memory and speed up the simulation, you can disable monitors and analysis groups which are not needed. Since this project file is very small, you may not notice the speed-up. For large files, however, you may find it significantly faster.

Once the simulation has run to completion, plot the profile monitor data with the Visualizer (right-click -> Visualize -> E) . In "Vector operation" choose "Y" which will select Ey. In "Scalar operations", choose ABS^2 which will plot intensity. You can plot the image in a new widow by clicking the "Plot in new window" button.

Go to the SETTING menu in the new figure and set the colorbar limits. To obtain the exact same plot as in the discussion and results section, set the colorbar min to 0, and the colorbar max to 5.

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