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This page contains 4 independent sections. The first section describes how to setup the cavity structure and the FDTD simulation region.  Section 2 describes the source and monitor setup, and some initial analysis.  Section 3 provides further information on using symmetry boundaries, and section 4 describes how to use the optimization feature of FDTD.

 

Create PC and check material index

Open a blank simulation file. For instructions see the FDTD Solutions GUI page in the Introduction to the Getting Started Examples.

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

tab

property

value


name

substrate

Geometry

x (μm)

0


y  (μm)

0


z  (μm)

0


x span (μm)

10


y span (μm)

10


z span (μm)

1

Material

index

2.0995

 

Press on arrow on the COMPONENTS button icon_components and select PHOTONIC CRYSTALS from the pull-down menu. This will open the object library window.

Select HEXAGONAL LATTICE PC H-CAVITY from the list and press the INSERT button.

Set the properties of the PC Cavity according to the following table.

tab

property

value

Properties

x (μm)

0


y (μm)

0


z (μm)

0


material

etch


H number

2


z span (μm)

1


n side

6


a (μm)

.575


radius

.194

 

Press the DUPLICATE button icon_duplicate to create a second copy of the hexagonal lattice PC cavity. Edit the properties according to the following table

tab

property

value


name

inner

Setup - Variables

x (μm)

0


y (μm)

0


z (μm)

0


H number

1


n side

2


radius (μm)

.100

 

Press on 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

simulation time (fs)

1500

Geometry

x (μm)

0


y (μm)

0


z (μm)

0


x span (μm)

12 * .575


y span (μm)

12 * .575 * sqrt(3) / 2


z span (μm)

3

Boundary conditions

z min bc

Symmetric

Advanced options

force symmetric x mesh

check


force symmetric y mesh

check


force symmetric z mesh

check

Note: Forcing a symmetric x mesh ensures that a mesh line lies at x=0, and therefore the mesh does not change when we switch the x min boundary condition from PML to symmetric or anti-symmetric. Strictly speaking we do not need this option for this particular simulation since we have set up the mesh in this example so that there will be a mesh line at x=0 anyways.

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

tab

property

value

General

dx (μm)

.575 / 8


dy (μm)

.575 * sqrt(3) / 2 / 8


override z mesh

uncheck

Geometry

x (μm)

0


y (μm)

0


z (μm)

0


x span (μm)

10


y span (μm)

10


z span (μm)

1

 

Press on the arrow on the MONITORS button icon_monitors and select the REFRACTIVE INDEX monitor from the pull-down menu. Set the properties according to the following table.

tab

property

value


name

index

Geometry

x (μm)

0


y (μm)

0


x span (μm)

10


y span (μm)

10

Get the index monitor data: you can use visualizer  Visualize->index preview to get the index distribution or verify the structure without running the simulation.

 

Add sources and monitors. Run simulation and get data.

 

Add sources and Q analysis object

Press the arrow on the on the SOURCES button icon_sources and select the DIPOLE source from the pull-down menu. Set the properties according to the following table:

tab

property

value


name

dipole1

General

dipole type

Magnetic dipole

Geometry

x (μm)

.1


y (μm)

.2

Frequency/Wavelength

frequency start (THz)

160


frequency stop (THz)

250

 

While the dipole is still selected, click the DUPLICATE button icon_duplicate on the toolbar (or, use the keyboard shortcut key D). Set the name to dipole2 and the x location of the dipole to 0.3 microns.

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

Insert the Q ANALYSIS analysis group. Set the location of the monitor to x = .4 μm, y = .2 μm and z = 0μm.

 

Run simulation and get data: Get Q analysis data

Press the Resources button icon_resources and check the number of processes (number of cores) for the local machine.  If you have additional computers on the network with FDTD installed as well as extra engine licenses, you can add them to the resource list.  Click "Add" and set the appropriate properties.

Press the "Run Tests" button to make sure the simulation engines on the resources are 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.  If there are any errors or warnings, they will appear in the "Result" field.

Run the simulation by pressing the RUN button icon_run_parallel. For more information about setting the parallel computing options see the Running simulations chapter.

To get the Q factor of the cavity, right-click on the Qanalysis group and select "runanalysis". Alternatively, open the edit window for the Qanalysis group, and under the Analysis->Script window, press the RUN ANALYSIS button to run the script.

 

Add profile monitor (now that we know the resonance frequencies for the structure)

Press the SWITCH TO LAYOUT icon_switch button to the objects tree window.

Press the arrow on the MONITORS button icon_monitors 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

2


minimum frequency (THz)

201


maximum frequency (THz)

209.5

Geometry

x (μm)

0


y (μm)

0


x span (μm)

10


y span (μm)

10

Spectral averaging and apodization

apodization

Full


apodization center (fs)

1000


apodization time width (fs)

250

 

Run simulation and get data: Get profile monitor data

Press the RUN icon_run_parallel button.

Once the simulation has run to completion, plot the profile monitor data by right-clicking on the monitor and selecting Visualize->E. Set the scalar operation to "Abs^2" to plot the electric field intensity.

This image corresponds to the result at the first frequency point. To plot the electric field intensity at other frequencies, simply move the frequency slider as shown below.

 

gs_cavity_profile_visualize_zoom55

 

To plot real(Ey), simply select "Y" for Vector operation and "Re" for Scalar operation.

 

SYMMETRY

To reduce memory requirement and speed up simulation, we can use the symmetry boundaries.

Press the SWITCH TO LAYOUT icon_switch .

Edit the FDTD simulation region. In the BOUNDARY CONDITIONS tab, set "x min bc" to be "Anti-Symmetric" and "y min bc" to be "Symmetric" to get the mode with specified symmetry.

Press the RUN FDTD button to re-run the simulation.

Visualize real(Ey) as shown in the visualizer screenshot above.

 

For more information on using symmetry boundaries, see the Symmetry boundaries page.

 

Optimize inner hole radius

Set the following variables in the Q analysis group:

tab

property

value

Setup - Variables

make_plots

0


number_resonances

1

Switch to/open the optimization and parameter sweep window

Press on the CREATE NEW OPTIMIZATION button icon_optimization. Set up the optimization as shown in the following screen shot. Warning: These settings will run Maximum Generations*Generation size = 20*10 = 200 simulations (set the "make plot" option in Qanalysis to 0 to avoid generating hundreds of figures). You can reduce the maximum generation size to run less simulations and therefore a faster optimization.

gs_pc_optimizationsettings_zoom87

Each generation of the optimization will create a temporary set of simulations to be run before being replaced by the next generation.  The job monitor will show the individual progress for each simulation of a generation set.  In the screenshot below, the simulation is using both Local Host and Laptop to run through the iterations in parallel. Note that in distributed mode, the source fsp file must be stored on the network, accessible by all the computing resources.

 

gs_pc_optimizationjobs_zoom65

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