# Gaussian Laser Beam and Photonic Crystal Cavity (photonicCrystalGaussSrcT.pre)

Keywords:

Gaussian Beam source, photonic crystal, transmission efficiency

## Problem description

This example illustrates how to model a Gaussian beam source that is illuminating a cavity inside a hexagonal photonic crystal lattice. The physical arrangement is shown in Fig. 269.

Fig. 269 Top view of the photonic lattice.

A Gaussian beam is launched from above into the simulation domain, which comprises three layers: a vacuum region above and a solid dielectric below, which together sandwich a central dielectric layer that contains a lattice of holes. This example includes two possible time signals for the Gaussian beam: WideBand or SingleFrequency, as shown in Fig. 269.

This simulation can be performed with a VSimEM license.

## Opening the Simulation

This example is accessed from within VSimComposer through the following steps:

• In the resulting Examples window, expand the VSim for Electromagnetics option.
• Expand the Photonics (Text-based setup) option.
• Select “Gaussian Laser Beam and Photonic Crystal Cavity (Text-based setup)” and press the Choose button.
• In the resulting dialog, create a new folder if desired, and press the Save button to create a copy of this example.

The basic variables of this problem should now be alterable via the text boxes in the left pane of the Setup Window, as shown in Fig. 270. For this documentation, a WideBand SimType will be used to demonstrate the functionality of this example.

Fig. 270 The Setup Window for the Gaussian Laser Beam and Photonic Crystal Cavity (Text-based setup) example

## Input File Features

The setup shown in Fig. 270 specifies the parameters of the simulation in SI units. For a visual reference of what these variables do, consult Fig. 269.

General simulation parameters:
• NDIM = Number of dimensions (3).
• SimType = {WideBand, SingleFrequency}.
• WideBand = Runs the simulation with a wideBand electromagnetic wave as the source in the simulation. The CyclesDrive parameter dictates how large this pulse is.
• SingleFrequency = Runs the simulation with a single frequency source. This frequency is determined by the waveLengthCtr parameter.
• SimCycles = Number of cycles run in simulation.
• CyclesDrive = Number of cycles to drive the wideband source.
• CyclesPerDump = Number of cyles per dump.
• NominalCellSize = The size of your cells, should be small enough to resolve your dielectric cavities in the crystal.
Photonic crystal specifications:
• HoleDiam = Diameter of each hole in crystal lattice.
• HolePitch = Distance between the center of each hole, all distances equal due to lattice.
• NumHolesY = Number of rows of holes.
• NumHolesZ = Number of columns of holes.
• HtVacuum = Height of vacuum, along x-axis.
• HtSi = Height of silicon layer, along x-axis.
• HtSiO2 = Height of silica layer, along x-axis.
• IndexRefrSiO2 = Index of refraction for silica.
• IndexRefrSi = Index of refraction for silicon.
• MarginY = How far the edge of the crystal is from your lattice in the x-direction.
• MarginZ = how far the edge of the crystal is from your lattice in the z-direction.
Source specifications:
• BeamPowr = Power of the beam.
• WaistDiam = Diameter of your Gaussian beam waist.
• {ThetaK,phiK,chiE} = The {polar angle, azimuthal, angle of polarization} respectively.
• WaveLengthCtr = The central wavelength of your wideband singal, and is the frequency used as the single frequency simulation type.
• WaveLengthBand = The wavelength width of your wide band signal, only used in wideBand simType.

## Running the Simulation

After performing the above actions, continue as follows:

• Proceed to the Run Window by pressing the Run button in the left column of buttons.
• MPI can be enabled to utilize multi-core systems.
• The default values of Number of Time Steps and Dump Periodicity are calculated using by the CyclesDrive and CyclesPerDump inputs in the Setup Window.
• To run the file, click on the Run button in the upper left corner of the Logs and Output Files pane. You will see the output of the run in the right pane. The run has completed when you see the output, “Engine completed successfully.” This is shown in Fig. 271.

Fig. 271 The Run Window for the Gaussian Laser Beam and Photonic Crystal Cavity (Text-based setup) example

## Visualizing the results

After performing the above actions, continue as follows:

• Proceed to the Visualize Window by pressing the Visualize button in the left column of buttons.

The input file specifies grid points at which to record field histories.

• In the top field of Visualization Controls, click the drop down menu and select History.

By default, there are 4 possible graphs viewable at one time in the Visualize Window. For each graph, one can select from the following fields: (0 = x, 1 = y, 2 = z).

• {E,B}_AtCtr_{0,1,2} = {0,0} in {y,z} plane in the middle of SiLayer.
• {E,B}_AtDet_{0,1,2} = Just above the upper x boundary.
• {E,B}_AtSrc_{0,1,2} = Just below the source.
• {E,B}_InCav_{0,1,2} = In the cavity off center.
• E_AtHole_{0,1,2} = In the middle of the second hole.
• E_AtMargin{Z,Y}_{0,1,2} = Just inside the z or y margin.
• poyntingFluxDet = Just above the upper x boundary.
• poyntingFluxSrc = Just below the source.

In each individual graph, one can choose the FFT option to view the frequency domain of your field. This can enable the analysis of the frequency response of the photonic crystal cavity. Below, Fig. 272 depicts four graphs of histories. The first two graphs are amplitude vs time, and the second two are a FFT of the first two on a log scale. The first and third graphs depict the history AtSrc_2, while the second and fourth graphs show the AtDet_2 history.

Fig. 272 The Visualize Window for the Gaussian Laser Beam and Photonic Crystal Cavity (Text-based setup) example

## Further Experiments

By using the wide-band source and examining the field strength detected below the crystal lattice, one may study the frequency response of this photonic crystal as the device geometry, dielectric constants, location and polarizations of the radiation source and detector change.