This section describes how to create a custom source field profile from monitor data obtained from another simulation.
The left screenshot shows a waveguide input coupler. Suppose we want to use this system to excite a small gold nano-particle located at the end of the coupler (lower figure).
In the left figure, we see the entire coupler (facet, coupler, and output waveguide). The taper and waveguide are 500nm high. The coupler input width is 8.5 microns. The waveguide tapers over 10 microns to a width of 500nm, where it still supports many modes at wavelengths around 500nm. The dielectric waveguide is assumed to have an index of 2. The operating wavelength is 500 nm.
In the right figure, we see a 40 nm radius gold sphere, 200nm past the end of the taper region.
To simulate the entire device, the simulation region must be about 20x20x10 wavelengths in size. This is a large simulation. To accurately model the gold nano-particle, a very small mesh will be required (approx 5nm). Even with a graded mesh, this will still make the simulation substantially slower.
A more efficient solution is to break this simulation into 2 parts:
1) Model the large taper without the nano-particle using the normal mesh settings (mesh accuracy 2). A profile monitor is used to record the field profile at the taper output.
2) Model a nano-particle and a small portion of the taper. The field profile from the initial simulation will be used as the source field profile for the second simulation.
Note: Reflection sensitive structure
It is important to note this method assumes that there is not much reflection back from the Gold defect and that the waves recorded at the field monitor are not perturbed by the defect. This would not be a valid approximation in all simulations, therefore care must be taken when using this method to divide up a simulation into multiple simulations.
Step 1: Model taper section. Record field profile at output
The simulation file usr_source_from_monitor1.fsp is used to model the taper section. In this file, the gold sphere has been removed (material properties set to air). A gaussian beam illuminates the facet. At the output of the taper section, a frequency profile monitor records the electric field at a wavelength of 500 nm.
The script usr_source_from_monitor1.lsf begins by running the simulation and plotting some initial results, as shown below. Next, it gets the E&H fields from the transmission monitor and packages them in a format that can be loaded into the Import source.
|E| in the taper
|E| field profile exported to the .fld file
Step 2: Create custom source and simulate nano-particle
The script file then loads usr_source_from_monitor2.fsp, which contains the waveguide and gold sphere portion of the problem. The script loads the field data from the initial simulation into the Import source. After importing the field profile, you can open the source properties and see the field profile as shown below. You should see the same field profile that was recorded by the monitor in the last simulation.
Note: Data interpolation
The field profile in the mat file is saved on the mesh of the first simulation. The mesh in the second simulation is different. In such situations, the field data will be automatically interpolated onto the new mesh.
After running the second simulation, the script plots some initial results. First, the transmission and reflection in the waveguide after injecting the created source are shown. The reflection is effectively zero at 500nm, but increases slightly away from the wavelength where the field was recorded. This is because a physically correct waveguide mode profile will change as a function of frequency. However, we are injecting the mode profile that is correct at 500nm to be injected over a range of frequencies. Therefore, we expect some source injection error to result in reflection due to mode mismatch when running broadband simulations with this technique. If better accuracy is required, then a series of single frequency simulations will be required. The transmission is less than 1 because we are only measuring transmission in the waveguide region and are ignoring light that is propagating away into the air and substrate.
The second plot uses a box of 6 monitors surrounding the gold particle to calculate the absorption spectrum in the gold. This plot shows that there are 2 absorption resonances.