This section provides a template for plasmonic simulations that are periodic in at least one direction. Please also see the common simulation considerations section for other important simulation tips that apply for all plasmonic simulations.For devices that are periodic (such as nanostructure arrays, multilayer devices), we often want to study the resonant transmission/reflection properties. This includes the total transmission/reflection, or the transmission/reflection into particular orders.
Plane wave sources should always be used for periodic simulations. When using sources such as dipoles and Gaussian beams with periodic boundaries, the sources will also be copied across each unit cell in the periodic directions, which is unphysical.
For sources at normal incidence, or for single frequency simulations with injection at normal incidence, the plane wave type should be set to "Bloch/Periodic".
For broadband simulations with injection at non-normal angles, the plane wave type can be set to "'BFAST". The BFAST technique can be used to inject light at a constant angle over all frequencies. For a complete description on how to set up a simulation with a plane wave source injected at an angle, please see plane waves - angled injection.
When using the "Bloch/Periodic" plane wave type, periodic boundaries should be used if the source is injected at normal incidence, and Bloch boundaries should be used when the source is injected at angled incidence in the directions where the structure is periodic. When using the "BFAST" plane wave type, the boundary conditions in periodic directions is automatically set to use the BFAST technique, so the boundary conditions in those directions do not need to be set. In the directions of periodicity, the simulation span must correspond to exactly 1 unit cell of the device.
When using the For directions that are not periodic, PML (absorbing) boundaries should be used.
Note that symmetry can still be used for periodic boundaries by setting the min/max settings to be the same. For example, the settings below correspond to a periodic simulation.
Monitors and analysis
One can place profile monitors anywhere in the near field region to study the near field enhancement. Once the simulation finishes running, simply right-click on the monitor and send the results to the Visualizer to study the near field results.
The best way to calculate the far field scattering pattern is to use one monitor, located above or below the structure (depending if you want scattering in the forward or backwards direction). Note that the monitor plane should always be placed in a homogeneous region in the near field. Once the simulation finishes running, you can calculate the far field projections from the monitor directly in the Results View window, or with far field projections script commands. Note that it is possible to use the projection functions to calculate the far field distribution of finite sized periodic arrays (even if the simulation is infinitely periodic). See far field projections - periodic structures for more detail.
In many cases, we are only interested the total transmission/reflection of the device. This can be easily calculated by placing power monitors above and below the structure, and then plotting the transmission in the Results View window. Note that power flowing towards the -x/-y/-z directions will carry a negative sign.
For structures that support multiple grating orders, we may want to look at the transmission/reflection into particular orders. FDTD has built-in grating projection calculations that can be used to calculate the direction and intensity of light reflected or transmitted through a periodic structure. The Object Library also provides a "grating_transmission" analysis group
can be used to find the transmission into any order. For a list of examples, see the gratings section of the applications gallery.
For some examples that use this simulation methodology for studying periodic structures, please see: