# Configuration

Configuration of the following FDTD simulation parameters:

# Wavelength

Definition of the global simulation wavelengths/frequencies. When new sources and monitors are created, these values are applied by default. However, you can override them in the component setup by unchecking "Set the range from the global wavelength".

# Simulation Domain

Definition of the simulation domain size.

# Center + size

Defines the FDTD simulation domain using a center position and size.

center: Center of the simulation domain in x, y, and z-directions.

Type: floating-point number

  • Unit: m, μm (default), or nm
  • Default: (0, 0, 0)

size: Size of the simulation domain in x, y, and z-directions.

Use Infinity to define a geometry extending to infinity in both directions along an axis.

Type: floating-point number

  • Unit: m, μm (default), or nm
  • Constraint: greater than or equal to 0
  • Required field

# Bounds

Constructs the simulation domain from minimum and maximum coordinate bounds.

min: Values of the x, y, and z coordinates defining the simulation domain lower corner.

Type: floating-point number

  • Unit: m, μm (default), or nm
  • Required field

max: Values of the x, y, and z coordinates defining the simulation domain upper corner.

Type: floating-point number

  • Unit: m, μm (default), or nm
  • Required field

# Run Time

run_time: Total electromagnetic evolution time in seconds.

The run time can be specified either manually as a time value or through a RunTimeSpec object that automatically determines the appropriate run time based on simulation parameters.

# Manual

When specified directly:

Type: floating-point number

  • Unit: sec
  • Constraint: greater than 0
  • Default: 1 ps
  • Required field

# Auto

When "Auto" is selected, the simulation will use RunTimeSpec and the run time is automatically computed based on the following formula:

‘run_time=source_factor×Tsrc_max+quality_factor×nmax×Lmaxc0‘`run\_time = source\_factor \times T_{src\_max} + quality\_factor \times n_{max} \times \frac{L_{max}}{c_0}`

Where:

  • source_factor: The contribution to the run time from the longest source (default: 3)
  • T_{src_max}: The longest time that a source is non-zero
  • quality_factor: Expected quality factor in the device
  • n_{max}: Maximum refractive index in the simulation
  • L_{max}: Distance along the largest dimension in the simulation
  • c_0: Speed of light in vacuum

Parameters:

  • Quality Factor: Quality factor expected in the device. This determines how long the simulation will run as it assumes a field decay time that scales proportionally to this value.
  • Source Factor: The contribution to the run time from the longest source is computed from the source_time length times source_factor. Larger values provide more buffer at the expense of potentially giving run_time values that are larger than needed. Default: 3

Note: If simulation shutoff is specified, simulation will terminate early when shutoff condition met.

# Grid Specification

Specifications for the simulation grid along each of the three directions.

See: GridSpec

# Background Medium

Simulation background medium.

Options:

# Boundary and Symmetry

# Boundary Condition

boundary_spec: Specification of boundary conditions along each dimension.

See: BoundarySpec

# Symmetry

symmetry: Defining reflection symmetry across a plane bisecting the simulation domain normal to the x-, y-, and z-axis, respectvely.

Note: The vectorial nature of the fields must be taken into account to correctly determine the symmetry value.

Options:

  • No symmetry (0 in Simulation JSON)
  • Even, i.e. 'PMC' symmetry (1 in Simulation JSON)
  • Odd, i.e. 'PEC' symmetry (-1 in Simulation JSON)

Default: No symmetry

# Shutoff Condition

shutoff: Ratio of the instantaneous integrated E-field intensity to the maximum value at which the simulation will automatically terminate time stepping. Used to prevent extraneous run time of simulations with fully decayed fields.

Note: Set to 0 to disable this feature.

Type: floating-point number

  • Unit: unitless
  • Constraint: greater than or equal to 0
  • Default: 1e-5

# Courant Factor

courant: Courant stability factor, controls time step to spatial step ratio. Lower values lead to more stable simulations for dispersive materials, but result in longer simulation times.

Type: floating-point number

  • Unit: unitless
  • Constraint: (0, 1]
  • Default: 0.99

# Subpixel Averaging

subpixel: Apply subpixel averaging methods of the permittivity on structure interfaces to result in much higher accuracy for a given grid size. The user can select different subpixel averaging methods on dielectric, metal, and PEC material interfaces.

Options:

  • True (default): set default averaging methods on dielectric, metal, and PEC material interfaces.
  • False: apply staircasing on all the material interfaces.
  • User Selected: user selected averaging methods for dielectric, metal, and PEC.

# Dielectric

dielectric: Subpixel averaging method applied to dielectric material interfaces.

Options:

  • PolarizedAveraging (default): Apply a polarized subpixel averaging method to dielectric boundaries.

Note: The algorithm is based on: A. Mohammadi, H. Nadgaran and M. Agio, "Contour-path effective permittivities for the two-dimensional finite-difference time-domain method," Optics express, 13(25), 10367-10381 (2005).

  • Staircasing: Apply staircasing scheme to material assignment of Yee grids on structure boundaries.

# Metal

metal: Subpixel averaging method applied to metallic structure interfaces. A material is considered as metallic if its real part of relative permittivity is less than 1 at the central frequency.

Options:

  • Staircasing (default): Apply staircasing scheme to material assignment of Yee grids on structure boundaries.

  • VolumetricAveraging: Apply volumetric averaging scheme to material properties of Yee grids on structure boundaries. The material property is averaged in the volume surrounding the Yee grid.

staircase_normal_component: Volumetric averaging works accurately if the electric field component is substantially tangential to the interface.

Options:

  • True (default): apply volumetric averaging only if the field component is largely tangential to the interface.
  • False: apply volumetric averaging regardless of how field component orients with the interface.

# PEC

pec: Subpixel averaging method applied to PEC structure interfaces.

Options:

  • PECConformal (default): Apply a subpixel averaging method known as conformal mesh scheme to PEC boundaries.

timestep_reduction: Reduction factor between 0 and 1 such that the simulation's time step size will be 1 - timestep_reduction times its default value. Accuracy can be improved with a smaller time step size, but simulation time increased as well.

Type: floating-point number

  • Unit: unitless
  • Constraint: (0, 1)
  • Default: 0.3

Note: The algorithm is based on: S. Dey and R. Mittra, "A locally conformal finite-difference time-domain (FDTD) algorithm for modeling three-dimensional perfectly conducting objects," IEEE Microwave and Guided Wave Letters, 7(9), 273 (1997) and S. Benkler, N. Chavannes and N. Kuster, "A new 3-D conformal PEC FDTD scheme with user-defined geometric precision and derived stability criterion," IEEE Transactions on Antennas and Propagation, 54(6), 1843 (2006).

  • Staircasing: Apply staircasing scheme to material assignment of Yee grids on structure boundaries.

Note: For PEC interface, the algorithm is based on: A. Taflove and S. C. Hagness, "Computational electromagnetics: the finite-difference time-domain method," Chapter 10.3 (2005).

  • HeuristicPECStaircasing: Apply a variant of staircasing scheme to PEC boundaries: the electric field grid is set to PEC if the field is substantially parallel to the interface.

# Normalization Index

normalize_index: Index of the source in the tuple of sources whose spectrum will be used to normalize the frequency-dependent data.

If None, the raw field data is returned unnormalized.

Type: integer

  • Unit: unitless
  • Constraint: greater than or equal to 0
  • Default: 0, which means the first source

# Structure Priority Mode

structure_priority_mode: This field only affects structures with priority set to None.

Options:

  • equal (default): the priority of structures is set to 0.
  • conductor: the priority of structures made of LossyMetalMedium is set to 90, PECMedium to 100, and others to 0.