# Python User Interface

This page is a listing of the functions exposed by the Python interface. For a gentler introduction, see Tutorial/Basics. Note that this page is not a complete listing of all functions. In particular, because of the SWIG wrappers, every function in the C++ interface is accessible from the Python module, but not all of these functions are documented or intended for end users. See also the instructions for parallel Meep.

The Python API functions and classes can be found in the meep module, which should be installed in your Python system by Meep's make install script. If you installed into a nonstandard location (e.g. your home directory), you may need to set the PYTHONPATH environment variable as documented in the Installation section. You typically import the meep module in Python with:

import meep as mp


## The Simulation Class

The Simulation class contains all the attributes that you can set to control various parameters of the Meep computation. The function signature of the Simulation constructor with its default values is listed here for convenience:

class Simulation(object):

def __init__(self,
cell_size,
resolution,
geometry=[],
sources=[],
eps_averaging=True,
dimensions=2,
boundary_layers=[],
symmetries=[],
verbose=False,
force_complex_fields=False,
default_material=mp.Medium(),
m=0,
k_point=False,
extra_materials=[],
material_function=None,
epsilon_func=None,
epsilon_input_file='',
progress_interval=4,
subpixel_tol=1e-4,
subpixel_maxeval=100000,
ensure_periodicity=False,
num_chunks=0,
courant=0.5,
accurate_fields_near_cylorigin=False,
filename_prefix='',
output_volume=None,
output_single_precision=False,


All Simulation attributes are described here in further detail. In brackets after each variable is the type of value that it should hold. The classes, complex datatypes like GeometricObject, are described in a later subsection. The basic datatypes, like integer, boolean, complex, and string are defined by Python. Vector3 is a meep class.

geometry [ list of GeometricObject class ]
Specifies the geometric objects making up the structure being simulated. When objects overlap, later objects in the list take precedence. Defaults to no objects (empty list).

sources [ list of Source class ]
Specifies the current sources to be present in the simulation. Defaults to none (empty list).

symmetries [ list of Symmetry class ]
Specifies the spatial symmetries (mirror or rotation) to exploit in the simulation. Defaults to none (empty list). The symmetries must be obeyed by both the structure and the sources. See also Exploiting Symmetry.

boundary_layers [ list of PML class ]
Specifies the PML absorbing boundary layers to use. Defaults to none.

cell_size [ Vector3 ]
Specifies the size of the computational cell which is centered on the origin of the coordinate system. Any sizes of 0 imply a reduced-dimensionality calculation. Strictly speaking, the dielectric function is taken to be uniform along that dimension. A 2d calculation is especially optimized. See dimensions below. Note: because Maxwell's equations are scale invariant, you can use any units of distance you want to specify the cell size: nanometers, microns, centimeters, whatever. However, it is usually convenient to pick some characteristic lengthscale of your problem and set that length to 1. See also Units. Required argument (no default).

default_material [Medium class ]
Holds the default material that is used for points not in any object of the geometry list. Defaults to air (ε=1). See also epsilon_input_file below.

material_function [ function ]
A Python function that takes a Vector3 and returns a Medium. See also Medium. Defaults to None.

epsilon_func [ function ]
A Python function that takes a Vector3 and returns the dielectric constant at that point. See also Medium. Defaults to None.

epsilon_input_file [string]
If this string is not empty (the default), then it should be the name of an HDF5 file whose first/only dataset defines a scalar, real-valued, frequency-independent dielectric function over some discrete grid. Alternatively, the dataset name can be specified explicitly if the string is in the form "filename:dataset". This dielectric function is then used in place of the ε property of default_material (i.e. where there are no geometry objects). The grid of the epsilon file dataset need not match the computational grid; it is scaled and/or linearly interpolated as needed to map the file onto the computational cell. The structure is warped if the proportions of the grids do not match. Note: the file contents only override the ε property of the default_material, whereas other properties (μ, susceptibilities, nonlinearities, etc.) of default_material are still used.

dimensions [integer]
Explicitly specifies the dimensionality of the simulation, if the value is less than 3. If the value is 3 (the default), then the dimensions are automatically reduced to 2 if possible when cell_size in the $z$ direction is 0. If dimensions is the special value of CYLINDRICAL, then cylindrical coordinates are used and the $x$ and $z$ dimensions are interpreted as $r$ and $z$, respectively. If dimensions is 1, then the cell must be along the $z$ direction and only $E_x$ and $H_y$ field components are permitted. If dimensions is 2, then the cell must be in the $xy$ plane.

m [number]
For CYLINDRICAL simulations, specifies that the angular $\phi$ dependence of the fields is of the form $e^{im\phi}$ (default is m=0). If the simulation cell includes the origin $r=0$, then m must be an integer.

accurate_fields_near_cylorigin [boolean]
For CYLINDRICAL simulations with |m| > 1, compute more accurate fields near the origin $r=0$ at the expense of requiring a smaller Courant factor. Empirically, when this option is set to True, a Courant factor of roughly $\min[0.5, 1 / (|m| + 0.5)]$ or smaller seems to be needed. Default is False, in which case the $D_r$, $D_z$, and $B_r$ fields within |m| pixels of the origin are forced to zero, which usually ensures stability with the default Courant factor of 0.5, at the expense of slowing convergence of the fields near $r=0$.

resolution [number]
Specifies the computational grid resolution in pixels per distance unit. Required argument. No default.

k_point [False or Vector3]
If False (the default), then the boundaries are perfect metallic (zero electric field). If a vector, then the boundaries are Bloch-periodic: the fields at one side are $\exp(i\mathbf{k}\cdot\mathbf{R})$ times the fields at the other side, separated by the lattice vector $\mathbf{R}$. The k_point vector is specified in Cartesian coordinates in units of 2π/distance. Note: this is different from MPB, equivalent to taking MPB's k_points through its function reciprocal->cartesian.

ensure_periodicity [boolean]
If True (the default) and if the boundary conditions are periodic (k_point is not False), then the geometric objects are automatically repeated periodically according to the lattice vectors which define the size of the computational cell.

eps_averaging [boolean]
If True (the default), then subpixel averaging is used when initializing the dielectric function. For details, see Section 3 ("Interpolation and the illusion of continuity") of Computer Physics Communications, Vol. 181, pp. 687-702, 2010. The input variables subpixel_maxeval (default 104) and subpixel_tol (default 10-4) specify the maximum number of function evaluations and the integration tolerance for subpixel averaging. Increasing/decreasing these, respectively, will cause a more accurate but slower computation of the average ε with diminishing returns for the actual FDTD error.

force_complex_fields [boolean]
By default, Meep runs its simulations with purely real fields whenever possible. It uses complex fields which require twice the memory and computation if the k_point is non-zero or if m is non-zero. However, by setting force_complex_fields to True, Meep will always use complex fields.

filename_prefix [string]
A string prepended to all output filenames. If empty (the default), then Meep uses the name of the current Python file, with ".py" replaced by "-" (e.g. foo.py uses a "foo-" prefix). See also Output File Names.

Courant [number]
Specify the Courant factor $S$ which relates the time step size to the spatial discretization: $cΔ t = SΔ x$. Default is 0.5. For numerical stability, the Courant factor must be at most $n_\textrm{min}/\sqrt{\textrm{# dimensions}}$, where $n_\textrm{min}$ is the minimum refractive index (usually 1), and in practice $S$ should be slightly smaller.

output_volume [Volume class ]
Specifies the default region of space that is output by the HDF5 output functions (below); see also the Volume class which manages meep::volume* objects. Default is None, which means that the whole computational cell is output. Normally, you should use the in_volume(...) function to modify the output volume instead of setting output_volume directly.

output_single_precision [boolean]
Meep performs its computations in double precision, and by default its output HDF5 files are in the same format. However, by setting this variable to True (default is False) you can instead output in single precision which saves a factor of two in space.

progress_interval [number]
Time interval (seconds) after which Meep prints a progress message. Default is 4 seconds.

extra_materials [ list of Medium class ]
By default, Meep turns off support for material dispersion, nonlinearities, and similar properties if none of the objects in geometry have materials with these properties — since they are not needed, it is faster to omit their calculation. This doesn't work however if you use a material_function: materials via a user-specified function of position instead of just geometric objects. If your material function only returns a nonlinear material, for example, Meep won't notice this unless you tell it explicitly via extra_materials. extra_materials is a list of materials that Meep should look for in the computational cell in addition to any materials that are specified by geometric objects. You should list any materials other than scalar dielectrics that are returned by material_function here.

load_structure [string]
If not empty, Meep will load the structure specified by this string. The file must have been created by meep.dump_structure. Defaults to an empty string. See Load and Dump Structure for more information.

The following require a bit more understanding of the inner workings of Meep to use. See also SWIG Wrappers.

structure [meep::structure*]
Pointer to the current structure being simulated; initialized by _init_structure which is called automatically by init_fields() which is called automatically by any of the run functions. The structure initailization is handled by the Simulation class, and most users will not need to call _init_structure.

fields [meep::fields*]
Pointer to the current fields being simulated; initialized by init_fields() which is called automatically by any of the run functions.

num_chunks [integer]
Minimum number of "chunks" (subarrays) to divide the structure/fields into (default 0). Actual number is determined by number of processors, PML layers, etcetera. Mainly useful for debugging.

## Predefined Variables

These are available directly via the meep package.

air, vacuum [Medium class ]
Two aliases for a predefined material type with a dielectric constant of 1.

perfect_electric_conductor or metal [Medium class ]
A predefined material type corresponding to a perfect electric conductor at the boundary of which the parallel electric field is zero. Technically, $\varepsilon = -\infty$.

perfect_magnetic_conductor [Medium class ]
A predefined material type corresponding to a perfect magnetic conductor at the boundary of which the parallel magnetic field is zero. Technically, $\mu = -\infty$.

inf [number]
A big number (10$^{20}$) to use for "infinite" dimensions of objects.

## Constants (Enumerated Types)

Several of the functions/classes in Meep ask you to specify e.g. a field component or a direction in the grid. These should be one of the following constants (which are available directly via the meep package):

direction constants
Specify a direction in the grid. One of X, Y, Z, R, P for $x$, $y$, $z$, $r$, $\phi$, respectively.

side constants
Specify particular boundary in some direction (e.g. $+x$ or $-x$). One of Low or High.

component constants
Specify a particular field or other component. One of Ex, Ey, Ez, Er, Ep, Hx, Hy, Hz, Hy, Hp, Hz, Bx, By, Bz, By, Bp, Bz, Dx, Dy, Dz, Dr, Dp, Dielectric, Permeability, for $E_x$, $E_y$, $E_z$, $E_r$, $E_\phi$, $H_x$, $H_y$, $H_z$, $H_r$, $H_\phi$, $B_x$, $B_y$, $B_z$, $B_r$, $B_\phi$, $D_x$, $D_y$, $D_z$, $D_r$, $D_\phi$, ε, μ, respectively.

derived_component constants
These are additional components which are not actually stored by Meep but are computed as needed, mainly for use in output functions. One of Sx, Sy, Sz, Sr, Sp, EnergyDensity, D_EnergyDensity, H_EnergyDensity for $S_x$, $S_y$, $S_z$, $S_r$, $S_\phi$ (components of the Poynting vector $\mathrm{Re}\,\mathbf{E}^* \times \mathbf{H}$), $(\mathbf{E}^* \cdot \mathbf{D} + \mathbf{H}^* \cdot \mathbf{B})/2$, $\mathbf{E}^* \cdot \mathbf{D}/2$, $\mathbf{H}^* \cdot \mathbf{B}/2$, respectively.

## Classes

Classes are complex datatypes with various properties which may have default values. Classes can be "subclasses" of other classes. Subclasses inherit all the properties of their superclass and can be used in any place the superclass is expected.

The meep package defines several types of classes. The most important of these is the Simulation class. Classes which are available directly from the meep package are constructed with:

meep.ClassName(prop1=val1, prop2=val2, ...)


The most numerous are the geometric object classes which are the same as those used in MPB. You can get a list of the available classes (and constants) in the python interpreter with:

import meep
[x for x in dir(meep) if x[0].isupper()]


More information, including their property types and default values, is available with the standard python help function: help(meep.ClassName).

The following are available directly via the meep package.

### Medium

This class is used to specify the materials that geometric objects are made of. It represents an electromagnetic medium which is possibly nonlinear and/or dispersive. See also Materials. To model a perfectly-conducting metal, use the predefined metal object, above. To model imperfect conductors, use a dispersive dielectric material. See also the predefined variables metal, perfect_electric_conductor, and perfect_magnetic_conductor above.

epsilon [number]
The frequency-independent isotropic relative permittivity or dielectric constant. Default is 1. You can also use index=n as a synonym for epsilon=n*n; note that this is not really the refractive index if you also specify μ, since the true index is $\sqrt{\mu\varepsilon}$.

Using epsilon=ep is actually a synonym for epsilon_diag=meep.Vector3(ep, ep, ep).

epsilon_diag and epsilon_offdiag [Vector3]
These properties allow you to specify ε as an arbitrary real-symmetric tensor by giving the diagonal and offdiagonal parts. Specifying epsilon_diag=Vector3(a, b, c) and/or epsilon_offdiag=Vector3(u, v, w) corresponds to a relative permittivity ε tensor \begin{pmatrix} a & u & v \\ u & b & w \\ v & w & c \end{pmatrix}

Default is the identity matrix ($a = b = c = 1$ and $u = v = w = 0$).

mu [number]
The frequency-independent isotropic relative permeability μ. Default is 1. Using mu=pm is actually a synonym for mu_diag=meep.Vector3(pm, pm, pm).

mu_diag and mu_offdiag [Vector3]
These properties allow you to specify μ as an arbitrary real-symmetric tensor by giving the diagonal and offdiagonal parts exactly as for ε above. Default is the identity matrix.

D_conductivity [number]
The frequency-independent electric conductivity $\sigma_D$. Default is 0. You can also specify a diagonal anisotropic conductivity tensor by using the property D_conductivity_diag which takes a Vector3 to give the $\sigma_D$ tensor diagonal. See also Conductivity.

B_conductivity [number]
The frequency-independent magnetic conductivity $\sigma_B$. Default is 0. You can also specify a diagonal anisotropic conductivity tensor by using the property B_conductivity_diag which takes a Vector3 to give the $\sigma_B$ tensor diagonal. See also Conductivity.

chi2 [number]
The nonlinear (Pockels) susceptibility $\chi^{(2)}$. Default is 0. See also Nonlinearity.

chi3 [number]
The nonlinear (Kerr) susceptibility $\chi^{(3)}$. Default is 0. See also Nonlinearity.

E_susceptibilities [ list of Susceptibility class ]
List of dispersive susceptibilities (see below) added to the dielectric constant ε in order to model material dispersion. Defaults to none (empty list). See also Material Dispersion.

H_susceptibilities [ list of Susceptibility class ]
List of dispersive susceptibilities (see below) added to the permeability μ in order to model material dispersion. Defaults to none (empty list). See also Material Dispersion.

material functions

Any function that accepts a Medium instance can also accept a user defined Python function. This allows you to specify the material as an arbitrary function of position. The function must have one argument, the position Vector3, and return the material at that point, which should be a Python Medium instance. This is accomplished by passing a function to the material_function keyword argument in the Simulation constructor, or the material keyword argument in any GeometricObject constructor.

Instead of the material or material_function arguments, you can also use the epsilon_func keyword argument to Simulation and GeometricObject, which takes a function of position that returns the dielectric constant at that point.

Important: If your material function returns nonlinear, dispersive (Lorentzian or conducting), or magnetic materials, you should also include a list of these materials in the extra_materials input variable (above) to let Meep know that it needs to support these material types in your simulation. For dispersive materials, you need to include a material with the same γn and μn values, so you can only have a finite number of these, whereas σn can vary continuously and a matching σn need not be specified in extra_materials. For nonlinear or conductivity materials, your extra_materials list need not match the actual values of σ or χ returned by your material function, which can vary continuously.

Complex ε and μ: you cannot specify a frequency-independent complex ε or μ in Meep where the imaginary part is a frequency-independent loss but there is an alternative. That is because there are only two important physical situations. First, if you only care about the loss in a narrow bandwidth around some frequency, you can set the loss at that frequency via the conductivity. Second, if you care about a broad bandwidth, then all physical materials have a frequency-dependent complex ε and/or μ, and you need to specify that frequency dependence by fitting to Lorentzian and/or Drude resonances via the LorentzianSusceptibility or DrudeSusceptibility classes below.

Dispersive dielectric and magnetic materials, above, are specified via a list of objects that are subclasses of type Susceptibility.

### Susceptibility

Parent class for various dispersive susceptibility terms, parameterized by an anisotropic amplitude σ. See Material Dispersion.

sigma [number]
The scale factor σ. You can also specify an anisotropic σ tensor by using the property sigma_diag which takes three numbers or a Vector3 to give the σ$_n$ tensor diagonal, and sigma_offdiag which specifies the offdiagonal elements (defaults to 0). That is, sigma_diag=meep.Vector3(a, b, c) and sigma_offdiag=meep.Vector3(u, v, w) corresponds to a σ tensor

\begin{pmatrix} a & u & v \\ u & b & w \\ v & w & c \end{pmatrix}

### LorentzianSusceptibility

Specifies a single dispersive susceptibility of Lorentzian (damped harmonic oscillator) form. See Material Dispersion, with the parameters (in addition to σ):

frequency [number]
The resonance frequency $f_n = \omega_n / 2\pi$.

gamma [number]
The resonance loss rate $γ_n / 2\pi$.

### DrudeSusceptibility

Specifies a single dispersive susceptibility of Drude form. See Material Dispersion, with the parameters (in addition to σ):

frequency [number]
The frequency scale factor $f_n = \omega_n / 2\pi$ which multiplies σ (not a resonance frequency).

gamma [number]
The loss rate $γ_n / 2\pi$.

Meep also supports a somewhat unusual polarizable medium, a Lorentzian susceptibility with a random noise term added into the damped-oscillator equation at each point. This can be used to directly model thermal radiation in both the far field and the near field. Note, however that it is more efficient to compute far-field thermal radiation using Kirchhoff's law of radiation, which states that emissivity equals absorptivity. Near-field thermal radiation can usually be computed more efficiently using frequency-domain methods, e.g. via SCUFF-EM.

### NoisyLorentzianSusceptibility or NoisyDrudeSusceptibility

Specifies a single dispersive susceptibility of Lorentzian (damped harmonic oscillator) or Drude form. See Material Dispersion, with the same sigma, frequency, and gamma parameters, but with an additional Gaussian random noise term (uncorrelated in space and time, zero mean) added to the P damped-oscillator equation.

noise_amp [number]
The noise has root-mean square amplitude σ $\times$ noise_amp.

### Vector3

Properties:

x, y, z [float or complex]
The x, y, and z components of the vector.

Vector3(x=0.0, y=0.0, z=0.0)
Create a new Vector3 with the given components. All three components default to zero.

v3 = v1 + v2
v3 = v1 - v2
v3 = v1.cross(v2)


Return the sum, difference, or cross product of the two vectors.

c = v1 * b
c = b * v1


If b is a Vector3, returns the dot product v1 and b. If b is a number, then v1 is scaled by the number.

v3 = v1.dot(v2)


Returns the dot product of v1 and v2.

v3 = v1.cross(v2)


Returns the cross product of v1 and v2.

v3 = v1.cdot(v2)


Returns the conjugated dot product: v1* dot v2.

v2 = v1.norm()


Returns the length math.sqrt(abs(v1.dot(v1))) of the given vector.

v2 = v1.unit()


Returns a unit vector in the direction of v1.

v1.close(v2, [tol])


Returns whether or not the corresponding components of the two vectors are within tol of each other. Defaults to 1e-7.

v1 == v2


Returns whether or not the two vectors are numerically equal. Beware of using this function after operations that may have some error due to the finite precision of floating-point numbers; use close instead.

v2 = v1.rotate(axis, theta)


Returns the vector v1 rotated by an angle theta (in radians) in the right-hand direction around the axis vector (whose length is ignored). You may find the python functions math.degrees and math.radians useful to convert angles between degrees and radians.

### GeometricObject

This class, and its descendants, are used to specify the solid geometric objects that form the dielectric structure being simulated. The base class is:

GeometricObject

Properties:

material [Medium class or function ]
The material that the object is made of (usually some sort of dielectric). Uses default Medium. If a function is supplied, it must take one argument and return a Python Medium.

epsilon_func [ function ]
A function that takes one argument (a Vector3) and returns the dielectric constant at that point. Can be used instead of material. Default is None.

center [Vector3]
Center point of the object. Defaults to (0, 0, 0).

Methods:

shift(vec [Vector3])
Shifts the objects center by vec. This can also be accomplished via the + operator: geomtric_obj + Vector3(10, 10, 10).

info(indent_by [integer])
Displays all properties and current values of a GeometricObject, indented by indent_by spaces(0 by default).

One normally does not create objects of type GeometricObject directly, however; instead, you use one of the following subclasses. Recall that subclasses inherit the properties of their superclass, so these subclasses automatically have the material and center properties and can be specified in a subclass's constructor via keyword arguemnts.

In a 2d calculation, only the intersections of the objects with the $xy$ plane are considered.

### Sphere

A sphere. Properties:

radius [number]
Radius of the sphere. No default value.

### Cylinder

A cylinder, with circular cross-section and finite height. Properties:

radius [number]
Radius of the cylinder's cross-section. No default value.

height [number]
Length of the cylinder along its axis. No default value.

axis [Vector3]
Direction of the cylinder's axis; the length of this vector is ignored. Defaults to point parallel to the $z$ axis.

### Cone

A cone, or possibly a truncated cone. This is actually a subclass of Cylinder, and inherits all of the same properties, with one additional property. The radius of the base of the cone is given by the radius property inherited from cylinder, while the radius of the tip is given by the new property, radius2. The center of a cone is halfway between the two circular ends.

radius2 [number]
Radius of the tip of the cone (i.e. the end of the cone pointed to by the axis vector). Defaults to zero (a "sharp" cone).

### Block

A parallelepiped (i.e., a brick, possibly with non-orthogonal axes).

size [Vector3]
The lengths of the block edges along each of its three axes. Not really a 3-vector, but it has three components, each of which should be nonzero. No default value.

e1, e2, e3 [Vector3]
The directions of the axes of the block; the lengths of these vectors are ignored. Must be linearly independent. They default to the three lattice directions.

### Ellipsoid

An ellipsoid. This is actually a subclass of Block, and inherits all the same properties, but defines an ellipsoid inscribed inside the block.

Here are some examples of geometric objects created using the above classes:

# A cylinder of infinite radius and height 0.25 pointing along the x axis,
# centered at the origin:
axis=meep.Vector3(1,0,0), material=meep.Medium(index=3.5))

# An ellipsoid with its long axis pointing along (1,1,1), centered on
# the origin (the other two axes are orthogonal and have equal
# semi-axis lengths):
ell = meep.Ellipsoid(center=meep.Vector3(0,0,0), size=meep.Vector3(0.8,0.2,0.2),
e1=Vector3(1,1,1), e2=Vector3(0,1,-1), e3=Vector3(-2,1,1),
material=meep.Medium(epsilon=13))

# A unit cube of material mat with a spherical air hole of radius 0.2 at
# its center, the whole thing centered at (1,2,3):
geometry=[meep.Block(center=Vector3(1,2,3), size=Vector3(1,1,1), material=meep.metal),


### Symmetry

This class is used for the symmetries input variable to specify symmetries which must preserve both the structure and the sources. Any number of symmetries can be exploited simultaneously but there is no point in specifying redundant symmetries: the computational cell can be reduced by at most a factor of 4 in 2d and 8 in 3d. See also Exploiting Symmetry. This is the base class of the specific symmetries below, so normally you don't create it directly. However, it has two properties which are shared by all symmetries:

direction [direction constant ]
The direction of the symmetry (the normal to a mirror plane or the axis for a rotational symmetry). e.g. X, Y, or Z (only Cartesian/grid directions are allowed). No default value.

phase [complex]
An additional phase to multiply the fields by when operating the symmetry on them. Default is +1, e.g. a phase of -1 for a mirror plane corresponds to an odd mirror. Technically, you are essentially specifying the representation of the symmetry group that your fields and sources transform under.

The specific symmetry sub-classes are:

Mirror
A mirror symmetry plane. Here, the direction is the direction normal to the mirror plane.

Rotate2
A 180° (twofold) rotational symmetry (a.k.a. $C_2$). Here, the direction is the axis of the rotation.

Rotate4
A 90° (fourfold) rotational symmetry (a.k.a. $C_4$). Here, the direction is the axis of the rotation.

### PML

This class is used for specifying the PML absorbing boundary layers around the cell, if any, via the boundary_layers input variable. See also Perfectly Matched Layers. boundary_layers can be zero or more PML objects, with multiple objects allowing you to specify different PML layers on different boundaries. The class represents a single PML layer specification, which sets up one or more PML layers around the boundaries according to the following properties.

thickness [number]
The spatial thickness of the PML layer which extends from the boundary towards the inside of the computational cell. The thinner it is, the more numerical reflections become a problem. No default value.

direction [direction constant ]
Specify the direction of the boundaries to put the PML layers next to. e.g. if X, then specifies PML on the $\pm x$ boundaries (depending on the value of side, below). Default is the special value ALL, which puts PML layers on the boundaries in all directions.

side [side constant ]
Specify which side, Low or High of the boundary or boundaries to put PML on. e.g. if side is Low and direction is X, then a PML layer is added to the $-x$ boundary. Default is the special value ALL, which puts PML layers on both sides.

strength [number]
A strength (default is 1.0) to multiply the PML absorption coefficient by. A strength of 2.0 will square the theoretical asymptotic reflection coefficient of the PML (making it smaller), but will also increase numerical reflections. Alternatively, you can change R_asymptotic, below.

R_asymptotic [number]
The asymptotic reflection in the limit of infinite resolution or infinite PML thickness, for refections from air (an upper bound for other media with index > 1). For a finite resolution or thickness, the reflection will be much larger, due to the discretization of Maxwell's equation. Default value is 10−15, which should suffice for most purposes. You want to set this to be small enough so that waves propagating within the PML are attenuated sufficiently, but making R_asymptotic too small will increase the numerical reflection due to discretization.

pml_profile [function]
By default, Meep turns on the PML conductivity quadratically within the PML layer — one doesn't want to turn it on suddenly, because that exacerbates reflections due to the discretization. More generally, with pml_profile one can specify an arbitrary PML "profile" function $f(u)$ that determines the shape of the PML absorption profile up to an overall constant factor. u goes from 0 to 1 at the start and end of the PML, and the default is $f(u) = u^2$. In some cases where a very thick PML is required, such as in a periodic medium (where there is technically no such thing as a true PML, only a pseudo-PML), it can be advantageous to turn on the PML absorption more smoothly. See Optics Express, Vol. 16, pp. 11376-92, 2008. For example, one can use a cubic profile $f(u) = u^3$ by specifying pml_profile=lambda u: u*u*u.

#### Absorber

Instead of a PML layer, there is an alternative class called Absorber which is a drop-in replacement for PML. For example, you can do boundary_layers=[meep.Absorber(thickness=2)] instead of boundary_layers=[meep.PML(thickness=2)]. All the parameters are the same as for PML, above. You can have a mix of PML on some boundaries and Absorber on others.

The Absorber class does not implement a perfectly matched layer (PML), however (except in 1d). Instead, it is simply a scalar electric and magnetic conductivity that turns on gradually within the layer according to the pml_profile (defaulting to quadratic). Such a scalar conductivity gradient is only reflectionless in the limit as the layer becomes sufficiently thick.

The main reason to use Absorber is if you have a case in which PML fails:

### Source

The Source class is used to specify the current sources via the Siumulation.sources attribute. Note that all sources in Meep are separable in time and space, i.e. of the form $\mathbf{J}(\mathbf{x},t) = \mathbf{A}(\mathbf{x}) \cdot f(t)$ for some functions $\mathbf{A}$ and $f$. Non-separable sources can be simulated, however, by modifying the sources after each time step. When real fields are being used (which is the default in many cases; see Simulation.force_complex_fields), only the real part of the current source is used.

Important note: These are current sources (J terms in Maxwell's equations), even though they are labelled by electric/magnetic field components. They do not specify a particular electric/magnetic field which would be what is called a "hard" source in the FDTD literature. There is no fixed relationship between the current source and the resulting field amplitudes; it depends on the surrounding geometry, as described in the FAQ and in Section 4.4 ("Currents and Fields: The Local Density of States") in Chapter 4 ("Electromagnetic Wave Source Conditions") of the book Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology.

Properties:

src [SourceTime class ]
Specify the time-dependence of the source (see below). No default.

component [component constant ]
Specify the direction and type of the current component: e.g. meep.Ex, meep.Ey, etcetera for an electric-charge current, and meep.Hx, meep.Hy, etcetera for a magnetic-charge current. Note that currents pointing in an arbitrary direction are specified simply as multiple current sources with the appropriate amplitudes for each component. No default.

center [Vector3]
The location of the center of the current source in the computational cell. No default.

size [Vector3]
The size of the current distribution along each direction of the computational cell. Default is (0,0,0): a point-dipole source.

amplitude [complex]
An overall complex amplitude multiplying the the current source. Default is 1.0.

amp_func [function]
A Python function of a single argument, that takes a Vector3 giving a position and returns a complex current amplitude for that point. The position argument is relative to the center of the current source, so that you can move your current around without changing your function. Default is None, meaning that a constant amplitude of 1.0 is used. Note that your amplitude function (if any) is multiplied by the amplitude property, so both properties can be used simultaneously.

As described in Section 4.2 ("Incident Fields and Equivalent Currents") in Chapter 4 ("Electromagnetic Wave Source Conditions") of the book Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology, it is also possible to supply a source that is designed to couple exclusively into a single waveguide mode (or other mode of some cross section or periodic region) at a single frequency, and which couples primarily into that mode as long as the bandwidth is not too broad. This is possible if you have MPB installed: Meep will call MPB to compute the field profile of the desired mode, and uses the field profile to produce an equivalent current source. Note: this feature does not work in cylindrical coordinates. To do this, instead of a source you should use an EigenModeSource:

### EigenModeSource

This is a subclass of Source and has all of the properties of Source above. However, you normally do not specify a component. Instead of component, the current source components and amplitude profile are computed by calling MPB to compute the modes of the dielectric profile in the region given by the size and center of the source, with the modes computed as if the source region were repeated periodically in all directions. If an amplitude and/or amp_func are supplied, they are multiplied by this current profile. The desired eigenmode and other features are specified by the following properties:

eig_band [integer]
The index n (1,2,3,...) of the desired band ωn(k) to compute in MPB where 1 denotes the lowest-frequency band at a given k point, and so on.

direction [meep.X, meep.Y, or meep.Z; default meep.AUTOMATIC], eig_match_freq [boolean; default True], eig_kpoint [Vector3]
By default (if eig_match_freq is True), Meep tries to find a mode with the same frequency ωn(k) as the src property (above), by scanning k vectors in the given direction using MPB's find_k functionality. Alternatively, if eig_kpoint is supplied, it is used as an initial guess and direction for k. By default, direction is the direction normal to the source region, assuming size is $d$–1 dimensional in a $d$-dimensional simulation (e.g. a plane in 3d). Alternatively if eig_match_freq is False, you can specify a particular k vector of the desired mode with eig_kpoint (in Meep units of 2π/$a$).

eig_parity [meep.NO_PARITY (default), meep.EVEN_Z, meep.ODD_Z, meep.EVEN_Y, meep.ODD_Y]
The parity (= polarization in 2d) of the mode to calculate, assuming the structure has $z$ and/or $y$ mirror symmetry in the source region. If the structure has both $y$ and $z$ mirror symmetry, you can combine more than one of these, e.g. EVEN_Z + ODD_Y. Default is NO_PARITY, in which case MPB computes all of the bands which will still be even or odd if the structure has mirror symmetry, of course. This is especially useful in 2d simulations to restrict yourself to a desired polarization.

eig_resolution [integer, defaults to same as Meep resolution ]
The spatial resolution to use in MPB for the eigenmode calculations. This defaults to the same resolution as Meep, but you can use a higher resolution in which case the structure is linearly interpolated from the Meep pixels.

eig_tolerance [number, defaults to 10–7 ]
The tolerance to use in the MPB eigensolver. MPB terminates when the eigenvalues stop changing to less than this fractional tolerance.

component [as above, but defaults to ALL_COMPONENTS]
Once the MPB modes are computed, equivalent electric and magnetic sources are created within Meep. By default, these sources include magnetic and electric currents in all transverse directions within the source region, corresponding to the mode fields as described in Section 4.2 ("Incident Fields and Equivalent Currents") in Chapter 4 ("Electromagnetic Wave Source Conditions") of the book Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology. If you specify a component property, however, you can include only one component of these currents if you wish. Most users won't need this feature.

eig_lattice_size [Vector3], eig_lattice_center [Vector3]
Normally, the MPB computational unit cell is the same as the source volume given by the size and center parameters. However, occasionally you want the unit cell to be larger than the source volume. For example, to create an eigenmode source in a periodic medium, you need to pass MPB the entire unit cell of the periodic medium, but once the mode is computed then the actual current sources need only lie on a cross section of that medium. To accomplish this, you can specify the optional eig_lattice_size and eig_lattice_center, which define a volume (which must enclose size and center) that is used for the unit cell in MPB with the dielectric function ε taken from the corresponding region in the Meep simulation.

Note that MPB only supports dispersionless non-magnetic materials but it does support anisotropic ε. Any nonlinearities, magnetic responses μ, conductivities σ, or dispersive polarizations in your materials will be ignored when computing the eigenmode source. PML will also be ignored.

The src_time object (Source.src), which specifies the time dependence of the source, can be one of the following three classes.

### ContinuousSource

A continuous-wave source proportional to $\exp(-i\omega t)$, possibly with a smooth (exponential/tanh) turn-on/turn-off.

frequency [number]
The frequency f in units of $c$/distance or ω in units of $2\pi c$/distance. See Units. No default value. You can instead specify wavelength=x or period=x, which are both a synonym for frequency=1/x; i.e. 1/ω in these units is the vacuum wavelength or the temporal period.

start_time [number]
The starting time for the source. Default is 0 (turn on at $t=0$).

end_time [number]
The end time for the source. Default is 1020 (never turn off).

width [number]
Roughly, the temporal width of the smoothing (technically, the inverse of the exponential rate at which the current turns off and on). Default is 0 (no smoothing). You can instead specify fwidth=x, which is a synonym for width=1/x (i.e. the frequency width is proportional to the inverse of the temporal width).

cutoff [number]
How many widths the current decays for before we cut it off and set it to zero. Default is 3.0. A larger value of cutoff will reduce the amount of high-frequency components that are introduced by the start/stop of the source, but will of course lead to longer simulation times.

### GaussianSource

A Gaussian-pulse source roughly proportional to $\exp(-i\omega t - (t-t_0)^2/2w^2)$. Technically, the "Gaussian" sources in Meep are the (discrete-time) derivative of a Gaussian, i.e. they are $(-i\omega)^{-1} \frac{\partial}{\partial t} \exp(-i\omega t - (t-t_0)^2/2w^2)$, but the difference between this and a true Gaussian is usually irrelevant.

frequency [number]
The center frequency $f$ in units of $c$/distance (or ω in units of $2\pi c$/distance). See Units. No default value. You can instead specify wavelength=x or period=x, which are both a synonym for frequency=1/x; i.e. 1/ω in these units is the vacuum wavelength or the temporal period.

width [number]
The width $w$ used in the Gaussian. No default value. You can instead specify fwidth=x, which is a synonym for width=1/x (i.e. the frequency width is proportional to the inverse of the temporal width).

start_time [number]
The starting time for the source; default is 0 (turn on at $t=0$). This is not the time of the peak. See below.

cutoff [number]
How many widths the current decays for before we cut it off and set it to zero — this applies for both turn-on and turn-off of the pulse. Default is 5.0. A larger value of cutoff will reduce the amount of high-frequency components that are introduced by the start/stop of the source, but will of course lead to longer simulation times. The peak of the Gaussian is reached at the time $t_0$=start_time + cutoff*width.

### CustomSource

A user-specified source function $f(t)$. You can also specify start/end times at which point your current is set to zero whether or not your function is actually zero. These are optional, but you must specify an end_time explicitly if you want run functions like until_after_sources to work, since they need to know when your source turns off.

src_func [function]
The function $f(t)$ specifying the time-dependence of the source. It should take one argument (the time in Meep units) and return a complex number.

start_time [number]
The starting time for the source. Default is -1020: turn on at $t=-\infty$. Note, however, that the simulation normally starts at $t=0$ with zero fields as the initial condition, so there is implicitly a sharp turn-on at $t=0$ whether you specify it or not.

end_time [number]
The end time for the source. Default is 1020 (never turn off).

### FluxRegion

A FluxRegion object is used with add_flux to specify a region in which Meep should accumulate the appropriate Fourier-transformed fields in order to compute a flux spectrum. It represents a region (volume, plane, line, or point) in which to compute the integral of the Poynting vector of the Fourier-transformed fields.

Properties:

center [Vector3]
The center of the flux region (no default).

size [Vector3]
The size of the flux region along each of the coordinate axes. Default is (0,0,0); a single point.

direction [direction constant ]
The direction in which to compute the flux (e.g. meep.X, meep.Y, etcetera). Default is AUTOMATIC, in which the direction is determined by taking the normal direction if the flux region is a plane (or a line, in 2d). If the normal direction is ambiguous (e.g. for a point or volume), then you must specify the direction explicitly (not doing so will lead to an error).

weight [complex]
A weight factor to multiply the flux by when it is computed. Default is 1.0.

Note that the flux is always computed in the positive coordinate direction, although this can effectively be flipped by using a weight of -1.0. This is useful, for example, if you want to compute the outward flux through a box, so that the sides of the box add instead of subtract.

### Volume

Many Meep functions require you to specify a volume in space, corresponding to the C++ type meep::volume. This class creates such a volume object, given the center and size properties (just like e.g. a Block object). If the size is not specified, it defaults to (0,0,0), i.e. a single point. Any method that accepts such a volume also accepts center and size keyword arguments. If these are specified instead of the volume, the library will construct a volume for you.

## Miscellaneous Functions

### Output File Names

The output filenames used by Meep, e.g. for HDF5 files, are automatically prefixed by the input variable filename_prefix. If filename_prefix is empty (the default), however, then Meep constructs a default prefix based on the current Python filename with ".py" replaced by "-": e.g. test.py implies a prefix of "test-". You can get this prefix by running:

Simulation.get_filename_prefix()
Return the current prefix string that is prepended, by default, to all file names.

If you don't want to use any prefix, then you should set filename_prefix to False.

In addition to the filename prefix, you can also specify that all the output files be written into a newly-created directory (if it does not yet exist). This is done by running:

Simulation.use_output_directory([dirname])
Put output in a subdirectory, which is created if necessary. If the optional argument dirname is specified, that is the name of the directory. Otherwise, the directory name is the current Python file name with ".py" replaced by "-out": e.g. test.py implies a directory of "test-out".

### Simulation Time

Simulation.meep_time()
Return the current simulation time in simulation time units (e.g. during a run function). This is not the wall-clock time.

Occasionally, e.g. for termination conditions of the form time < T?, it is desirable to round the time to single precision in order to avoid small differences in roundoff error from making your results different by one timestep from machine to machine (a difference much bigger than roundoff error); in this case you can call Simulation.round_time() instead, which returns the time rounded to single precision.

### Field Computations

Meep supports a large number of functions to perform computations on the fields. Most of them are accessed via the lower-level C++/SWIG interface. Some of them are based on the following simpler, higher-level versions. They are accessible as methods of a Simulation instance.

get_field_point(c, pt)
Given a component or derived_component constant c and a Vector3 pt, returns the value of that component at that point.

get_epsilon_point(pt)
Equivalent to get_field_point(meep.Dielectric, pt).

add_dft_fields(cs, freq_min, freq_max, nfreq, where=None, center=None, size=None)
Given a list of field components cs, compute the Fourier transform of these fields for nfreq equally spaced frequencies covering the frequency range freq_min to freq_max over the Volume specified by where (default to the entire computationall cell). The volume can also be specified via the center and size arguments.

flux_in_box(dir, box=None, center=None, size=None)
Given a direction constant, and a meep.Volume, returns the flux (the integral of $\Re [\mathbf{E}^* \times \mathbf{H}]$) in that volume. Most commonly, you specify a volume that is a plane or a line, and a direction perpendicular to it, e.g. flux_in_box(d=meep.X,meep.Volume(center=meep.Vector3(0,0,0),size=meep.Vector3(0,1,1))). If the center and size arguments are provided instead of box, meep will construct the appropriate volume for you.

electric_energy_in_box(box=None, center=None, size=None)
Given a meep.Volume, returns the integral of the electric-field energy $\mathbf{E}^* \cdot \mathbf{D}/2$ in the given volume. If the volume has zero size along a dimension, a lower-dimensional integral is used. If the center and size arguments are provided instead of box, meep will construct the appropriate volume for you.

magnetic_energy_in_box(box=None, center=None, size=None)
Given a meep.Volume, returns the integral of the magnetic-field energy $\mathbf{H}^* \cdot \mathbf{B}/2$ in the given volume. If the volume has zero size along a dimension, a lower-dimensional integral is used. If the center and size arguments are provided instead of box, meep will construct the appropriate volume for you.

field_energy_in_box(box=None, center=None, size=None)
Given a meep.Volume, returns the integral of the electric- and magnetic-field energy $\mathbf{E}^* \cdot \mathbf{D}/2 + \mathbf{H}^* \cdot \mathbf{B}/2$ in the given volume. If the volume has zero size along a dimension, a lower-dimensional integral is used. If the center and size arguments are provided instead of box, meep will construct the appropriate volume for you.

modal_volume_in_box(box=None, center=None, size=None)
Given a meep.Volume, returns the instantaneous modal volume according to the Purcell-effect definition: integral (ε|E|^2) / maximum (ε|E|^2). If no volume argument is provided, the entire computational cell is used by default. If the center and size arguments are provided instead of box, meep will construct the appropriate volume for you.

Note that if you are at a fixed frequency and you use complex fields (via Bloch-periodic boundary conditions or fields_complex=True), then one half of the flux or energy integrals above corresponds to the time average of the flux or energy for a simulation with real fields.

Often, you want the integration box to be the entire computational cell. A useful function to return this box, which you can then use for the box arguments above, is Simulation.total_volume().

One versatile feature is that you can supply an arbitrary function $f(\mathbf{x},c_1,c_2,\ldots)$ of position $\mathbf{x}$ and various field components $c_1,\ldots$ and ask Meep to integrate it over a given volume, find its maximum, or output it (via output_field_function, described later). This is done via the functions:

integrate_field_function(cs, func, where=None, center=None, size=None)
Returns the integral of the complex-valued function func over the Volume specified by where (defaults to entire computational cell) for the meep::fields contained in the Simulation instance that calls this method. func is a function of position (a Vector3, its first argument) and zero or more field components specified by cs: a list of component constants. func can be real- or complex-valued. The volume can optionally be specified via the center and size arguments.

If any dimension of where is zero, that dimension is not integrated over. In this way you can specify 1d, 2d, or 3d integrals.

max_abs_field_function(cs, func, where=None, center=None, size=None)
As integrate_field_function, but returns the maximum absolute value of func in the volume where instead of its integral.

The integration is performed by summing over the grid points with a simple trapezoidal rule, and the maximum is similarly over the grid points. See Field Functions for examples of how to call integrate_field_function and max_abs_field_function. See Synchronizing the Magnetic and Electric Fields if you want to do computations combining the electric and magnetic fields. The volume can optionally be specified via the center and size arguments.

Occasionally, one wants to compute an integral that combines fields from two separate simulations (e.g. for nonlinear coupled-mode calculations). This functionality is supported in Meep, as long as the two simulations have the same computational cell, the same resolution, the same boundary conditions and symmetries (if any), and the same PML layers (if any).

integrate2_field_function(fields2, cs1, cs2, func, where=None, center=None, size=None)
Similar to integrate_field_function, but takes additional parameters fields2 and cs2. fields2 is a meep::fields* object similar to the global fields variable (see below) specifying the fields from another simulation. cs1 is a list of components to integrate with from the meep::fields instance in Simulation.fields, as for integrate_field_function, while cs2 is a list of components to integrate from fields2. Similar to integrate_field_function, func is a function that returns an number given arguments consisting of: the position vector, followed by the values of the components specified by cs1 (in order), followed by the values of the components specified by cs2 (in order). The volume can optionally be specified via the center and size arguments.

To get two fields in memory at once for integrate2_field_function, the easiest way is to run one simulation within a given Python file, then save the results in another fields variable, then run a second simulation. This would look something like:

...set up and run first simulation...
fields2 = sim.fields # save the fields in a variable
sim.fields = None    # prevent the fields from getting deallocated by reset-meep
sim.reset_meep()
...set up and run second simulation...


It is also possible to timestep both fields simultaneously (e.g. doing one timestep of one simulation then one timestep of another simulation, and so on, but this requires you to call much lower-level functions like fields_step().

Once the fields/simulation have been initialized, you can change the values of various parameters by using the following functions (which are attributes of the Simulation class):

reset_meep()
Reset all of Meep's parameters, deleting the fields, structures, etcetera, from memory as if you had not run any computations.

restart_fields()
Restart the fields at time zero, with zero fields. Does not reset the Fourier transforms of the flux planes, which continue to be accumulated.

change_k_point(k)
Change the k_point (the Bloch periodicity).

change_sources(new_sources)
Change the Sources input variable to new_sources, and changes the sources used for the current simulation.

set_materials(geometry=None, default_material=None)
This can be called in a step function, and is useful for changing the geometry or default material as a function of time.

### Flux Spectra

Given a bunch of FluxRegion objects (see above), you can tell Meep to accumulate the Fourier transforms of the fields in those regions in order to compute flux spectra. See also the Introduction and Tutorial/Basics. These are attributes of the Simulation class. The most important function is:

add_flux(fcen, df, nfreq, FluxRegions...)
Add a bunch of FluxRegions to the current simulation (initializing the fields if they have not yet been initialized), telling Meep to accumulate the appropriate field Fourier transforms for nfreq equally spaced frequencies covering the frequency range fcen-df/2 to fcen+df/2. Return a flux object, which you can pass to the functions below to get the flux spectrum, etcetera.

As described in the tutorial, you normally use add_flux via statements like:

transmission = sim.add_flux(...)
to store the flux object in a variable. add_flux initializes the fields if necessary, just like calling run, so you should only call it after initializing your Simulation object which includes specifying geometry, sources, boundary_layers, etcetera. You can create as many flux objects as you want, e.g. to look at powers flowing in different regions or in different frequency ranges. Note, however, that Meep has to store (and update at every time step) a number of Fourier components equal to the number of grid points intersecting the flux region multiplied by the number of electric and magnetic field components required to get the Poynting vector multiplied by nfreq, so this can get quite expensive (in both memory and time) if you want a lot of frequency points over large regions of space.

Once you have called add_flux, the Fourier transforms of the fields are accumulated automatically during time-stepping by the run functions. At any time, you can ask for Meep to print out the current flux spectrum via:

display_fluxes(fluxes...)
Given a number of flux objects, this displays a comma-separated table of frequencies and flux spectra, prefixed by "flux1:" or similar (where the number is incremented after each run). All of the fluxes should be for the same fcen/df/nfreq. The first column are the frequencies, and subsequent columns are the flux spectra.

You might have to do something lower-level if you have multiple flux regions corresponding to different frequency ranges, or have other special needs. display_fluxes(f1, f2, f3) is actually equivalent to display_csv("flux", get_flux_freqs(f1), get_fluxes(f1), get_fluxes(f2), get_fluxes(f3)), where display_csv takes a bunch of lists of numbers and prints them as a comma-separated table, and we are calling two lower-level functions:

get_flux_freqs(flux)
Given a flux object, returns a list of the frequencies that it is computing the spectrum for.

get_fluxes(flux)
Given a flux object, returns a list of the current flux spectrum that it has accumulated.

As described in Tutorial/Basics, for a reflection spectrum you often want to save the Fourier-transformed fields from a "normalization" run and then load them into another run to be subtracted. This can be done via:

save_flux(filename, flux)
Save the Fourier-transformed fields corresponding to the given flux object in an HDF5 file of the given name without the ".h5" suffix (the current filename-prefix is prepended automatically).

load_flux(filename, flux)
Load the Fourier-transformed fields into the given flux object (replacing any values currently there) from an HDF5 file of the given name without the ".h5" suffix (the current filename-prefix is prepended automatically). You must load from a file that was saved by save_flux in a simulation of the same dimensions (for both the computational cell and the flux regions) with the same number of processors.

load_minus_flux(filename, flux)
As load_flux, but negates the Fourier-transformed fields after they are loaded. This means that they will be subtracted from any future field Fourier transforms that are accumulated.

scale_flux_fields(s, flux)
Scale the Fourier-transformed fields in flux by the complex number s. e.g. load_minus_flux is equivalent to load_flux followed by scale_flux_fields with s=-1.

get_eigenmode_coefficients(flux, direction, volume, bands, alpha, vgrp)
Given a flux object, direction, volume object, and list of band indices, return the eigenmode coefficients in alpha and group velocities in vgrp. The flux object must be created using add_flux. See Tutorial/Mode Decomposition for an example.

### Force Spectra

Very similar to flux spectra, you can also compute force spectra: forces on an object as a function of frequency, computed by Fourier transforming the fields and integrating the vacuum Maxwell stress tensor

over a surface $S$ via $\mathbf{F} = \int_S \sigma d\mathbf{A}$. We recommend that you normally only evaluate the stress tensor over a surface lying in vacuum, as the interpretation and definition of the stress tensor in arbitrary media is often problematic (the subject of extensive and controversial literature). It is fine if the surface encloses an object made of arbitrary materials, as long as the surface itself is in vacuum.

Most commonly, you will want to normalize the force spectrum in some way, just as for flux spectra. Most simply, you could divide two different force spectra to compute the ratio of forces on two objects. Often, you will divide a force spectrum by a flux spectrum, to divide the force $F$ by the incident power $P$ on an object, in order to compute the useful dimensionless ratio $Fc$/$P$ where $c=1$ in Meep units. For example, it is a simple exercise to show that the force $F$ on a perfectly reflecting mirror with normal-incident power $P$ satisfies $Fc$/$P=2$, and for a perfectly absorbing (black) surface $Fc$/$P=1$.

The usage is similar to the flux spectra: you define a set of ForceRegion objects telling Meep where it should compute the Fourier-transformed fields and stress tensors, and call add_force to add these regions to the current simulation over a specified frequency bandwidth, and then use display_forces to display the force spectra at the end. There are also save_force, load_force, and load_minus_force functions that you can use to subtract the fields from two simulation, e.g. in order to compute just the force from scattered fields, similar to the flux spectra. These types and functions are defined as follows:

ForceRegion

A region (volume, plane, line, or point) in which to compute the integral of the stress tensor of the Fourier-transformed fields. Its properties are:

center [ Vector3 ]
The center of the force region (no default).

size [ Vector3 ]
The size of the force region along each of the coordinate axes. Default is (0,0,0) (a single point).

direction [ direction constant ]
The direction of the force that you wish to compute (e.g. X, Y, etcetera). Unlike FluxRegion, you must specify this explicitly, because there is not generally any relationship between the direction of the force and the orientation of the force region.

weight [ complex ]
A weight factor to multiply the force by when it is computed. Default is 1.0.

In most circumstances, you should define a set of ForceRegions whose union is a closed surface lying in vacuum and enclosing the object that is experiencing the force.

Simulation.add_force(fcen, df, nfreq, ForceRegions...)
Add a bunch of ForceRegions to the current simulation (initializing the fields if they have not yet been initialized), telling Meep to accumulate the appropriate field Fourier transforms for nfreq equally spaced frequencies covering the frequency range fcen-df/2 to fcen+df/2. Return a force object, which you can pass to the functions below to get the force spectrum, etcetera.

As for flux regions, you normally use add_force via statements like:

Fx = sim.add_force(...)


to store the flux object in a variable. add_force initializes the fields if necessary, just like calling run, so you should only call it after initializing your Simulation object which includes specifiying geometry, sources, boundary_layers, etcetera. You can create as many force objects as you want, e.g. to look at forces on different objects, in different directions, or in different frequency ranges. Note, however, that Meep has to store (and update at every time step) a number of Fourier components equal to the number of grid points intersecting the force region, multiplied by the number of electric and magnetic field components required to get the stress vector, multiplied by nfreq, so this can get quite expensive (in both memory and time) if you want a lot of frequency points over large regions of space.

Once you have called add_force, the Fourier transforms of the fields are accumulated automatically during time-stepping by the run functions. At any time, you can ask for Meep to print out the current force spectrum via:

display_forces(forces...)
Given a number of force objects, this displays a comma-separated table of frequencies and force spectra, prefixed by "force1:" or similar (where the number is incremented after each run). All of the forces should be for the same fcen/df/nfreq. The first column are the frequencies, and subsequent columns are the force spectra.

You might have to do something lower-level if you have multiple force regions corresponding to different frequency ranges, or have other special needs. display_forces(f1, f2, f3) is actually equivalent to display_csv("force", get_force_freqs(f1), get_forces(f1), get_forces(f2), get_forces(f3)), where display_csv takes a bunch of lists of numbers and prints them as a comma-separated table, and we are calling two lower-level functions:

get_force_freqs(flux)
Given a force object, returns a list of the frequencies that it is computing the spectrum for.

get_forces(flux)
Given a force object, returns a list of the current force spectrum that it has accumulated.

As described in Tutorial/Basics, to compute the force from scattered fields often want to save the Fourier-transformed fields from a "normalization" run and then load them into another run to be subtracted. This can be done via:

save_force(filename, force)
Save the Fourier-transformed fields corresponding to the given force object in an HDF5 file of the given name without the ".h5" suffix (the current filename-prefix is prepended automatically).

load_force(filename, force)
Load the Fourier-transformed fields into the given force object (replacing any values currently there) from an HDF5 file of the given name without the ".h5" suffix (the current filename-prefix is prepended automatically). You must load from a file that was saved by save_force in a simulation of the same dimensions for both the computational cell and the force regions with the same number of processors.

load_minus_force(filename, force)
As load_force, but negates the Fourier-transformed fields after they are loaded. This means that they will be subtracted from any future field Fourier transforms that are accumulated.

### LDOS spectra

Meep can also calculate the LDOS (local density of states) spectrum, as described in Tutorial/Local Density of States. To do this, you simply pass the following step function to your run command:

dft_ldos(fcen, df, nfreq)
Compute the power spectrum of the sources (usually a single point dipole source), normalized to correspond to the LDOS, in a frequency bandwith df centered at fcen, at nfreq frequency points.

The resulting spectrum is outputted as comma-delimited text, prefixed by ldos:,, and is also stored in the dft_ldos_data global variable after the run is complete.

Analytically, the per-polarization LDOS is exactly proportional to the power radiated by an $\ell$-oriented point-dipole current, $p(t)$, at a given position in space. For a more mathematical treatment of the theory behind the LDOS, we refer you to the relevant discussion in Section 4.4 ("Currents and Fields: The Local Density of States") in Chapter 4 ("Electromagnetic Wave Source Conditions") of the book Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology, but for now we simply give the definition:

where the $|\hat{p}(\omega)|^2$ normalization is necessary for obtaining the power exerted by a unit-amplitude dipole (assuming linear materials), and hats denote Fourier transforms. It is this quantity that is computed by the dft_ldos command for a single dipole source. For a volumetric source, the numerator and denominator are both integrated over the current volume, but "LDOS" computation is less meaningful in this case.

### Near-to-Far-Field Spectra

Meep can compute a near-to-far-field transformation in the frequency domain as described in Tutorial/Near-to-Far Field Spectra: given the fields on a "near" bounding surface inside the computational cell, it can compute the fields arbitrarily far away using an analytical transformation, assuming that the "near" surface and the "far" region lie in a single homogeneous non-periodic 2d or 3d region. That is, in a simulation surrounded by PML that absorbs outgoing waves, the near-to-far-field feature can compute the fields outside the computational cell as if the outgoing waves had not been absorbed (i.e. in the fictitious infinite open volume). Moreover, this operation is performed on the Fourier-transformed fields: like the flux and force spectra above, you specify a set of desired frequencies, Meep accumulates the Fourier transforms, and then Meep computes the fields at each frequency for the desired far-field points.

This is based on the principle of equivalence: given the Fourier-transformed tangential fields on the "near" surface, Meep computes equivalent currents and convolves them with the analytical Green's functions in order to compute the fields at any desired point in the "far" region. For details, see Section 4.2.1 ("The Principle of Equivalence") in Chapter 4 ("Electromagnetic Wave Source Conditions") of the book Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology.

There are three steps to using the near-to-far-field feature: first, define the "near" surface(s) as a set of surfaces capturing all outgoing radiation in the desired direction(s); second, run the simulation, typically with a pulsed source, to allow Meep to accumulate the Fourier transforms on the near surface(s); third, tell Meep to compute the far fields at any desired points (optionally saving the far fields from a grid of points to an HDF5 file). To define the near surfaces, use:

add_near2far(fcen, df, nfreq, Near2FarRegions...)
Add a bunch of Near2FarRegions to the current simulation (initializing the fields if they have not yet been initialized), telling Meep to accumulate the appropriate field Fourier transforms for nfreq equally-spaced frequencies covering the frequency range fcen-df/2 to fcen+df/2. Return a near2far object, which you can pass to the functions below to get the far fields.

Each Near2FarRegion is identical to FluxRegion except for the name: in 3d, these give a set of planes (important: all these "near surfaces" must lie in a single homogeneous material with isotropic ε and μ — and they should not lie in the PML regions) surrounding the source(s) of outgoing radiation that you want to capture and convert to a far field. Ideally, these should form a closed surface, but in practice it is sufficient for the Near2FarRegions to capture all of the radiation in the direction of the far-field points. Important: as for flux computations, each Near2FarRegion should be assigned a weight of ±1 indicating the direction of the outward normal relative to the +coordinate direction. So, for example, if you have six regions defining the six faces of a cube, i.e. the faces in the +x, -x, +y, -y, +z, and -z directions, then they should have weights +1, -1, +1, -1, +1, and -1 respectively. Note that, neglecting discretization errors, all near-field surfaces that enclose the same outgoing fields are equivalent and will yield the same far fields with a discretization-induced difference that vanishes with increasing resolution etc.

After the simulation run is complete, you can compute the far fields. This is usually for a pulsed source so that the fields have decayed away and the Fourier transforms have finished accumulating.

get_farfield(near2far, x)
Given a Vector3 point x which can lie anywhere outside the near-field surface, including outside the computational cell and a near2far object, returns the computed (Fourier-transformed) "far" fields at x as list of length 6nfreq, consisting of fields (Ex1,Ey1,Ez1,Hx1,Hy1,Hz1,Ex2,Ey2,Ez2,Hx2,Hy2,Hz2,...) for the frequencies 1,2,…,nfreq.

output_farfields(near2far, fname, resolution, where=None, center=None, size=None)
Given an HDF5 file name fname (does not include the .h5 suffix), a Volume given by where (may be 0d, 1d, 2d, or 3d), and a resolution (in grid points / distance unit), outputs the far fields in where (which may lie outside the computational cell) in a grid with the given resolution (which may differ from the FDTD grid resolution) to the HDF5 file as a set of twelve array datasets ex.r, ex.i, ..., hz.r, hz.i, giving the real and imaginary parts of the Fourier-transformed $E$ and $H$ fields on this grid. Each dataset is an nx×ny×nz×nfreq 4d array of space×frequency although dimensions that =1 are omitted. The volume can optionally be specified via center and size.

Note that far fields have the same units and scaling as the Fourier transforms of the fields, and hence cannot be directly compared to time-domain fields. In practice, it is easiest to use the far fields in computations where overall scaling (units) cancel out or are irrelevant, e.g. to compute the fraction of the far fields in one region vs. another region.

For a scattered-field computation, you often want to separate the scattered and incident fields. Just as is described in Tutorial/Basics for flux computations, you can do this by saving the Fourier-transformed incident from a "normalization" run and then load them into another run to be subtracted. This can be done via:

save_near2far(filename, near2far)
Save the Fourier-transformed fields corresponding to the given near2far object in an HDF5 file of the given name (without the ".h5" suffix). The current filename-prefix is prepended automatically.

load_near2far(filename, near2far)
Load the Fourier-transformed fields into the given near2far object (replacing any values currently there) from an HDF5 file of the given name without the ".h5" suffix (the current filename-prefix is prepended automatically). You must load from a file that was saved by save_near2far in a simulation of the same dimensions for both the computational cell and the near2far regions with the same number of processors.

load_minus_near2far(filename, near2far)
As load_near2far, but negates the Fourier-transformed fields after they are loaded. This means that they will be subtracted from any future field Fourier transforms that are accumulated.

scale_near2far_fields(s, near2far)
Scale the Fourier-transformed fields in near2far by the complex number s. e.g. load_minus_near2far is equivalent to load_near2far followed by scale_near2far_fields with s=-1.

These functions dump the raw ε data to disk and load it back for doing multiple simulations with the same materials but different sources etc. The only prerequisite is that the dump/load simulations have the same chunks (i.e. the same grid, number of processors, and PML). Currently only stores ε and μ, and not nonlinear coefficients or polarizability.

Simulation.dump_structure(fname)
Dumps the structure to the file fname.

Simulation.load_structure(fname)
Loads a structure from the file fname. A file name to load can also be passed to the Simulation constructor via the load_structure keyword argument.

### Frequency-Domain Solver

Meep contains a frequency-domain solver that computes the fields produced in a geometry in response to a continuous-wave (CW) source. This is based on an iterative linear solver instead of time-stepping. For details, see Section 5.3 ("Frequency-domain solver") of Computer Physics Communications, Vol. 181, pp. 687-702, 2010. Benchmarking results have shown that in many instances, such as cavities (e.g., ring resonators) with long-lived resonant modes, this solver converges much faster than simply running an equivalent time-domain simulation with a continuous-wave source, time-stepping until all transient effects from the source turn-on have disappeared, especially if the fields are desired to a very high accuracy. To use it, simply define a ContinuousSrc with the desired frequency and initialize the fields and geometry via init_fields():

sim = meep.Simulation(...)
sim.init_fields()
sim.solve_cw(tol, maxiters, L)


The first two parameters to the frequency-domain solver are the tolerance tol for the iterative solver (10−8, by default) and a maximum number of iterations maxiters (104, by default). Finally, there is a parameter $L$ that determines a tradeoff between memory and work per step and convergence rate of the iterative algorithm, biconjugate gradient stabilized (BiCGSTAB-L), that is used; larger values of $L$ will often lead to faster convergence at the expense of more memory and more work per iteration. Default is $L=2$, and normally a value ≥ 2 should be used.

The frequency-domain solver supports arbitrary geometries, PML, boundary conditions, symmetries, parallelism, conductors, and arbitrary nondispersive materials. Lorentz-Drude dispersive materials are not currently supported in the frequency-domain solver, but since you are solving at a known fixed frequency rather than timestepping, you should be able to pick conductivities etcetera in order to obtain any desired complex ε and μ at that frequency.

The frequency-domain solver requires you to use complex-valued fields, via force_complex_fields=True.

After solve_cw completes, it should be as if you had just run the simulation for an infinite time with the source at that frequency. You can call the various field-output functions and so on as usual at this point.

## Run and Step Functions

The actual work in Meep is performed by run functions, which time-step the simulation for a given amount of time or until a given condition is satisfied. These are attributes of the Simulation class.

The run functions, in turn, can be modified by use of step functions: these are called at every time step and can perform any arbitrary computation on the fields, do outputs and I/O, or even modify the simulation. The step functions can be transformed by many modifier functions, like at_beginning, during_sources, etcetera which cause them to only be called at certain times, etcetera, instead of at every time step.

A common point of confusion is described in The Run Function Is Not A Loop. Please read this article if you want to make Meep do some customized action on each time step, as many users make the same mistake. What you really want to in that case is to write a step function, as described below.

### Run Functions

run(step_functions..., until=condition/time)
Run the simulation until a certain time or condition, calling the given step functions (if any) at each timestep. The keyword argument until is either a number, in which case it is an additional time (in Meep units) to run for, or it is a function (of no arguments) which returns True when the simulation should stop.

run(step_functions..., until_after_sources=condition/time)
Run the simulation until all sources have turned off, calling the given step functions (if any) at each timestep. The keyword argument until_after_sources is either a number, in which case it is an additional time (in Meep units) to run for after the sources are off, or it is a function (of no arguments). In the latter case, the simulation runs until the sources are off and condition returns True.

In particular, a useful value for until_after_sources or until is often stop_when_field_decayed, which is demonstrated in Tutorial/Basics:

stop_when_fields_decayed(dT, c, pt, decay_by)
Return a condition function, suitable for passing to until/until_after_sources, that examines the component c (e.g. Ex, etc.) at the point pt (a Vector3) and keeps running until its absolute value squared has decayed by at least decay_by from its maximum previous value. In particular, it keeps incrementing the run time by dT (in Meep units) and checks the maximum value over that time period — in this way, it won't be fooled just because the field happens to go through 0 at some instant.

Note that, if you make decay_by very small, you may need to increase the cutoff property of your source(s), to decrease the amplitude of the small high-frequency components that are excited when the source turns off. High frequencies near the Nyquist frequency of the grid have slow group velocities and are absorbed poorly by PML.

Finally, another run function, useful for computing ω(k) band diagrams, is:

run_k_points(T, k_points)
Given a list of Vector3, k_points of k vectors, runs a simulation for each k point (i.e. specifying Bloch-periodic boundary conditions) and extracts the eigen-frequencies, and returns a list of the complex frequencies. In particular, you should have specified one or more Gaussian sources. It will run the simulation until the sources are turned off plus an additional $T$ time units. It will run Harminv at the same point/component as the first Gaussian source and look for modes in the union of the frequency ranges for all sources. Returns a list of lists of frequencies (one list of frequencies for each k). Also prints out a comma-delimited list of frequencies, prefixed by freqs:, and their imaginary parts, prefixed by freqs-im:. See Tutorial/Resonant Modes and Transmission in a Waveguide Cavity.

### Predefined Step Functions

Several useful step functions are predefined by Meep. These are available directly via the meep package but require a Simulation instance as an argument.

#### Output Functions

The most common step function is an output function, which outputs some field component to an HDF5 file. Normally, you will want to modify this by one of the at_* functions, below, as outputting a field at every time step can get quite time- and storage-consuming.

Note that although the various field components are stored at different places in the Yee lattice, when they are outputted they are all linearly interpolated to the same grid: to the points at the centers of the Yee cells, i.e. $(i+0.5,j+0.5,k+0.5)\cdotΔ$ in 3d.

output_epsilon()
Output the dielectric function (relative permittivity) ε. Note that this only outputs the frequency-independent part of ε (the $\omega\to\infty$ limit).

output_mu()
Output the relative permeability function μ. Note that this only outputs the frequency-independent part of μ (the $\omega\to\infty$ limit).

output_dft(dft_fields, fname, where=None, center=None, size=None)
Output the Fourier-transformed fields in dft_fields (created by add_dft_fields) to an HDF5 file with name fname (does not include the .h5 suffix). The Volume where defaults to the entire computational cell. The volume can also be specified via the center and size arguments.

output_hpwr()
Output the magnetic-field energy density $\mathbf{H}^* \cdot \mathbf{B} / 2$

output_dpwr()
Output the electric-field energy density $\mathbf{E}^* \cdot \mathbf{D} / 2$

output_tot_pwr()
Output the total electric and magnetic energy density. Note that you might want to wrap this step function in synchronized_magnetic to compute it more accurately. See Synchronizing the Magnetic and Electric Fields.

output_Xfield_x(), output_Xfield_y(), output_Xfield_z(), output_Xfield_r(), output_Xfield_p()
Output the $x$, $y$, $z$, $r$, or $\phi$ component respectively, of the field X, where X is either h, b, e, d, or s for the magnetic, electric, displacement, or Poynting field, respectively. If the field is complex, outputs two datasets, e.g. ex.r and ex.i, within the same HDF5 file for the real and imaginary parts, respectively. Note that for outputting the Poynting field, you might want to wrap the step function in synchronized_magnetic to compute it more accurately. See Synchronizing the Magnetic and Electric Fields.

output_Xfield()
Outputs all the components of the field X, where X is either h, b, e, d, or s as above, to an HDF5 file. That is, the different components are stored as different datasets within the same file.

output_png(component, h5topng_options)
Output the given field component (e.g. Ex, etc.) as a PNG image, by first outputting the HDF5 file, then converting to PNG via h5topng, then deleting the HDF5 file. The second argument is a string giving options to pass to h5topng (e.g. "-Zc bluered"). See also Tutorial/Basics.

It is often useful to use the h5topng -C or -A options to overlay the dielectric function when outputting fields. To do this, you need to know the name of the dielectric-function .h5 file which must have been previously output by output_epsilon. To make this easier, a built-in shell variable $EPS is provided which refers to the last-output dielectric-function .h5 file. So, for example output_png(meep.Ez,"-C$EPS") will output the $E_z$ field and overlay the dielectric contours.

By default, output_png deletes the .h5 file when it is done. To preserve the .h5 file requires output_png(component, h5topng_options, rm_h5=False).

More generally, it is possible to output an arbitrary function of position and zero or more field components, similar to the integrate_field_function described above. This is done by:

output_field_function(name, cs, func)
Output the field function func to an HDF5 file in the datasets named name*.r and name*.i for the real and imaginary parts. Similar to integrate_field_function, func is a function of position (a Vector3) and the field components corresponding to cs: a list of component constants.

output_real_field_function(name, cs, func)
As output_field_function, but only outputs the real part of func to the dataset given by the string name.

See also Field Function Examples. See also Synchronizing the Magnetic and Electric Fields if you want to do computations combining the electric and magnetic fields.

#### Array Slices

The output functions described above write the data for the fields and materials for the entire computational volume to an HDF5 file. This is useful for post-processing as you can later read in the HDF5 file to obtain field/material data as a NumPy array. However, in some cases it is convenient to bypass the disk altogether to obtain the data directly in the form of a NumPy array without writing/reading HDF5 files. Additionally, you may want the field/material data on just a subregion (or slice) of the entire volume. This functionality is provided by the get_array method which takes as input a subregion of the computational volume and the field/material component. The method returns a NumPy array containing values of the field/material at the current simulation time.

 get_array(vol=None, center=None, size=None component=meep.Ez, cmplx=False, arr=None)


with the following input parameters:

• vol: Volume; the orthogonal subregion/slice of the computational volume. The return value of get_array has the same dimensions as the Volume's size attribute. If None (default), then a size and center must be specified.

• center, size : Vector3; if both are specified, the library will construct an apporpriate Volume. This is a convenience feature and alternative to supplying a Volume.

• component: field/material component (i.e., mp.Ex, mp.Hy, mp.Sz, mp.Dielectric, etc). Defaults to mp.Ez.

• cmplx: boolean; if True, return complex-valued data otherwise return real-valued data (default).

• arr: optional field to pass a pre-allocated NumPy array of the correct size, which will be overwritten with the field/material data instead of allocating a new array. Normally, this will be the array returned from a previous call to get_array for a similar slice, allowing one to re-use arr (e.g., when fetching the same slice repeatedly at different times).

get_dft_array(dft_obj, component, num_freq)
Returns the Fourier-transformed fields as a NumPy array.

• dft_obj: a dft_flux, dft_force, dft_fields, or dft_near2far object obtained from calling the appropriate add function (e.g., meep.add_flux).

• component: A field component (e.g., meep.Ez)

• num_freq: The number of frequencies

#### Harminv

The following step function collects field data from a given point and runs Harminv on that data to extract the frequencies, decay rates, and other information.

Harminv(c, pt, fcen, df, [maxbands])
Harminv is implemented as a class whose constructor returns a step function that collects data from the field component c (e.g. $E_x$, etc.) at the given point pt (a Vector3). Then, at the end of the run, it uses Harminv to look for modes in the given frequency range (center fcen and width df), printing the results to standard output (prefixed by harminv:) as comma-delimited text, and also storing them to the variable Harminv.modes. The optional argument maxbands is the maximum number of modes to search for. Defaults to 100.

Important: normally, you should only use Harminv to analyze data after the sources are off. Wrapping it in after_sources(meep.Harminv(...)) is sufficient.

In particular, Harminv takes the time series $f(t)$ corresponding to the given field component as a function of time and decomposes it (within the specified bandwidth) as:

The results are stored in the list Harminv.modes, which is a list of tuples holding the frequency, amplitude, and error of the modes. Given one of these tuples (e.g., first_mode = harminv_instance.modes[0]), you can extract its various components:

freq
The complex frequency ω (in the usual Meep $2\pi c$ units).

freq.real
The real part of the frequency ω.

freq.decay
The imaginary part of the frequency ω.

Q
The dimensionless lifetime, or quality factor defined as $-\mathrm{Re}\,\omega / 2 \mathrm{Im}\,\omega$.

amp
The complex amplitude $a$.

err
A crude measure of the error in the frequency (both real and imaginary)...if the error is much larger than the imaginary part, for example, then you can't trust the $Q$ to be accurate. Note: this error is only the uncertainty in the signal processing, and tells you nothing about the errors from finite resolution, finite cell size, and so on.

For example, [m.freq for m in harminv_instance.modes] gives a list of the real parts of the frequencies. Be sure to save a reference to the Harminv instance if you wish to use the results after the simulation:

sim = meep.Simulation(...)
h = meep.Harminv(...)
sim.run(meep.after_sources(h))
# do something with h.modes


### Step-Function Modifiers

Rather than writing a brand-new step function every time we want to do something a bit different, the following "modifier" functions take a bunch of step functions and produce new step functions with modified behavior. See also Tutorial/Basics for examples.

#### Miscellaneous Step-Function Modifiers

combine_step_funcs(step_functions...)
Given zero or more step functions, return a new step function that on each step calls all of the passed step functions.

synchronized_magnetic(step_functions...)
Given zero or more step functions, return a new step function that on each step calls all of the passed step functions with the magnetic field synchronized in time with the electric field. See Synchronizing the Magnetic and Electric Fields.

#### Controlling When a Step Function Executes

when_true(condition, step_functions...)
Given zero or more step functions and a condition function condition (a function of no arguments), evaluate the step functions whenever condition returns True.

when_false(condition, step_functions...)
Given zero or more step functions and a condition function condition (a function of no arguments), evaluate the step functions whenever condition returns False.

at_every(dT, step_functions...)
Given zero or more step functions, evaluates them at every time interval of $dT$ units (rounded up to the next time step).

after_time(T, step_functions...)
Given zero or more step functions, evaluates them only for times after a $T$ time units have elapsed from the start of the run.

before_time(T, step_functions...)
Given zero or more step functions, evaluates them only for times before a $T$ time units have elapsed from the start of the run.

at_time(T, step_functions...)
Given zero or more step functions, evaluates them only once, after a $T$ time units have elapsed from the start of the run.

after_sources(step_functions...)
Given zero or more step functions, evaluates them only for times after all of the sources have turned off.

after_sources_and_time(T, step_functions...)
Given zero or more step functions, evaluates them only for times after all of the sources have turned off, plus an additional $T$ time units have elapsed.

during_sources(step_functions...)
Given zero or more step functions, evaluates them only for times before all of the sources have turned off.

at_beginning(step_functions...)
Given zero or more step functions, evaluates them only once, at the beginning of the run.

at_end(step_functions...)
Given zero or more step functions, evaluates them only once, at the end of the run.

#### Modifying HDF5 Output

in_volume(v, step_functions...)
Given zero or more step functions, modifies any output functions among them to only output a subset (or a superset) of the computational cell, corresponding to the meep::volume* v (created by the Volume function).

in_point(pt, step_functions...)
Given zero or more step functions, modifies any output functions among them to only output a single point of data, at pt (a Vector3).

to_appended(filename, step_functions...)
Given zero or more step functions, modifies any output functions among them to append their data to datasets in a single newly-created file named filename (plus an .h5 suffix and the current filename prefix). They append by adding an extra dimension to their datasets, corresponding to time.

with_prefix(prefix, step_functions...)
Given zero or more step functions, modifies any output functions among them to prepend the string prefix to the file names (much like filename_prefix, above).

### Writing Your Own Step Functions

A step function can take two forms. The simplest is just a function with one argument (the simulation instance), which is called at every time step unless modified by one of the modifier functions above. e.g.

def my_step(sim):
print("Hello world!")


If one then does sim.run(my_step, until=100), Meep will run for 100 time units and print "Hello world!" at every time step.

This suffices for most purposes. However, sometimes you need a step function that opens a file, or accumulates some computation, and you need to clean up (e.g. close the file or print the results) at the end of the run. For this case, you can write a step function of two arguments: the second argument will either be step when it is called during time-stepping, or finish when it is called at the end of the run:

def my_step(sim, todo):
if todo == 'step':
# do something
elif todo == 'finish':
# do something else

# access simulation attributes
sim.fields ...etc.


## Low-Level Functions

By default, Meep initializes C++ objects like meep::structure and meep::fields in the Simulation object based on attributes like sources and geometry. Theses objects are then accessible via simulation_instance.structure and simulation_instance.fields. Given these, you can then call essentially any function in the C++ interface, because all of the C++ functions are automatically made accessible to Python by the wrapper-generator program SWIG.

### Initializing the Structure and Fields

The structure and fields variables are automatically initialized when any of the run functions is called, or by various other functions such as add_flux. To initialize them separately, you can call Simulation.init_fields() manually, or Simulation._init_structure(k_point) to just initialize the structure.

If you want to time step more than one field simultaneously, the easiest way is probably to do something like:

sim = Simulation(cell_size, resolution).init_fields()
my_fields = sim.fields
sim.fields = None
sim.reset_meep()


and then change the geometry etc. and re-run sim.init_fields(). Then you'll have two field objects in memory.

### SWIG Wrappers

If you look at a function in the C++ interface, then there are a few simple rules to infer the name of the corresponding Python function.

• First, all functions in the meep:: namespace are available in the Meep Python module from the top-level meep package.
• Second, any method of a class is accessible via the standard Python class interface. For example, meep::fields::step, which is the function that performs a time-step, is exposed to Python as fields_instance.step() where a fields instance is usually accessible from Simulation.fields.
• C++ constructors are called using the normal Python class instantiation. E.g., fields = meep.fields(...) returns a new meep::fields object. Calling destructors is not necessary because objects are automatically garbage collected.

Some argument type conversion is performed automatically, e.g. types like complex numbers are converted to complex<double>, etcetera. Vector3 vectors are converted to meep::vec, but to do this we need to know the dimensionality of the problem in C++. The problem dimensions are automatically initialized by Simulation._init_structure, but if you want to pass vector arguments to C++ before that time you should call Simulation.require_dimensions(), which infers the dimensions from the cell_size, k_point, and dimensions variables.