# Materials in Meep

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If you specify a Lorentzian resonance <math>\omega_n</math> at too high a frequency relative to the time discretization <math>\Delta t</math>, the simulation becomes unstable. Essentially, the problem is that you are trying to model a <math>\mathbf{P}_n</math> that oscillates too fast compared with the time discretization for the discretization to work properly. If this happens, you have three options: increase the resolution (which increases the resolution in both space and time), decrease the Courant factor (which decreases <math>\Delta t</math> compared to <math>\Delta x</math>), or use a different model function for your dielectric response. | If you specify a Lorentzian resonance <math>\omega_n</math> at too high a frequency relative to the time discretization <math>\Delta t</math>, the simulation becomes unstable. Essentially, the problem is that you are trying to model a <math>\mathbf{P}_n</math> that oscillates too fast compared with the time discretization for the discretization to work properly. If this happens, you have three options: increase the resolution (which increases the resolution in both space and time), decrease the Courant factor (which decreases <math>\Delta t</math> compared to <math>\Delta x</math>), or use a different model function for your dielectric response. | ||

- | Roughly speaking, the <math>\mathbf{P}_n</math> equation becomes unstable for <math>\omega_n \Delta t / 2 > 1</math>. (Note that, in Meep frequency units, you specify <math>\omega_n/2\pi</math>, so this quantity should be less than <math>1/\pi \Delta t</math>.) A future version of Meep will check a necessary stability criterion automatically and halt with an error message if it is violated. | + | Roughly speaking, the <math>\mathbf{P}_n</math> equation becomes unstable for <math>\omega_n \Delta t / 2 > 1</math>. (Note that, in Meep frequency units, you specify <math>f_n = \omega_n/2\pi</math>, so this quantity should be less than <math>1/\pi \Delta t</math>.) A future version of Meep will check a necessary stability criterion automatically and halt with an error message if it is violated. |

== Loss and gain == | == Loss and gain == |

## Revision as of 18:21, 6 September 2009

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The material structure in Maxwell's equations is determined by the relative permittivity ε(**x**) and the relative permeability μ(**x**).

However, ε is not only a function of position. In general, it also depends on frequency (*material dispersion*) and on the electric field **E** itself (*nonlinearity*). It may also depend on the orientation of the field (*anisotropy*). Material dispersion, in turn, is generally associated with absorption loss in the material, or possibly *gain*. All of these effects can be simulated in Meep, with certain restrictions.

Similarly for the relative permeability μ(**x**), for which dispersion, nonlinearity, and anisotropy are all supported as well.

In this section, we describe the form of the equations and material properties that Meep can simulate. The actual interface with which you specify these properties is described in the Meep reference.

## Contents |

## Material dispersion

Physically, material dispersion arises because the polarization of the material does not respond instantaneously to an applied field **E**, and this is essentially the way that it is implemented in FDTD. In particular, is expanded to:

where is the *instantaneous* dielectric function (the infinite-frequency response) and **P** is the remaining frequency-dependent *polarization* density in the material. **P**, in turn, has its *own* time-evolution equation, and the exact form of this equation determines the frequency-dependence ε(ω). [Note that Meep's definition of ω uses a sign convention exp( − *i*ω*t*) for the time dependence.] In particular, Meep supports any material dispersion of the form of a sum of harmonic resonances, plus a term from the frequency-independent electric conductivity:

where σ_{D} is the electric conductivity, ω_{n} and γ_{n} are user-specified constants, and the is a user-specified function of position giving the strength of the *n*-th resonance. The σ parameters can be anisotropic tensors (although currently only diagonal tensors are supported), while the frequency-independent term can be an arbitrary tensor (not necessarily diagonal). This corresponds to evolving **P** via the equations:

That is, we must store and evolve a set of auxiliary fields along with the electric field in order to keep track of the polarization **P**. Essentially any ε(ω) could be modeled by including enough of these polarization fields — Meep allows you to specify any number of these, limited only by computer memory and time (which must increase with the number of polarization terms you require).

Note that the conductivity σ_{D} corresponds to an imaginary part of ε given by (not including the harmonic-resonance terms) . When you specify frequency in Meep units, however, you are specifying *f* without the 2π, so the imaginary part of ε is .

## Numerical stability

If you specify a Lorentzian resonance ω_{n} at too high a frequency relative to the time discretization Δ*t*, the simulation becomes unstable. Essentially, the problem is that you are trying to model a that oscillates too fast compared with the time discretization for the discretization to work properly. If this happens, you have three options: increase the resolution (which increases the resolution in both space and time), decrease the Courant factor (which decreases Δ*t* compared to Δ*x*), or use a different model function for your dielectric response.

Roughly speaking, the equation becomes unstable for ω_{n}Δ*t* / 2 > 1. (Note that, in Meep frequency units, you specify *f*_{n} = ω_{n} / 2π, so this quantity should be less than 1 / πΔ*t*.) A future version of Meep will check a necessary stability criterion automatically and halt with an error message if it is violated.

## Loss and gain

If γ above is nonzero, then the dielectric function ε(ω) becomes *complex*, where the imaginary part is associated with absorption loss in the material if it is positive, or gain if it is negative. Alternatively, a dissipation loss or gain may be added by a positive or negative conductivity, respectively—this is often convenient if you only care about the imaginary part of ε in a narrow bandwidth, and is described in detail below.

If you look at Maxwell's equations, then plays exactly the same role as a current . Just as is the rate of change of mechanical energy (the power expended by the electric field on moving the currents), therefore, the rate at which energy is lost to absorption for is given by:

- absorption rate

Meep can keep track of this energy for the Lorentzian polarizability terms (but not for the conductivity terms), which for gain gives the amount of energy expended in amplifying the field. (This feature may be removed in a future Meep version.)

## Conductivity and complex ε

Often, you only care about the absorption loss in a narrow bandwidth, where you just want to set the imaginary part of ε (or μ) to some known experimental value, in the same way that you often just care about setting a dispersionless real ε that is the correct value in your bandwidth of interest.

One approach to this problem would be allowing you to specify a constant (frequency-independent) imaginary part of ε, but this has the disadvantage of requiring the simulation to employ complex fields (doubling the memory and time requirements), and also tends to be numerically unstable. Instead, the approach in Meep is for you to set the conductivity σ_{D} (or σ_{B} for an imaginary part of μ), chosen so that is the correct value at your frequency ω of interest. (Note that, in Meep, you specify *f* = ω / 2π instead of ω for the frequency, however, so you need to include the factor of 2π when computing the corresponding imaginary part of ε!) Conductivities can be implemented with purely real fields, so they are not nearly as expensive as implementing a frequency-independent complex ε or μ.

For example, suppose you want to simulate a medium with at a frequency 0.42 (in your Meep units), and you only care about the material in a narrow bandwidth around this frequency (i.e. you don't need to simulate the full experimental frequency-dependent permittivity). Then, in Meep, you could use `(make medium (epsilon 3.4) (D-conductivity (/ (* 2 pi 0.42 0.101) 3.4))`

; i.e. and .

**Note**: the "conductivity" in Meep is slightly different from the conductivity you might find in a textbook, because (for computational convenience) it appears as in our Maxwell equations rather than the more-conventional ; this just means that our definition is different from the usual electric conductivity by a factor of ε. Also, just as Meep uses the dimensionless relative permittivity for ε, it uses nondimensionalized units of 1/*a* (where *a* is your unit of distance) for the conductivities σ_{D,B}. If you have the electric conductivity σ in SI units and want to convert to σ_{D} in Meep units, you can simply use the formula: (where *a* is your unit of distance in meters, *c* is the vacuum speed of light in m/s, is the SI vacuum permittivity, and is the real relative permittivity).

## Nonlinearity

In general, ε can be changed anisotropically by the **E** field itself, with:

where the *ij* is the index of the change in the 3×3 ε tensor and the χ terms are the nonlinear susceptibilities. The χ^{(2)} sum is the Pockels effect and the χ^{(3)} sum is the Kerr effect. (If the above expansion is frequency-independent, then the nonlinearity is *instantaneous*; more generally, Δε would depend on some average of the fields at previous times.)

Currently, Meep supports instantaneous, isotropic Pockels and Kerr nonlinearities, corresponding to a frequency-independent and , respectively. Thus,

Here, "diag(**E**)" indicates the diagonal 3×3 matrix with the components of **E** along the diagonal.

Normally, for nonlinear systems you will want to use real fields **E**. (This is usually the default. However, Meep uses complex fields if you have Bloch-periodic boundary conditions with a non-zero Bloch wavevector **k**, or in cylindrical coordinates with . In the C++ interface, real fields must be explicitly specified.)

For complex fields in nonlinear systems, the physical interpretration of the above equations is unclear because one cannot simply obtain the physical solution by taking the real part any more. In particular, Meep simply *defines* the meaning of the nonlinearity for complex fields as follows: the real and imaginary parts of the fields do not interact nonlinearly. That is, the above equation should be taken to hold for the real and imaginary parts (of **E** and **D**) *separately*. (e.g. |**E**|^{2} is the squared magnitude of the *real* part of **E** for when computing the real part of **D**, and conversely for the imaginary part.)

**Note**: The behavior for complex fields was changed for Meep 0.10. Also, in Meep 0.9 there was a bug: when you specified χ^{(3)}in the interface, you were actually specifying . This was fixed in Meep 0.10.

For a discussion of how to relate χ^{(3)} in Meep to experimental Kerr coefficients, see Units and nonlinearity in Meep.

## Magnetic permeability μ

All of the above features that are supported for the electric permittivity ε are also supported for the magnetic permeability μ. That is, Meep supports μ with dispersion from (magnetic) conductivity and Lorentzian resonances, as well as magnetic χ^{(2)} and χ^{(3)} nonlinearities. The description of these is exactly the same as above, so we won't repeat it here — just take the above descriptions and replace ε, **E**, **D**, and σ_{D} by μ, **H**, **B**, and σ_{B}, respectively.