# Boundary conditions

## Perfectly Matched Layer: open boundary condition for electromagnetic waves

For the transverse electric (TE) case, the original Berenger’s Perfectly Matched Layer (PML) paper [1] writes

(26)$\varepsilon _{0}\frac{\partial E_{x}}{\partial t}+\sigma _{y}E_{x} = \frac{\partial H_{z}}{\partial y}$
(27)$\varepsilon _{0}\frac{\partial E_{y}}{\partial t}+\sigma _{x}E_{y} = -\frac{\partial H_{z}}{\partial x}$
(28)$\mu _{0}\frac{\partial H_{zx}}{\partial t}+\sigma ^{*}_{x}H_{zx} = -\frac{\partial E_{y}}{\partial x}$
(29)$\mu _{0}\frac{\partial H_{zy}}{\partial t}+\sigma ^{*}_{y}H_{zy} = \frac{\partial E_{x}}{\partial y}$
(30)$H_{z} = H_{zx}+H_{zy}$

This can be generalized to

(31)$\varepsilon _{0}\frac{\partial E_{x}}{\partial t}+\sigma _{y}E_{x} = \frac{c_{y}}{c}\frac{\partial H_{z}}{\partial y}+\overline{\sigma }_{y}H_{z}$
(32)$\varepsilon _{0}\frac{\partial E_{y}}{\partial t}+\sigma _{x}E_{y} = -\frac{c_{x}}{c}\frac{\partial H_{z}}{\partial x}+\overline{\sigma }_{x}H_{z}$
(33)$\mu _{0}\frac{\partial H_{zx}}{\partial t}+\sigma ^{*}_{x}H_{zx} = -\frac{c^{*}_{x}}{c}\frac{\partial E_{y}}{\partial x}+\overline{\sigma }_{x}^{*}E_{y}$
(34)$\mu _{0}\frac{\partial H_{zy}}{\partial t}+\sigma ^{*}_{y}H_{zy} = \frac{c^{*}_{y}}{c}\frac{\partial E_{x}}{\partial y}+\overline{\sigma }_{y}^{*}E_{x}$
(35)$H_{z} = H_{zx}+H_{zy}$

For $$c_{x}=c_{y}=c^{*}_{x}=c^{*}_{y}=c$$ and $$\overline{\sigma }_{x}=\overline{\sigma }_{y}=\overline{\sigma }_{x}^{*}=\overline{\sigma }_{y}^{*}=0$$, this system reduces to the Berenger PML medium, while adding the additional constraint $$\sigma _{x}=\sigma _{y}=\sigma _{x}^{*}=\sigma _{y}^{*}=0$$ leads to the system of Maxwell equations in vacuum.

### Propagation of a Plane Wave in an APML Medium

We consider a plane wave of magnitude ($$E_{0},H_{zx0},H_{zy0}$$) and pulsation $$\omega$$ propagating in the APML medium with an angle $$\varphi$$ relative to the x axis

(36)$E_{x} = -E_{0}\sin \varphi \: e^{i\omega \left( t-\alpha x-\beta y\right) }$
(37)$E_{y} = E_{0}\cos \varphi \: e^{i\omega \left( t-\alpha x-\beta y\right) }$
(38)$H_{zx} = H_{zx0} \: e^{i\omega \left( t-\alpha x-\beta y\right) }$
(39)$H_{zy} = H_{zy0} \: e^{i\omega \left( t-\alpha x-\beta y\right) }$

where $$\alpha$$ and $$\beta$$ are two complex constants to be determined.

Introducing Eqs. (36), (37), (38) and (39) into Eqs. (31), (32), (33) and (34) gives

(40)$\varepsilon _{0}E_{0}\sin \varphi -i\frac{\sigma _{y}}{\omega }E_{0}\sin \varphi = \beta \frac{c_{y}}{c}\left( H_{zx0}+H_{zy0}\right) +i\frac{\overline{\sigma }_{y}}{\omega }\left( H_{zx0}+H_{zy0}\right)$
(41)$\varepsilon _{0}E_{0}\cos \varphi -i\frac{\sigma _{x}}{\omega }E_{0}\cos \varphi = \alpha \frac{c_{x}}{c}\left( H_{zx0}+H_{zy0}\right) -i\frac{\overline{\sigma }_{x}}{\omega }\left( H_{zx0}+H_{zy0}\right)$
(42)$\mu _{0}H_{zx0}-i\frac{\sigma ^{*}_{x}}{\omega }H_{zx0} = \alpha \frac{c^{*}_{x}}{c}E_{0}\cos \varphi -i\frac{\overline{\sigma }^{*}_{x}}{\omega }E_{0}\cos \varphi$
(43)$\mu _{0}H_{zy0}-i\frac{\sigma ^{*}_{y}}{\omega }H_{zy0} = \beta \frac{c^{*}_{y}}{c}E_{0}\sin \varphi +i\frac{\overline{\sigma }^{*}_{y}}{\omega }E_{0}\sin \varphi$

Defining $$Z=E_{0}/\left( H_{zx0}+H_{zy0}\right)$$ and using Eqs. (40) and (41), we get

(44)$\beta = \left[ Z\left( \varepsilon _{0}-i\frac{\sigma _{y}}{\omega }\right) \sin \varphi -i\frac{\overline{\sigma }_{y}}{\omega }\right] \frac{c}{c_{y}}$
(45)$\alpha = \left[ Z\left( \varepsilon _{0}-i\frac{\sigma _{x}}{\omega }\right) \cos \varphi +i\frac{\overline{\sigma }_{x}}{\omega }\right] \frac{c}{c_{x}}$

Adding $$H_{zx0}$$ and $$H_{zy0}$$ from Eqs. (42) and (43) and substituting the expressions for $$\alpha$$ and $$\beta$$ from Eqs. (44) and (45) yields

\begin{split}\begin{aligned} \frac{1}{Z} & = \frac{Z\left( \varepsilon _{0}-i\frac{\sigma _{x}}{\omega }\right) \cos \varphi \frac{c^{*}_{x}}{c_{x}}+i\frac{\overline{\sigma }_{x}}{\omega }\frac{c^{*}_{x}}{c_{x}}-i\frac{\overline{\sigma }^{*}_{x}}{\omega }}{\mu _{0}-i\frac{\sigma ^{*}_{x}}{\omega }}\cos \varphi \nonumber \\ & + \frac{Z\left( \varepsilon _{0}-i\frac{\sigma _{y}}{\omega }\right) \sin \varphi \frac{c^{*}_{y}}{c_{y}}-i\frac{\overline{\sigma }_{y}}{\omega }\frac{c^{*}_{y}}{c_{y}}+i\frac{\overline{\sigma }^{*}_{y}}{\omega }}{\mu _{0}-i\frac{\sigma ^{*}_{y}}{\omega }}\sin \varphi \end{aligned}\end{split}

If $$c_{x}=c^{*}_{x}$$, $$c_{y}=c^{*}_{y}$$, $$\overline{\sigma }_{x}=\overline{\sigma }^{*}_{x}$$, $$\overline{\sigma }_{y}=\overline{\sigma }^{*}_{y}$$, $$\frac{\sigma _{x}}{\varepsilon _{0}}=\frac{\sigma ^{*}_{x}}{\mu _{0}}$$ and $$\frac{\sigma _{y}}{\varepsilon _{0}}=\frac{\sigma ^{*}_{y}}{\mu _{0}}$$ then

(46)$Z = \pm \sqrt{\frac{\mu _{0}}{\varepsilon _{0}}}$

which is the impedance of vacuum. Hence, like the PML, given some restrictions on the parameters, the APML does not generate any reflection at any angle and any frequency. As for the PML, this property is not retained after discretization, as shown subsequently.

Calling $$\psi$$ any component of the field and $$\psi _{0}$$ its magnitude, we get from Eqs. (36), (44), (45) and (46) that

(47)$\psi =\psi _{0} \: e^{i\omega \left( t\mp x\cos \varphi /c_{x}\mp y\sin \varphi /c_{y}\right) }e^{-\left( \pm \frac{\sigma _{x}\cos \varphi }{\varepsilon _{0}c_{x}}+\overline{\sigma }_{x}\frac{c}{c_{x}}\right) x} e^{-\left( \pm \frac{\sigma _{y}\sin \varphi }{\varepsilon _{0}c_{y}}+\overline{\sigma }_{y}\frac{c}{c_{y}}\right) y}.$

We assume that we have an APML layer of thickness $$\delta$$ (measured along $$x$$) and that $$\sigma _{y}=\overline{\sigma }_{y}=0$$ and $$c_{y}=c.$$ Using (47), we determine that the coefficient of reflection given by this layer is

\begin{split}\begin{aligned} R_{\mathrm{APML}}\left( \theta \right) & = e^{-\left( \sigma _{x}\cos \varphi /\varepsilon _{0}c_{x}+\overline{\sigma }_{x}c/c_{x}\right) \delta }e^{-\left( \sigma _{x}\cos \varphi /\varepsilon _{0}c_{x}-\overline{\sigma }_{x}c/c_{x}\right) \delta },\nonumber \\ & = e^{-2\left( \sigma _{x}\cos \varphi /\varepsilon _{0}c_{x}\right) \delta }, \end{aligned}\end{split}

which happens to be the same as the PML theoretical coefficient of reflection if we assume $$c_{x}=c$$. Hence, it follows that for the purpose of wave absorption, the term $$\overline{\sigma }_{x}$$ seems to be of no interest. However, although this conclusion is true at the infinitesimal limit, it does not hold for the discretized counterpart.

### Discretization

In the following we set $$\varepsilon_0 = \mu_0 = 1$$. We discretize Eqs. (26), (27), (28), and (29) to obtain

$\frac{E_x|^{n+1}_{j+1/2,k,l}-E_x|^{n}_{j+1/2,k,l}}{\Delta t} + \sigma_y \frac{E_x|^{n+1}_{j+1/2,k,l}+E_x|^{n}_{j+1/2,k,l}}{2} = \frac{H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j+1/2,k-1/2,l}}{\Delta y}$
$\frac{E_y|^{n+1}_{j,k+1/2,l}-E_y|^{n}_{j,k+1/2,l}}{\Delta t} + \sigma_x \frac{E_y|^{n+1}_{j,k+1/2,l}+E_y|^{n}_{j,k+1/2,l}}{2} = - \frac{H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j-1/2,k+1/2,l}}{\Delta x}$
$\frac{H_{zx}|^{n+3/2}_{j+1/2,k+1/2,l}-H_{zx}|^{n+1/2}_{j+1/2,k+1/2,l}}{\Delta t} + \sigma^*_x \frac{H_{zx}|^{n+3/2}_{j+1/2,k+1/2,l}+H_{zx}|^{n+1/2}_{j+1/2,k+1/2,l}}{2} = - \frac{E_y|^{n+1}_{j+1,k+1/2,l}-E_y|^{n+1}_{j,k+1/2,l}}{\Delta x}$
$\frac{H_{zy}|^{n+3/2}_{j+1/2,k+1/2,l}-H_{zy}|^{n+1/2}_{j+1/2,k+1/2,l}}{\Delta t} + \sigma^*_y \frac{H_{zy}|^{n+3/2}_{j+1/2,k+1/2,l}+H_{zy}|^{n+1/2}_{j+1/2,k+1/2,l}}{2} = \frac{E_x|^{n+1}_{j+1/2,k+1,l}-E_x|^{n+1}_{j+1/2,k,l}}{\Delta y}$

and this can be solved to obtain the following leapfrog integration equations

\begin{split}\begin{aligned} E_x|^{n+1}_{j+1/2,k,l} & = \left(\frac{1-\sigma_y \Delta t/2}{1+\sigma_y \Delta t/2}\right) E_x|^{n}_{j+1/2,k,l} + \frac{\Delta t/\Delta y}{1+\sigma_y \Delta t/2} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j+1/2,k-1/2,l}\right) \\ E_y|^{n+1}_{j,k+1/2,l} & = \left(\frac{1-\sigma_x \Delta t/2}{1+\sigma_x \Delta t/2}\right) E_y|^{n}_{j,k+1/2,l} - \frac{\Delta t/\Delta x}{1+\sigma_x \Delta t/2} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j-1/2,k+1/2,l}\right) \\ H_{zx}|^{n+3/2}_{j+1/2,k+1/2,l} & = \left(\frac{1-\sigma^*_x \Delta t/2}{1+\sigma^*_x \Delta t/2}\right) H_{zx}|^{n+1/2}_{j+1/2,k+1/2,l} - \frac{\Delta t/\Delta x}{1+\sigma^*_x \Delta t/2} \left(E_y|^{n+1}_{j+1,k+1/2,l}-E_y|^{n+1}_{j,k+1/2,l}\right) \\ H_{zy}|^{n+3/2}_{j+1/2,k+1/2,l} & = \left(\frac{1-\sigma^*_y \Delta t/2}{1+\sigma^*_y \Delta t/2}\right) H_{zy}|^{n+1/2}_{j+1/2,k+1/2,l} + \frac{\Delta t/\Delta y}{1+\sigma^*_y \Delta t/2} \left(E_x|^{n+1}_{j+1/2,k+1,l}-E_x|^{n+1}_{j+1/2,k,l}\right) \end{aligned}\end{split}

If we account for higher order $$\Delta t$$ terms, a better approximation is given by

\begin{split}\begin{aligned} E_x|^{n+1}_{j+1/2,k,l} & = e^{-\sigma_y\Delta t} E_x|^{n}_{j+1/2,k,l} + \frac{1-e^{-\sigma_y\Delta t}}{\sigma_y \Delta y} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j+1/2,k-1/2,l}\right) \\ E_y|^{n+1}_{j,k+1/2,l} & = e^{-\sigma_x\Delta t} E_y|^{n}_{j,k+1/2,l} - \frac{1-e^{-\sigma_x\Delta t}}{\sigma_x \Delta x} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j-1/2,k+1/2,l}\right) \\ H_{zx}|^{n+3/2}_{j+1/2,k+1/2,l} & = e^{-\sigma^*_x\Delta t} H_{zx}|^{n+1/2}_{j+1/2,k+1/2,l} - \frac{1-e^{-\sigma^*_x\Delta t}}{\sigma^*_x \Delta x} \left(E_y|^{n+1}_{j+1,k+1/2,l}-E_y|^{n+1}_{j,k+1/2,l}\right) \\ H_{zy}|^{n+3/2}_{j+1/2,k+1/2,l} & = e^{-\sigma^*_y\Delta t} H_{zy}|^{n+1/2}_{j+1/2,k+1/2,l} + \frac{1-e^{-\sigma^*_y\Delta t}}{\sigma^*_y \Delta y} \left(E_x|^{n+1}_{j+1/2,k+1,l}-E_x|^{n+1}_{j+1/2,k,l}\right) \end{aligned}\end{split}

More generally, this becomes

\begin{split}\begin{aligned} E_x|^{n+1}_{j+1/2,k,l} & = e^{-\sigma_y\Delta t} E_x|^{n}_{j+1/2,k,l} + \frac{1-e^{-\sigma_y\Delta t}}{\sigma_y \Delta y}\frac{c_y}{c} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j+1/2,k-1/2,l}\right) \\ E_y|^{n+1}_{j,k+1/2,l} & = e^{-\sigma_x\Delta t} E_y|^{n}_{j,k+1/2,l} - \frac{1-e^{-\sigma_x\Delta t}}{\sigma_x \Delta x}\frac{c_x}{c} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j-1/2,k+1/2,l}\right) \\ H_{zx}|^{n+3/2}_{j+1/2,k+1/2,l} & = e^{-\sigma^*_x\Delta t} H_{zx}|^{n+1/2}_{j+1/2,k+1/2,l} - \frac{1-e^{-\sigma^*_x\Delta t}}{\sigma^*_x \Delta x}\frac{c^*_x}{c} \left(E_y|^{n+1}_{j+1,k+1/2,l}-E_y|^{n+1}_{j,k+1/2,l}\right) \\ H_{zy}|^{n+3/2}_{j+1/2,k+1/2,l} & = e^{-\sigma^*_y\Delta t} H_{zy}|^{n+1/2}_{j+1/2,k+1/2,l} + \frac{1-e^{-\sigma^*_y\Delta t}}{\sigma^*_y \Delta y}\frac{c^*_y}{c} \left(E_x|^{n+1}_{j+1/2,k+1,l}-E_x|^{n+1}_{j+1/2,k,l}\right) \end{aligned}\end{split}

If we set

\begin{split}\begin{aligned} c_x & = c \: e^{-\sigma_x\Delta t} \frac{\sigma_x \Delta t}{1-e^{-\sigma_x\Delta t}} \\ c_y & = c \: e^{-\sigma_y\Delta t} \frac{\sigma_y \Delta t}{1-e^{-\sigma_y\Delta t}} \\ c^*_x & = c \: e^{-\sigma^*_x\Delta t} \frac{\sigma^*_x \Delta t}{1-e^{-\sigma^*_x\Delta t}} \\ c^*_y & = c \: e^{-\sigma^*_y\Delta t} \frac{\sigma^*_y \Delta t}{1-e^{-\sigma^*_y\Delta t}}\end{aligned}\end{split}

then this becomes

\begin{split}\begin{aligned} E_x|^{n+1}_{j+1/2,k,l} & = e^{-\sigma_y\Delta t} \left[ E_x|^{n}_{j+1/2,k,l} + \frac{\Delta t}{\Delta y} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j+1/2,k-1/2,l}\right) \right] \\ E_y|^{n+1}_{j,k+1/2,l} & = e^{-\sigma_x\Delta t} \left[ E_y|^{n}_{j,k+1/2,l} - \frac{\Delta t}{\Delta x} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j-1/2,k+1/2,l}\right) \right] \\ H_{zx}|^{n+3/2}_{j+1/2,k+1/2,l} & = e^{-\sigma^*_x\Delta t} \left[ H_{zx}|^{n+1/2}_{j+1/2,k+1/2,l} - \frac{\Delta t}{\Delta x} \left(E_y|^{n+1}_{j+1,k+1/2,l}-E_y|^{n+1}_{j,k+1/2,l}\right) \right] \\ H_{zy}|^{n+3/2}_{j+1/2,k+1/2,l} & = e^{-\sigma^*_y\Delta t} \left[ H_{zy}|^{n+1/2}_{j+1/2,k+1/2,l} + \frac{\Delta t}{\Delta y} \left(E_x|^{n+1}_{j+1/2,k+1,l}-E_x|^{n+1}_{j+1/2,k,l}\right) \right] \end{aligned}\end{split}

When the generalized conductivities are zero, the update equations are

\begin{split}\begin{aligned} E_x|^{n+1}_{j+1/2,k,l} & = E_x|^{n}_{j+1/2,k,l} + \frac{\Delta t}{\Delta y} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j+1/2,k-1/2,l}\right) \\ E_y|^{n+1}_{j,k+1/2,l} & = E_y|^{n}_{j,k+1/2,l} - \frac{\Delta t}{\Delta x} \left(H_z|^{n+1/2}_{j+1/2,k+1/2,l}-H_z|^{n+1/2}_{j-1/2,k+1/2,l}\right) \\ H_{zx}|^{n+3/2}_{j+1/2,k+1/2,l} & = H_{zx}|^{n+1/2}_{j+1/2,k+1/2,l} - \frac{\Delta t}{\Delta x} \left(E_y|^{n+1}_{j+1,k+1/2,l}-E_y|^{n+1}_{j,k+1/2,l}\right) \\ H_{zy}|^{n+3/2}_{j+1/2,k+1/2,l} & = H_{zy}|^{n+1/2}_{j+1/2,k+1/2,l} + \frac{\Delta t}{\Delta y} \left(E_x|^{n+1}_{j+1/2,k+1,l}-E_x|^{n+1}_{j+1/2,k,l}\right) \end{aligned}\end{split}

as expected.

## Perfect Electrical Conductor

This boundary can be used to model a dielectric or metallic surface. For the electromagnetic solve, at PEC, the tangential electric field and the normal magnetic field are set to 0. In the guard-cell region, the tangential electric field is set equal and opposite to the respective field component in the mirror location across the PEC boundary, and the normal electric field is set equal to the field component in the mirror location in the domain across the PEC boundary. Similarly, the tangential (and normal) magnetic field components are set equal (and opposite) to the respective magnetic field components in the mirror locations across the PEC boundary.

The PEC boundary condition also impacts the deposition of charge and current density. On the boundary the charge density and parallel current density is set to zero. If a reflecting boundary condition is used for the particles, density overlapping with the PEC will be reflected back into the domain (for both charge and current density). If absorbing boundaries are used, an image charge (equal weight but opposite charge) is considered in the mirror location accross the boundary, and the density from that charge is also deposited in the simulation domain. Fig. 29 shows the effect of this. The left boundary is absorbing while the right boundary is reflecting.

[1]

J. P. Berenger. A Perfectly Matched Layer For The Absorption Of Electromagnetic-Waves. Journal of Computational Physics, 114(2):185–200, Oct 1994.