#### Abstract

The diffraction of light is considered for a plane screen with an open bounded aperture. The corresponding solution behind the screen is given explicitly in terms of the Fourier transforms of the tangential components of the electric boundary field in the aperture. All components of the electric as well as the magnetic field vector are considered. We introduce solutions with global finite energy behind the screen and describe them in terms of two boundary potential functions. This new approach leads to a decoupling of the vectorial boundary equations in the aperture in the case of global finite energy. For the physically admissible solutions, that is, the solutions with local finite energy, we derive a characterisation in terms of the electric boundary fields.

#### 1. Introduction

This paper deals with the classical diffraction problem for electromagnetic waves passing a bounded aperture in an ideally conducting plane screen. We treat the problem within the exact theory, that is, we consider the corresponding solutions of the time harmonic Maxwell equations that fulfil the correct boundary conditions on the screen.

The problem of diffraction of electromagnetic waves by an infinite slit has been treated by the Fourier method in the papers [1, 2]. In [1] especially representations of the solutions that fulfil a certain energy condition have been given in terms of distributional electric boundary fields satisfying special regularity properties. In [2] mapping properties of the corresponding boundary operators between Sobolev spaces have been studied. These Sobolev spaces have been chosen such that the corresponding diffraction solutions satisfy the correct physical energy condition.

While the slit problem treated in [1, 2] can be decoupled into two scalar problems by considering two kinds of polarisations of the electromagnetic field, in the case of a bounded aperture such a decoupling is not possible in general. However, for the latter case we derive a new kind of decoupling of the vectorial system which can be performed if and only if the condition of global finite energy in part (b) of Definition 2.5 is fulfilled, see Theorem 3.2.

Finally we study the condition of local finite energy which covers all physically admissible solutions. Here we give a characterisation of solutions with local finite energy in terms of a regularity property of the electric boundary fields, see Theorem 4.1. This is done in a self-contained way by using the Paley-Wiener theorem for distributions defined on the bounded aperture as well as a special contour integration method in the Fourier domain.

#### 2. Electromagnetic Diffraction by an Aperture in a Plane Screen

We start with an informal physical description of the electromagnetic diffraction problem and fix some notations which will be used in the sequel. Then we will develop a more general mathematical frame with boundary distributions in Sobolev spaces in order to obtain diffraction solutions satisfying physical energy conditions.

Monochromatic light waves with a fixed wavenumber satisfy the first-order system of Maxwell-Helmholtz equations In the whole paper we consider a real wavenumber , although the results can be generalised to the case of a complex wavenumber with and . We assume that the electromagnetic field with components , , consists of functions defined in the upper half-space

The diffraction problem is considered for an open bounded aperture
in the screen plane . In the sequel we will suppress the notation of the third component 0 for the points in the screen plane, and interpret as well as the *screen * as subsets of . For describing the whole electromagnetic field in terms of its boundary values, for the moment we assume that these are functions, given for by

The screen is assumed to be an ideal conducting wall. This implies the physical boundary conditions In the general case the boundary fields (2.5) have to be replaced by appropriate distributions, and the limit will be performed in the Fourier domain instead of the half-space .

For this purpose we write again , with and fixed , and assume that each field component represents a tempered distribution in with Fourier transform Then we obtain from the first-order Maxwell-Helmholtz equations the following Fourier transformed Maxwell-Helmholtz system: for all and fixed we have Here we have replaced the partial derivative with respect to by the ordinary derivative .

We define With the two-dimensional Laplace operator we have For the action of in the Fourier domain we obtain multiplication by the matrix We replace (2.8) with the ordinary differential equations and the two algebraic conditions For all fixed we supplement the system of differential equations (2.12) by the initial conditions and put Then the general solution of the homogeneous linear system (2.12) is with the unit matrix .

But the terms and are exponentially increasing for fixed and . For avoiding that the Fourier transformed fields are also exponentially increasing we have to require the following algebraic conditions for : By using (2.17), for and we can replace (2.13) with From the general solution and (2.17), (2.18) we obtain the following decay conditions for : With , and there holds the important and well-known Sommerfeld-Weyl integral representation The left-hand side in (2.20) is the singular solution of the three-dimensional Helmholtz equation . For this reason it is natural to require the algebraic conditions (2.17), (2.18) also in the case , such that (2.19) is generally valid for and , .

Distributions with compact support in the screen plane are tempered, and it follows from the Paley-Wiener theorem that is a smooth function which has polynomial growth on . This is used in the following theorem, which results if we regard (2.17), (2.18), and (2.19) and apply the Fourier inversion formula for to each component and .

Theorem 2.1. *Let there be given with support in the bounded region . Then the following functions constitute a -solution of the Maxwell-Helmholtz system (2.1) in the upper half-space , ;
*

*Proof. *The calculation of the partial derivatives of and can be interchanged with integration. This can be used to check the Maxwell-Helmholtz equations independent from the previous representations of the Fourier-transforms and in terms of and .

*Definition 2.2. *The electromagnetic field in the half-space behind the screen is completely determined by the electric boundary components , . We call , , the half-space solution determined by the boundary distributions with compact support in .

*Remark 2.3. *Assume that are smooth functions with compact support in and that , is the corresponding electromagnetic field with the components given in Theorem 2.1. Then we obtain from Fourierโs inversion formula that for all

*Remark 2.4. *Assume again that are smooth functions with compact support in . Then we obtain from Theorem 2.1 and the Sommerfeld-Weyl integral (2.20) for all and all that
Equations (2.23) involve the boundary conditions (2.6) for the electric field components on the ideal conducting plane screen . In order to obtain a coupled system of boundary integro-differential equations we pass for to the limit and obtain
In general the electric boundary fields and are unknown distributions with compact support in , whereas and are given distributions in the aperture . In order to select physical admissible solutions of the diffraction problem we need some conditions for its electromagnetic energy content, especially in local volume elements . Recall that .

*Definition 2.5. *Let , , be the half-space solution determined by the boundary distributions with compact support in .(a)The solution is called *physical admissible* if and only if it satisfies the *local energy condition*for every bounded domain .(b)The solution satisfies the stronger *global energy condition* if and only if
for some , and therewith for all , with the layer.

In the following two sections we determine the solutions with global as well as those with local finite energy in terms of an appropriate functional analytical setting for the electric boundary fields and .

#### 3. The Global Energy Condition

Throughout the rest of this paper let the open aperture be a finite union of nonempty bounded Lipschitz domains in the screen plane, such that the compact sets are pairwise disjoint.

By , , we denote the Sobolev space of tempered distributions , for which the Fourier transform is locally integrable with (cf. [3], Chapterโโ8.8). is the norm on the Banach space .

For the Sobolev space is given by Here denotes the support of the distribution in the compact set . The space is equipped with the norm of , which makes it into a Banach space (cf. [4, Chapterโโ3]). The Lipschitz property of guarantees that is the closure of in . For a more general result see [4, Theoremโโ3.29].

Theorem 3.1. *Let , , be the half-space solution determined by the boundary distributions with compact support in . Then the diffraction solution has global finite energy if and only if**
and for all with .*

*Proof. *In order to perform the energy evaluation with Parsevalโs theorem we define with the Fourier transformed electromagnetic boundary fields , , the quantity
It follows from a lengthy calculation with for the values of the Fourier transformed boundary fields as well as from the algebraic relations (2.17), (2.18), which are valid for , that

With , we conclude from Parsevalโs theorem, (2.19) and the definition (2.15) of for
Using for all with the estimate
and observing that we have uniformly in for appropriate constants
we conclude from (3.6) that is finite if and only if
Note that with and also has support in .

Assume first that and that for a certain with . Since have compact support, we conclude from the Paley-Wiener theorem that are smooth and especially continuous. Thus in a bounded domain
and hence because of (2.18) is not integrable. Due to (3.1) this violates the necessary condition in (3.9).

For showing the other direction of the equivalence stated in the theorem we assume that
for all with . In order to prove (3.9) it remains to show that . Since , have compact support in , we obtain from the Paley-Wiener theorem that the Fourier transforms , can be continuated to entire functions. We also denote these entire functions by . We define the entire function by
Using (3.12) and (2.18), it follows for example from Theorem A.1 in the appendix that is a locally integrable function.

From the first equation in (2.18) and the Cauchy-Schwarz inequality we obtain the estimate
Now follows from the assumption , because is locally integrable and is bounded for sufficiently large .

Next we present a characterisation of the global finite energy solutions in terms of two boundary potential functions. In the following we use the vectorial differential operator (2.9) satisfying (2.10).

Theorem 3.2. *Boundary potential functions*(a)*Given are two functions , in the following called boundary potential functions, satisfying the regularity condition
Define
Then the corresponding electromagnetic field , , determined by and according to Theorem 2.1, has global finite energy. *(b)*Let , be given such that the half-space solution in Theorem 2.1 has global finite energy. If the set is connected, then the boundary fields and can be represented by two boundary potential functions satisfying (3.15) and (3.16) in part (a).*(c)*From the assumptions of part (a), or alternatively part (b), one obtains in the distributional sense
on the whole screen plane with
*

*Proof. *Part (a) follows in the Fourier domain by representing , , and in terms of and as
These equations and the assumptions imply that , and regarding uniformly in for a certain constant also . Since , have compact support in like , , the proof of part (a) follows from the fact that condition (3.9) is equivalent to .

For proving part (b) we assume that the diffraction solution corresponding to has global finite energy. We conclude from Theorem 3.1 and Theorem A.1 in the appendix that we obtain entire functions by
From (3.20) we get for that
Thus for the inverse Fourier transform of lies in .

Now we show that and have their support in and hence, by reason of (3.21), . To this aim we choose some such that
Since are supported in , we have . Therefore we obtain from the Paley-Wiener theorem and (3.20) that and are also supported in , because , , as , , are of exponential type not larger than .

Equations (3.20) can be resolved with respect to , . This gives (3.16). Together with (2.10) it follows that in the complement of there holds
Now we make use of the fact that the complement of is connected. Since for , Holmgrenโs unique continuation principle, applied to the two scalar Helmholtz equations in (3.23), implies that , have their support in .

Finally, since for , from (3.20) we obtain for
The asymptotic behaviour of this expression for shows the validity of the regularity condition (3.15).

The convolution integrals in part (c) can be rewritten in the Fourier domain by using the Sommerfeld-Weyl integral representation (2.20) in the limit . The resulting relations in the Fourier domain follow from the representations of the components and in Theorem 2.1.

The validity of (3.18) is a consequence of the algebraic relations between , , , and .

*Remark 3.3. *Consider the Sobolev spaces given for by
with the restriction of the tempered distribution to the subspace of the Schwartz space , where is the closure of the set in with respect to the topology of [5], ยง1 in Section 5.

is a Banach space with respect to the norm given by
The magnetic boundary fields in part (c) of Theorem 3.2 corresponding to global finite energy solutions may also be reinterpreted as distributions restricted to . In this case we obtain
In general the conditions (3.27) are weaker than the conditions (3.18) in Theorem 3.2, and we assume that they are fulfilled for diffraction solutions with *local* finite energy.

The conditions (3.27), where and are considered only in the aperture , reflects the physical fact that and are only prescribed in . Namely, and are the tangential magnetic components of the incoming electromagnetic wave in the aperture .

#### 4. The Local Energy Condition

In this section we derive the following characterisation for the diffraction solutions of local finite energy.

Theorem 4.1. *Let be and . Let , , be defined as in Theorem 2.1. Then the diffraction solution , , has local finite energy if and only if
*

For proving Theorem 4.1 firstly we formulate the subsequent Lemma 4.2. Then, using this lemma, we give the proof of the theorem. Afterwards we prove the lemma.

In the sequel we will make use of the following notations. For we define the open ball by For and the open cylinder is defined by

Lemma 4.2. *Let be and . Let , , be the diffraction solution given in Theorem 2.1. Let be with and let be and . Then one has the following equivalences and implications.*

(a)*. *(b)*. *(c)*. *(d)*. *(e)*. *(f)*. *

*Proof of Theorem 4.1. *Let there be given with support in and let , , be defined as in Theorem 2.1.

Firstly, we assume that the diffraction solution , , has local finite energy and show the validity of (4.1). Since the diffraction solution has local finite energy especially we have . From the parts (a) and (b) of Lemma 4.2 we find that . From the parts (c) and (d) of the lemma we thus obtain . Now the validity of (4.1) follows from
(cf. (3.9)).

For proving the other direction, we assume that relation (4.1) is valid. Because of (4.4) from the parts (c) and (d) of Lemma 4.2 it follows that . Regarding the other three field components, from the parts (e) and (f) of the lemma we obtain . Since and can be chosen arbitrarily large, the diffraction solution , , has local finite energy.

We have yet to prove Lemma 4.2.

*Proof of Lemma 4.2. *Let the assumptions of Lemma 4.2 be fulfilled.*Proof of Part (a)*

Let be . Obviously we have only to show the validity of the implication

We set and . For we consider the rotation matrix
and define the function by for and . We show that
regardless of whether or not.

From (4.7) we have
where is the image of the set under the linear mapping , that is, the set rotated by the angle .

Since , by considering a sufficient number of angles , from (4.8) we find that there is a polygon with . Thus
and therefore (4.5) holds true.

It remains to prove (4.7). To this end we use a contour integration technique which one of the authors had already used earlier in the treatment of diffraction problems [6, 7].

From the representation of given in Theorem 2.1 we conclude that
where the function is given by
By assumption, it holds that . Thus, from the Paley-Wiener theorem and the fact that is an orthogonal matrix it follows that
for some constants and .

We split the inner integral in (4.10) into one integral over the interval and one over . Firstly we consider the integral over .

For we define the curve by
Let be the closed contour consisting of the parts , and . By Cauchyโs theorem we have
here for nonreal values of the function is defined by analytic continuation of the function , where and , and .

Because of (4.12), for it holds that
From the last two equations we obtain

Now we treat the integral over . Let be the principal branch of the square root function (branch cut ). For we set is thought to be defined on the negative real half-axis by continuation from the *lower* complex half-plane. In what follows, the function is defined by . Regarding the integration variable we distinguish the two cases and .

Firstly we assume that . We have
By contour integration analogous as in the derivation of (4.16), applied to the next-to-last integral, eventually we get

In the case it holds that for , and contour integration yields

In the following, for or we set , where the square root is chosen in the way that and . This definition is in accordance with (2.15). In this notation, from (4.18) and (4.19) we obtain
Together with (4.16) we thus find

Now we show that each of the three addends in the right-hand side of (4.21), considered as a function of , lies in . Then, because of the representation (4.10), the stated relation (4.7) is proved.

We begin with the first addend. Let , let be the characteristic function of the set and let denote the inverse Fourier transform. There is a constant , which does not depend on , such that
From this it follows that the first addend in the right-hand side of (4.21) is quadratically integrable over the set and thus especially quadratically integrable over .

Now we come to the second addend. We define by . From (4.12) we find that there is a constant with
Since for and the quantity is a real number and therefore for , we conclude that
Thus we have
Because the inverse Fourier transform (here with regard to the variable ) is isometric on , we see that
It follows that
The substitution in the innermost integral now shows that the second addend in the right-hand side of (4.21) indeed lies in .

Regarding the third addend, from (4.12) we find
Now let be as above. By reason of
we conclude that there are constants such that
This estimate corresponds to formula (4.24), used in the case of the second addend. Continuing as in this latter case, it is seen that the third addend in the right-hand side of (4.21) lies in too.*Proof of Part (b)*

The preceding proof of part (a) is based on the fact that for it holds that
The relation (4.31), which is equivalent to the representation of given in Theorem 2.1, has led to (4.10).

Since the third magnetic component fulfills conditions which are analogous to (4.31) and (4.32), the proof of part (b) of the lemma is completely analogous to the proof of part (a). The condition for that is analogous to (4.31) is given in (2.19); note that (2.19) also holds for . The condition is a direct consequence of
following from the second equation in (2.18), and .*Proof of Parts (c) and (d)*

These parts of the lemma can be proved along the lines of the proof of Theorem 3.1. For example, with respect to , , one has
The condition (3.12), used in the proof of Theorem 3.1 because the factor is not locally square integrable, is not needed for the special field components , , and .*Proof of Part (e)*

From the first equation in (2.18) and the first equation in (2.19) we obtain
Thus for we have
Now we define the function by
and the function by
Using (4.36), we find
with
From (4.31) we see that there is a constant such that
For it holds that
Therefore with some constant we have for and
From this we get from the Cauchy-Schwarz inequality that