Green’s functions in homogeneous uniaxial media

The electric and magnetic Green’s dyadic functions for a uniaxial medium can be written as [72]

G^EE(r,r⁰) =

"

c² ω²⊥

∇ ⊗ ∇+_k⁻¹

#

g_e(r−r⁰)−F(r−r⁰) (2.20) and

G^HH(r,r⁰) =

"

c²

ω²∇ ⊗ ∇+⊥1

#

g_o(r−r⁰)+⊥F(r−r⁰). (2.21) The scalar Green’s functions for the ordinary and extraordinary modes are given by (x≡r−r⁰)

g_o/e(x) = 1 4π

e^{iωc√⊥xo/e}

x_o/e , (2.22)

where x²_e= _k

⊥(x×e_k)²+ (x·e_k)² and x²_o=x²:=|x|². (2.23) Here e_k is the unit vector parallel to the optical axis, i. e. in our case e_k =e_z.

Moreover, we can split the dyad F into its extraordinary and

ordinary part [72] These expressions can now be brought into a form which is much more convenient for further calculations. To this end, we make a Fourier expansion of the scalar Green’s functions (by Fourier transforming the differential equations governing g_o/e, see Ref. [72]):

g_o(x) =

Taking the poles for outgoing solutions into account in the residue theorem we can integrate overk_k and express the scalar Green’s functions as introducing the integral operators

Iˆ_e= where the components of the wavevectors parallel to the optical axis are defined as

k_k^e=

We have used the quantities x_k = e_k ·x = z−z⁰ and x_⊥ = x −xke_k. k_⊥ = kx ky 0^T is the part of the wavevector perpendicular toe_kand k_⊥² =kk⊥k²=k_x²+k²_y. Outgoing waves are propagating away from the source. The sign ofk^e/o_k must be chosen such that the plane wave ∝e^ikkxk propagates in positive x_k-direction (because this is the direction away from the source).

“Propagation” refers to the energy flow, not the phase which can propagate in a different direction.

Now, we further introduce a polarization basis by defining

a^e/o_s ≡ k^e/o×e_k

where it is assumed thatk^e/o_k is real. This basis accompanied by the wavevector

basis for realk^e/o_k with the completeness relation

a_s^e/o⊗a^e/o_s +a^e/o_p ⊗a_p^e/o+ k^e/o⊗k^e/o

k^e/o2 =1. (2.37) The undetermined signs in k^e/o and a^e/o_p are chosen depending on the observation point. The first choice is forxk>0, the half-space in which waves propagate in positive x_k-direction (thus k_k-component of k^e/o must be positive); the second choice for xk <0, the halfspace in which waves propagate in negative xk -direction (thus k_k-component must be negative).

If the wavevector componentk_k becomes complex, as in dissipa-tive media or for evanescent modes, the above introduced vectors a_p^e/oandk^e/o are not orthogonal to each other anymore (a^e/o_s re-mains orthogonal to both). However, relation (2.37) rere-mains valid and the vectors still form a basis.

From the definition ofa^e/o_s it follows thata^e_s =a^o_s ≡a_s and that (independent of k_k)

(k×e_k)⊗(k×e_k)

(k×e_k)² =a_s⊗a_s. (2.38) This last relation allows us to rewrite the dyad F. We find the expressions

Fe= 1

_kIˆe[a_s⊗a_s], (2.39) Fo = ˆIo[a_s⊗a_s]. (2.40)

Finally, using the expansions forg_eandg_o together with the def-inition of the polarization bases and the completeness relation, we can bring the electric and magnetic Green’s dyad in the final form. We obtain

G^EE=G^EEo +G^EEe

= ˆIo[a_s⊗a_s] + ˆIe

h

a^e_p⊗a^e_p+Bk^e⊗k^e+Cek⊗e_kⁱ (2.41) and

G^HH=G^HHo +G^HHe = ˆI_o^h⊥a^o_p⊗a^o_pⁱ+ ˆI_e[⊥a_s⊗a_s] (2.42) introducing the anisotropy coefficients

B≡

ω²

c²_k−k^e2

k^e2ω_c2²_k and C≡ ⊥

k

−1. (2.43) Remembering the definition of the ˆI operators Green’s functions (2.41) and (2.42) are represented here as a superposition of or-dinary and extraoror-dinary plane monochromatic waves.

Note, that due to the second derivation in Eqs. (2.20) and (2.21) with respect to x_k the nabla operator ∇ not only provides an ik^e/o but we additionally obtain the two singular terms [73]

G^EEsing= 1 _k

c² ω²Iˆe

h2ik^e_kⁱe_k⊗e_kδ(xk)

=−1 _k

c²

ω²e_k⊗e_kδ(x),

(2.44)

G^HHsing = c² ω²Iˆo

h2ik^o_kⁱe_k⊗e_kδ(x_k)

=−c²

ω²e_k⊗e_kδ(x),

(2.45)

where we have introduced the Dirac delta function δ(x)≡

∞

Z Z Z

−∞

e^±ik·x d³k

(2π)³. (2.46) For the calculation of the heat flux these terms do not contribute since in this case x 6= 0 (i. e. r 6= r⁰) as will be seen later. On the other hand, these terms do also not contribute to the density of states when neglecting losses, since in this case these terms are purely real. Therefore, we will neglect these terms in the following.

With the expressions for the electric and magnetic Green’s dyad and k^e/o×a_p^e/o=k^e/oa_s it can be easily proven that

G^HH=−c²

ω²∇ ×G^EE× ∇⁰. (2.47) Remembering Maxwell’s equations the hybrid Green’s functions are found to be [72]

G^HE = 1

iωµ₀∇ ×G^EE, (2.48) G^EH = −1

iω₀⁻¹∇ ×G^HH. (2.49) As can be seen later,G^EH is not needed for the materials under

consideration, so we focus on G^HE here. However, G^EH can be calculated the same way and due to reciprocity G^EH(r,r⁰) =

µ0

0G^HET(r,r⁰) holds [72].

We find

G^HE=G^HEo +G^HEe

= Iˆ_o

ωµ0 [(k^o×a_s)⊗a_s] + Iˆ_e ωµ0

h(k^e×a^e_p)⊗a^e_p +B(k^e×k^e)⊗k^e+C(k^e×e_k)⊗e_kⁱ

= Iˆo

ωµ₀

h−k^oa^o_p⊗a_sⁱ+ Iˆe

ωµ₀

hk^ea_s⊗a_p^e+Ck⊥a_s⊗e_kⁱ.

(2.50)

Obviously, for an electric source the electric field of ordinary modes is polarized along a_s and the magnetic field of ordinary modes along a_p^o. However, although the magnetic field of ex-traordinary modes is polarized alonga_s their electric field is not polarized purely in a^e_p-direction but has also components ink^e -direction. Indeed, in uniaxial, nonmagnetic materials only for ordinary modes electric fields E / D, magnetic fields H / B and wavevector k^o form an orthogonal set of vectors (orthog-onal assuming real k). For extraordinary modes only electric displacement field D, magnetic fieldsH /B and wavevector k^e are orthogonal which can be seen from the corresponding Green’s

function

G^DE=G^DEo +G^DEe =0G^EE=0Iˆo[⊥a_s⊗a_s] +0Iˆe

"

k^e2 _k^ω_c2²

a^e_p⊗diag(k, k, ⊥)a^e_p

# (2.51)

or directly from Maxwell’s equations.

Furthermore, by definitionE,Hand (complex) Poynting vector S = ¹₂E×H^∗ are also orthogonal. For extraordinary modes neitherE andDnorS andk^e are parallel but point in different directions.

As a final remark, let us look at the polarization state. Propa-gating modes in a lossless medium have real wavevectorsk^e/o. a_s and a^e/o_p are also real. Electric and magnetic fields are linearly polarized and in-phase.

When k^e/o becomes complex, as for evanescent modes or in dis-sipative materials, the situation changes. a_s is still real. SoEof ordinary modes andHof extraordinary modes are linearly polar-ized. But becausea^e/o_p is complex, electric field of extraordinary modes and magnetic field of ordinary modes are elliptically po-larized.

The modes used here have solely real tangential wavevectors k_⊥. It is as if they are excited by light impinging from a loss-less medium on an interface with the uniaxial medium under consideration. They should not be confused with modes which have ak^e/o that can be split into a real direction and a complex amplitude. Such modes represent a different basis with different

properties. E. g. in the isotropic case Eand Hare both linearly polarized but not in-phase.

As will be seen later, the setups under consideration are all 1-dimensional and infinitely extended in the transverse direction.

Hence, all problems exhibit a translational symmetry in trans-verse direction and modes with real transtrans-verse wavevectors k_⊥ are a rational choice for our mode basis. Complex k_⊥ lead to different field amplitudes along the transverse direction which is difficult to interpret in a system with translational symmetry.

So our choice of basis is not only a consequence of the derivation but will allow a physical interpretation of the thermal radiation equations.

Please notice that we use the names “ordinary” and “extraor-dinary” for the two fundamental polarization states throughout the thesis independent of the material, to keep it consistent.

For isotropic materials the states are usually called “s- and p-polarized” instead. For lossy materials some people could expect another mode basis (with complexk_⊥) when talking of ordinary and extraordinary modes.

2.3.2 Green’s functions for uniaxial medium–vacuum

Im Dokument 1.1 Near-field radiative heat transfer (Seite 43-52)