leaky modes of dielectric cavities - university of arizonaย ยท 2017. 9. 19.ย ยท the dielectric is...
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Leaky Modes of Dielectric Cavities Masud Mansuripurโ , Miroslav Kolesikโ , and Per Jakobsenโก
โ College of Optical Sciences, The University of Arizona, Tucson โกDepartment of Mathematics and Statistics, University of Tromsรธ, Norway
[Published in Spintronics IX, edited by H.-J. Drouhin, J.-E. Wegrowe, and M. Razeghi, Proceedings of SPIE 9931, 99310B ~ 1:20 (2016).]
Abstract. In the absence of external excitation, light trapped within a dielectric medium generally decays by leaking out โ and also by getting absorbed within the medium. We analyze the leaky modes of a parallel-plate slab, a solid glass sphere, and a solid glass cylinder, by examining those solutions of Maxwellโs equations (for dispersive as well as non-dispersive media) which admit of a complex-valued oscillation frequency. Under certain circumstances, these leaky modes constitute a complete set into which an arbitrary distribution of the electromagnetic field residing inside a dielectric body can be expanded. We provide completeness proofs, and also present results of numerical calculations that illustrate the relationship between the leaky modes and the resonances of dielectric cavities formed by a simple parallel-plate slab, a glass sphere, and a glass cylinder.
1. Introduction. A parallel plate dielectric slab, a solid glass sphere, and a solid glass cylinder are examples of material bodies which, when continually illuminated, accept and accommodate some of the incident light, eventually reaching a steady state where the rate of the incoming light equals that of the outgoing. By properly tuning the frequency of the incident light, one can excite resonances, arriving at conditions under which the optical intensity inside the dielectric host exceeds, often by a large factor, that of the incident light beam. If now the incident beam is suddenly terminated, the light trapped within the host begins to leak out, and, eventually, that portion of the electromagnetic (EM) energy which is not absorbed by the host, returns to the surrounding environment.
The so-called leaky modes of a dielectric body are characterized by their unique complex-valued frequency ๐๐๐๐ = ๐๐๐๐โฒ + i๐๐๐๐โณ, where the index ๐๐ identifies individual modes [1-5]. The imaginary part ๐๐๐๐โณ of each such frequency signifies the decay rate of the leaky mode. In the following sections, we analyze the EM structure of the leaky modes of dielectric slabs, spheres, and cylinders, and examine the conditions under which an initial field distribution can be decomposed into a superposition of leaky modes. We also present numerical results where the resonance conditions and quality factors (๐๐-factor = |๐๐๐๐โฒ | |๐๐๐๐โณ|โ ) of certain cavities are computed; the correspondence between these and the leaky-mode frequencies is subsequently explored.
The present paperโs contribution to the mathematics of open systems is a completeness proof for leaky modes of dispersive media under certain special circumstances. These include the cases of (i) a dielectric slab initially illuminated at normal incidence, (ii) a solid dielectric sphere under arbitrary illumination, and (iii) a solid dielectric cylinder illuminated perpendicular to the cylinder axis. Our completeness proof, while relatively simple, is self-contained in the sense that it does not rely on any general theorems as is the case, for instance, with quantum mechanical proofs of completeness that rely on the completeness of the Hamiltonian eigenstates.
We begin by analyzing in Sec.2 a non-dispersive dielectric slab illuminated at normal incidence. The analysis is then extended in Sec.3 to the case of a dispersive slab, where we introduce a general methodology for proving the completeness of leaky modes under special circumstances. Numerical results showing the connection between the resonances of the slab (when illuminated by a tunable source) and the leaky mode frequencies are presented in Sec.4. The next section describes the leaky modes of a dielectric slab illuminated at oblique incidence.
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Here we find that, although leaky modes exist and can be readily evaluated by numerical means, the proof of completeness encounters a roadblock due to certain mathematical difficulties.
In Sec.6 we discuss the leaky modes of a dispersive dielectric sphere, and demonstrate their completeness for a general class of initial conditions. Numerical results that show the circumstances under which a solid glass sphere resonates with an incident EM field, and also the correspondence between the resonance lines and the leaky-mode frequencies are the subjects of Sec.7. We proceed to extend our methodology to dispersive dielectric cylinders in Sec.8, where we derive the characteristic equation for leaky modes under general circumstances, and provide a completeness proof for these leaky modes in certain special cases where the direction of illumination is perpendicular to the cylinder axis. Numerical results that show strong similarities between the resonances of dielectric cylinders and those of dielectric spheres are presented in Sec.9. The final section provides a summary of the paper followed by concluding remarks.
2. Leaky modes of a parallel-plate dielectric slab. Figure 1 shows a transparent slab of thickness ๐๐ and refractive index ๐๐, placed in contact with a perfect reflector. At first, we assume the dielectric is free from dispersion, so that, across a broad range of frequencies, ๐๐ remains constant. (The analysis will be extended in the following section to cover dispersive media as well.) Inside the slab, a standing wave is initially set up by a normally-incident plane-wave (not shown), which oscillates at a frequency ๐๐0 and is linearly-polarized along the ๐ฅ๐ฅ-axis, having counter-propagating ๐ธ๐ธ-field amplitudes ยฑ๐ธ๐ธ0๐๐๏ฟฝ. The incident beam is abruptly terminated at ๐ก๐ก = 0, causing the field inside the slab to leak out and, eventually, to vanish.
Each leaky mode of this simple cavity may be described in terms of plane-waves having complex frequencies ๐๐๐๐ = ๐๐๐๐โฒ + i๐๐๐๐โณ and corresponding ๐๐-vectors ยฑ(๐๐๐๐๐๐ ๐๐โ )๐๐๏ฟฝ inside the dielectric slab, and (๐๐๐๐ ๐๐โ )๐๐๏ฟฝ outside. Denoting by ยฑ๐ธ๐ธ0๐๐๐๐๏ฟฝ the amplitudes of counter-propagating plane-waves inside the slab, and by ๐ธ๐ธ1๐๐๐๐๏ฟฝ the amplitude of the leaked plane-wave in the free-space region outside, we write expressions for the ๐ฌ๐ฌ and ๐ฏ๐ฏ field distributions of each leaky mode, and proceed to obtain the leaky mode frequencies by matching the boundary conditions at the exit facet of the cavity located at ๐ง๐ง = ๐๐.
When the incident beam is abruptly terminated at ๐ก๐ก = 0, the EM field at ๐ก๐ก > 0 may be described as a superposition of leaky modes. Each leaky mode consists of two counter-propagating plane-waves inside the dielectric slab, and a third plane-wave propagating in the free-space region outside. Considering that the ๐ธ๐ธ-field at the surface of the perfect conductor (located at ๐ง๐ง = 0) must vanish, the ๐ฌ๐ฌ and ๐ฏ๐ฏ fields inside and outside the slab may be written as
๐ฌ๐ฌin(๐๐, ๐ก๐ก) = ๐ธ๐ธ0๐๐๐๐๏ฟฝ exp๏ฟฝi๐๐๐๐(๐๐๐ง๐ง โ ๐๐๐ก๐ก) ๐๐โ ๏ฟฝ โ ๐ธ๐ธ0๐๐๐๐๏ฟฝ exp๏ฟฝโi๐๐๐๐(๐๐๐ง๐ง + ๐๐๐ก๐ก) ๐๐โ ๏ฟฝ, (1a)
๐ฏ๐ฏin(๐๐, ๐ก๐ก) = ๏ฟฝ๐๐๐ธ๐ธ0๐๐ ๐๐0โ ๏ฟฝ๐๐๏ฟฝ exp๏ฟฝi๐๐๐๐(๐๐๐ง๐ง โ ๐๐๐ก๐ก) ๐๐โ ๏ฟฝ + ๏ฟฝ๐๐๐ธ๐ธ0๐๐ ๐๐0โ ๏ฟฝ๐๐๏ฟฝ exp๏ฟฝโi๐๐๐๐(๐๐๐ง๐ง + ๐๐๐ก๐ก) ๐๐โ ๏ฟฝ. (1b)
๐ฌ๐ฌout(๐๐, ๐ก๐ก) = ๐ธ๐ธ1๐๐๐๐๏ฟฝ exp๏ฟฝi๐๐๐๐(๐ง๐ง โ ๐๐๐ก๐ก) ๐๐โ ๏ฟฝ, (2a)
๐ฏ๐ฏout(๐๐, ๐ก๐ก) = ๏ฟฝ๐ธ๐ธ1๐๐ ๐๐0โ ๏ฟฝ๐๐๏ฟฝ exp๏ฟฝi๐๐๐๐(๐ง๐ง โ ๐๐๐ก๐ก) ๐๐โ ๏ฟฝ. (2b)
z
x
๐ธ๐ธ0๐๐
d
n
Mirr
or
โ๐ธ๐ธ0๐๐
๐ธ๐ธ1๐๐
Fig.1. Dielectric slab of thickness ๐๐ and refractive index ๐๐, coated on its left facet with a perfect reflector. Also shown is a leaky mode, having amplitude ๐ธ๐ธ0๐๐ inside the slab and ๐ธ๐ธ1๐๐ outside.
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Here ๐๐0 = ๏ฟฝ๐๐0 ๐๐0โ is the impedance of free space. It is not difficult to verify that each of the plane-waves appearing in the above equations satisfies Maxwellโs equations for real- as well as complex-valued ๐๐๐๐. In general, the refractive index ๐๐ of the dielectric material should be a function of ๐๐๐๐, although in the present section we are going to assume that the material is free from dispersion. Note that the two plane-waves inside the slab have equal magnitudes but a relative phase of ๐๐, so that the net ๐ธ๐ธ-field at the mirror surface (located at ๐ง๐ง = 0) vanishes. The boundary conditions at the exit facet of the dielectric slab (located at ๐ง๐ง = ๐๐) now yield
๐ธ๐ธ0๐๐ exp๏ฟฝi๐๐๐๐๐๐๐๐ ๐๐โ ๏ฟฝ โ ๐ธ๐ธ0๐๐ exp๏ฟฝโi๐๐๐๐๐๐๐๐ ๐๐โ ๏ฟฝ = ๐ธ๐ธ1๐๐ exp๏ฟฝi๐๐๐๐๐๐ ๐๐โ ๏ฟฝ, (3a)
๐๐๐ธ๐ธ0๐๐ exp๏ฟฝi๐๐๐๐๐๐๐๐ ๐๐โ ๏ฟฝ + ๐๐๐ธ๐ธ0๐๐ exp๏ฟฝโi๐๐๐๐๐๐๐๐ ๐๐โ ๏ฟฝ = ๐ธ๐ธ1๐๐ exp๏ฟฝi๐๐๐๐๐๐ ๐๐โ ๏ฟฝ. (3b)
Dividing Eq.(3a) by Eq.(3b) eliminates both ๐ธ๐ธ0๐๐ and ๐ธ๐ธ1๐๐, yielding the following constraint on acceptable values of ๐๐๐๐: exp๏ฟฝi2๐๐๐๐๐๐๐๐ ๐๐โ ๏ฟฝ = โ(๐๐ + 1) (๐๐ โ 1)โ . (4)
Assuming that ๐๐ > 1, it is clear from Eq.(4) that the imaginary part of ๐๐๐๐ must be negative. Acceptable values of ๐๐๐๐ = ๐๐๐๐โฒ + i๐๐๐๐โณ may now be found from Eq.(4), as follows:
๐๐๐๐โฒ = (2๐๐+1)๐๐๐๐2๐๐๐๐
and ๐๐๐๐โณ = โ๏ฟฝ ๐๐2๐๐๐๐
๏ฟฝ ln ๏ฟฝ๐๐+1๐๐โ1
๏ฟฝ. (5)
The index ๐๐ appearing in the above expression for ๐๐๐๐โฒ is an arbitrary integer (zero, positive, or negative), which uniquely identifies individual modes of the leaky cavity. Note that, in the absence of dispersion, the imaginary part of ๐๐๐๐ is independent of the mode number ๐๐; as such we shall henceforth remove the subscript ๐๐ from ๐๐๐๐โณ, and proceed to write it simply as ๐๐โณ. Thus the various modes are seen to have different oscillation frequencies, ๐๐๐๐โฒ , but amplitudes that obey the same temporal decay factor, exp(๐๐โณ๐ก๐ก).
The beam that leaks out of the cavity and into the free-space region ๐ง๐ง > ๐๐, is seen to grow exponentially along the ๐ง๐ง-axis, in accordance with the expression ๐ธ๐ธ1๐๐ exp[โ๐๐โณ(๐ง๐ง โ ๐๐๐ก๐ก) ๐๐โ ], but of course this exponential growth terminates at ๐ง๐ง = ๐๐๐ก๐ก, where the leaked beam meets up with the tail end of the beam that was originally reflected from the front facet of the device (i.e., prior to the abrupt termination of the incident beam at ๐ก๐ก = 0). The EM energy in the region ๐๐ < ๐ง๐ง < ๐๐๐ก๐ก is just the energy that has leaked out of the dielectric slab, with the exponential decrease of the amplitude in time compensating the expansion of the region โilluminatedโ by the leaked beam.
Inside the dielectric slab, where 0 โค ๐ง๐ง โค ๐๐, the individual mode profiles of the EM field may be written as
๐ฌ๐ฌ(๐๐, ๐ก๐ก) = ๐ธ๐ธ0๐๐๐๐๏ฟฝ ๏ฟฝexp๏ฟฝโ(๐๐โณ โ i๐๐๐๐โฒ )๐๐๐ง๐ง ๐๐โ ๏ฟฝ โ exp๏ฟฝ(๐๐โณ โ i๐๐๐๐โฒ )๐๐๐ง๐ง ๐๐โ ๏ฟฝ๏ฟฝ exp[(๐๐โณ โ i๐๐๐๐โฒ )๐ก๐ก], (6a)
๐ฏ๐ฏ(๐๐, ๐ก๐ก) = ๏ฟฝ๐๐๐ธ๐ธ0๐๐ ๐๐0โ ๏ฟฝ๐๐๏ฟฝ ๏ฟฝexp๏ฟฝโ๏ฟฝ๐๐โณ โ i๐๐๐๐โฒ ๏ฟฝ๐๐๐ง๐ง ๐๐โ ๏ฟฝ+ exp๏ฟฝ๏ฟฝ๐๐โณ โ i๐๐๐๐โฒ ๏ฟฝ๐๐๐ง๐ง ๐๐โ ๏ฟฝ๏ฟฝ exp[(๐๐โณ โ i๐๐๐๐โฒ )๐ก๐ก]. (6b)
The EM field residing in the dielectric slab at ๐ก๐ก = 0 may be expressed as a superposition of the leaky modes of Eq.(6), so that each mode would evolve in time, oscillating in accordance with its own phase-factor exp(โi๐๐๐๐โฒ ๐ก๐ก), while declining in magnitude in accordance with the (common) amplitude-decay-factor exp(๐๐โณ๐ก๐ก). Unfolding the modal field profile of Eq.(6) around the ๐ง๐ง = 0 plane, then writing the unfolded field at ๐ก๐ก = 0 as a distribution over the interval โ๐๐ โค ๐ง๐ง โค ๐๐, we will have
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๐ฌ๐ฌ(๐๐, ๐ก๐ก = 0) = ๐ธ๐ธ0๐๐๐๐๏ฟฝ exp๏ฟฝโ(๐๐โณ โ i๐๐๐๐โฒ )๐๐๐ง๐ง ๐๐โ ๏ฟฝ, (7a)
๐ฏ๐ฏ(๐๐, ๐ก๐ก = 0) = ๏ฟฝ๐๐๐ธ๐ธ0๐๐ ๐๐0โ ๏ฟฝ๐๐๏ฟฝ exp๏ฟฝโ๏ฟฝ๐๐โณ โ i๐๐๐๐โฒ ๏ฟฝ๐๐๐ง๐ง ๐๐โ ๏ฟฝ. (7b)
Substitution for ๐๐๐๐โฒ and ๐๐โณ from Eq.(5) into Eq.(7) now yields
๐ฌ๐ฌ(๐๐, ๐ก๐ก = 0) = ๐ธ๐ธ0๐๐๐๐๏ฟฝ [(๐๐ + 1) (๐๐ โ 1)โ ]๐ง๐ง 2๐๐โ exp(i๐๐๐ง๐ง 2๐๐โ ) exp(i๐๐๐๐๐ง๐ง ๐๐โ ), (8a)
๐ฏ๐ฏ(๐๐, ๐ก๐ก = 0) = ๏ฟฝ๐๐๐ธ๐ธ0๐๐ ๐๐0โ ๏ฟฝ๐๐๏ฟฝ [(๐๐ + 1) (๐๐ โ 1)โ ]๐ง๐ง 2๐๐โ exp(i๐๐๐ง๐ง 2๐๐โ ) exp(i๐๐๐๐๐ง๐ง ๐๐โ ). (8b)
It is thus clear that any initial field distribution inside the dielectric slab can be unfolded around the ๐ง๐ง = 0 plane, multiplied by [(๐๐ + 1) (๐๐ โ 1)โ ]โ๐ง๐ง 2๐๐โ exp(โi๐๐๐ง๐ง 2๐๐โ ), then expanded in a Fourier series to create a superposition of the leaky modes given by Eq.(6).
3. Effects of dispersion. Suppose now that the dielectric material is dispersive. The simplest case would involve a medium whose electric and magnetic dipoles behave as single Lorentz oscillators, each having its own resonance frequency ๐๐๐๐, plasma frequency ๐๐๐๐, and damping coefficient ๐พ๐พ. The electric and magnetic susceptibilities of the material will then be given by
๐๐๐๐(๐๐) = ๐๐๐๐๐๐2
๐๐๐๐๐๐2 โ ๐๐2 โ i๐พ๐พ๐๐๐๐
, ๐๐๐๐(๐๐) = ๐๐๐๐๐๐2
๐๐๐๐๐๐2 โ ๐๐2 โ i๐พ๐พ๐๐๐๐
. (9)
The corresponding refractive index, now a function of the frequency ๐๐, will be
๐๐(๐๐) = โ๐๐๐๐ = ๏ฟฝ(1 + ๐๐๐๐)(1 + ๐๐๐๐) = ๏ฟฝ1 +๐๐๐๐๐๐2
๐๐๐๐๐๐2 โ๐๐2โi๐พ๐พ๐๐๐๐
ร ๏ฟฝ1 +๐๐๐๐๐๐2
๐๐๐๐๐๐2 โ๐๐2โi๐พ๐พ๐๐๐๐
= ๏ฟฝ(๐๐โฮฉ1๐๐)(๐๐โฮฉ2๐๐)(๐๐โฮฉ3๐๐)(๐๐โฮฉ4๐๐) ร ๏ฟฝ(๐๐โฮฉ1๐๐)(๐๐โฮฉ2๐๐)
(๐๐โฮฉ3๐๐)(๐๐โฮฉ4๐๐) , (10a)
where ฮฉ1,2 = ยฑ๏ฟฝ๐๐๐๐2 + ๐๐๐๐2 โ ยผ๐พ๐พ2 โ ยฝi๐พ๐พ, (10b)
ฮฉ3,4 = ยฑ๏ฟฝ๐๐๐๐2 โ ยผ๐พ๐พ2 โ ยฝi๐พ๐พ. (10c)
Assuming that ๐พ๐พ โช ๐๐๐๐, the poles and zeros of both ๐๐(๐๐) and ๐๐(๐๐) will be located in the lower-half of the complex ๐๐-plane, as shown in Fig.2. The dashed line-segments in the figure represent branch-cuts that are needed to uniquely specify each square-root function appearing on the right-hand side of Eq.(10a). For the sake of simplicity, we shall further assume that the branch-cuts of โ๐๐ and those of โ๐๐ do not overlap. Whenever ๐๐ crosses (i.e., goes from immediately above to immediately below) one of these four branch-cuts, the refractive index ๐๐(๐๐) is multiplied by โ1. Note also that, in the limit when |๐๐| โ โ (along any straight line originating at ๐๐ = 0), the complex entities ๐๐(๐๐), ๐๐(๐๐), and the refractive index ๐๐(๐๐) will all approach 1.0, while 1 โ ๐๐2(๐๐) approaches (๐๐๐๐๐๐2 + ๐๐๐๐๐๐2 ) ๐๐2โ .
Now, with reference to Fig.1, consider a plane-wave of frequency ๐๐๐๐ and amplitude ๐ธ๐ธ1๐๐ propagating along the ๐ง๐ง-axis in the free-space region outside the cavity, while the EM field inside the cavity is given by ๐ฌ๐ฌ(๐ง๐ง, ๐ก๐ก) = 2i๐ธ๐ธ0๐๐๐๐๏ฟฝ sin[๐๐(๐๐๐๐)๐๐๐๐๐ง๐ง ๐๐โ ] exp(โi๐๐๐๐๐ก๐ก), (11a)
๐ฏ๐ฏ(๐ง๐ง, ๐ก๐ก) = 2๐๐0โ1[๐๐(๐๐๐๐) ๐๐(๐๐๐๐)โ ]๐ธ๐ธ0๐๐๐๐๏ฟฝ cos[๐๐(๐๐๐๐)๐๐๐๐๐ง๐ง ๐๐โ ] exp(โi๐๐๐๐๐ก๐ก). (11b)
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In the absence of an incident beam, the matching of boundary conditions at ๐ง๐ง = ๐๐ yields
2i๐ธ๐ธ0๐๐ sin[๐๐(๐๐๐๐)๐๐๐๐๐๐ ๐๐โ ] = ๐ธ๐ธ1๐๐exp (i๐๐๐๐๐๐ ๐๐โ ), (12a)
2๐๐0โ1[๐๐(๐๐๐๐) ๐๐(๐๐๐๐)โ ]๐ธ๐ธ0๐๐ cos[๐๐(๐๐๐๐)๐๐๐๐๐๐ ๐๐โ ] = ๐๐0โ1๐ธ๐ธ1๐๐exp (i๐๐๐๐๐๐ ๐๐โ ). (12b)
The above equations are simultaneously satisfied if and only if ๐๐๐๐ happens to be a zero of the following function: ๐น๐น(๐๐) = ๐๐(๐๐) cos[๐๐(๐๐)๐๐๐๐ ๐๐โ ] โ i๐๐(๐๐) sin[๐๐(๐๐)๐๐๐๐ ๐๐โ ]. (13)
We expect the zeros ๐๐๐๐ of ๐น๐น(๐๐) to be confined to the lower-half of the complex ๐๐-plane, because, when the incident beam is set to zero, the time-dependence factor exp(โi๐๐๐๐๐ก๐ก) of the corresponding leaky modes inside the cavity can only decrease with time. Also, considering that ๐๐(โ๐๐๐๐โ) = ๐๐โ(๐๐๐๐) and ๐๐(โ๐๐๐๐โ) = ๐๐โ(๐๐๐๐), the zeros of ๐น๐น(๐๐) always appear in pairs such as ๐๐๐๐ and โ๐๐๐๐โ . Trivial leaky modes occur at ๐๐๐๐ = ฮฉ1๐๐ and ฮฉ1๐๐ (with their twins occurring at โ๐๐๐๐โ = ฮฉ2๐๐ and ฮฉ2๐๐), where ๐๐(ฮฉ1,2) = 0. Substitution into Eq.(11) reveals that, for these trivial leaky modes, which are associated with the zeros of the refractive index ๐๐(๐๐), both ๐ธ๐ธ and ๐ป๐ป fields inside and outside the cavity vanish. Finally, with reference to the complex ๐๐-plane of Fig.2, note that when ๐๐ crosses (i.e., moves from immediately above to immediately below) one of the branch-cuts, ๐๐(๐๐) gets multiplied by โ1, which causes ๐น๐น(๐๐) of Eq.(13) to switch sign.
Fig.2. Locations in the ๐๐-plane of the poles and zeros of ๐๐(๐๐), whose square root contributes to the refractive index ๐๐(๐๐) in accordance with Eq.(10). A similar set of poles and zeros, albeit at different locations in the ๐๐-plane, represents ๐๐(๐๐). The dashed lines connecting pairs of adjacent poles and zeros constitute branch-cuts for the function ๐๐(๐๐). In accordance with the Cauchy-Goursat theorem [6], the integral of a meromorphic function, such as ๐๐(๐๐), over a circle of radius ๐ ๐ ๐๐ is 2๐๐i times the sum of the residues of the function at the poles of ๐๐(๐๐) that reside within the circle.
Our goal is to express an initial field distribution inside the cavity at ๐ก๐ก = ๐ก๐ก0, say,
๐ฌ๐ฌ(๐ง๐ง, ๐ก๐ก0) = 2i๐ธ๐ธ0๐๐๏ฟฝ sin[๐๐(๐๐0)๐๐0๐ง๐ง ๐๐โ ] exp(โi๐๐0๐ก๐ก0), (14a)
๐ฏ๐ฏ(๐ง๐ง, ๐ก๐ก0) = 2๐๐0โ1[๐๐(๐๐0) ๐๐(๐๐0)โ ]๐ธ๐ธ0๐๐๏ฟฝ cos[๐๐(๐๐0)๐๐0๐ง๐ง ๐๐โ ] exp(โi๐๐0๐ก๐ก0) ; (0 โค ๐ง๐ง โค ๐๐), (14b) as a superposition of leaky modes that decay with the passage of time. To this end, we construct the function ๐บ๐บ(๐๐) which incorporates the ๐ธ๐ธ-field profile, namely, sin[๐๐(๐๐)๐๐๐ง๐ง ๐๐โ ], the real-valued frequency ๐๐0 of the initial distribution, and the function ๐น๐น(๐๐) of Eq.(13), as follows:
๐บ๐บ(๐๐) = sin[๐๐(๐๐)๐๐๐ง๐ง ๐๐โ ](๐๐ โ ๐๐0)๐น๐น(๐๐) = sin[๐๐(๐๐)๐๐๐ง๐ง ๐๐โ ]
(๐๐ โ ๐๐0){๐๐(๐๐) cos[๐๐(๐๐)๐๐๐๐ ๐๐โ ]โ i๐๐(๐๐) sin[๐๐(๐๐)๐๐๐๐ ๐๐โ ]}ยท (15)
Note that, when ๐๐ crosses a branch-cut, both the numerator and the denominator of ๐บ๐บ(๐๐) switch signs, thus ensuring that the function as a whole remains free of branch-cuts. Let us now examine the behavior of ๐บ๐บ(๐๐) around a large circle of radius ๐ ๐ ๐๐ centered at the origin of the ๐๐-
ร ร
๐๐โฒ
๐๐โณ
ฮฉ1๐๐ ฮฉ2๐๐ ฮฉ3๐๐ ฮฉ4๐๐
๐ ๐ ๐๐
6
plane, such as that in Fig.2. Since ๐๐(๐๐) โ 1 โ (๐๐๐๐๐๐ ๐๐โ )2 and ๐๐(๐๐) โ 1 โ ยฝ ๏ฟฝ๐๐๐๐๐๐2 + ๐๐๐๐๐๐2 ๏ฟฝ ๐๐2โ everywhere on the circle as ๐ ๐ ๐๐ โ โ, the limit of ๐บ๐บ(๐๐) will be
lim๐ ๐ ๐๐โโ ๐บ๐บ(๐๐) = lim|๐๐|โโexp(i๐๐๐๐๐ง๐ง ๐๐โ ) โ exp(โi๐๐๐๐๐ง๐ง ๐๐โ )
i(๐๐ โ ๐๐0)[(๐๐โ๐๐)exp(i๐๐๐๐๐๐ ๐๐โ ) + (๐๐+๐๐)exp(โi๐๐๐๐๐๐ ๐๐โ )] = 0. (16)
Recognizing that, for all points within the cavity (i.e., ๐ง๐ง < ๐๐), the function ๐บ๐บ(๐๐) approaches zero exponentially as |๐๐| โ โ, we conclude that the integral of ๐บ๐บ(๐๐) over a large circle of radius ๐ ๐ ๐๐ vanishes. The Cauchy-Goursat theorem of complex analysis [6] then ensures that all the residues of ๐บ๐บ(๐๐) in the complex-plane add up to zero, that is,
sin[๐๐(๐๐0)๐๐0๐ง๐ง ๐๐โ ]๐น๐น(๐๐0) + ๏ฟฝ sin[๐๐(๐๐๐๐)๐๐๐๐๐ง๐ง ๐๐โ ]
(๐๐๐๐ โ ๐๐0)๐น๐นโฒ๏ฟฝ๐๐๐๐๏ฟฝ๐๐= 0. (17)
Consequently, the expansion of the initial ๐ธ๐ธ-field profile inside the cavity expressed as a sum over all the leaky modes is given by
sin[๐๐(๐๐0)๐๐0๐ง๐ง ๐๐โ ] = ๏ฟฝ ๐น๐น(๐๐0)(๐๐0 โ ๐๐๐๐)๐น๐นโฒ๏ฟฝ๐๐๐๐๏ฟฝ๐๐
sin[๐๐(๐๐๐๐)๐๐๐๐๐ง๐ง ๐๐โ ]. (18)
A similar method may be used to arrive at an expansion of the initial ๐ป๐ป-field distribution in terms of the same leaky modes as in Eq.(18). In this case, the function ๐บ๐บ(๐๐) must be chosen as
๐บ๐บ(๐๐) = ๐๐(๐๐)cos[๐๐(๐๐)๐๐๐ง๐ง ๐๐โ ](๐๐ โ ๐๐0)๐๐(๐๐)๐น๐น(๐๐) ยท (19)
Once again, since the integral of the above ๐บ๐บ(๐๐) around a large circle of radius ๐ ๐ ๐๐ (centered at the origin) approaches zero, the residues of ๐บ๐บ(๐๐) in the present case also must add up to zero. Consequently, the procedure for expanding the initial ๐ป๐ป-field distribution via Eq.(19) is similar to that used previously to expand the initial ๐ธ๐ธ-field via Eq.(15).
4. Numerical results. Figure 3 shows the ratio of the ๐ธ๐ธ-field amplitude inside a dielectric slab to the incident ๐ธ๐ธ-field, plotted versus the excitation frequency ๐๐ normalized by ๐๐0 = 1.885 ร1015 rad secโ (corresponding to the free space wavelength ๐๐0 = 1.0 ยตm). The 500 nm-thick slab has refractive index ๐๐ = 3.75 + 0.0116i at ๐๐ = ๐๐0, and, as shown in Fig.1, is coated on one of its facets by a perfect reflector. It is assumed that ๐๐(๐๐) = 1.0, and that the permittivity ๐๐(๐๐) follows a single Lorentz oscillator model with resonance frequency ๐๐๐๐ = 4๐๐0, damping coefficient ๐พ๐พ = 0.1๐๐0, and plasma frequency ๐๐๐๐ = 14๐๐0. In the interval between the pole and zero of the refractive index, namely, [ฮฉ3๐๐ ,ฮฉ1๐๐] (see Fig.2), the field amplitude inside the cavity is seen to be vanishingly small. Outside this โforbiddenโ range of frequencies, the field has resonance peaks at specific frequencies, and the ๐ธ๐ธinside ๐ธ๐ธincidentโ ratio between adjacent peaks and valleys can vary by as much as a factor of 4.
The contour plots of Fig.4 show, within the complex ๐๐-plane, the zeros of Re[๐น๐น(๐๐)] in red and the zeros of Im[๐น๐น(๐๐)] in blue. Both the real and imaginary parts of ๐๐ are normalized by the reference
Fig. 3. Ratio of the ๐ธ๐ธ-field amplitude inside a dielectric slab to the incident ๐ธ๐ธ-field, plotted versus normalized frequency ๐๐ ๐๐0โ .
7
frequency ๐๐0. The points where the contours cross each other โ several of them circled in the plot โ represent the zeros of ๐น๐น(๐๐), which we have denoted by ๐๐๐๐ = ๐๐๐๐โฒ + i๐๐๐๐โณ and referred to as leaky-mode frequencies. The resonance peaks seen in Fig.3 occur at or near the frequencies ๐๐ = ๐๐๐๐โฒ of the various leaky modes. The region of the ๐๐-plane depicted in Fig.4(a) contains all the leaky-mode frequencies to the left of ฮฉ3๐๐; a large number of such frequencies are seen to accumulate in the vicinity of ๐๐ = ฮฉ3๐๐, where the coupling of the incident light to the cavity is weak, and the damping within the slab is dominated by absorption losses. The region of the ๐๐-plane depicted in Fig.4(b) contains all the leaky-mode frequencies to the right of ฮฉ1๐๐. The imaginary part ๐๐๐๐โณ of these frequencies acquires large negative values as the real part ๐๐๐๐โฒ of the corresponding leaky frequency increases. No leaky modes reside in the upper-half of the ๐๐-plane, nor are there any leaky modes in the strip between ฮฉ1๐๐ and ฮฉ3๐๐.
Fig. 4. Contour plots in the ๐๐-plane, showing the zeros of Re[๐น๐น(๐๐)] in red and the zeros of Im[๐น๐น(๐๐)] in blue. Both the real and imaginary parts of ๐๐ are normalized by the reference frequency ๐๐0. (a) Region of ๐๐-plane to the left of ฮฉ3๐๐. (b) Region of ๐๐-plane to the right of ฮฉ1๐๐. The points where the contours cross each other (several of them circled) are the zeros of ๐น๐น(๐๐).
A comparison of Figs.3 and 4 reveals the close relationship between the leaky mode frequencies and the resonances of the dielectric slab. Resonances occur at or near the frequencies ๐๐ = ๐๐๐๐โฒ , and the height and width of a resonance line are, by and large, determined by the decay rate ๐๐๐๐โณ of the corresponding leaky mode โ unless the leaky mode frequency happens to be so close to the pole(s) of the refractive index ๐๐(๐๐) that the strong absorption within the medium would suppress the resonance. It must be emphasized that the presence of a gap in the frequency domain (such as that between ๐๐ = ฮฉ3๐๐ and ๐๐ = ๐บ๐บ1๐๐ in the present example) should not prevent the leaky modes from forming a basis, because, as an ensemble, the leaky modes are expected (on physical grounds) to carry all the spatial frequencies needed to capture the various features of arbitrary initial ๐ธ๐ธ-field and ๐ป๐ป-field distributions.
5. Leaky modes propagating at oblique angle relative to the surface normal. The diagram in Fig.5 shows a leaky mode of a dielectric slab, whose ๐๐-vector has a component ๐๐๐ฅ๐ฅ along the ๐ฅ๐ฅ-axis. Here ๐๐๐ฅ๐ฅ is assumed to be a real-valued and positive constant. Although the following discussion is confined to the case of transverse magnetic (TM) polarization, the analysis is straightforward and can be readily extended to the case of transverse electric (TE) polarization as well. Inside the slab depicted in Fig.5, ๐๐๐ง๐ง0 = ๏ฟฝ๐๐๐๐(๐๐ ๐๐โ )2 โ ๐๐๐ฅ๐ฅ2 while the ๐ฏ๐ฏ and ๐ฌ๐ฌ field amplitudes are ๐ป๐ป0๐๐๐๐๏ฟฝ and ๐ฌ๐ฌ0๐๐ = โ(๐๐๐ฅ๐ฅ๐๐๏ฟฝ ยฑ ๐๐๐ง๐ง0๐๐๏ฟฝ) ร ๐ป๐ป0๐๐๐๐๏ฟฝ (๐๐0๐๐๐๐)โ , respectively. The total EM field inside the slab is given by
๐ฏ๐ฏin(๐๐, ๐ก๐ก) = ๐ป๐ป0๐๐๏ฟฝ{exp[i(๐๐๐ฅ๐ฅ๐ฅ๐ฅ โ ๐๐๐ง๐ง0๐ง๐ง)] + exp[i(๐๐๐ฅ๐ฅ๐ฅ๐ฅ + ๐๐๐ง๐ง0๐ง๐ง)]} exp(โi๐๐๐ก๐ก). (20a)
8
Fig. 5. Dielectric slab of thickness ๐๐ and refractive index ๐๐(๐๐) = โ๐๐๐๐, supporting a TM polarized leaky mode. Inside the slab, the ๐๐-vectors of the counter-propagating plane-waves are ๐๐๐ฅ๐ฅ๐๐๏ฟฝ ยฑ ๐๐๐ง๐ง0๐๐๏ฟฝ, while their field amplitudes are ๐ฏ๐ฏ = ๐ป๐ป0๐๐๐๐๏ฟฝ and ๐ฌ๐ฌ = (ยฑ๐๐๐ง๐ง0๐๐๏ฟฝ โ ๐๐๐ฅ๐ฅ๐๐๏ฟฝ)๐ป๐ป0๐๐ (๐๐0๐๐๐๐)โ . The plane-wave that leaves the slab resides in free space; its ๐๐-vector is ๐๐๐ฅ๐ฅ๐๐๏ฟฝ + ๐๐๐ง๐ง1๐๐๏ฟฝ, and its field amplitudes are ๐ฏ๐ฏ = ๐ป๐ป1๐๐๐๐๏ฟฝ and ๐ฌ๐ฌ = (๐๐๐ง๐ง1๐๐๏ฟฝ โ ๐๐๐ฅ๐ฅ๐๐๏ฟฝ)๐ป๐ป1๐๐ (๐๐0๐๐)โ .
๐ฌ๐ฌin(๐๐, ๐ก๐ก) = [๐ป๐ป0 ๐๐0๐๐(๐๐)๐๐โ ]{โ(๐๐๐ง๐ง0๐๐๏ฟฝ + ๐๐๐ฅ๐ฅ๐๐๏ฟฝ) exp[i(๐๐๐ฅ๐ฅ๐ฅ๐ฅ โ ๐๐๐ง๐ง0๐ง๐ง)]
+(๐๐๐ง๐ง0๐๐๏ฟฝ โ ๐๐๐ฅ๐ฅ๐๐๏ฟฝ) exp[i(๐๐๐ฅ๐ฅ๐ฅ๐ฅ + ๐๐๐ง๐ง0๐ง๐ง)]} exp(โi๐๐๐ก๐ก). (20b)
Outside the slab, ๐๐๐ง๐ง1 = ๏ฟฝ(๐๐ ๐๐โ )2 โ ๐๐๐ฅ๐ฅ2, ๐ฏ๐ฏ1๐๐ = ๐ป๐ป1๐๐๐๐๏ฟฝ and ๐ฌ๐ฌ1๐๐ = โ(๐๐๐ฅ๐ฅ๐๐๏ฟฝ + ๐๐๐ง๐ง1๐๐๏ฟฝ) ร ๐ป๐ป1๐๐๐๐๏ฟฝ (๐๐0๐๐)โ . The total EM field in the free-space region outside the slab, where ๐ง๐ง > ๐๐, is thus given by
๐ฏ๐ฏout(๐๐, ๐ก๐ก) = ๐ป๐ป1๐๐๏ฟฝ exp[i(๐๐๐ฅ๐ฅ๐ฅ๐ฅ + ๐๐๐ง๐ง1๐ง๐ง)] exp(โi๐๐๐ก๐ก). (21a)
๐ฌ๐ฌout(๐๐, ๐ก๐ก) = (๐ป๐ป1 ๐๐0๐๐โ )(๐๐๐ง๐ง1๐๐๏ฟฝ โ ๐๐๐ฅ๐ฅ๐๐๏ฟฝ) exp[i(๐๐๐ฅ๐ฅ๐ฅ๐ฅ + ๐๐๐ง๐ง1๐ง๐ง)] exp(โi๐๐๐ก๐ก). (21b)
Matching the boundary conditions at ๐ง๐ง = ๐๐, we find
๐ป๐ป0[exp(โi๐๐๐ง๐ง0๐๐) + exp(i๐๐๐ง๐ง0๐๐)] = ๐ป๐ป1 exp(i๐๐๐ง๐ง1๐๐). (22a)
๐๐๐ง๐ง0๐ป๐ป0[โ exp(โi๐๐๐ง๐ง0๐๐) + exp(i๐๐๐ง๐ง0๐๐)] = ๐๐(๐๐)๐๐๐ง๐ง1๐ป๐ป1 exp(i๐๐๐ง๐ง1๐๐). (22b)
The above equations are simultaneously satisfied when the frequency ๐๐ satisfies the following characteristic equation:
๐น๐น(๐๐) = ๐๐(๐๐)๏ฟฝ(๐๐ ๐๐โ )2 โ ๐๐๐ฅ๐ฅ2 cos๏ฟฝ๏ฟฝ๐๐๐๐(๐๐ ๐๐โ )2 โ ๐๐๐ฅ๐ฅ2 ๐๐๏ฟฝ
โi๏ฟฝ๐๐๐๐(๐๐ ๐๐โ )2 โ ๐๐๐ฅ๐ฅ2 sin๏ฟฝ๏ฟฝ๐๐๐๐(๐๐ ๐๐โ )2 โ ๐๐๐ฅ๐ฅ2 ๐๐๏ฟฝ = 0. (23)
As before, the zeros of ๐น๐น(๐๐) represent the leaky-mode frequencies associated with initial conditions whose dependence on the ๐ฅ๐ฅ-coordinate is given by the phase-factor exp(i๐๐๐ฅ๐ฅ๐ฅ๐ฅ). In the ๐๐-plane, the function ๐น๐น(๐๐) of Eq.(23) has a branch-cut on the real-axis between the zeros of ๐๐๐ง๐ง1, i.e., ยฑ๐๐๐๐๐ฅ๐ฅ. It also has other branch-cuts associated with the poles and zeros of ๐๐๐ง๐ง0, as was the case for normal incidence discussed in Sec.3. The branch-cut on the real-axis is troublesome, as it cannot be easily eliminated in order to render ๐น๐น(๐๐) analytic. While, on physical grounds, we believe that an arbitrary initial distribution can still be expressed as a superposition of leaky modes whose frequencies are the roots of Eq.(23), the aforementioned branch-cut residing on the real-axis in the ๐๐-plane prevents us from proving the completeness of such leaky modes. We must remain content with the fact that the computed roots of Eq.(23) reside in the lower-half of the ๐๐-plane, and that these roots have properties that are expected of leaky modes whose ๐ฅ๐ฅ-dependence is given by exp(i๐๐๐ฅ๐ฅ๐ฅ๐ฅ). The quest for a completeness proof, however, must continue, and the methods of the preceding sections, which worked so well in the case of normal incidence on a dielectric slab, must somehow be extended to encompass the case of oblique incidence.
z
x
๐ธ๐ธ0๐๐
d
n
Mirr
or
๐ธ๐ธ0๐๐
๐ธ๐ธ1๐๐
๐ป๐ป1๐๐ ๐ป๐ป0๐๐
9
6. Leaky modes of a solid dielectric sphere. The vector spherical harmonics of the EM field within a homogeneous, isotropic, linear medium having permeability ๐๐0๐๐(๐๐) and permittivity ๐๐0๐๐(๐๐) are found by solving Maxwellโs equations in spherical coordinates [7]. The electric and magnetic field profiles for Transverse Electric (TE) and Transverse Magnetic (TM) modes of the EM field are found to beโ
๐๐ = 0 TE mode (๐ธ๐ธ๐๐ = 0):
๐ฌ๐ฌ(๐๐, ๐ก๐ก) = ๐ธ๐ธ0๐ฝ๐ฝโ+ยฝ(๐๐๐๐)โ๐๐๐๐
๐๐โ1(cos๐๐) exp(โi๐๐๐ก๐ก)๐๐๏ฟฝ . (24)
๐ฏ๐ฏ(๐๐, ๐ก๐ก) = ๐ธ๐ธ0๐๐0๐๐(๐๐)๐๐๐๐
๏ฟฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)iโ๐๐๐๐
[cot ๐๐ ๐๐โ1(cos๐๐) โ sin ๐๐ ๏ฟฝฬ๏ฟฝ๐โ1(cos๐๐)]๐๐๏ฟฝ
โ๐๐๐๐๐ฝ๐ฝโฬ+ยฝ(๐๐๐๐)+ยฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)iโ๐๐๐๐
๐๐โ1(cos๐๐)๐ฝ๐ฝ๏ฟฝ๏ฟฝ exp(โi๐๐๐ก๐ก). (25)
๐๐ โ 0 TE mode (๐ธ๐ธ๐๐ = 0):
๐ฌ๐ฌ(๐๐, ๐ก๐ก) = ๐ธ๐ธ0 ๏ฟฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)โ๐๐๐๐
๐๐โ๐๐(cos๐๐)sin๐๐
๐ฝ๐ฝ๏ฟฝ + ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)i๐๐โ๐๐๐๐
sin๐๐ ๏ฟฝฬ๏ฟฝ๐โ๐๐(cos๐๐)๐๐๏ฟฝ๏ฟฝ exp[i(๐๐๐๐ โ ๐๐๐ก๐ก)]. (26)
๐ฏ๐ฏ(๐๐, ๐ก๐ก) = โ ๐ธ๐ธ0๐๐0๐๐(๐๐)๐๐๐๐
๏ฟฝโ(โ+1)๐ฝ๐ฝโ+ยฝ(๐๐๐๐)๐๐โ๐๐๐๐
๐๐โ๐๐(cos ๐๐)๐๐๏ฟฝ โ ๐๐๐๐๐ฝ๐ฝโฬ+ยฝ(๐๐๐๐)+ยฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)๐๐โ๐๐๐๐
sin๐๐ ๏ฟฝฬ๏ฟฝ๐โ๐๐(cos ๐๐)๐ฝ๐ฝ๏ฟฝ
โ๐๐๐๐๐ฝ๐ฝโฬ+ยฝ(๐๐๐๐)+ยฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)iโ๐๐๐๐
๐๐โ๐๐(cos๐๐)sin๐๐
๐๐๏ฟฝ๏ฟฝ exp[i(๐๐๐๐ โ ๐๐๐ก๐ก)]. (27)
๐๐ = 0 TM mode (๐ป๐ป๐๐ = 0):
๐ฌ๐ฌ(๐๐, ๐ก๐ก) = โ ๐ป๐ป0๐๐0๐๐(๐๐)๐๐๐๐
๏ฟฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)iโ๐๐๐๐
๏ฟฝcot ๐๐ ๐๐โ1(cos๐๐) โ sin๐๐ ๏ฟฝฬ๏ฟฝ๐โ1(cos๐๐)๏ฟฝ๐๐๏ฟฝ
โ๐๐๐๐๐ฝ๐ฝโฬ+ยฝ(๐๐๐๐)+ยฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)iโ๐๐๐๐
๐๐โ1(cos๐๐)๐ฝ๐ฝ๏ฟฝ๏ฟฝ exp(โi๐๐๐ก๐ก). (28)
๐ฏ๐ฏ(๐๐, ๐ก๐ก) = ๐ป๐ป0๐ฝ๐ฝโ+ยฝ(๐๐๐๐)โ๐๐๐๐
๐๐โ1(cos ๐๐) exp(โi๐๐๐ก๐ก)๐๐๏ฟฝ . (29)
๐๐ โ 0 TM mode (๐ป๐ป๐๐ = 0):
๐ฌ๐ฌ(๐๐, ๐ก๐ก) = ๐ป๐ป0๐๐0๐๐(๐๐)๐๐๐๐
๏ฟฝโ(โ+1)๐ฝ๐ฝโ+ยฝ(๐๐๐๐)๐๐โ๐๐๐๐
๐๐โ๐๐(cos๐๐)๐๐๏ฟฝ โ ๐๐๐๐๐ฝ๐ฝโฬ+ยฝ(๐๐๐๐)+ยฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)๐๐โ๐๐๐๐
sin๐๐ ๏ฟฝฬ๏ฟฝ๐โ๐๐(cos๐๐)๐ฝ๐ฝ๏ฟฝ
โ๐๐๐๐๐ฝ๐ฝโฬ+ยฝ(๐๐๐๐)+ยฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)iโ๐๐๐๐
๐๐โ๐๐(cos๐๐)sin๐๐
๐๐๏ฟฝ๏ฟฝ exp[i(๐๐๐๐ โ ๐๐๐ก๐ก)]. (30)
๐ฏ๐ฏ(๐๐, ๐ก๐ก) = ๐ป๐ป0 ๏ฟฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)โ๐๐๐๐
๐๐โ๐๐(cos๐๐)sin๐๐
๐ฝ๐ฝ๏ฟฝ + ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)i๐๐โ๐๐๐๐
sin๐๐ ๏ฟฝฬ๏ฟฝ๐โ๐๐(cos ๐๐)๐๐๏ฟฝ๏ฟฝ exp[i(๐๐๐๐ โ ๐๐๐ก๐ก)]. (31)
In the above equations, the Bessel function ๐ฝ๐ฝ๐๐(๐ง๐ง) and its derivative with respect to ๐ง๐ง, ๐ฝ๐ฝ๏ฟฝฬ๏ฟฝ๐(๐ง๐ง), could be replaced by a Bessel function of the second kind, ๐๐๐๐(๐ง๐ง), and its derivative, ๏ฟฝฬ๏ฟฝ๐๐๐(๐ง๐ง), or by Hankel functions of type 1 or type 2, namely, โ๐๐
(1,2)(๐ง๐ง), and corresponding derivatives โฬ๐๐(1,2)(๐ง๐ง).
โ For a given ๐๐, the TM mode may be obtained from the corresponding TE mode by substituting ๐ฌ๐ฌ for ๐ฏ๐ฏ, and โ๐ฏ๐ฏ for ๐ฌ๐ฌ, keeping in mind that ๐๐๐๐ = ๐๐๐๐ ๏ฟฝ๐๐0๐๐0๐๐(๐๐)๐๐(๐๐)โ , and that the ๐ธ๐ธ ๐ป๐ปโ amplitude ratio for each mode is always given by ๏ฟฝ๐๐0๐๐(๐๐) ๐๐0๐๐(๐๐)โ .
10
The (complex) field amplitudes are denoted by ๐ธ๐ธ0 and ๐ป๐ป0. In our spherical coordinate system, the point ๐๐ is at a distance ๐๐ from the origin, its polar and azimuthal angles being ๐๐ and ๐๐. The oscillation frequency is ๐๐, and the wave-number ๐๐ is defined as ๐๐(๐๐) = ๐๐(๐๐)๐๐0, where ๐๐0 = ๐๐ ๐๐โ , and ๐๐(๐๐) = ๏ฟฝ๐๐(๐๐)๐๐(๐๐) is the refractive index of the host medium. The integers โ โฅ 1, and ๐๐ (ranging from โโ to +โ) specify the polar and azimuthal mode numbers. ๐๐โ๐๐(๐๐) is an associated Legendre function, while ๏ฟฝฬ๏ฟฝ๐โ๐๐(๐๐) is its derivative with respect to ๐๐. Finally, the various Bessel functions of half-integer order are defined by the following formulas [8]:
๐ฝ๐ฝโ+ยฝ(๐ง๐ง) = ๏ฟฝ 2๐๐๐ง๐ง๏ฟฝsin(๐ง๐ง โยฝโ๐๐)๏ฟฝ (โ1)๐๐(โ+2๐๐)!
(2๐๐)!(โโ2๐๐)!๏ฟฝ 12๐ง๐ง๏ฟฝ2๐๐
โโ 2โ โ
๐๐=0
+ cos(๐ง๐ง โยฝโ๐๐)๏ฟฝ (โ1)๐๐(โ+2๐๐+1)!(2๐๐+1)!(โโ2๐๐โ1)!
๏ฟฝ 12๐ง๐ง๏ฟฝ2๐๐+1
โ(โโ1) 2โ โ
๐๐=0
๏ฟฝ. (32)
๐๐โ+ยฝ(๐ง๐ง) = (โ1)โโ1๏ฟฝ 2๐๐๐ง๐ง๏ฟฝcos(๐ง๐ง + ยฝโ๐๐)๏ฟฝ (โ1)๐๐(โ+2๐๐)!
(2๐๐)!(โโ2๐๐)!๏ฟฝ 12๐ง๐ง๏ฟฝ2๐๐
โโ 2โ โ
๐๐=0
โ sin(๐ง๐ง + ยฝโ๐๐)๏ฟฝ (โ1)๐๐(โ+2๐๐+1)!(2๐๐+1)!(โโ2๐๐โ1)!
๏ฟฝ 12๐ง๐ง๏ฟฝ2๐๐+1
โ(โโ1) 2โ โ
๐๐=0
๏ฟฝ. (33)
โโ+ยฝ(1) (๐ง๐ง) = ๏ฟฝ 2
๐๐๐ง๐งexp{i[๐ง๐ง โ ยฝ(โ + 1)๐๐]}๏ฟฝ (โ+๐๐)!
๐๐!(โโ๐๐)!๏ฟฝ i2๐ง๐ง๏ฟฝ๐๐
โ
๐๐=0
. (34)
Note that โ๐ง๐ง๐ฝ๐ฝโ+ยฝ(๐ง๐ง) is an even function of ๐ง๐ง when โ = 1, 3, 5,โฏ, and an odd function when โ = 2, 4, 6,โฏ. This fact will be needed later on, when we try to argue that certain branch-cuts in the complex ๐๐-plane are inconsequential.
Consider now a solid dielectric sphere of radius ๐ ๐ , relative permeability ๐๐(๐๐), and relative permittivity ๐๐(๐๐). Inside the particle, the radial dependence of the TE mode is governed by a Bessel function of the first kind, ๐ธ๐ธ0 ๐ฝ๐ฝโ+ยฝ(๐๐๐๐), and its derivative. The refractive index of the spherical particle being ๐๐(๐๐) = ๏ฟฝ๐๐(๐๐)๐๐(๐๐), the corresponding wave-number inside the particle is ๐๐(๐๐) = ๐๐(๐๐)๐๐0 = ๐๐(๐๐)๐๐ ๐๐โ . The particle is surrounded by free space, which is host to an outgoing spherical harmonic whose radial dependence is governed by a type 1 Hankel function, ๐ธ๐ธ1โโ+ยฝ
(1) (๐๐0๐๐), and its derivative. Invoking the Bessel function identity ๐ง๐ง๐ฝ๐ฝ๏ฟฝฬ๏ฟฝ๐(๐ง๐ง) = ๐๐๐ฝ๐ฝ๐๐(๐ง๐ง) โ๐ง๐ง๐ฝ๐ฝ๐๐+1(๐ง๐ง) โ which applies to ๐๐๐๐(๐ง๐ง) and โ๐๐
(1,2)(๐ง๐ง) as wellโ we find, upon matching the boundary conditions at ๐๐ = ๐ ๐ , that the following two equations must be simultaneously satisfied:
๐ธ๐ธ0๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ )๏ฟฝ๐๐๐๐0๐ ๐
= ๐ธ๐ธ1โโ+ยฝ(1) (๐๐0๐ ๐ )
๏ฟฝ๐๐0๐ ๐ , (35a)
๐ธ๐ธ0[(โ+1)๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ ) โ ๐๐๐๐0๐ ๐ ๐ฝ๐ฝโ+3 2โ (๐๐๐๐0๐ ๐ )]
๐๐(๐๐)๏ฟฝ๐๐๐๐0๐ ๐ =
๐ธ๐ธ1[(โ+1)โโ+ยฝ(1) (๐๐0๐ ๐ ) โ ๐๐0๐ ๐ โโ+3 2โ
(1) (๐๐0๐ ๐ )]
๏ฟฝ๐๐0๐ ๐ ยท (35b)
G&R 8.466-1
G&R 8.461-1
G&R 8.461-2 G&R 8.465-1
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Streamlining the above equations, we arrive at
๏ฟฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ ) โโ๐๐โโ+ยฝ
(1) (๐๐0๐ ๐ )
(โ + 1)๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ ) โ ๐๐๐๐0๐ ๐ ๐ฝ๐ฝโ+3 2โ (๐๐๐๐0๐ ๐ ) โ๐๐โ๐๐๏ฟฝ(โ + 1)โโ+ยฝ(1) (๐๐0๐ ๐ ) โ ๐๐0๐ ๐ โโ+3 2โ
(1) (๐๐0๐ ๐ )๏ฟฝ๏ฟฝ ๏ฟฝ๐ธ๐ธ0
๐ธ๐ธ1๏ฟฝ = 0. (36)
A non-trivial solution for ๐ธ๐ธ0 and ๐ธ๐ธ1 thus exists if and only if the determinant of the coefficient matrix in Eq.(36) vanishes, that is,
๐น๐น(๐๐) = ๐๐๐๐0๐ ๐ โโ+ยฝ(1) (๐๐0๐ ๐ )๐ฝ๐ฝโ+3 2โ (๐๐๐๐0๐ ๐ ) + [(๐๐ โ 1)(โ + 1)โโ+ยฝ
(1) (๐๐0๐ ๐ ) โ ๐๐๐๐0๐ ๐ โโ+3 2โ(1) (๐๐0๐ ๐ )] ๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ ) = 0. (37)
This is the characteristic equation for leaky TE modes, whose solutions comprise the entire set of leaky frequencies ๐๐๐๐. The index ๐๐ is used here to enumerate the various leaky-mode frequencies. For TM modes, ๐๐(๐๐) in Eq.(37) must be replaced by ๐๐(๐๐). Equation (37) must be solved numerically for complex frequencies ๐๐๐๐; these being characteristic frequencies of the particleโs leaky modes, one expects (on physical grounds) to find all the roots ๐๐๐๐ of ๐น๐น(๐๐) in the lower-half of the complex plane. Note that โ๐๐๐น๐น(๐๐) is an even function of ๐๐ when โ = 1, 3, 5,โฏ, and an odd function when โ = 2, 4, 6,โฏ. This is because successive Bessel functions ๐ฝ๐ฝโ+ยฝ and ๐ฝ๐ฝโ+3 2โ alternate between odd and even parities. Note also that ๐น๐น(๐๐) vanishes at the zeros of ๐๐(๐๐), that is, ๐น๐น(ฮฉ1) = ๐น๐น(ฮฉ2) = 0. Finally, when ๐๐ โ 0, ๐น๐น(๐๐) approaches a constant (see the Appendix), and when |๐๐| โ โ, ๐๐(๐๐) โ 1 โ (๐๐๐๐๐๐ ๐๐โ )2 and ๐๐(๐๐) โ 1 โ (๐๐๐๐๐๐ ๐๐โ )2, thus allowing the asymptotic behavior of ๐น๐น(๐๐) to be determined from Eqs.(32) and (34).
Our goal is to express an initial field distribution inside the particle (e.g., one of the spherical harmonic waveforms given by Eqs.(24)-(31), which oscillate at a real-valued frequency ๐๐0) as a superposition of leaky modes, each having its own complex frequency ๐๐๐๐. To this end, we must form a meromorphic function ๐บ๐บ(๐๐) incorporating the following attributes:
i) The function ๐น๐น(๐๐) of Eq.(37) appears in the denominator of ๐บ๐บ(๐๐), thus causing the zeros of ๐น๐น(๐๐) to act as poles for ๐บ๐บ(๐๐).
ii) A desired initial waveform, say, ๐ฝ๐ฝโ+ยฝ[๐๐๐๐(๐๐)๐๐ ๐๐โ ], appearing in the numerator of ๐บ๐บ(๐๐). iii) The real-valued frequency ๐๐0 associated with the initial waveform acting as a pole for ๐บ๐บ(๐๐). iv) In the limit when |๐๐| โ โ, ๐บ๐บ(๐๐) โ 0 exponentially, so that โฎ๐บ๐บ(๐๐)๐๐๐๐ over a circle of large
radius ๐ ๐ ๐๐ vanishes.
A simple (although by no means the only) such function is
๐บ๐บ(๐๐) = ๐๐3 2โ exp(i๐ ๐ ๐๐ ๐๐โ )๐ฝ๐ฝโ+ยฝ(๐๐๐๐)(๐๐โ๐๐0)๐น๐น(๐๐) ยท (38)
With reference to Eq.(32), note that the pre-factor 1 โ๐๐โ of the Bessel function in the numerator of ๐บ๐บ(๐๐) cancels the corresponding pre-factor that accompanies the denominator. The remaining part of the Bessel function in the numerator will then have the same parity with respect to ๐๐(๐๐) as the function that appears in the denominator. Consequently, switching the sign of ๐๐(๐๐) does not alter ๐บ๐บ(๐๐), indicating that the branch-cuts associated with ๐๐(๐๐) in the complex ๐๐-plane do not introduce discontinuities into ๐บ๐บ(๐๐). The presence of โ๐๐ exp(i๐ ๐ ๐๐ ๐๐โ ) in the numerator of ๐บ๐บ(๐๐) is intended to eliminate the undesirable features of the Hankel functions appearing in the denominator. The function ๐บ๐บ(๐๐) is thus analytic everywhere except at the poles,
12
where its denominator vanishes. The poles, of course, consist of ๐๐ = ๐๐0, which is the frequency of the initial EM field residing inside the spherical particle at ๐ก๐ก = 0, and ๐๐ = ๐๐๐๐, which are the leaky-mode frequencies found by solving Eq.(37) โ or its TM mode counterpart.
In the limit |๐๐| โ โ, where ๐๐(๐๐) โ 1 โ (๐๐๐๐๐๐ ๐๐โ )2 and ๐๐(๐๐) โ 1 โ (๐๐๐๐๐๐ ๐๐โ )2, we find that ๐บ๐บ(๐๐) approaches zero exponentially. Thus, the vanishing of โฎ๐บ๐บ(๐๐)๐๐๐๐ around a circle of large radius ๐ ๐ ๐๐ means that all the residues of ๐บ๐บ(๐๐) must add up to zero, that is,
๐๐03 2โ exp(i๐ ๐ ๐๐0 ๐๐โ )๐ฝ๐ฝโ+ยฝ[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ]
๐น๐น(๐๐0) + โ ๐๐๐๐3 2โ exp(i๐ ๐ ๐๐๐๐ ๐๐โ )๐ฝ๐ฝโ+ยฝ[๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ ]
(๐๐๐๐โ ๐๐0)๐น๐นโฒ๏ฟฝ๐๐๐๐๏ฟฝ๐๐ = 0. (39)
The initial field distribution ๐ฝ๐ฝโ+ยฝ[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ] may thus be expanded as the following superposition of all the leaky modes:
๐ฝ๐ฝโ+ยฝ[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ] = โ ๐๐๐๐3 2โ exp[i๐ ๐ (๐๐๐๐โ ๐๐0) ๐๐โ ]๐น๐น(๐๐0)
๐๐03 2โ ๏ฟฝ๐๐0โ ๐๐๐๐๏ฟฝ๐น๐นโฒ๏ฟฝ๐๐๐๐๏ฟฝ
ร๐๐ ๐ฝ๐ฝโ+ยฝ[๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ ]. (40)
To incorporate into the initial distribution the denominator โ๐๐๐๐, which accompanies all the field components in Eqs.(24)-(31), we modify Eq.(40) โ albeit triviallyโ as follows:
๐ฝ๐ฝโ+ยฝ[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ]๏ฟฝ๐๐0๐๐(๐๐0)๐๐ ๐๐โ
= โ๐๐๐๐3 2โ ๏ฟฝ๐๐๐๐๐๐(๐๐๐๐)exp๏ฟฝi๐ ๐ ๏ฟฝ๐๐๐๐โ ๐๐0๏ฟฝ ๐๐โ ๏ฟฝ๐น๐น(๐๐0)
๐๐03 2โ ๏ฟฝ๐๐0๐๐(๐๐0)๏ฟฝ๐๐0โ ๐๐๐๐๏ฟฝ๐น๐นโฒ๏ฟฝ๐๐๐๐๏ฟฝ
๐๐ ร ๐ฝ๐ฝโ+ยฝ[๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ ]
๏ฟฝ๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โยท (41)
Taking advantage of the flexibility of ๐บ๐บ(๐๐), we now extend the same treatment to the remaining components of the EM field. For instance, if we choose
๐บ๐บ(๐๐) = โ๐๐exp(i๐ ๐ ๐๐ ๐๐โ ) ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)(๐๐โ๐๐0)๐๐(๐๐)๐น๐น(๐๐) , (42)
then ๐บ๐บ(๐๐) โ 0 exponentially in the limit when |๐๐| โ โ, resulting in a vanishing integral around the circle of large radius ๐ ๐ ๐๐ in the ๐๐-plane. We thus arrive at an alternative form of Eq.(41), which is useful for expanding the field component ๐ป๐ป๐๐ appearing in Eqs.(25) and (27), that is,
๐ฝ๐ฝโ+ยฝ[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ]๐๐(๐๐0)๐๐๐๐0๏ฟฝ๐๐0๐๐(๐๐0)๐๐ ๐๐โ
= โ๐๐๐๐3 2โ ๏ฟฝ๐๐๐๐๐๐(๐๐๐๐)exp[i๐ ๐ (๐๐๐๐โ ๐๐0) ๐๐โ ]๐น๐น(๐๐0)
๐๐03 2โ ๏ฟฝ๐๐0๐๐(๐๐0)(๐๐0โ ๐๐๐๐)๐น๐นโฒ(๐๐๐๐)
ร ๐ฝ๐ฝโ+ยฝ[๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ ]
๐๐(๐๐๐๐)๐๐๐๐๐๐๏ฟฝ๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ๐๐ ยท (43)
Finally, if we choose
๐บ๐บ(๐๐) = โ๐๐exp(i๐ ๐ ๐๐ ๐๐โ )[๐๐๐๐๐ฝ๐ฝโฬ+ยฝ(๐๐๐๐)+ยฝ๐ฝ๐ฝโ+ยฝ(๐๐๐๐)](๐๐โ๐๐0)๐๐(๐๐)๐น๐น(๐๐) , (44)
it continues to be meromorphic (i.e., free of branch-cuts), and will have a vanishing integral over a large circle of radius ๐ ๐ ๐๐ in the limit when ๐ ๐ ๐๐ โ โ. The relevant expansion of the field components ๐ป๐ป๐๐ and ๐ป๐ป๐๐ appearing in Eqs.(25) and (27) will then be
[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ]๐ฝ๐ฝโฬ+ยฝ[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ]+ยฝ๐ฝ๐ฝโ+ยฝ[๐๐0๐๐(๐๐0)๐๐ ๐๐โ ]๐๐(๐๐0)๐๐๐๐0๏ฟฝ๐๐0๐๐(๐๐0)๐๐ ๐๐โ
= โ๐๐๐๐
3 2โ ๏ฟฝ๐๐๐๐๐๐(๐๐๐๐)exp[i๐ ๐ (๐๐๐๐ โ ๐๐0) ๐๐โ ]๐น๐น(๐๐0)
๐๐03 2โ ๏ฟฝ๐๐0๐๐(๐๐0)๏ฟฝ๐๐0 โ ๐๐๐๐๏ฟฝ๐น๐นโฒ(๐๐๐๐)๐๐ ร
[๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ ]๐ฝ๐ฝโฬ+ยฝ[๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ ]+ยฝ๐ฝ๐ฝโ+ยฝ[๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โ ]
๐๐(๐๐๐๐)๐๐๐๐๐๐๏ฟฝ๐๐๐๐๐๐(๐๐๐๐)๐๐ ๐๐โยท (45)
13
In this way, one can expand into a superposition of leaky modes the various ๐ฌ๐ฌ and ๐ฏ๐ฏ field components that comprise an initial distribution. It will then be possible to follow each leaky mode as its phase evolves while its amplitude decays with the passage of time.
7. Numerical results for a solid glass sphere. Figure 6 shows the resonances of a dielectric sphere of radius ๐ ๐ = 50๐๐0 and refractive index ๐๐ = 1.5 at and around the reference frequency ๐๐0 = 1.216 ร 1015 rad secโ , which corresponds to the free-space wavelength ๐๐0 = 1.55 ๐๐๐๐. In this and subsequent figures, the frequency ๐๐ is normalized by ๐๐0. The contours of real and imaginary parts of the characteristic equation ๐น๐น(๐๐) = 0 can be plotted in the complex ๐๐-plane, as was done for a dielectric slab in Fig.4. Where the contours cross each other, the function ๐น๐น(๐๐) vanishes, indicating the existence of a leaky mode at the crossing frequency ๐๐๐๐ = ๐๐๐๐โฒ + i๐๐๐๐โณ. The ratio |๐๐๐๐โฒ | |๐๐๐๐โณ|โ is a measure of the ๐๐-factor of the spherical cavity at the excitation frequency ๐๐ = ๐๐๐๐โฒ . Shown in Fig.6 are the computed ๐๐-factors of the spherical cavity for both TE and TM modes at the various resonance frequencies corresponding to โ = 340. (Note that the characteristic equation does not depend on ๐๐, which indicates that, for a given integer โ, the modes associated with all ๐๐ between โโ and โ are degenerate.) The lowest resonance frequency occurs at ๐๐ โ 0.78๐๐0. The large values of ๐๐ seen in Fig.6 are a consequence of the fact that the refractive index ๐๐ is assumed to be purely real; later, when absorption is incorporated into the model via the imaginary part of ๐๐, the ๐๐-factors will drop to more reasonable values.
Fig. 6. Computed ๐๐-factor versus the resonance frequency for a dielectric sphere of radius ๐ ๐ = 50๐๐0 and refractive index ๐๐ = 1.5 in the vicinity of ๐๐0 = 1.216 ร 1015 rad secโ . On the horizontal axis, the frequency ๐๐ is normalized by ๐๐0. The leaky mode frequencies ๐๐๐๐ = ๐๐๐๐โฒ + i๐๐๐๐โณ are solutions of ๐น๐น(๐๐) =0, which have been found numerically. The ratio |๐๐๐๐โฒ | |๐๐๐๐โณ|โ is used as a measure of the ๐๐-factor of the spherical cavity at the excitation frequency ๐๐ = ๐๐๐๐โฒ . The figure shows computed ๐๐-factors for both TE and TM modes at the various resonance frequencies of the dielectric sphere corresponding to โ = 340.
The direct method of determining the resonances of the spherical cavity involves the computation of the ratio ๐ธ๐ธinside/๐ธ๐ธincident for an incident Hankel function of type 2 (incoming wave) and a fixed mode number โ. Once again, the results are independent of the azimuthal mode number ๐๐, as the modes associated with ๐๐ = โโ to โ are all degenerate. Figure 7 shows plots of ๐ธ๐ธinside/๐ธ๐ธincident for the spherical cavity of radius ๐ ๐ = 50๐๐0, refractive index ๐๐ = 1.5, and mode number โ = 340, at and around ๐๐0 = 1.216 ร 1015 rad secโ ; the results for both TE and TM modes are presented in the figure. The resonances are seen to be strong with narrow linewidths. Note that, outside the resonance peaks and especially at lower frequencies, the coupling of the incident beam to the cavity is extremely weak. The TE and TM modes are quite similar in their coupling efficiencies and resonant line-shapes, their major difference being the slight shift of TM resonances toward higher frequencies, as can be seen in Fig.7(c). Figure 7(d) is a magnified view of the line-shape for a single TE resonant line centered at ๐๐ = 1.002068๐๐0.
14
Fig. 7. Plots of the amplitude ratio of the ๐ธ๐ธ-field inside the dielectric sphere (๐ ๐ = 50๐๐0,๐๐ = 1.5) to the incident ๐ธ๐ธ-field for the โ = 340 spherical harmonic. The horizontal axis represents the excitation frequency ๐๐ normalized by ๐๐0 = 1.216 ร 1015 rad secโ . (a) TE mode. (b) TM mode. Note that the cutoff frequency for both modes is ๐๐ โ 0.78๐๐0, below which no resonances are excited. Above the cutoff, in between adjacent resonances, the field amplitude inside the cavity drops to exceedingly small values. The occurrence of extremely large resonance peaks in these plots is due to the assumed value of the refractive index ๐๐ being purely real. (c) Close-up view of the resonance lines of the glass ball for the โ = 340 spherical harmonic, showing the TM resonances being slightly shifted away from the TE resonance lines. (d) Magnified view of an individual TE resonance line centered at ๐๐๐ ๐ = 1.002068๐๐0.
To gain an appreciation for the effect of the mode number โ on the resonant behavior of our spherical cavity, we show in Fig.8 the computed ratio ๐ธ๐ธinside/๐ธ๐ธincident for โ = 10, 20 and 25. It is observed that, with an increasing mode number โ, the lowest accessible resonance moves to higher frequencies, and that the ๐๐-factor associated with individual resonance lines tends to rise.
Finally, Fig.9 shows the computed ๐๐-factors (๐๐ = |๐๐๐๐โฒ | |๐๐๐๐โณ|โ ) for a spherical cavity having ๐ ๐ = 50๐๐0, ๐๐ = ๐๐โฒ + i๐๐โณ, and โ = 340. Setting ๐๐โฒ = 1.5 allows a comparison between the results depicted in Fig.6, where ๐๐โณ = 0, and those in Fig.9, which correspond to ๐๐โณ = 10โ8 (blue), 10โ7 (red), and 10โ6 (black). These positive values of ๐๐โณ account for the presence of small amounts of absorption within the spherical cavity. Compared to the case of ๐๐โณ = 0, the resonance frequencies in Fig.9 have not changed by much, but the ๐๐-factors of the various resonances are seen to have declined substantially. As expected, the greatest drop in the ๐๐-factor is associated with the largest value of ๐๐โณ.
(d)
(a) (b)
(c)
15
8. Leaky modes of a solid dielectric cylinder. In a cylindrical coordinate system, within a linear, isotropic, homogeneous medium having permeability ๐๐0๐๐(๐๐), permittivity ๐๐0๐๐(๐๐), and refractive index ๐๐(๐๐) = โ๐๐๐๐, Maxwellโs equations have solutions of the form ๐ฌ๐ฌ(๐๐, ๐ก๐ก) =๐ฌ๐ฌ(๐๐) exp[i(๐๐๐๐ + ๐๐๐ง๐ง๐ง๐ง โ ๐๐๐ก๐ก)] and ๐ฏ๐ฏ(๐๐, ๐ก๐ก) = ๐ฏ๐ฏ(๐๐) exp[i(๐๐๐๐ + ๐๐๐ง๐ง๐ง๐ง โ ๐๐๐ก๐ก)]. Here the azimuthal mode-number ๐๐ could be a positive, zero, or negative integer, the real-valued ๐๐๐ง๐ง is the propagation constant along the ๐ง๐ง-axis, and ๐๐ is the oscillation frequency [7]. The radial propagation constant will then be ๐๐๐๐ = ๏ฟฝ[๐๐(๐๐)๐๐ ๐๐โ ]2 โ ๐๐๐ง๐ง2, and the various field components will depend on the radial distance ๐๐ from the ๐ง๐ง-axis as follows:
๐๐ = 0 TE mode (๐ธ๐ธ๐ง๐ง = 0):
๐ฌ๐ฌ(๐๐) = ๐ธ๐ธ0 ๐ฝ๐ฝ1(๐๐๐๐๐๐)๐๐๏ฟฝ . (46)
๐ฏ๐ฏ(๐๐) = โ๐ธ๐ธ0[๐๐๐ง๐ง ๐ฝ๐ฝ1(๐๐๐๐๐๐)๐๐๏ฟฝ + i๐๐๐๐๐ฝ๐ฝ0(๐๐๐๐๐๐)๐๐๏ฟฝ] [๐๐0๐๐(๐๐)๐๐]โ . (47)
๐๐ โ 0 TE mode (๐ธ๐ธ๐ง๐ง = 0):
๐ฌ๐ฌ(๐๐) = ๐ธ๐ธ0๏ฟฝi(๐๐ ๐๐๐๐๐๐โ )๐ฝ๐ฝ|๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ โ ๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ๏ฟฝ. (48)
๐ฏ๐ฏ(๐๐) = ๐ธ๐ธ0๏ฟฝ๐๐๐ง๐ง ๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ + i(๐๐๐๐๐ง๐ง ๐๐๐๐๐๐โ )๐ฝ๐ฝ|๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ โ i๐๐๐๐๐ฝ๐ฝ|๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ๏ฟฝ [๐๐0๐๐(๐๐)๐๐]โ . (49)
๐๐ = 0 TM mode (๐ป๐ป๐ง๐ง = 0):
๐ฌ๐ฌ(๐๐) = ๐ป๐ป0[๐๐๐ง๐ง ๐ฝ๐ฝ1(๐๐๐๐๐๐)๐๐๏ฟฝ + i๐๐๐๐ ๐ฝ๐ฝ0(๐๐๐๐๐๐)๐๐๏ฟฝ] [๐๐0๐๐(๐๐)๐๐]โ . (50)
๐ฏ๐ฏ(๐๐) = ๐ป๐ป0 ๐ฝ๐ฝ1(๐๐๐๐๐๐)๐๐๏ฟฝ . (51)
๐๐ โ 0 TM mode (๐ป๐ป๐ง๐ง = 0):
๐ฌ๐ฌ(๐๐) = ๐ป๐ป0๏ฟฝ๐๐๐ง๐ง ๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ + i(๐๐๐๐๐ง๐ง ๐๐๐๐๐๐โ )๐ฝ๐ฝ|๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ โ i๐๐๐๐๐ฝ๐ฝ|๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ๏ฟฝ [๐๐0๐๐(๐๐)๐๐]โ . (52)
๐ฏ๐ฏ(๐๐) = โ๐ป๐ป0๏ฟฝi(๐๐ ๐๐๐๐๐๐โ )๐ฝ๐ฝ|๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ โ ๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐๐๐)๐๐๏ฟฝ๏ฟฝ. (53)
Fig. 8. Dependence on excitation frequency ๐๐ of the amplitude ratio of the ๐ธ๐ธ-field inside the glass sphere to the incident ๐ธ๐ธ-field for TE spherical harmonics having โ = 10 (black), โ = 20 (blue), and โ = 25 (red).
Fig. 9. Similar to Fig. 6, except that the refractive index ๐๐ = ๐๐โฒ + i๐๐โณ of the dielectric sphere is now allowed to have a small nonzero imaginary part, ๐๐โณ, representing absorption within the material.
16
In the above equations, ๐ฝ๐ฝ๐๐(๐ง๐ง) is a Bessel function of the first kind, integer-order ๐๐, and ๐ฝ๐ฝ๏ฟฝฬ๏ฟฝ๐(๐ง๐ง) is its derivative with respect to ๐ง๐ง. One could also replace these with ๐๐๐๐(๐ง๐ง) and ๏ฟฝฬ๏ฟฝ๐๐๐(๐ง๐ง), the Bessel function of the second kind and its derivative, or with โ๐๐
(1,2)(๐ง๐ง) and โฬ๐๐(1,2)(๐ง๐ง), the
Hankel functions of type 1 and 2, and their corresponding derivatives. Useful identities include:
๐ฝ๐ฝ๐๐(๐ง๐ง) = (๐ง๐ง 2โ )๐๐๏ฟฝ (โ1)๐๐(๐ง๐ง 2โ )2๐๐
๐๐! ฮ(๐๐+๐๐+1)
โ
๐๐=0, (|arg(๐ง๐ง)| < ๐๐). (54)
๐ง๐ง๐ฝ๐ฝ๏ฟฝฬ๏ฟฝ๐(๐ง๐ง) = ๐๐๐ฝ๐ฝ๐๐(๐ง๐ง) โ ๐ง๐ง๐ฝ๐ฝ๐๐+1(๐ง๐ง). (55)
๐ฝ๐ฝ๐๐๏ฟฝ๐๐i๐๐๐๐๐ง๐ง๏ฟฝ = ๐๐i๐๐๐๐๐๐๐ฝ๐ฝ๐๐(๐ง๐ง). (56)
๐ฝ๐ฝ๐๐(๐ง๐ง) ~ ๏ฟฝ2 (๐๐๐ง๐ง)โ cos(๐ง๐ง โ ยฝ๐๐๐๐ โ ยผ๐๐), (|๐ง๐ง| โซ 1 and |arg(๐ง๐ง)| < ๐๐). (57)
โ๐๐(1)(๐ง๐ง)~๏ฟฝ2 (๐๐๐ง๐ง)โ exp[i(๐ง๐ง โ ยฝ๐๐๐๐ โ ยผ๐๐)], (|๐ง๐ง| โซ 1 and |arg(๐ง๐ง)| < ๐๐). (58)
๐ง๐งโฬ๐๐(1)(๐ง๐ง) = ๐๐โ๐๐
(1)(๐ง๐ง) โ ๐ง๐งโ๐๐+1(1) (๐ง๐ง). (59)
โ๐๐(1)๏ฟฝ๐๐i๐๐๐ง๐ง๏ฟฝ = โ๐๐โi๐๐๐๐โ๐๐
(2)(๐ง๐ง). (60)
Consider now an infinitely-long, right-circular cylinder having radius ๐ ๐ and optical constants ๐๐(๐๐) and ๐๐(๐๐), surrounded by free space. The radial propagation constants inside and outside the cylinder are denoted by ๐๐๐๐0 = ๏ฟฝ๐๐๐๐(๐๐ ๐๐โ )2 โ ๐๐๐ง๐ง2 and ๐๐๐๐1 = ๏ฟฝ(๐๐ ๐๐โ )2 โ ๐๐๐ง๐ง2, respectively. For the ๐๐ = 0 leaky TE mode, the boundary conditions at ๐๐ = ๐ ๐ impose the following constraints:
๐ธ๐ธ0 ๐ฝ๐ฝ1(๐๐๐๐0๐ ๐ ) = ๐ธ๐ธ1โ1(1)(๐๐๐๐1๐ ๐ ), (61)
[๐ธ๐ธ0๐๐๐๐0 ๐๐(๐๐)โ ] ๐ฝ๐ฝ0(๐๐๐๐0๐ ๐ ) = ๐ธ๐ธ1๐๐๐๐1โ0(1)(๐๐๐๐1๐ ๐ ). (62)
Therefore, for a leaky ๐๐ = 0 TE mode to exist, the following characteristic equation must be satisfied:
๐น๐น(๐๐) = ๐๐(๐๐)๐๐๐๐1โ0(1)(๐๐๐๐1๐ ๐ ) ๐ฝ๐ฝ1(๐๐๐๐0๐ ๐ )โ ๐๐๐๐0 ๐ฝ๐ฝ0(๐๐๐๐0๐ ๐ )โ1
(1)(๐๐๐๐1๐ ๐ ) = 0. (63)
The corresponding equation for the leaky ๐๐ = 0 TM modes is similar, with ๐๐(๐๐) replacing ๐๐(๐๐). Of course, for a real-valued frequency ๐๐, if |๐๐๐ง๐ง| happens to be between ๐๐ ๐๐โ and ๐๐(๐๐)๐๐ ๐๐โ , the EM field surrounding the dielectric cylinder will be evanescent, in which case the confined mode within the cylinder will not be leaky. Given that ๐พ๐พ๐๐(๐ง๐ง) = ยฝ๐๐(i)๐๐+1โ๐๐
(1)(i๐ง๐ง), where ๐พ๐พ๐๐(๐ง๐ง) is a modified Bessel function of imaginary argument, one may rewrite Eq.(63) as follows:
๐๐(๐๐)๏ฟฝ๐๐๐ง๐ง2 โ (๐๐ ๐๐โ )2 ๐พ๐พ0๏ฟฝ๏ฟฝ๐๐๐ง๐ง2 โ (๐๐ ๐๐โ )2 ๐ ๐ ๏ฟฝ ๐ฝ๐ฝ1๏ฟฝ๐๐๐๐0๐ ๐ ๏ฟฝ+ ๐๐๐๐0 ๐ฝ๐ฝ0๏ฟฝ๐๐๐๐0๐ ๐ ๏ฟฝ๐พ๐พ1๏ฟฝ๏ฟฝ๐๐๐ง๐ง2 โ (๐๐ ๐๐โ )2 ๐ ๐ ๏ฟฝ = 0. (64)
When ๐๐(๐๐) and ๐๐(๐๐) are real, Eq.(64) will have real-valued solutions for ๐๐, which represent the guided ๐๐ = 0 modes of the cylinder. In general, however, the time-averaged Poynting vector associated with the field outside the cylinder will have a nonzero component along ๐๐๏ฟฝ, indicating that Eq.(63) has solutions in the form of complex frequencies ๐๐ whose imaginary parts are negative.
G&R 8.476-1
G&R 8.402
G&R 8.472-2
G&R 8.476-8
G&R 8.451-1
G&R 8.451-3
G&R 8.472-2
17
Another special case occurs when ๐๐๐ง๐ง = 0, in which case the boundary conditions at ๐๐ = ๐ ๐ impose the following constraints on TE modes:
๐ธ๐ธ0 ๐ฝ๐ฝ|ฬ๐๐|(๐๐๐ ๐ ๐๐ ๐๐โ ) = ๐ธ๐ธ1โฬ|๐๐|(1)(๐ ๐ ๐๐ ๐๐โ ), (65)
[๐ธ๐ธ0๐๐(๐๐) ๐๐(๐๐)โ ] ๐ฝ๐ฝ|๐๐|(๐๐๐ ๐ ๐๐ ๐๐โ ) = ๐ธ๐ธ1โ|๐๐|(1)(๐ ๐ ๐๐ ๐๐โ ). (66)
The corresponding characteristic equation will then be
๐น๐น(๐๐) = ๐๐(๐๐)๐ฝ๐ฝ|ฬ๐๐|(๐๐๐ ๐ ๐๐ ๐๐โ )โ|๐๐|(1)(๐ ๐ ๐๐ ๐๐โ ) โ ๐๐(๐๐)๐ฝ๐ฝ|๐๐|(๐๐๐ ๐ ๐๐ ๐๐โ )โฬ|๐๐|
(1)(๐ ๐ ๐๐ ๐๐โ ) = 0. (67)
This equation is valid for positive, zero, and negative values of the azimuthal mode number ๐๐. It is also valid for TM modes provided that ๐๐(๐๐) is replaced with ๐๐(๐๐). Note that, for ๐๐ = 0, we have ๐ฝ๐ฝ0ฬ(๐ง๐ง) = โ๐ฝ๐ฝ1(๐ง๐ง) and โฬ0
(1)(๐ง๐ง) = โโ1(1)(๐ง๐ง), confirming that Eq.(63) reduces to Eq.(67)
when ๐๐๐ง๐ง = 0. In general, we expect the solutions of Eq.(67) to be complex frequencies ๐๐ having negative imaginary parts.
In the general case when ๐๐ โ 0 and ๐๐๐ง๐ง โ 0, the boundary conditions at ๐๐ = ๐ ๐ can be satisfied only for a superposition of TE and TM modes. Listed below are the continuity equations for ๐ธ๐ธ๐ง๐ง, ๐ป๐ป๐ง๐ง, ๐ธ๐ธ๐๐, and ๐ป๐ป๐๐. The continuity of ๐ท๐ท๐๐ is guaranteed by those of ๐ป๐ป๐ง๐ง and ๐ป๐ป๐๐, while the continuity of ๐ต๐ต๐๐ is guaranteed by those of ๐ธ๐ธ๐ง๐ง and ๐ธ๐ธ๐๐.โ
๐ธ๐ธ๐ง๐ง: ๐ป๐ป0๐๐๐๐0 ๐ฝ๐ฝ|๐๐|(๐๐๐๐0๐ ๐ ) = ๐ป๐ป1๐๐(๐๐)๐๐๐๐1โ|๐๐|(1)(๐๐๐๐1๐ ๐ ), (68)
๐ป๐ป๐ง๐ง: ๐ธ๐ธ0๐๐๐๐0 ๐ฝ๐ฝ|๐๐|(๐๐๐๐0๐ ๐ ) = ๐ธ๐ธ1๐๐(๐๐)๐๐๐๐1โ|๐๐|(1)(๐๐๐๐1๐ ๐ ), (69)
๐ธ๐ธ๐๐: ๐ธ๐ธ0 ๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐0๐ ๐ ) โ i ๏ฟฝ ๐๐๐๐๐ง๐ง๐๐0๐๐๐ ๐ ๐๐๐๐๐๐0
๏ฟฝ๐ป๐ป0 ๐ฝ๐ฝ|๐๐|(๐๐๐๐0๐ ๐ ) = ๐ธ๐ธ1โฬ|๐๐|(1)(๐๐๐๐1๐ ๐ ) โ i ๏ฟฝ ๐๐๐๐๐ง๐ง
๐๐0๐ ๐ ๐๐๐๐๐๐1๏ฟฝ๐ป๐ป1โ|๐๐|
(1)(๐๐๐๐1๐ ๐ ), (70)
๐ป๐ป๐๐: ๐ป๐ป0 ๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐0๐ ๐ ) + i ๏ฟฝ ๐๐๐๐๐ง๐ง๐๐0๐๐๐ ๐ ๐๐๐๐๐๐0
๏ฟฝ๐ธ๐ธ0 ๐ฝ๐ฝ|๐๐|(๐๐๐๐0๐ ๐ ) = ๐ป๐ป1โฬ|๐๐|(1)(๐๐๐๐1๐ ๐ ) + i ๏ฟฝ ๐๐๐๐๐ง๐ง
๐๐0๐ ๐ ๐๐๐๐๐๐1๏ฟฝ๐ธ๐ธ1โ|๐๐|
(1)(๐๐๐๐1๐ ๐ ). (71)
The characteristic equation that ensures the existence of a non-trivial solution to the above equations is thus found to be
๏ฟฝ๐๐(๐๐)๐๐๐๐0
๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐0๐ ๐ )
๐ฝ๐ฝ|๐๐|(๐๐๐๐0๐ ๐ )โ 1
๐๐๐๐1 โฬ|๐๐|
(1)(๐๐๐๐1๐ ๐ )
โ|๐๐|(1)(๐๐๐๐1๐ ๐ )
๏ฟฝ ร ๏ฟฝ๐๐(๐๐)๐๐๐๐0
๐ฝ๐ฝ|ฬ๐๐|(๐๐๐๐0๐ ๐ )
๐ฝ๐ฝ|๐๐|(๐๐๐๐0๐ ๐ ) โ1๐๐๐๐1
โฬ|๐๐|
(1)๏ฟฝ๐๐๐๐1๐ ๐ ๏ฟฝ
โ|๐๐|(1)(๐๐๐๐1๐ ๐ )
๏ฟฝ = ๏ฟฝ ๐๐๐๐๐ง๐ง๐ ๐ ๐๐ ๐๐โ
๏ฟฝ 1๐๐๐๐02 โ 1
๐๐๐๐12 ๏ฟฝ๏ฟฝ
2ยท (72)
The values of ๐๐ that satisfy Eq.(72) are the leaky mode frequencies corresponding to the azimuthal mode number ๐๐ and the propagation constant ๐๐๐ง๐ง along the ๐ง๐ง-axis. Note that, upon setting ๐๐ = 0 in Eq.(72), the right-hand side of the equation vanishes. The two terms on the left-hand side will then be decoupled, representing the ๐๐ = 0 TE and TM modes, respectively; this is consistent with the characteristic function given in Eq.(63). Similarly, setting ๐๐๐ง๐ง = 0 in Eq.(72) causes the right-hand side of the equation to vanish, thus, once again, decoupling the TE and TM modes, which are represented by the two terms on the left-hand side of the equation. โ ๐ท๐ท๐๐: ๐ป๐ป0 ๐ฝ๐ฝ|ฬ๐๐|๏ฟฝ๐๐๐๐0๐ ๐ ๏ฟฝ + i ๏ฟฝ๐๐0๐๐ ๐๐๐๐
๐ ๐ ๐๐๐ง๐ง๐๐๐๐0๏ฟฝ ๐ธ๐ธ0 ๐ฝ๐ฝ|๐๐|๏ฟฝ๐๐๐๐0๐ ๐ ๏ฟฝ = ๐ป๐ป1โฬ|๐๐|
(1)๏ฟฝ๐๐๐๐1๐ ๐ ๏ฟฝ + i ๏ฟฝ ๐๐0๐๐๐๐๐ ๐ ๐๐๐ง๐ง๐๐๐๐1
๏ฟฝ ๐ธ๐ธ1โ|๐๐|(1)๏ฟฝ๐๐๐๐1๐ ๐ ๏ฟฝ.
๐ต๐ต๐๐: ๐ธ๐ธ0 ๐ฝ๐ฝ|ฬ๐๐|๏ฟฝ๐๐๐๐0๐ ๐ ๏ฟฝ โ i ๏ฟฝ๐๐0๐๐ ๐๐๐๐๐ ๐ ๐๐๐ง๐ง๐๐๐๐0
๏ฟฝ๐ป๐ป0 ๐ฝ๐ฝ|๐๐|(๐๐๐๐0๐ ๐ ) = ๐ธ๐ธ1โฬ|๐๐|(1)๏ฟฝ๐๐๐๐1๐ ๐ ๏ฟฝ โ i ๏ฟฝ ๐๐0๐๐๐๐
๐ ๐ ๐๐๐ง๐ง๐๐๐๐1๏ฟฝ๐ป๐ป1โ|๐๐|
(1)๏ฟฝ๐๐๐๐1๐ ๐ ๏ฟฝ.
18
Given that, under the circumstances, ๐๐๐๐0 = ๐๐(๐๐)๐๐ ๐๐โ and ๐๐๐๐1 = ๐๐ ๐๐โ , it is easy to verify that Eq.(72) is consistent with Eq.(67).
In the remainder of this section, our attention will be confined to TE modes with ๐๐๐ง๐ง = 0, for which the characteristic equation is given by Eq.(67). Our goal is to expand an initial field distribution residing within the cylinder at ๐ก๐ก = ๐ก๐ก0, say, one of the mode profiles listed in Eqs.(46)-(53) having ๐๐๐ง๐ง = 0 and a real-valued oscillation frequency ๐๐0, as a superposition of leaky modes whose complex frequencies ๐๐ = ๐๐๐๐ are solutions of the characteristic equation ๐น๐น(๐๐) = 0 given by Eq.(67). To this end, we form the function ๐บ๐บ(๐๐), as follows:
๐บ๐บ(๐๐) =๐๐exp(i๐๐๐๐1๐ ๐ )๐ฝ๐ฝ๐๐(๐๐๐๐0๐๐) (๐๐๐๐0๐๐)๏ฟฝ
(๐๐โ๐๐0)๐น๐น(๐๐) ยท (73)
For odd (even) values of ๐๐, the numerator and the denominator of Eq.(73) are both even (odd) functions of the radial ๐๐-vector ๐๐๐๐0. Consequently, ๐บ๐บ(๐๐) does not switch signs when ๐๐ crosses (i.e., goes from above to below) a branch-cut associated with ๐๐๐๐0(๐๐). One must be careful here about ๐๐๐๐1 = ๏ฟฝ(๐๐ ๐๐โ )2 โ ๐๐๐ง๐ง2, which appears within the function ๐น๐น(๐๐); see Eqs.(63) and (72). The branch-cut associated with ๐๐๐๐1(๐๐) is on the real-axis, spanning the interval Re(๐๐) โ [โ๐๐๐๐๐ง๐ง , ๐๐๐๐๐ง๐ง]. When crossing this branch-cut, ๐๐๐๐1 switches from a positive to a negative imaginary value, or vice-versa. The function ๐บ๐บ(๐๐) must remain insensitive to this sign-change as well. Unfortunately, we do not know how to handle this problem, and that is why we have limited the scope of the present discussion to the case of ๐๐๐ง๐ง = 0, for which the problem associated with the branch-cut residing on the real-axis disappears. (This problem is similar to that encountered in Sec.5 in the case of oblique incidence on a dielectric slab.)
In the limit when |๐๐| โ โ, the function ๐บ๐บ(๐๐) approaches zero (exponentially), thus ensuring the vanishing of โฎ๐บ๐บ(๐๐)๐๐๐๐ over a large circle of radius ๐ ๐ ๐๐. Consequently, the residues of ๐บ๐บ(๐๐) evaluated at all its poles must add up to zero, that is,
๐๐0 exp(i๐๐0๐ ๐ ๐๐โ ) ๐น๐น(๐๐0)
ร ๐ฝ๐ฝ๐๐[๐๐(๐๐0)๐๐0๐๐ ๐๐โ ]๐๐(๐๐0)๐๐0๐๐ ๐๐โ
+ ๏ฟฝ ๐๐๐๐exp(i๐๐๐๐๐ ๐ ๐๐โ ) (๐๐๐๐โ ๐๐0)๐น๐นโฒ(๐๐๐๐)
ร ๐ฝ๐ฝ๐๐๏ฟฝ๐๐(๐๐๐๐)๐๐๐๐๐๐ ๐๐โ ๏ฟฝ๐๐(๐๐๐๐)๐๐๐๐๐๐ ๐๐โ
= 0๐๐
. (74)
Rearranging the various terms of the above equation, we arrive at
๐ฝ๐ฝ๐๐[๐๐(๐๐0)๐๐0๐๐ ๐๐โ ] = ๏ฟฝ ๐๐(๐๐0)๐น๐น(๐๐0)exp๏ฟฝi(๐๐๐๐โ ๐๐0)๐ ๐ ๐๐โ ๏ฟฝ(๐๐0โ ๐๐๐๐)๐๐(๐๐๐๐)๐น๐นโฒ(๐๐๐๐)๐๐
ร ๐ฝ๐ฝ๐๐๏ฟฝ๐๐(๐๐๐๐)๐๐๐๐๐๐ ๐๐โ ๏ฟฝ. (75)
The above expansion may be used to represent the field components ๐ธ๐ธ๐๐(๐๐) of TE modes and ๐ป๐ป๐๐(๐๐) of TM modes; see Eqs.(48) and (53). To expand the field component ๐ธ๐ธ๐๐(๐๐) of a TE mode or ๐ป๐ป๐๐(๐๐) of a TM mode, the function ๐บ๐บ(๐๐) must be modified as follows:
๐บ๐บ(๐๐) =๐๐exp(i๐๐๐๐1๐ ๐ )๐ฝ๐ฝ๏ฟฝฬ๏ฟฝ๐(๐๐๐๐0๐๐)
(๐๐โ๐๐0)๐น๐น(๐๐) ยท (76)
In a similar vein, the appropriate forms of ๐บ๐บ(๐๐) for the remaining TE field components are
๐บ๐บ(๐๐) =exp(i๐๐๐๐1๐ ๐ )๐ฝ๐ฝ๏ฟฝฬ๏ฟฝ๐(๐๐๐๐0๐๐)
(๐๐โ๐๐0)๐๐(๐๐)๐น๐น(๐๐) ยท (77)
๐บ๐บ(๐๐) =exp(i๐๐๐๐1๐ ๐ )๐ฝ๐ฝ๐๐(๐๐๐๐0๐๐) (๐๐๐๐0๐๐)๏ฟฝ
(๐๐โ๐๐0)๐๐(๐๐)๐น๐น(๐๐) ยท (78)
19
๐บ๐บ(๐๐) =exp(i๐๐๐๐1๐ ๐ )๐๐๐๐0๐ฝ๐ฝ๐๐(๐๐๐๐0๐๐)
(๐๐โ๐๐0)๐๐(๐๐)๐น๐น(๐๐) ยท (79)
For TM modes, one must substitute ๐๐(๐๐) for ๐๐(๐๐) in Eqs.(77)-(79).
9. Numerical results for a cylindrical cavity. Figure 10 provides a comparison between the resonances of a dielectric sphere (๐ ๐ = 50๐๐0, ๐๐ = 1.5) and those of a similar dielectric cylinder (๐ ๐ = 50๐๐0, ๐ฟ๐ฟ = โ,๐๐ = 1.5,๐๐๐ง๐ง = 0). The excitation frequency ๐๐ is normalized by ๐๐0 =1.216 ร 1015 rad secโ , which corresponds to the free-space wavelength ๐๐0 = 1.55 ๐๐๐๐. In Fig.10(a) the computed cavity ๐๐-factors are plotted versus ๐๐ ๐๐0โ for the โ = 340 TM spherical harmonic and the ๐๐ = 340 TM cylindrical harmonic. Figure 10(b) compares the ๐ธ๐ธ-field amplitude ratio (i.e., ๐ธ๐ธ-field inside the cavity to the incident ๐ธ๐ธ-field) of the โ = 25 TM spherical harmonic with that of the ๐๐ = 25 TM cylindrical harmonic; this near agreement between the ๐ธ๐ธ-field ratios for the two cavities persists for larger values of โ and ๐๐. The resonances of the cylindrical rod are seen to be very similar to those of the glass sphere.
Fig. 10. A comparison between the resonances of a dielectric sphere (๐ ๐ = 50๐๐0,๐๐ = 1.5) and a dielectric cylinder (๐ ๐ = 50๐๐0, ๐ฟ๐ฟ = โ,๐๐ = 1.5,๐๐๐ง๐ง = 0). In (a) the cavity ๐๐-factors are plotted versus ๐๐ ๐๐0โ for the โ = 340 TM spherical and the ๐๐ = 340 TM cylindrical harmonics. In (b) the ratio ๐ธ๐ธinside/๐ธ๐ธincident is plotted versus ๐๐ ๐๐0โ for the TM modes of the sphere (โ = 25) and the cylinder (๐๐ = 25).
We thus observe that, in their general behavior, cylindrical cavities are quite similar to spherical cavities. This is not unexpected, considering that, for large โ and ๐๐, the EM field inside the sphere is more or less confined to a narrow band at the equator, and that the geometry of the equatorial region of a sphere is not too different from that of a cylinder. Of course, our calculations pertaining to the dielectric cylinder have been based on the assumption that the EM field is uniformly distributed along the cylinder axis, which is necessary if the results are to be compared with those for a spherical cavity at large โ values. Had we chosen, instead, to couple the light to the cylinder within a narrow strip (i.e., by illuminating a belt around the cylinder having a narrow spread along ๐ง๐ง), the light, once inside the cylinder, would have walked away from the strip due to diffraction effects. That would have caused a reduction in the ๐๐-factor of the cylinder compared to that of a spherical cavity with a similar diameter at a large value of โ.
10. Concluding remarks. Leaky modes of dielectric cavities contain a wealth of information about their resonant behavior, including the lifetimes associated with the light trapped inside the cavity immediately after the source of excitation is turned off. We have proved the completeness of these leaky modes under special circumstances, although completeness under more general conditions remains to be demonstrated. Our completeness proof rigorously accounts for realistic dispersion effects, including absorption losses and the existence of branch-cuts associated with the Lorentz oscillator model. Our numerical results have intimated the close connection between resonant behavior and the leaky eigen-modes of dielectric slabs, spheres, and cylinders. The fact
(a) (b)
20
that spherical harmonics with large โ values, and also cylindrical harmonics with large ๐๐ values, are associated with high-๐๐ resonances hints at the importance of EM angular momentum in relation to the long lifetimes of the modes trapped inside these cavities. In other words, there appears to be a connection between the strength of the circular motion of EM energy inside a cavity and the time it takes for this energy to leak out. These connections will be explored in a forthcoming publication.
Appendix
We show that ๐น๐น(๐๐) of Eq.(37) approaches a constant when ๐๐ โ 0. In the limit ๐ง๐ง โ 0, we have
๐ฝ๐ฝ๐๐(๐ง๐ง) โ (๐ง๐ง 2โ )๐๐
ฮ(๐๐+1)ยท (A1)
๐๐๐๐(๐ง๐ง) โ (๐ง๐ง 2โ )๐๐
tan(๐๐๐๐)ฮ(1+๐๐) โ(๐ง๐ง 2โ )โ๐๐
sin(๐๐๐๐)ฮ(1โ๐๐) ; (๐๐ โ an integer). (A2)
Therefore, when ๐๐ โ 0, we will have
๐น๐น(๐๐) = ๐๐๐๐0๐ ๐ โโ+ยฝ(1) (๐๐0๐ ๐ )๐ฝ๐ฝโ+3 2โ (๐๐๐๐0๐ ๐ ) + [(๐๐ โ 1)(โ + 1)โโ+ยฝ
(1) (๐๐0๐ ๐ ) โ ๐๐๐๐0๐ ๐ โโ+3 2โ(1) (๐๐0๐ ๐ )] ๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ )
= ๐๐๐๐0๐ ๐ ๐ฝ๐ฝโ+ยฝ(๐๐0๐ ๐ )๐ฝ๐ฝโ+3 2โ (๐๐๐๐0๐ ๐ ) + [(๐๐ โ 1)(โ + 1)๐ฝ๐ฝโ+ยฝ(๐๐0๐ ๐ ) โ ๐๐๐๐0๐ ๐ ๐ฝ๐ฝโ+3 2โ (๐๐0๐ ๐ )] ๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ )
+i๐๐๐๐0๐ ๐ ๐๐โ+ยฝ(๐๐0๐ ๐ )๐ฝ๐ฝโ+3 2โ (๐๐๐๐0๐ ๐ ) + i[(๐๐ โ 1)(โ + 1)๐๐โ+ยฝ(๐๐0๐ ๐ ) โ ๐๐๐๐0๐ ๐ ๐๐โ+3 2โ (๐๐0๐ ๐ )] ๐ฝ๐ฝโ+ยฝ(๐๐๐๐0๐ ๐ )
โ i(โ1)โ+1๐๐๐๐0๐ ๐ ฮ(ยฝโโ)ฮ(โ+5 2โ )
(ยฝ๐๐0๐ ๐ )โ(โ+ยฝ)(ยฝ๐๐๐๐0๐ ๐ )โ+3 2โ
+ iฮ(โ+3 2โ )
๏ฟฝ(โ1)โ+1(๐๐โ1)(โ+1)ฮ(ยฝโโ)
(ยฝ๐๐0๐ ๐ )โ(โ+ยฝ) โ (โ1)โ๐๐๐๐0๐ ๐ ฮ(โยฝโโ)
(ยฝ๐๐0๐ ๐ )โ(โ+3 2โ )๏ฟฝ (ยฝ๐๐๐๐0๐ ๐ )โ+ยฝ
โ i(โ1)โ+1๐๐โ+ยฝ
ฮ(โ+3 2โ ) ๏ฟฝ(๐๐โ1)(โ+1)ฮ(ยฝโโ) + 2๐๐
ฮ(โยฝโโ)๏ฟฝยท (A3)
Consequently, ๐น๐น(๐๐) has no poles at ๐๐ = 0, which indicates that, in the vicinity of ๐๐ = 0, the function ๐บ๐บ(๐๐) is not singular.
Acknowledgement. This work has been supported in part by the AFOSR grant No. FA9550-13-1-0228.
References
1. G. Garcia-Calderon and R. Peierls, โResonant states and their uses,โ Nuclear Physics A265, 443-460 (1976). 2. H. A. Haus and D. A. B. Miller, โAttenuation of cutoff modes and leaky modes of dielectric slab structures,โ IEEE
Journal of Quantum Electronics QE-22, 310-318 (1986). 3. P. T. Leung, S. Y. Liu, and K. Young, โCompleteness and orthogonality of quasinormal modes in leaky optical
cavities,โ Phys. Rev. A 49, 3057-67 (1994). 4. P. T. Leung, S. Y. Liu, and K. Young, โCompleteness and time-independent perturbation of the quasinormal
modes of an absorptive and leaky cavity,โ Phys. Rev. A 49, 3982-89 (1994). 5. J. Hu and C. R. Menyuk, โUnderstanding leaky modes: slab waveguide revisited,โ Advances in Optics and
Photonics 1, 58-106 (2009). 6. F. B. Hildebrand, Advanced Calculus for Applications, 2nd edition, Prentice-Hall, New Jersey (1976). 7. J. D. Jackson, Classical Electrodynamics, 3rd edition, Wiley, New York (1999). 8. I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products, 7th ed., Academic, New York (2007). โ A general representation of Bessel functions of the first kind, order ๐๐, is ๐ฝ๐ฝ๐๐(๐ง๐ง) = (๐ง๐ง 2โ )๐๐ โ (โ1)๐๐(๐ง๐ง 2โ )2๐๐
๐๐! ฮ(๐๐+๐๐+1)โ๐๐=0 (G&R
8.440). Considering that, for spherical harmonics, ๐๐ = โ + ยฝ โฅ 3 2โ , it is seen that ๐ฝ๐ฝโ+ยฝ(๐ง๐ง) ๐ง๐งโ โ 0 when ๐ง๐ง โ 0.
G&R 8.440
G&R 8.443
๐๐0 = ๐๐ ๐๐โ
0 0
๐๐(0) = ๏ฟฝ๐๐(0)๐๐(0)