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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 26 — Dec. 21, 2009
  • pp: 24112–24129
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Solving dielectric and plasmonic waveguide dispersion relations on a pocket calculator

Rohan D. Kekatpure, Aaron C. Hryciw, Edward S. Barnard, and Mark L. Brongersma  »View Author Affiliations


Optics Express, Vol. 17, Issue 26, pp. 24112-24129 (2009)
http://dx.doi.org/10.1364/OE.17.024112


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Abstract

We present a robust iterative technique for solving complex transcendental dispersion equations routinely encountered in integrated optics. Our method especially befits the multilayer dielectric and plasmonic waveguides forming the basis structures for a host of contemporary nanophotonic devices. The solution algorithm ports seamlessly from the real to the complex domain—i.e., no extra complexity results when dealing with leaky structures or those with material/metal loss. Unlike several existing numerical approaches, our algorithm exhibits markedly-reduced sensitivity to the initial guess and allows for straightforward implementation on a pocket calculator.

© 2009 Optical Society of America

1. Introduction

With an ever-improving ability to fabricate miniature functional components, the field of nanophotonics is in a phase of explosive growth. Not only are novel phenomena being routinely observed in wavelength- and sub-wavelength-scale components, but these concepts are being rapidly translated into exotic devices for applications ranging from solar energy and information technology, to biology [1

1. M. L. Brongersma and P. G. Kik, eds., Surface Plasmon Nanophotonics, vol. 131 of Springer series in optical sciences (Springer, 2007).

, 2

2. R. Zia, J. A. Schuller, and M. L. Brongersma, “Plasmonics: The next chip-scale technology,” Materials Today 9, 20–27 (2006). [CrossRef]

, 3

3. R. A. Pala, J. S. White, E. S. Barnard, J. Liu, and M. L. Brongersma, “Design of plasmonic thin-film solar cells with broadband absorption enhancements,” Adv. Mater. 21, 1–6 (2009). [CrossRef]

]. At the heart of this development is the ability of nanostructures to confine, guide, and scatter light in ways that are not achievable with bulk materials. It follows that understanding and improving the performance of these nanostructures requires a detailed knowledge of the electromagnetic modes they support.

The allowed electromagnetic modes in microphotonic structures are determined by solving Maxwell’s equations for the given geometry. The last step in this procedure is the application of boundary conditions at the interfaces, yielding the dispersion equations which must be solved to obtain the allowed modes. These dispersion equations are in general transcendental and except in a few simple cases, their solutions cannot be expressed in terms of elementary mathematical functions.

In this paper, we present an easy-to-implement iterative procedure for solving complex transcendental dispersion equations that is relatively insensitive to initial guess. Our method applies to rectangular multilayer dielectric and plasmonic waveguides which may have either material or leakage loss. We first use a simple numerical example to illustrate the use of this technique and its convergence behavior. We then successively apply the procedure to find modes of dielectric slab waveguides, photonic wire waveguides, and plasmonic waveguides.

Fig. 1. Graphs of the left- (red) and the right- (blue) hand sides of Eq. (4). The red dot indicates the approximate location of the solution near x⋍0.5.

Papers in the past have discussed the use of iterative techniques for solving transcendental equations in general [17

17. J. P. McKelvey, “Simple iterative procedures for solving transcendental equations with the electronic slide rule,” Am. J. Phys. 43, 331–334 (1975). [CrossRef]

]. Our aim in this paper is to demonstrate how this method can be applied to automate the design and analysis of waveguide structures of significant contemporary interest.

2. Iterative technique: A simple example

Although the general philosophy of iterative methods is well known, we will use a simple example to introduce our terminology, illustrate the procedure use, and build an intuitive understanding of convergence issues. Suppose we need to solve the equation:

F(x)=0.
(1)

where F(x) is a combination of elementary mathematical functions. We reformulate this as an iterative problem by converting Eq. (1) to:

x=f(x).
(2)

where f is obtained by manipulating the parent function F. Next, we start with an initial guess x 1 and obtain a sequence according to:

xn+1=f(xn).
(3)

If the sequence defined by Eq. (3) converges, then the limiting value is the solution to Eq. (2). We will illustrate the method by a simple example. Consider the transcendental equation

sinx=1x.
(4)

Figure 1 shows the graphs of the left-hand side (LHS) and right-hand side (RHS) of Eq. (4), which intersect around x⋍0.5. To find the root more accurately by the iterative method, we recast the equation in a form similar to Eq. (2) by choosing f (x)=1-sin x, setting up an iteration scheme following Eq. (3):

xn+1=1sinxn.
(5)

Fig. 2. Solution of the example transcendental equation via the iterative method. The graphs (a), (d), and (g) show the left- (magenta) and the right- (gray) hand sides of Eq. (5), (6), and (7). The red dot indicates the position of the initial guess. Convergence/divergence behavior of the real [(b), (e), (h)] and imaginary [(c), (f), (i)] parts of the iterates.

Before we proceed to apply the iterative technique to practical waveguide problems, it is useful to gain an intuitive understanding of how the above scheme converges to the solution. This is important since, for any given F(x), there are usually multiple ways to choose f (x) which differ in their convergence behavior. For example, an equally-legitimate way of setting up an iteration scheme for Eq. (4) would be to choose f (x)=sin-1(1-x) and obtain the sequence {xn} according to:

xn+1=sin1(1xn).
(6)

The behavior of successive iterates is shown in Fig. 2(d–f). Regardless of how close the initial guess is to the actual solution, the iterative scheme diverges (spirals away) from the intersection point. Although the real part of the solution appears to converge after about 25 iterations, the overall complex solution does not converge, as seen from the undamped oscillations in the imaginary part of the solution. Thus, this iteration scheme, although derived from the same parent equation, does not converge. This behavior is typical for transcendental equations involving trigonometric functions and is therefore encountered for waveguide problems, as we will show in section 4.2.

On the other hand, convergence of Eq. (4) can be improved considerably by choosing an alternative iterative form as follows. Squaring both sides of Eq. (4) yields:

sin2x=1cos2x=(1x)2=12x+x2.
(7)

from which we can obtain yet another sequence of {xn} of iterations according to:

xn+1=12(xn2+cos2xn).
(8)

Figures 2(g–i) depict the convergence of succesive iterates of Eq. (8). By comparing Fig. 2(b) with Fig. 2(h) it is apparent that Eq. (8) converges to the solution significantly faster than Eq. (5).

Fig. 3. Criterion for convergence of the iterative solution. The magenta line in both figures is the curve y=x and the blue lines are two different cases of y=f (x). The solution (a) diverges for |f′(x)|≥1 and (d) converges for |f′(x)|<1. (b), (c), (e), and (f) show how the convergence/divergence is reflected in the behavior of the real and the imaginary parts of the successive iterates.

The preceding examples illustrate the crucial importance of choosing appropriate iterative forms. Arriving at such a form necessitates an understanding of the convergence of the iterative scheme. Figure 3 pictorially shows the criterion for convergence of the iterative scheme in Eq. (2). Whether the iteration spirals toward the intersection point (the solution) or away from it is governed by the relative slopes of the intersecting curves. If f (x) is steeper than x (i.e., if |f′(x)|>1), the successive computations and re-substitutions spiral away from the intersection and the solution diverges. On the other hand, if f (x) rises gently compared to x (i.e., if |f′(x)|<1), then the scheme spirals toward the intersection and the solution converges. For convergent functions, the rate of convergence is governed by the magnitude of |f′(x)|. The RHS of Eq. (8) is much flatter (|f′(x)|≪1) near the solution compared to the RHS of Eq. (5), which results in the latter’s considerably slower convergence. While the preceding justification is not a rigorous proof, it provides a basis for choosing the manipulations required to obtain convergent forms of the practically-useful equations we consider in the forthcoming sections [18

18. J. Dugundji and A. Granas, Fixed Point Theory (Springer-Verlag, 2003).

].

3. Dispersion equation of a general asymmetric three-layer structure

The design of many integrated photonics systems of practical importance—including both novel plasmonic waveguide architectures as well as conventional dielectric waveguides—can often be reduced to solving for the effective mode index and electromagnetic field distributions in a three-layer planar structure. As such, we focus our analysis on a generalized three-layer slab waveguide, as shown in Fig. 4(a). The central layer is called the core and has a complex relative permittivity εf. The bottom and top cladding layers are called the substrate (with permittivity εs) and cover (with permittivity εc), respectively.

Fig. 4. Geometries and modes of three-layer infinite slab waveguide structures. Parts (e–i) plot the typical magnetic field profiles.

Depending on which materials comprise the three layers, it is possible to classify the most commonly-encountered waveguides into three basic types. The dielectric waveguides, shown in Fig. 4(b), have all three layers made of dielectrics, with εf>εs,εc. The structures in Figs. 4(c) and (d) are two basic configurations of plasmonic waveguides. The first, shown in Fig. 4(c), consists of an dielectric sandwiched between two (possibly different) metals and is commonly known as a metal-dielectric-metal (MDM) waveguide. The second structure, shown in Fig. 4(d), consists of a metal layer between two (possibly different) dielectrics; it is commonly known as an dielectric-metal-dielectric (DMD) waveguide.

Figures 4(e–i) schematically show the possible modes of a general three-layer slab waveguide. These modes are distinguished according to several criteria: mode number (number of zero-crossings in the field), polarization (transverse electric or transverse magnetic), field symmetry (even or odd), and field profile in the core (sinusoidal or hyperbolic). Of the possible field profiles shown in Fig. 4, the mode shown in (e), having sinusoidal core-field variation with no zero-crossings, is exclusive to dielectric waveguides and is known as the fundamental mode. Modes (h) and (i), having hyperbolic core-field variation, are exclusive to plasmonic waveguides and are known as the gap-plasmon modes. Modes (f) and (g), having sinusoidal core-field variation with ≥1 zero-crossings, are common to both dielectric and plasmonic waveguides.

This rich variety of modes arises out of the solutions of the dispersion equation written for an asymmetric three-layer slab waveguide. In principle, a single dispersion equation can completely describe all the modes of both dielectric and plasmonic waveguides. However, because the nominally-assumed core-field distributions for the two types are different (sinusoidal for dielectric waveguides and hyperbolic for plasmonic waveguides), we choose to write two separate equations for the two types [19

19. C. R. Pollock, Fundamentals of Optoelectronics (McGraw-Hill Professional Publishing, 2003).

, 20

20. J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B 33, 5186–5201 (1986). [CrossRef]

]:

tan(kh)=k(pγc+qγs)k2pqγcγsfordielectricwaveguides.
(9)
tanh(κh)=κ(pαc+qαs)κ2pqαcαsforplasmonicwaveguides.
(10)

For convenience, we have collected the symbol definitions and their expressions in Table 1. Using these definitions, Eqs. (9) and (10) can be cast explicitly as transcendental equations in a single complex variable k or κ. Furthermore, it is evident that Eq. (9) transforms to Eq. (10) for purely imaginary values of k (i.e., k, κ ∊ ℝ). Having solved for k or κ, the mode’s complex effective index n eff may be calculated using relations in Table 1.

Table 1. Definitions of various quantities and their expressions in terms of the material parameters and the perpendicular core wavevector k or κ.

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Fig. 5. Categories of three-layer slab waveguides for the purposes of iterative solution.

We will now begin the description of the actual iterative solution procedure. We saw in section 1 that the convergence of this technique depends crucially on the iteration function. As a result, the iterative form for determining modes with sinusoidal core-fields [Fig. 4(e–g)] is different from the one needed for determining modes with hyperbolic core-fields [Fig. 4(h–i)]. Therefore, we split our description into two broad categories: dielectric waveguides and plasmonic waveguides. These two categories are further divided depending upon the degree of confinement (strong or weak) for dielectric waveguides, and core/cladding type (MDM or DMD) for plasmonic waveguides. Figure 5 depicts the division of the waveguide modes that we have made for the purposes of describing our iterative solution process. In the forthcoming sections, we will obtain and test rapidly-convergent iterative forms for calculating the mode indices of these four sub-categories of three-layer asymmetric slab waveguides. These useful forms will be boxed for the convenience of the reader.

4. Modes of dielectric waveguides

4.1. Strong-confinement dielectric waveguides

Using the half-angle identity tan(z/2)=cotz±1+cot2z, Eq. (9) can be transformed to:

tan(kh/2)=(pqγcγsk2)±GcGsk(pγc+qγs).
(11)

Inverting the tangent function in Eq. (11) gives us the convergent iterative form for strong-confinement waveguides as:

kn+1=2h{Mπ+tan1[(pqγcnγsnkn2)±GcnGsnkn(pγcn+qγsn)]}
(12)

Before embarking on actual computation, we need to specify the parity (+ or - sign), order (M), and the polarization (p and q) of the desired mode. For TE polarization (p=q=1) and even parity (+ sign), the effective indices of successive even TE modes may be simply calculated by setting M=0,1, …m and iterating according to Eq. (12); this yields the TE0, TE2, … TE2m modes. For TE polarization with odd parity (- sign), setting M=1,2, …,m yields the TE1, TE3, … TE2m-1 modes. Note that for even modes, the mode order begins with M=0, whereas for odd modes, it begins with M=1. The procedure for obtaining TM mode indices is identical, with proper input of (p,q) as shown in Table 1.

To illustrate the use of the iterative method, we choose a 1-µm-thick silicon-on-insulator (SOI) slab waveguide operating at 1550 nm as a test structure. The refractive indices of the silicon film, oxide substrate, and air cover are assumed to be √εf=3.50,√εs=1.45, and √εc=1.00 respectively. Using the TE0 mode as an example, Fig. 6 depicts how Eq. (12) converges to the solution. Fig. 6(b) and (c) show the real and imaginary values of the first twenty iterates of Eq. (12). Although the iteration is carried out in terms of k, we have chosen to depict the convergence in terms of the effective index n eff=β/k 0 since this is a more familiar quantity in waveguide analysis. Figure 6(a) shows the LHS and RHS of Eq. (12) and how the iterative scheme converges to the solution. Notice that the slope condition mentioned in section 1 is satisfied for this particular case.

To examine the iterative technique in contrast with standard numerical techniques, we solved for the modes of the same structure using the Newton’s method. Table 2 compares the two solutions. Newton’s method calculations were performed by feeding Eq. (11) to the FindRoot function with an initial guess dependent on the mode order, parity, and polarization. To obtain the initial guesses, we plotted the left- and right-hand sides of Eq. (11) and made a manual estimate based on the intersection of the graphs. Given the multiple solutions inherent in Eq. (11), these initial guesses needed to be fairly close to the actual solution for the Newton’s method to return the correct solution. On the other hand, all iterative solutions in Table 1 were initiated with the same initial guess k 1=Ks. Moreover, the convergence of the iterative technique is completely insensitive to the exact value of this initial guess: we obtained the same effective

Table 2. Comparison of mode indices for a silicon-on-insulator (SOI) slab waveguide operating at 1550 nm computed using the iterative method and the Newton’s method as implemented by the FindRoot function in Mathematica™

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indices for initial guesses ranging from Ks to 105 Ks (including complex values). While a precise mathematical characterization of the convergence behavior is outside the scope of our paper, this exercise does suggest the robustness of the iterative technique with respect to the accuracy of the initial guess.

Fig. 6. Iterative solution of a strong-confinement waveguide using Eq. (12). (a) The convergence plot depicting the normalized LHS (magenta) and RHS (gray) of Eq. (12); f (k) on the y-axis of of (a) refers to the function on the right hand side. The convergence of the real and imaginary parts of the effective index are shown in (b) and (c), respectively.

4.1.1. Symmetric strong-confinement waveguides

If the relative permittivities of the cover and the substrate are equal, then the structure is called a symmetric waveguide. Since several important photonic device structures employ symmetric waveguides, we now give explicit iterative forms for calculating their mode indices. These iterative forms arise out of the simplifications in Eq. (12) due to the equality of εs and εc:

kn+1=2h[Mπ+tan1(pKc2/kn21)]forevenmodes.
(13a)
kn+1=2h[Mπcot1(pKc2/kn21)]foroddmodes.
(13b)

We also note that, like Eq. (12), M assumes values starting from 0 for even modes and 1 for odd modes.

4.2. Weak-confinement dielectric waveguides

Equation (12) converges for a very wide range of refractive indices, wavelengths, and waveguide thicknesses which are commonly used in practical integrated photonics designs. It performs

Table 3. Comparison of mode indices for an Al0.1Ga0.9As/GaAs/air slab waveguide operating at 1550 nm computed using the iterative method and the Newton’s method as implemented by the FindRoot function in Mathematica

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poorly, however, when applied to the weak-confinement single-mode waveguide structures routinely fabricated out of III–V (e.g., GaAs/AlGaAs) or organic materials. In the context of multimode waveguides, the strong-confinement formula encounters convergence problems when used to calculate indices of modes near the waveguide cutoff. We highlight this limitation and its remedy through the following example.

Consider a 1-µm-thick air-clad GaAs waveguide on an Al0.1Ga0.9As substrate operating at 1550 nm. The refractive indices of GaAs and Al0.1Ga0.9As are assumed to be √εf=3.300 and √εs=3.256 respectively. We attempt to find the fundamental TE mode iteratively by inputting the corresponding parameters to Eq. (12) (M=0, p=q=1, and + sign). Figures 7(b) and (c) show the behavior of the real and imaginary parts of the calculated effective index for first 50 iterates. The real part of the effective index initially diverges to reach a maximum at iteration number 30 and begins to converge thereafter. Starting from the thirtieth iteration, the imaginary part oscillates between ±0.01. These oscillations do not damp out even if we increase the number of iterations to 104. A graphical solution using Mathematica, however, indicates that this structure supports TE0 and TM0 modes with indices of approximately 3.266 and 3.263, respectively. We conclude that the iterative scheme in Eq. (12) fails to converge for this example.

This failure to converge can be understood by examining the behavior of the LHS and RHS of Eq. (12) near the intersection point, as shown in Fig. 7(a). Near the solution, |f1(κ)|>1. As a result, the iterates begin to diverge away from the intersection point. In fact, |f1 (κ)| continues to increase away from the intersection, causing a rapidly-increasing divergence. However, at κ⋍0.537k 0, |f1(κ)| abruptly becomes <1, causing the real part of the iteration to converge. This convergence of the real part is misleading, however since the imaginary part shows undamped oscillations. The behavior of the strong confinement formula in this case is similar to the example problem in section 1 illustrated in Fig. 2(d–f).

In summary, the basic cause of the failure of convergence is the breakdown of the slope condition mentioned in section 1. This is remedied by recasting Eq. (9) into a form which satisfies the convergence condition. To this end, we use the identity tan2 z=1/cos2 z-1, rewriting Eq. (9) as:

k2±GcGscos(kh)=pqγcγs.
(14)

The convergent form suitable for weak confinement is obtained by solving for k contained in the γcγs term on the right hand side of Eq. (14):

kn+1=(pqKcKs)2(GcnGsn)2cos2(knh)+(p2q21)kn4p2q2(Kc2+Ks2)2GcnGsncos(knh)
(15)

Fig. 7. (a–c) Iterative solution of a weak-confinement waveguide using Eq. (12). (a) The LHS and RHS of Eq. (12); f (k) on the y-axis denotes the RHS of Eq. (12). Notice the apparent convergence of the real part (b) and the oscillatory divergence in the imaginary part (c) of the effective index. (d–f) Solution of the same weak-confinement waveguide problem using Eq. (15). (d) The LHS and RHS of Eq. (15); f (k) on the y-axis denotes the RHS of Eq. (15). (e) and (f) show the convergence of the real and imaginary parts of the effective index.

Solving for the mode indices of the previously-considered GaAs/Al0.1Ga0.9As waveguide using Eq. (15) yields the effective indices of the TE0 and TM0 modes in excellent agreement with the Newton’s method, as seen from Table 3. The convergence behavior of the effective index is plotted in Fig. 7(d–f). The convergence of Eq. (15) improves as the core–cladding index contrast decreases; similarly, the convergence of Eq. (12) improves for increasing core–cladding index contrast.

4.2.1. Symmetric weak-confinement dielectric waveguides

Similarly to strong-confinement waveguides, having identical cover and substrate indices considerably simplifies the iterative form for weakly-confined slab waveguides, which can be written as:

kn+1=Kc1+p2tan2(knh/2)forevenmodes.
(16a)
kn+1=Kc1+p2cot2(knh/2)foroddmodes.
(16b)

4.3. Extension to photonic wire waveguides

Waveguides in which light is confined in two dimensions and propagates in the third dimension are known as photonic wire (PW) waveguides; optical fibers, as well as ridge, rib, and channel waveguides are examples. Because they offer stronger light confinement compared to slab waveguides, PW waveguides are commonly employed in on-chip optical devices where a small size is essential for achieving several critical device specifications (e.g., speed, low power, integration density, etc.).

Fig. 8. (a) Typical geometry of a SOI photonic wire waveguide. (b) The two steps used in determining the mode index of the PW waveguide using the effective index method. The procedure is illustrated here for a Ey-polarized mode; steps are similar for a Hy-polarized mode.

As an example, suppose that we need to determine the effective index of the fundamental Ey-polarized mode of a sub-micron PW waveguide shown in Fig. 9 (a) with h=450nm and w=300nm. In the first step, we solve for the TE mode equation for the structure shown in Fig. 8(b). Since SOI waveguide constitutes a strong-confinement system, the first step is readily accomplished by iteration of Eq. (12) for the TE condition (namely, M=0, p=q=1). This yields the first effective index as n′=3.073930677459340. In the second step, we consider an x-confined slab with core-index n′ and iterate Eq. (12) with the fundamental TM mode condition. This yields the effective index of the PW waveguide as n″=2.652766507502340.

Fig. 9. (a) Photonic wire waveguide structure used for evaluation of the effective index method (EIM). (b) The relative error in the calculation of the mode index using EIM as a function of the waveguide width. Relative error is defined as |niter-nFEM|/nFEM. The electric field of the fundamental mode of a SOI photonic wire waveguide calculated using (c) the finite element method and (d) the EIM. The operating wavelength is 1550 nm for all calculations.

To test the relative accuracy of the iterative method, we use the finite element method (FEM) to compute the mode index. For the waveguide with the above dimensions, FEM gives n″=2.612594. It is evident that even for sub-mircron dimensions, the relative error defined as |niter-nFEM|/nFEM is only 1.5%. Figure 9(b) shows the variation of the relative error with waveguide dimensions for a fixed width-to-height aspect ratio of 1.5, from which we note the exponential drop in the relative error with increasing waveguide size. The reason for the decrease of accuracy with decreasing size lies in the increased interaction of the electro-magnetic fields with the corner regions of the waveguide. By decomposing a two-dimensional mode-solving problem into two one-dimensional problems, we effectively chose to neglect the interaction of the fields with the waveguide corners. This assumption becomes decreasingly accurate with decreasing waveguide size. Figure 9(c) and (d) show the y-component of the electric field, calculated using FEM and the iterative method, for the sub-micron waveguide considered above. Note the strong electric field at the waveguide corners in Fig. 9(c) calculated using FEM, and its absence in Fig. 9(d) calculated using the iterative method. In addition to highlighting this difference for small waveguides, this exercise illustrates the well-known fact that while EIM successfully calculates the effective indices, it does not guarantee satisfaction of the boundary conditions. It should therefore not be used to calculate fields for wavelength-sized structures.

In spite of this limitation, the EIM remains an intuitive way of accomplishing designs which rely primarily on the knowledge of mode indices. Examples of such designs include several contemporary device structures such as SOI ring resonators, Bragg gratings, Mach-Zehnder interferometers, and directional couplers. By offering a simple means of solving the slab waveguide dispersion equation, the iterative method greatly enhances the efficacy and implementation speed of EIM.

5. Modes of plasmonic waveguides

Propagation loss and waveguide leakage arise routinely in sub-wavelength-scale metallic waveguides. Intense research efforts are currently underway in the field of metal-based optics, also known as plasmonics (see, e.g., [1

1. M. L. Brongersma and P. G. Kik, eds., Surface Plasmon Nanophotonics, vol. 131 of Springer series in optical sciences (Springer, 2007).

] and references contained therein). Extended metal structures support surface plasmon-polariton (SPP) modes that are electromagnetic waves strongly coupled to collective electron oscillations in the metal. To exploit the strong light localization achievable in plasmonic structures, a variety of waveguide configurations have recently been proposed and demonstrated [1

1. M. L. Brongersma and P. G. Kik, eds., Surface Plasmon Nanophotonics, vol. 131 of Springer series in optical sciences (Springer, 2007).

]. However, because of its simplicity, the MDM geometry remains a canonical structure for achieving and studying strong light confinement. As such, a convenient technique for determining the optical modes of MDM waveguides is highly valuable.

We begin the description of the iterative solution technique for plasmonic waveguides by referring to the division of slab waveguide types illustrated in Fig. 4(c) and (d). We first consider the modes of an MDM-type plasmonic waveguide followed by a treatment of the DMD-type.

5.1. Metal-dielectric-metal waveguides

A MDM waveguide supports gap-plasmon and TM-like waveguide modes. Even and odd gap-plasmon modes are practically important, as they offer the strongest sub-wavelength field confinement. However, TM-like modes are also of theoretical value and are necessary for gaining a complete understanding of reflection and transmission phenomena in MDM waveguides and antennas [16

16. S. E. Kocabaş, G. Veronis, D. A. B. Miller, and S. Fan, “Modal analysis and coupling in metal-insulator-metal waveguides,” Phys. Rev. B 79, 035120 (2009). [CrossRef]

]. Effective indices for both these types of modes can be conveniently determined using the iterative technique, as we show in the following.

5.1.1. Gap-plasmon modes

We start with our master plasmonic Eq. (10) and rearrange it as:

κ2+2Sκcoth(κh)+pqαcαs=0.
(17)

κn+1=Sncoth(κnh)±Sn2coth2(κnh)pqαcnαsn
(18)

This simple-looking equation can calculate even (+ sign) and odd (- sign) gap-plasmon mode indices of a wide variety of deep sub-wavelength asymmetric plasmonic waveguides. We illustrate the use of Eq. (18) through two examples. Our first structure is a 50-nm-thick gold-silica-silver slab waveguide operating at 1550 nm. The relative permittivities of gold, silica, and silver are assumed to be -95.92-i10.97, 2.1025, and -143.49-i9.52, respectively. The effective index of the even gap-plasmon mode, calculated using the + sign in Eq. (18), is given in Table 4. Figure 10 shows plots of the LHS and RHS of Eq. (18) and the convergence of the real and imaginary parts of the effective index.

The thin 50-nm slab considered above does not support an odd (antisymmetric) gap-plasmon mode. However, increasing the silica thickness to 3µm allows the structure to support gap-plasmon modes of both symmetries. We calculate the effective indices iteratively using Eq. (18) with + and - signs for odd and even gap-plasmon modes respectively. Once again, the iterative method agrees with the Newton’s method (Table 4).

5.1.2. TM-like waveguide modes

The other class of modes supported by a MDM waveguide are the TM-like waveguide modes with profiles as shown in Fig. 4(f) and (g). For these modes, κ is purely imaginary and plasmon Eq. (10) transforms to dielectric Eq. (9). The iterative form for obtaining the indices of these modes is identical to Eq. (12), with only a few slight differences as regards its implementation.

Fig. 10. (a) Normalized left- and right-hand sides of Eq. (18); f (κ) refers to the RHS. Convergence of the real (b) and the imaginary (c) parts of the effective index for the fundamental gap-plasmon mode.

Table 4. Effective indices of various modes of MDM waveguides operating at 1550 nm obtained using the iterative method, with a comparison to the solutions calculated using the Newton’s method as implemented by the FindRoot function in Mathematica.

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For the case of dielectric waveguides, the mode index M assumed integer values starting from 0 for even modes and 1 for odd modes. For MDM waveguides, this is reversed: M assumes integer values starting from 1 for even modes and 0 for odd modes. This is a consequence of the signs of p and q being reversed for MDM waveguides due to the negative permittivity of the metal “claddings.” Additionally, because of the large index difference between metals and dielectrics, the TM-like modes of MDM waveguides are almost always calculated using the strong-confinement formula in Eq. (12).

5.1.3. Symmetric MDM waveguides

Many contemporary high-confinement architectures employ the symmetricMDMwaveguide as their skeleton structure. Equality of the relative permittivities of the substrate and cover further simplify Eq. (18) for the gap-plasmon modes. We can express the resulting iterative forms as:

κn+1=pκn2+Kc2tanh(κnh/2)forevengapplasmon.
(19a)
κn+1=pκn2+Kc2coth(κnh/2)foroddgapplasmon.
(19b)

5.2. Dielectric-metal-dielectric waveguides

The second type of plasmonic waveguide structure commonly encountered in practice is an DMD waveguide, as shown in Fig. 4(d). In theory, metal films of arbitrary thickness in a homogenous dielectric medium (including air) or in Kretschmann-type coupling configurations [21

21. H. Raether, “Surface plasmons on smooth and rough surfaces and on gratings,” Springer Tracts Mod. Phys. 111, 1–133 (1988).

] are examples of DMD waveguides. In practice, we refer to metallic waveguides as DMD-type only if the SPP modes on the two metal-dielectric interfaces are coupled. Because of the rapid decay of the fields (with distance) inside metals, such a mode-coupling is possible only for thin (h<100nm) metal films.

To obtain the iterative form for determining mode indices of DMD waveguides, we write Eq. (10) as:

tanhκh=2Aκκ2+A2B2,
(20)

where A and B are defined in Table 1. Considering this as a quadratic equation in A and B leads us to the desired iterative forms. We start by identifying the lower-index dielectric as the substrate and using initial guesses of A 0=k 0, B 0=0, and κ 0=k 0. We then iterate to obtain five different quantities successively, using the following equations in the order shown:

an=κncothκnh±Bn2+κn2csch2(κnh),
(21a)
bn=an2+κn2+2anκncoth(κnh),
(21b)
κn+1=(an+bn)2/p2+Qc2,
(21c)
An+1=(pζc,n+1+qξs,n+1)/2,
(21d)
Bn+1=(pζc,n+1qζs,n+1)/2.
(21e)

We will now illustrate the use of Eq. (21) through specific examples. Consider the operation of a 50-nm-thick silver film on a silica substrate at 1550 nm. The relative permittivities of silver and silica are the same as assumed previously; the top cover is air (εc=1). We start by specifying A 0,B 0, and κ 0, and obtain their successive values according to Eq. (21). The convergence of An,Bn, and κn is shown in Fig. 11; Table 5 compares the solution obtained using the iterative scheme with that computed using the Newton’s method. In our computations, the Newton’s method was unable to return the complex mode index unless we gave a very precise

Table 5. Effective indices of various modes of an DMD waveguide operating at 1550 nm obtained using the iterative method and their comparison with solutions obtained using the Newton’s method as implemented by the FindRoot function in Mathematica.

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initial guess for both the real and the imaginary parts. On the other hand, the iterative method computed the complex mode index regardless of the initial guess.

Fig. 11. Convergence of the real and imaginary parts of κ, A, and B in Eq. (21) for the case of a 50 nm thick silver-silica-silver waveguide operating at 1550 nm.

5.2.1. Symmetric DMD waveguides

Symmetric DMD waveguides appear in the form of idealized waveguide geometries such as metal films in homogenous dielectric media (including air). Fortunately, for the symmetric case, the iterative scheme consists of a single equation which can be written as:

κn+1=Qc1p2tanh2(κnh/2)forevenplasmonmode.
(22a)
κn+1=Qc1p2coth2(κnh/2)foroddplasmonmode.
(22b)

These equations do not follow automatically from Eq. (21) but instead have to be derived separately by considering the dispersion equation for the symmetric DMD waveguide. Eq. (22) are preferable to Eq. (21) for analyzing symmetric DMD structures owing to better convergence behavior and an obvious ease in programming.

6. Extension to multilayer structures

7. Conclusion

We have presented a robust and an easy-to-implement iterative technique for determining complex propagation constants of asymmetric dielectric and plasmonic waveguides. At the heart of our procedure are the closed-form iteration functions, namely the boxed Eqs. (12), (15), (18), and (21). In addition to the programming ease, the iterative technique has an inherent ability to give arbitrary-precision answers—a feat difficult to achieve using graphical or curve-fitting algorithms such as the reflection-pole method. Because of its insensitivity to initial guess, it is our hope that this technique will help facilitate design tasks that require rapid and automated calculation of mode indices for a variety of nanophotonic structures having a rectangular geometry.

Acknowledgements

We thank Dr. E. Kocabaş and Prof. D. A. B. Miller for several helpful discussions. We also gratefully acknowledge the financial support from the Si-based Laser Initiative of the Multi-disciplinary University Research Initiative (MURI) under the Air Force Aerospace Research Award No. FA9550-06-1-0470 and the MARCO Interconnect Focus Center.

References and links

1.

M. L. Brongersma and P. G. Kik, eds., Surface Plasmon Nanophotonics, vol. 131 of Springer series in optical sciences (Springer, 2007).

2.

R. Zia, J. A. Schuller, and M. L. Brongersma, “Plasmonics: The next chip-scale technology,” Materials Today 9, 20–27 (2006). [CrossRef]

3.

R. A. Pala, J. S. White, E. S. Barnard, J. Liu, and M. L. Brongersma, “Design of plasmonic thin-film solar cells with broadband absorption enhancements,” Adv. Mater. 21, 1–6 (2009). [CrossRef]

4.

W. H. Press, S. A. Teukolsky, W. J. Vetterling, and B. P. Flannery, Numerical recipes in C++, The art of scientific computing (Cambridge University Press, 2002), 2nd ed.

5.

A. W. Snyder and J. Love, Optical waveguide theory (Science Paperbacks, 1983).

6.

J. Takahara, S. Yamagishi, H. Taki, A. Morimoto, and T. Kobayashi, “Guiding of a one-dimensional guiding of a one-dimensional optical beam with nanometer diameter,” Opt. Lett. 22, 475–477 (1997). [CrossRef] [PubMed]

7.

J.-C. Weeber, Y. Lacroute, and A. Dereux, “Optical near-field distributions of surface plasmon waveguide modes,” Phys. Rev. B 68, 115401 (2003). [CrossRef]

8.

R. Zia, A. Chandran, and M. L. Brongersma, “Dielectric waveguide model for guided surface polaritons,” Opt. Lett. 30, 1473–1475 (2005). [CrossRef] [PubMed]

9.

R. Zia, J. A. Schuller, and M. L. Brongersma, “Near-field characterization of guided polariton propagation and cutoff in surface plasmon waveguides,” Phys. Rev. B 74, 165415 (2006). [CrossRef]

10.

R. Zia, M. D. Selker, and M. L. Brongersma, “Leaky and bound modes of surface plasmon waveguides,” Phys. Rev. B 71, 165431 (2005). [CrossRef]

11.

G. Veronis and S. Fan, “Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides,” Appl. Phys. Lett. 87, 131102 (2005). [CrossRef]

12.

G. Veronis and S. Fan, “Guided subwavelength plasmonic mode supported by a slot in a thin metal film,” Opt. Lett. 30, 3359–3361 (2005). [CrossRef]

13.

S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.-Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength waveguide components including interferometers and ring resonators,” Nature 440, 508–511 (2006). [CrossRef] [PubMed]

14.

E. Anemogiannis, E. N. Glytsis, and T. K. Gaylord, “Determination of guided and leaky modes in lossless and lossy planar multilayer optical waveguides: reflection pole method and wavevector density method,” J. Lightwave Technol. 17, 929–941 (1999). [CrossRef]

15.

R. Zia, M. D. Selker, P. B. Catrysse, and M. L. Brongersma, “Geometries and materials for subwavelength surface plasmon modes,” J. Opt. Soc. Am. A 21, 2442–2446 (2004). [CrossRef]

16.

S. E. Kocabaş, G. Veronis, D. A. B. Miller, and S. Fan, “Modal analysis and coupling in metal-insulator-metal waveguides,” Phys. Rev. B 79, 035120 (2009). [CrossRef]

17.

J. P. McKelvey, “Simple iterative procedures for solving transcendental equations with the electronic slide rule,” Am. J. Phys. 43, 331–334 (1975). [CrossRef]

18.

J. Dugundji and A. Granas, Fixed Point Theory (Springer-Verlag, 2003).

19.

C. R. Pollock, Fundamentals of Optoelectronics (McGraw-Hill Professional Publishing, 2003).

20.

J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B 33, 5186–5201 (1986). [CrossRef]

21.

H. Raether, “Surface plasmons on smooth and rough surfaces and on gratings,” Springer Tracts Mod. Phys. 111, 1–133 (1988).

OCIS Codes
(000.4430) General : Numerical approximation and analysis
(240.6680) Optics at surfaces : Surface plasmons
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Physical Optics

History
Original Manuscript: October 13, 2009
Revised Manuscript: December 11, 2009
Manuscript Accepted: December 14, 2009
Published: December 17, 2009

Citation
Rohan D. Kekatpure, Aaron C. Hryciw, Edward S. Barnard, and Mark L. Brongersma, "Solving dielectric and plasmonic waveguide dispersion relations on a pocket calculator," Opt. Express 17, 24112-24129 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-26-24112


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References

  1. M. L. Brongersma and P. G. Kik, eds., Surface Plasmon Nanophotonics, Springer series in Optical Sciences (Springer, 2007) Vol. 131.
  2. R. Zia, J. A. Schuller, and M. L. Brongersma, "Plasmonics: The next chip-scale technology," Maters. Today 9, 20-27 (2006). [CrossRef]
  3. R. A. Pala, J. S. White, E. S. Barnard, J. Liu, and M. L. Brongersma, "Design of plasmonic thin-film solar cells with broadband absorption enhancements," Adv. Mater. 21, 1-6 (2009). [CrossRef]
  4. W. H. Press, S. A. Teukolsky,W. J. Vetterling, and B. P. Flannery, Numerical recipes in C++, The art of scientific computing (Cambridge University Press, 2002), 2nd ed.
  5. A. W. Snyder and J. Love, Optical Waveguide Theory (Science Paperbacks, 1983).
  6. J. Takahara, S. Yamagishi, H. Taki, A. Morimoto, and T. Kobayashi, "Guiding of a one-dimensional guiding of a one-dimensional optical beam with nanometer diameter," Opt. Lett. 22, 475-477 (1997). [CrossRef] [PubMed]
  7. J.-C. Weeber, Y. Lacroute, and A. Dereux, "Optical near-field distributions of surface plasmon waveguide modes," Phys. Rev. B 68, 115401 (2003). [CrossRef]
  8. R. Zia, A. Chandran, and M. L. Brongersma, "Dielectric waveguide model for guided surface polaritons," Opt. Lett. 30, 1473-1475 (2005). [CrossRef] [PubMed]
  9. R. Zia, J. A. Schuller, and M. L. Brongersma, "Near-field characterization of guided polariton propagation and cutoff in surface plasmon waveguides," Phys. Rev. B 74, 165415 (2006). [CrossRef]
  10. R. Zia, M. D. Selker, and M. L. Brongersma, "Leaky and bound modes of surface plasmon waveguides," Phys. Rev. B 71, 165431 (2005). [CrossRef]
  11. G. Veronis and S. Fan, "Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides," Appl. Phys. Lett. 87, 131102 (2005). [CrossRef]
  12. G. Veronis and S. Fan, "Guided subwavelength plasmonic mode supported by a slot in a thin metal film," Opt. Lett. 30, 3359-3361 (2005). [CrossRef]
  13. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.-Y. Laluet, and T. W. Ebbesen, "Channel plasmon subwavelength waveguide components including interferometers and ring resonators," Nature 440, 508-511 (2006). [CrossRef] [PubMed]
  14. E. Anemogiannis, E. N. Glytsis, and T. K. Gaylord, "Determination of guided and leaky modes in lossless and lossy planar multilayer optical waveguides: reflection pole method and wavevector density method," J. Lightwave Technol. 17, 929-941 (1999). [CrossRef]
  15. R. Zia, M. D. Selker, P. B. Catrysse, and M. L. Brongersma, "Geometries and materials for subwavelength surface plasmon modes," J. Opt. Soc. Am. A 21, 2442-2446 (2004). [CrossRef]
  16. S. E. Kocabas¸, G. Veronis, D. A. B. Miller, and S. Fan, "Modal analysis and coupling in metal-insulator-metal waveguides," Phys. Rev. B 79, 035120 (2009). [CrossRef]
  17. J. P. McKelvey, "Simple iterative procedures for solving transcendental equations with the electronic slide rule," Am. J. Phys. 43, 331-334 (1975). [CrossRef]
  18. J. Dugundji and A. Granas, Fixed Point Theory (Springer-Verlag, 2003).
  19. C. R. Pollock, Fundamentals of Optoelectronics (McGraw-Hill Professional Publishing, 2003).
  20. J. J. Burke, G. I. Stegeman, and T. Tamir, "Surface-polariton-like waves guided by thin, lossy metal films," Phys. Rev. B 33, 5186-5201 (1986). [CrossRef]
  21. Q1Q2. H. Raether, "Surface plasmons on smooth and rough surfaces and on gratings," Springer Tracts Mod. Phys. 111, 1-133 (1988).

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