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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 17 — Aug. 13, 2012
  • pp: 18665–18678
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Analysis of long-range surface plasmon polaritons in nonlinear plasmonic waveguides using pseudospectral method

Chia-Chien Huang  »View Author Affiliations


Optics Express, Vol. 20, Issue 17, pp. 18665-18678 (2012)
http://dx.doi.org/10.1364/OE.20.018665


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Abstract

A full-vectorial pseudospectral method is reported for solving the mode characteristics of nonlinear dielectric and plasmonic waveguides. The coupled equations are formulated in terms of transverse magnetic-field components, and self-consistent solutions are obtained through an iterative procedure. The proposed scheme applies in a saturable medium with biaxial anisotropy of practical interest. The accuracy and efficiency of this scheme are demonstrated by solving the mode bistability of a nonlinear dielectric optical waveguide, analyzed by the well-known finite-element-method-based imaginary-distance beam propagation method. Furthermore, the relationship between geometry and input power is studied by analyzing the power dispersion curve of the long-range surface plasmon polariton modes of a nonlinear plasmonic waveguide.

© 2012 OSA

1. Introduction

The intensity-dependent properties of nonlinear dielectric optical waveguides enrich propagation phenomena and induce effects that are entirely different than those observed for materials with linear permittivity responses. As a result, many functional photonic devices such as waveguide modulators [1

1. T. H. Wood, “Multiple quantum well (MQW) waveguide modulators,” J. Lightwave Technol. 6(6), 743–757 (1988). [CrossRef]

], optical switches [2

2. S. Radic, N. George, and G. P. Agrawal, “Optical switching in λ/4-shifted nonlinear periodic structures,” Opt. Lett. 19(21), 1789–1791 (1994). [CrossRef] [PubMed]

], wavelength auto-routers [3

3. Y. D. Wu, “New all-optical wavelength auto-router based on spatial solitons,” Opt. Express 12(18), 4172–4177 (2004). [CrossRef] [PubMed]

], and logic gates [4

4. T. Fujisawa and M. Koshiba, “All-optical logic gates based on nonlinear slot-waveguide couplers,” J. Opt. Soc. Am. B 23(4), 684–691 (2006). [CrossRef]

] are based on nonlinear mechanisms such as low threshold power, self-focusing, and bistability. In recent years, the dispersion relations of surface plasmon polaritons (SPPs) surrounded by a nonlinear dielectric medium [5

5. A. R. Davoyan, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear plasmonic slot waveguides,” Opt. Express 16(26), 21209–21214 (2008). [CrossRef] [PubMed]

10

10. A. Degiron and D. R. Smith, “Nonlinear long-range plasmonic waveguides,” Phys. Rev. A 82(3), 033812 (2010). [CrossRef]

] have been reported to offer additional factors for tailoring the mode characteristics of SPPs. In Ref [5

5. A. R. Davoyan, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear plasmonic slot waveguides,” Opt. Express 16(26), 21209–21214 (2008). [CrossRef] [PubMed]

], Davoyan et al. studied a metal-dielectric slot waveguide and predicted the symmetry-breaking bifurcation of symmetry modes with critical power that depends on the slot width. In Refs [6

6. I. D. Rukhlenko, A. Pannipitiya, and M. Premaratne, “Dispersion relation for surface plasmon polaritons in metal/nonlinear-dielectric/metal slot waveguides,” Opt. Lett. 36(17), 3374–3376 (2011). [CrossRef] [PubMed]

,7

7. I. D. Rukhlenko, A. Pannipitiya, M. Premaratne, and G. Agrawal, “Exact dispersion relation for nonlinear plasmonic waveguides,” Phys. Rev. B 84(11), 113409 (2011). [CrossRef]

], Rukhlenko et al. derived an exact dispersion relation for SPPs using an exact field decomposition of transverse magnetic (TM) modes, and showed that it enables backward-propagating modes. For enhancing nanoscale nonlinear effects, Davoyan et al. [8

8. A. R. Davoyan, I. V. Shadrivov, A. A. Zharov, D. K. Gramotnev, and Y. S. Kivshar, “Nonlinear nanofocusing in tapered plasmonic waveguides,” Phys. Rev. Lett. 105(11), 116804 (2010). [CrossRef] [PubMed]

] proposed nonlinear nanofocusing in a tapered plasmonic waveguide. In Ref [9

9. J. R. Salgueiro and Y. S. Kivshar, “Nonlinear plasmonic directional couplers,” Appl. Phys. Lett. 97(8), 081106 (2010). [CrossRef]

], the authors proposed a nonlinear plasmonic directional coupler and showed how it substantially differs from a nonlinear dielectric coupler using the finite difference time domain (FDTD) approach. However, these studies [5

5. A. R. Davoyan, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear plasmonic slot waveguides,” Opt. Express 16(26), 21209–21214 (2008). [CrossRef] [PubMed]

9

9. J. R. Salgueiro and Y. S. Kivshar, “Nonlinear plasmonic directional couplers,” Appl. Phys. Lett. 97(8), 081106 (2010). [CrossRef]

] concerned only planar structures, not practical two-dimensional (2D) channel waveguides. Until now, thin metal strips surrounded by Kerr-type nonlinear materials [10

10. A. Degiron and D. R. Smith, “Nonlinear long-range plasmonic waveguides,” Phys. Rev. A 82(3), 033812 (2010). [CrossRef]

] have been the only devices studied with regard to the mode properties of long-range (LR) SPPs. These studies used the commercial eigenmode solver (COMSOL Multiphysics), which is based on the finite-element method (FEM).

2. Mathematical formulations

The magnetic vector field (H) of a monochromatic light wave with time dependence of exp(jωt) propagating along the z direction obeys the wave equation
×([ε]1×H)ω2μ0H=0,
(1)
where ω is the angular frequency, μ0 is the vacuum permeability, and [ε] is the nonlinear permittivity tensor. For the biaxial nature of materials, the nonlinear permittivity tensor can be expressed as
[ε]=ε0[εr]=ε0[ε˜x000ε˜y000ε˜z],
(2)
ε˜i=εi+(c0ε0εin¯)f(Ex,Ey,Ez),(i=x,y,z),
(3)
where ε0 is the vacuum permittivity, [εr] is the relative permittivity tensor, εi is the linear relative permittivity, c0 is the speed of light in vacuum, n¯is the nonlinearity coefficient, and the function f (Ex, Ey, Ez) expresses the flux-dependent behavior of the nonlinear medium. Considering the stationary guided modes of the nonlinear medium, the magnetic-field components are assumed to be of the form exp(−jßz), where β = k0ne is the propagation constant along the z direction, k0=ω(μ0ε0)1/2 is the wave number in vacuum, and ne is the effective refractive index of the guided modes. Using the divergence relationH=0, the magnetic-field component Hz in Eq. (1) can be represented by two transverse magnetic components Hx and Hy to obtain the following full vector eigenvalue equation:
[PxxPxyPyxPyy][HxHy]=β2[HxHy],
(4)
where the differential operators Pxx, Pxy, Pyx, and Pyy are defined as
Pxx=2x2+ε˜yε˜z2y2+ε˜yy(1ε˜z)y+k02ε˜y,
(5a)
Pxy=(1ε˜yε˜z)2xyε˜yy(1ε˜z)x,
(5b)
Pyx=(1ε˜xε˜z)2xyε˜xx(1ε˜z)y,
(5c)
Pyy=ε˜xε˜z2x2+2y2+ε˜xx(1ε˜z)x+k02ε˜x.
(5d)
In the proposed scheme, we divide the computational window at interfaces between different materials into several subdomains with uniform or continuous refractive-index profiles, and then assemble these subdomains using physical interfacial conditions. The interfacial conditions used include continuous normal and tangential components of the magnetic fields Hx and Hy at each intra-element boundary, and the other two interfacial conditions are obtained from the divergence free condition of the magnetic field vectorH=0and the Ampere’s law×H=[ε]E, where E is the electric field vector. To represent the mode fields with only Hx and Hy components, we express the condition H=0as
Hz=1jβ(Hxx+Hyy),
(6)
where Hz is the longitudinal component of the magnetic field. For the condition ×H=[ε]E, the longitudinal component of electric field Ez is expressed as
Ez=jωε0ε˜z(HxyHyx).
(7)
Equations (6) and (7) are used as interfacial conditions according to the continuities of Hz and Ez at interfaces.

To compute the flux-dependent behavior of the nonlinear medium, the electric-field components that are needed to compute the nonlinear relative permittivities in Eq. (3) can be derived by Maxwell’s equations:
Ex=neZ0ε˜x[Hy1β2y(Hxx+Hyy)],
(8a)
Ey=neZ0ε˜y[Hx1β2x(Hxx+Hyy)],
(8b)
Ez=jneZ0ε˜z[1βy(HxyHyx)],
(8c)
where Z0 denotes the intrinsic impedance in vacuum (Z0 = 377 Ω). The input power along the z direction is given by
P=12(ExHy*EyHx*)dxdy=neZ02[1ε˜y|Hx|2+1ε˜x|Hy|21ε˜yβ2x(Hxx+Hyy)Hx*1ε˜xβ2y(Hxx+Hyy)Hy*]dxdy,
(9)
where * denotes the complex conjugate.

3. Computational schemes

In the PSM, the computational window is divided into several subdomains at interfaces between different materials with uniform or continuous refractive-index profiles. In each subdomain, the transverse magnetic-field components are expanded as follows into a product of suitable basis functions comprising the Lagrange-type interpolations θ(x) in the x direction, ψ(y) in the y direction, and the Hx and Hy field components denoted Hx,pq and Hy,pq at the (nx + 1) × (ny + 1) grid (collocation) points:
Hx(x,y)=p=0nxq=0nyθp(x)ψq(y)Hx,pq,
(10a)
Hy(x,y)=p=0nxq=0nyθp(x)ψq(y)Hy,pq.
(10b)
Here, θp(xm)=δmp, ψp(ym)=δmp, and δmp denotes the Kronecker delta. For an arbitrary interior rectangular subdomain, the (nx + 1) × (ny + 1) collocation points are illustrated in Fig. 1(a)
Fig. 1 (a) Mesh division of an arbitrary interior subdomain and (b) mesh division of a problem with two subdomains (labeled 1 and 2).
(where nx = ny = 10 is illustrated).

Substituting Eqs. (10a) and (10b) into Eq. (4), we find that Eq. (4) is satisfied exactly at the specific (nx + 1) × (ny + 1) collocation points determined by the chosen basis functions. The operators Pxx, Pxy, Pyx, and Pyy in Eq. (4) are
Pxx=i=0nxj=0ny[p=0nxq=0ny{θp(2)(x)ψq(y)+ε˜yε˜zθp(x)ψq(2)(y)+ε˜yy(1ε˜z)θp(x)ψq(1)(y)+k02ε˜yθp(x)ψq(y)}]x=xi,y=yj,
(11a)
Pxy=i=0nxj=0ny[p=0nxq=0ny{(1ε˜yε˜z)θp(1)(x)ψq(1)(y)ε˜yy(1ε˜z)θp(1)(x)ψq(y)}]x=xi,y=yj,
(11b)
Pyx=i=0nxj=0ny[p=0nxq=0ny{(1ε˜xε˜z)θp(1)(x)ψq(1)(y)ε˜xx(1ε˜z)θp(x)ψq(1)(y)}]x=xi,y=yj,
(11c)
Pyy=i=0nxj=0ny[p=0nxq=0ny{ε˜xε˜zθp(2)(x)ψq(y)+θp(x)ψq(2)(y)+ε˜xx(1ε˜z)θp(1)(x)ψq(y)+k02ε˜xθp(x)ψq(y)}]x=xi,y=yj,
(11d)
where θp(h)(x)andψq(h)(y)indicate the hth-order derivatives of θp(x) with respect to x and ψq(y) with respect to y, respectively. After obtaining a matrix eigenvalue equation for each subdomain, a global matrix equation is then obtained by assembling all the matrix equations of all subdomains. Assuming that the computational window is divided into m subdomains, the pattern of the global matrix forms the following block-diagonal matrix:
[Q10000Q200000000Qm][H1H2Hm]=β2[H1H2Hm],
(12)
where
Qr=[PxxrPxyrPyxrPyyr],Ηr=[HxrHyr],(r=1,2,3,,m).
(13)
In Eq. (12), the linear equations at the interface points shared by more than two subdomains have to be removed by imposing physical interface conditions, including the continuous normal and tangential components of the magnetic fields Hx and Hy boundary, and the continuities of Hz and Ez in Eqs. (6) and (7), respectively. Accordingly, the number of grid points at each subdomain is reduced to (nx − 1) × (ny − 1). For brevity, we formulate the coupling of Hx and Hy using only two subdomains (labeled 1 and 2), as shown in Fig. 1(b). At interface boundary points [i.e., (xnx(1),yi)=(x0(2),yi), where i = 0, 1, 2, …, ny, shown by the red circles] located in the vertical red line and shared by two subdomains, the condition of continuous normal and tangential components of the magnetic fields Hx and Hy is implicitly satisfied, and the continuity of Hz in Eq. (6) gives
[Rx(1)Ry(1)][Hx(1)Hy(1)]=[Rx(2)Ry(2)][Hx(2)Hy(2)],
(14)
where
Rs(1)=[Rs(1)(ψ0(y0(1)))Rs(1)(ψ1(y0(1)))...Rs(1)(ψny(y0(1)))Rs(1)(ψ0(y1(1)))Rs(1)(ψ1(y1(1)))...Rs(1)(ψny(y1(1)))..................Rs(1)(ψ0(yny(1)))Rs(1)(ψ1(yny(1)))...Rs(1)(ψny(yny(1)))],s=x,y,
(15a)
Rx(1)(ψi(y0(1)))=[θ0(1)(xnx(1))ψi(y0(1))θ1(1)(xnx(1))ψi(y0(1))...θnx(1)(xnx(1))ψi(y0(1))],Ry(1)(ψi(y0(1)))=[θ0(xnx(1))ψi(1)(y0(1))θ1(xnx(1))ψi(1)(y0(1))...θnx(xnx(1))ψi(1)(y0(1))],
(15b)
Rs(2)=[Rs(2)(ψ0(y0(2)))Rs(2)(ψ1(y0(2)))...Rs(2)(ψny(y0(2)))Rs(2)(ψ0(y1(2)))Rs(2)(ψ1(y1(2)))...Rs(2)(ψny(y1(2)))..................Rs(2)(ψ0(yny(2)))Rs(2)(ψ1(yny(2)))...Rs(2)(ψny(yny(2)))],s=x,y,
(15c)
Rx(2)(ψi(y0(2)))=[θ0(1)(x0(2))ψi(y0(2))θ1(1)(x0(2))ψi(y0(2))...θnx(1)(x0(2))ψi(y0(2))],Ry(2)(ψi(y0(2)))=[θ0(x0(2))ψi(1)(y0(2))θ1(x0(2))ψi(1)(y0(2))...θnx(x0(2))ψi(1)(y0(2))],
(15d)
and the continuity of Ez in Eq. (7) gives
[Ux(1)Uy(1)][Hx(1)Hy(1)]=[Ux(2)Uy(2)][Hx(2)Hy(2)],
(16)
where
Us(1)=[Us(1)(ψ0(y0(1)))Us(1)(ψ1(y0(1)))...Us(1)(ψny(y0(1)))Us(1)(ψ0(y1(1)))Us(1)(ψ1(y1(1)))...Us(1)(ψny(y1(1)))..................Us(1)(ψ0(yny(1)))Us(1)(ψ1(yny(1)))...Us(1)(ψny(yny(1)))],s=x,y,
(17a)
Ux(1)(ψi(y0(1)))=1ε˜z(1)[θ0(xnx(1))ψi(1)(y0(1))θ1(xnx(1))ψi(1)(y0(1))...θnx(xnx(1))ψi(1)(y0(1))],Uy(1)(ψi(y0(1)))=1ε˜z(1)[θ0(1)(xnx(1))ψi(y0(1))θ1(1)(xnx(1))ψi(y0(1))...θnx(1)(xnx(1))ψi(y0(1))],
(17b)
Us(2)=[Us(2)(ψ0(y0(2)))Us(2)(ψ1(y0(2)))...Us(2)(ψny(y0(2)))Us(2)(ψ0(y1(2)))Us(2)(ψ1(y1(2)))...Us(2)(ψny(y1(2)))..................Us(2)(ψ0(yny(2)))Us(2)(ψ1(yny(2)))...Us(2)(ψny(yny(2)))],s=x,y,
(17c)
Ux(2)(ψi(y0(2)))=1ε˜z(2)[θ0(x0(2))ψi(1)(y0(2))θ1(x0(2))ψi(1)(y0(2))...θnx(x0(2))ψi(1)(y0(2))],Uy(2)(ψi(y0(2)))=1ε˜z(2)[θ0(1)(x0(2))ψi(y0(2))θ1(1)(x0(2))ψi(y0(2))...θnx(1)(x0(2))ψi(y0(2))].
(17d)
In Eqs. (14) and (16), Hx(1) and Hy(1)denote the magnetic field components of Hx and Hy, respectively, at the collocation point in subdomain 1, including the boundary points. Through Eqs. (14) and (16), the field unknowns at interface boundary points appearing in Eq. (12) can be represented by interior collocation points inside subdomains 1 and 2. After considering the physical interface conditions, the final matrix equation is no longer in a block-diagonal form but becomes a matrix with approximately half full elements.

  • (i) For a given input power P, specify the mode effective index and mode field of the corresponding linear condition as initial values.
  • (ii) Solve the final matrix equation to find the new ne and H, and then normalize the numerically calculated fieldH=[Hx,Hy]T to beH¯=[H¯x,H¯y]Tby setting
    [1ε˜y|H¯x|2+1ε˜x|H¯y|21ε˜yβ2x(H¯xx+H¯yy)H¯x*1ε˜xβ2y(H¯xx+H¯yy)H¯y*]dxdy=1,
    (20)

    where T denotes the transpose. The actual magnetic field can be expressed as Hactual=2P/(neZ0)H¯.

  • (iii) Once we obtain the new ne and Hactual, the electric-field components are calculated by Eqs. 8(a)8(c), and then the updated nonlinear permittivities are obtained through Eq. (3).
  • (iv) Repeat steps (ii) and (iii) until the criterion (nes+1nes)<106 is satisfied, where s denotes the s-th iteration.

4. Simulation results and discussion

To verify the numerical accuracy and effectiveness of the proposed PSM, we calculate the power dispersion relation of a nonlinear strip dielectric optical waveguide, which has been solved by Obayya et al. using the FEM-based IDBPM [14

14. S. S. A. Obayya, B. M. A. Rahman, K. T. V. Grattan, and H. A. E. Mikati, “Full vectorial finite–element solution of nonlinear bistable optical waveguides,’,” IEEE J. Quantum Electron. 38(8), 1120–1125 (2002). [CrossRef]

]. Furthermore, for a nonlinear plasmonic waveguide composed of a thin metal film surrounded by a saturable nonlinear substrate and linear cladding, we analyze how the stationary LR–SPP mode depends on the geometrical parameters that vary according to the power level.

4.1 A nonlinear strip dielectric optical waveguide

A strip waveguide with width w = 2 μm and thickness t = 1.2 μm is illustrated in Fig. 2(a)
Fig. 2 Schematic of (a) cross section of nonlinear strip waveguide with a nonlinear substrate and (b) division of its computational domain.
. The permittivities of the linear core and linear cladding regions are εco = 1.572 and εcl = 1.552, respectively, at the operating wavelength λ = 0.515 μm.

The nonlinear permittivity of the saturable nonlinear substrate is considered as
ε˜sub=εsub+Δεsat[1exp(γ|E|2Δεsat)],
(21)
where εsub = 1.552, γ = c0ε0εsubn¯, n¯=109 m2/W (MBAA liquid crystal), and Δεsat=0.31 [14

14. S. S. A. Obayya, B. M. A. Rahman, K. T. V. Grattan, and H. A. E. Mikati, “Full vectorial finite–element solution of nonlinear bistable optical waveguides,’,” IEEE J. Quantum Electron. 38(8), 1120–1125 (2002). [CrossRef]

]. By the PSM, the geometry structure of the waveguide is divided into 9 subdomains, as shown in Fig. 2(b), and the mode fields in each subdomain are expanded with suitable basis functions. For example, subdomains 1, 3, 7, and 9 are semi-infinite in both x and y directions; thus the mode fields are all expanded with LGFs. As for subdomains 2 and 8, the mode profiles in the x direction are expanded with Chebyshev polynomials and those in the y direction are expanded with LGFs. Considering the fundamental quasi-transverse-electric (quasi-TE) modeH11y, the dispersion curve calculated by the present scheme with 15 basis-function terms in each subdomain is shown in Fig. 3
Fig. 3 Effective index versus input power for fundamental quasi-TE modeH11y.
along with that obtained by the FEM-based IDBPM [14

14. S. S. A. Obayya, B. M. A. Rahman, K. T. V. Grattan, and H. A. E. Mikati, “Full vectorial finite–element solution of nonlinear bistable optical waveguides,’,” IEEE J. Quantum Electron. 38(8), 1120–1125 (2002). [CrossRef]

]. To satisfy the convergent criterion of 10−6, the total unknowns needed by the present PSM are 2× (3×15 − 2) × (3×15 − 2) = 3698. The numerical results calculated by the two schemes agree well and are shown in Fig. 3. This result demonstrates the accuracy and effectiveness of the proposed PSM. For lower input power, the effective index increases slightly as the input power increases. Once the power exceeds the threshold Pth = 83 μW, the bistable phenomenon begins, as shown by the sharp jump of the effective index (see Fig. 3). The transition of the mode behavior is due to self-focusing within the nonlinear substrate and leads to power shifting from the core region to the substrate region.

To display mode switching, the major (Hy) and minor (Hx) magnetic fields of the stationary mode at a power of P = 80μW on the lower branch of the dispersion curve are shown in Fig. 4(a)
Fig. 4 (a) Major magnetic field component (Hy) and (b) minor field component (Hx) of H11ymode for an input power of P = 80 μW on the lower branch of the power dispersion curve.
and 4(b), respectively. The power of the Hy is confined mainly in the linear core region, and the amplitude of the Hy is approximately 10 times greater than that of the Hx.

In addition, Figs. 5(a)
Fig. 5 (a) Major magnetic field component (Hy) and (b) minor field component (Hx) of H11ymode for an input power of P = 80 μW on the upper branch of the power dispersion curve.
and 5(b) show the Hy and Hx, respectively, at P = 80 μW for the upper branch of the dispersion curve. The profile of the Hy shifts into the nonlinear substrate region, and the amplitude of the Hy is of the order 103 greater than that of the Hx. The result validates the view that the minor field can be ignored while the mode is operated in the upper branch. The bistability of the nonlinear waveguide is often used to construct all-optical switching devices.

4.2 Fundamental long-range surface plasmon polariton mode of a nonlinear strip plasmonic waveguide

The low-loss LR–SPP mode of a thin metal strip bounded by a linear medium has been extensively studied [24

24. P. Berini, “Plasmon polariton modes guided by a metal film of finite width,” Opt. Lett. 24(15), 1011–1013 (1999). [CrossRef] [PubMed]

30

30. P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon. 1(3), 484–588 (2009). [CrossRef]

], but the configuration in which the nonlinear medium surrounds the metal strip is mostly unexplored. To date, to the best of our knowledge, only a single report exists [10

10. A. Degiron and D. R. Smith, “Nonlinear long-range plasmonic waveguides,” Phys. Rev. A 82(3), 033812 (2010). [CrossRef]

] that investigates the stationary LR–SPPs and that considers Kerr-type nonlinearities. Most reports [5

5. A. R. Davoyan, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear plasmonic slot waveguides,” Opt. Express 16(26), 21209–21214 (2008). [CrossRef] [PubMed]

9

9. J. R. Salgueiro and Y. S. Kivshar, “Nonlinear plasmonic directional couplers,” Appl. Phys. Lett. 97(8), 081106 (2010). [CrossRef]

] dealing with nonlinear SPPs focus on planar plasmonic waveguide structures. To fill this void, herein, we report the characteristics of the fundamental LR–SPP mode in a plasmonic waveguide composed of a metal strip of width w and thickness t bound between a nonlinear substrate and a linear cladding media. The waveguide structure is shown in Fig. 1(a) except that the core region is a metal strip. When operated at the telecommunication wavelength λ = 1.55 μm, the permittivities of the metal (Au) strip and the linear cladding are εf = –132 – 12.65i and εcl = 1.752, respectively. While considering the more practical condition in which the nonlinear coefficient of real Kerr-type media is much lower and saturates, the permittivity of the substrate is assumed to be saturable following the form of Eq. (21) with Δεsat = 0.31 but with a different value εsub = 1.752. Figures 6(a)
Fig. 6 Real (a) and imaginary (b) parts of the effective index of the fundamental LR–SPP mode as a function of power for width w = 4 μm and various thicknesses.
and 6(b) show, for width w = 4 μm and thickness t = 20, 50, and 80 nm, the variations of real and imaginary parts, respectively, of the effective index as a function of power using 15 terms of the basis functions in each subdomain.

As the input power increases, both the real and imaginary parts of the effective index increase monotonically if the refractive index is not saturated. Figures 7(a)
Fig. 7 Magnetic-field profiles |Hx| of the fundamental LR–SPP mode for the following guided powers: (a) P = 0, (b) P = 50, and (c) P = 100 μW for thickness t = 80nm and width w = 4 μm.
7(c) show, for t = 80 nm, the mode patterns (|Hx| field, the dominate component) for P = 0, 50, and 100 μW, respectively. The monotonic increase in the real and imaginary parts of the effective index with power reveals a better mode confinement and higher loss, respectively. Note that the degree of mode asymmetry increases significantly and the mode maximum is increasingly localized at the metal-cladding interface. The increase of refractive index with power in the nonlinear substrate region, which results from the self-focusing effect, resembles the fundamental LR–SPP modes that are supported in asymmetric structures [25

25. P. Berini, “Plasmon-polariton modes guided by a metal film of finite width bounded by different dielectrics,” Opt. Express 7(10), 329–335 (2000). [CrossRef] [PubMed]

,27

27. P. Berini, “Plasmon-polariton modes guided thin lossy metal films of finite width: bound modes of asymmetric structures,” Phys. Rev. B 63(12), 125417 (2001). [CrossRef]

]. Similar results are also found for an infinitely wide metal film surrounded by the same medium [23

23. C. C. Huang, “Pseudospectral mode solver for analyzing nonlinear optical waveguides,” Opt. Express 20(12), 13014–13029 (2012). [CrossRef] [PubMed]

,31

31. G. I. Stegeman and C. T. Seaton, “Nonlinear surface plasmons guided by thin metal films,” Opt. Lett. 9(6), 235–237 (1984). [CrossRef] [PubMed]

,32

32. J. Ariyasu, C. T. Seaton, G. I. Stegeman, A. A. Maradudin, and R. F. Wallis, “Nonlinear surface polaritons guided by meal films,” J. Appl. Phys. 58(7), 2460–2466 (1985). [CrossRef]

]. In contrast, in dielectric waveguides, the highest power is localized in the nonlinear substrate, as shown in the first example. These results validate the view that the nonlinear LR–SPP mode exhibits mode focusing, but not mode switching, as observed in nonlinear dielectric waveguides (at least for a realistic power intensity).

Reducing the thickness to t = 20 nm, the variation in the imaginary part of the effective index with respect to power becomes rather small, as shown in Fig. 6(b). The mode patterns at powers P = 0, 50, and 100 μW are shown in Figs. 8(a)
Fig. 8 Magnetic-field profiles |Hx| of fundamental LR–SPP mode for guided powers (a) P = 0, (b) P = 50, and (c) P = 100 μW for thickness t = 20 nm and width w = 4 μm.
, 8(b), and 8(c), respectively. The degree of mode asymmetry in this case is smaller than that for greater thicknesses.

To analyze the impact of a decreased width, we calculate the power dispersion curves for w = 2, 4, and 8 μm and thickness t = 50 nm. The real and imaginary parts of the effective index of refraction are shown in Figs. 9(a)
Fig. 9 Real (a) and imaginary (b) parts of effective index of fundamental LR–SPP mode as a function of power at the thickness t = 50 nm and different widths.
and 9(b), respectively, as functions of power. The variations in the real part of the effective index with respect o the input power are analogous for these widths; however, for w = 8 μm, the variations in the imaginary parts exhibit fairly different features. The energy losses for w = 2 and 4 μm increase linearly with the input power, whereas it increases exponentially for w = 8 μm.

To gain insight into the results, the mode patterns for w = 2 μm at P = 0, 50, and 100 μW are shown in Figs. 10(a)
Fig. 10 Magnetic-field profiles |Hx| of fundamental LR–SPP mode for guided powers (a) P = 0, (b) P = 50, and (c) P = 100 μW for width w = 2 μm and thickness t = 50 nm.
10(c), respectively. The variation in mode patterns with respect to power also exhibits mode asymmetry and is increasingly localized at the metal-cladding interface, which is similar to the results shown above for w = 4 μm.

To understand the sharp variation in energy loss with respect to input power, the mode patterns for w = 8 μm at P = 0, 50, and 100 μW are shown in Figs. 11(a)
Fig. 11 Magnetic-field profiles |Hx| of fundamental LR–SPP mode for guided powers (a) P = 0, (b) P = 50, and (c) P = 100 μW for width w = 8 μm and thickness t = 50 nm.
11(c), respectively. We observe that the mode patterns are different from that for narrower widths. As the input power increases, the mode profiles localized in the substrate region are better confined and the mode intensities localized in the cladding region decrease significantly. Therefore, the net result is an exponential increase in the energy loss. The abovementioned results indicate that no bistable phenomenon occurs in the nonlinear fundamental LR–SPP mode of a two-dimensional metal strip waveguide at a reasonable input power intensity of P = 100 μW for the geometry sizes considered in this study. The guided mode found in a nonlinear plasmonic waveguide may be regarded as a hybrid mode composed of a SPP mode and a dielectric waveguide mode. This view is similar to that proposed by Oulton et al. [33

33. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics 2(8), 496–500 (2008). [CrossRef]

] for a hybrid optical waveguide composed of a dielectric nanowire separated from a metal surface by a dielectric gap. The coupling between the LR–SPP mode and the dielectric waveguide mode supported by the sufficiently strong self-focusing effect may lead to mode switching if the input power is increased without any limit. Although mode switching is not observed for P = 100 μW, it is observed in a planar plasmonic structure at a certain thicknesses of the metal film and for reasonable input power [23

23. C. C. Huang, “Pseudospectral mode solver for analyzing nonlinear optical waveguides,” Opt. Express 20(12), 13014–13029 (2012). [CrossRef] [PubMed]

].

5. Conclusion

Acknowledgments

The author would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 99-2112-M-005-005-MY3.

References and links

1.

T. H. Wood, “Multiple quantum well (MQW) waveguide modulators,” J. Lightwave Technol. 6(6), 743–757 (1988). [CrossRef]

2.

S. Radic, N. George, and G. P. Agrawal, “Optical switching in λ/4-shifted nonlinear periodic structures,” Opt. Lett. 19(21), 1789–1791 (1994). [CrossRef] [PubMed]

3.

Y. D. Wu, “New all-optical wavelength auto-router based on spatial solitons,” Opt. Express 12(18), 4172–4177 (2004). [CrossRef] [PubMed]

4.

T. Fujisawa and M. Koshiba, “All-optical logic gates based on nonlinear slot-waveguide couplers,” J. Opt. Soc. Am. B 23(4), 684–691 (2006). [CrossRef]

5.

A. R. Davoyan, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear plasmonic slot waveguides,” Opt. Express 16(26), 21209–21214 (2008). [CrossRef] [PubMed]

6.

I. D. Rukhlenko, A. Pannipitiya, and M. Premaratne, “Dispersion relation for surface plasmon polaritons in metal/nonlinear-dielectric/metal slot waveguides,” Opt. Lett. 36(17), 3374–3376 (2011). [CrossRef] [PubMed]

7.

I. D. Rukhlenko, A. Pannipitiya, M. Premaratne, and G. Agrawal, “Exact dispersion relation for nonlinear plasmonic waveguides,” Phys. Rev. B 84(11), 113409 (2011). [CrossRef]

8.

A. R. Davoyan, I. V. Shadrivov, A. A. Zharov, D. K. Gramotnev, and Y. S. Kivshar, “Nonlinear nanofocusing in tapered plasmonic waveguides,” Phys. Rev. Lett. 105(11), 116804 (2010). [CrossRef] [PubMed]

9.

J. R. Salgueiro and Y. S. Kivshar, “Nonlinear plasmonic directional couplers,” Appl. Phys. Lett. 97(8), 081106 (2010). [CrossRef]

10.

A. Degiron and D. R. Smith, “Nonlinear long-range plasmonic waveguides,” Phys. Rev. A 82(3), 033812 (2010). [CrossRef]

11.

K. Hayata and M. Koshiba, “Full vectorial analysis of nonlinear-optical waveguides,” J. Opt. Soc. Am. B 5(12), 2494–2501 (1988). [CrossRef]

12.

R. D. Ettinger, F. A. Fernandez, B. M. A. Rahman, and J. B. Davies, “Vector finite element solutions of saturable nonlinear strip-loaded optical waveguides,” IEEE Photon. Technol. Lett. 3(2), 147–149 (1991). [CrossRef]

13.

X. H. Wang and G. K. Gambrell, “Full vectorial simulation of bistability phenomena in nonlinear-optical channel waveguides,” J. Opt. Soc. Am. B 10(6), 1090–1095 (1993). [CrossRef]

14.

S. S. A. Obayya, B. M. A. Rahman, K. T. V. Grattan, and H. A. E. Mikati, “Full vectorial finite–element solution of nonlinear bistable optical waveguides,’,” IEEE J. Quantum Electron. 38(8), 1120–1125 (2002). [CrossRef]

15.

K. Hayata and M. Koshiba, “Full vectorial analysis of nonlinear-optical waveguides,” J. Lightwave Technol. 20, 1876–1884 (2002).

16.

J. P. Boyd, Chebyshev and Fourier Spectral Methods (Springer–Verlag, 2nd edition, 2001).

17.

C. C. Huang, C. C. Huang, and J. Y. Yang, “A full-vectorial pseudospectral modal analysis of dielectric optical waveguides with stepped refractive index profiles,” IEEE J. Sel. Top. Quantum Electron. 11(2), 457–465 (2005). [CrossRef]

18.

P. J. Chiang, C. P. Yu, and H. C. Chang, “Full-vectorial optical waveguide mode solvers using multidomain pseudospectral frequency-domain (PSFD) formulations,” IEEE J. Quantum Electron. 44(1), 56–66 (2008). [CrossRef]

19.

C. C. Huang, “Simulation of optical waveguides by novel full-vectorial pseudospectral-based imaginary-distance beam propagation method,” Opt. Express 16(22), 17915–17934 (2008). [CrossRef] [PubMed]

20.

J. B. Xiao and X. H. Sun, “Full-vectorial mode solver for anisotropic optical waveguides using multidomain spectral collocation method,” Opt. Commun. 283(14), 2835–2840 (2010). [CrossRef]

21.

C. C. Huang, “Numerical investigation of mode characteristics of nanoscale surface plasmon-polaritons using a pseudospectral scheme,” Opt. Express 18(23), 23711–23726 (2010). [CrossRef] [PubMed]

22.

C. C. Huang, “Solving the full anisotropic liquid crystal waveguides by using an iterative pseudospectral-based eigenvalue method,” Opt. Express 19(4), 3363–3378 (2011). [CrossRef] [PubMed]

23.

C. C. Huang, “Pseudospectral mode solver for analyzing nonlinear optical waveguides,” Opt. Express 20(12), 13014–13029 (2012). [CrossRef] [PubMed]

24.

P. Berini, “Plasmon polariton modes guided by a metal film of finite width,” Opt. Lett. 24(15), 1011–1013 (1999). [CrossRef] [PubMed]

25.

P. Berini, “Plasmon-polariton modes guided by a metal film of finite width bounded by different dielectrics,” Opt. Express 7(10), 329–335 (2000). [CrossRef] [PubMed]

26.

P. Berini, “Plasmon-polariton modes guided thin lossy metal films of finite width: bound modes of symmetric structures,” Phys. Rev. B 61(15), 10484–10503 (2000). [CrossRef]

27.

P. Berini, “Plasmon-polariton modes guided thin lossy metal films of finite width: bound modes of asymmetric structures,” Phys. Rev. B 63(12), 125417 (2001). [CrossRef]

28.

S. J. Al-Bader, “Optical transmission on metallic wires-fundamental modes,” IEEE J. Quantum Electron. 40(3), 325–329 (2004). [CrossRef]

29.

A. Degiron and D. R. Smith, “Numerical simulations of long-range plasmons,” Opt. Express 14(4), 1611–1625 (2006). [CrossRef] [PubMed]

30.

P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon. 1(3), 484–588 (2009). [CrossRef]

31.

G. I. Stegeman and C. T. Seaton, “Nonlinear surface plasmons guided by thin metal films,” Opt. Lett. 9(6), 235–237 (1984). [CrossRef] [PubMed]

32.

J. Ariyasu, C. T. Seaton, G. I. Stegeman, A. A. Maradudin, and R. F. Wallis, “Nonlinear surface polaritons guided by meal films,” J. Appl. Phys. 58(7), 2460–2466 (1985). [CrossRef]

33.

R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics 2(8), 496–500 (2008). [CrossRef]

OCIS Codes
(130.2790) Integrated optics : Guided waves
(190.4390) Nonlinear optics : Nonlinear optics, integrated optics
(230.7380) Optical devices : Waveguides, channeled
(240.4350) Optics at surfaces : Nonlinear optics at surfaces

ToC Category:
Optics at Surfaces

History
Original Manuscript: June 28, 2012
Manuscript Accepted: July 23, 2012
Published: July 31, 2012

Citation
Chia-Chien Huang, "Analysis of long-range surface plasmon polaritons in nonlinear plasmonic waveguides using pseudospectral method," Opt. Express 20, 18665-18678 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-17-18665


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References

  1. T. H. Wood, “Multiple quantum well (MQW) waveguide modulators,” J. Lightwave Technol.6(6), 743–757 (1988). [CrossRef]
  2. S. Radic, N. George, and G. P. Agrawal, “Optical switching in λ/4-shifted nonlinear periodic structures,” Opt. Lett.19(21), 1789–1791 (1994). [CrossRef] [PubMed]
  3. Y. D. Wu, “New all-optical wavelength auto-router based on spatial solitons,” Opt. Express12(18), 4172–4177 (2004). [CrossRef] [PubMed]
  4. T. Fujisawa and M. Koshiba, “All-optical logic gates based on nonlinear slot-waveguide couplers,” J. Opt. Soc. Am. B23(4), 684–691 (2006). [CrossRef]
  5. A. R. Davoyan, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear plasmonic slot waveguides,” Opt. Express16(26), 21209–21214 (2008). [CrossRef] [PubMed]
  6. I. D. Rukhlenko, A. Pannipitiya, and M. Premaratne, “Dispersion relation for surface plasmon polaritons in metal/nonlinear-dielectric/metal slot waveguides,” Opt. Lett.36(17), 3374–3376 (2011). [CrossRef] [PubMed]
  7. I. D. Rukhlenko, A. Pannipitiya, M. Premaratne, and G. Agrawal, “Exact dispersion relation for nonlinear plasmonic waveguides,” Phys. Rev. B84(11), 113409 (2011). [CrossRef]
  8. A. R. Davoyan, I. V. Shadrivov, A. A. Zharov, D. K. Gramotnev, and Y. S. Kivshar, “Nonlinear nanofocusing in tapered plasmonic waveguides,” Phys. Rev. Lett.105(11), 116804 (2010). [CrossRef] [PubMed]
  9. J. R. Salgueiro and Y. S. Kivshar, “Nonlinear plasmonic directional couplers,” Appl. Phys. Lett.97(8), 081106 (2010). [CrossRef]
  10. A. Degiron and D. R. Smith, “Nonlinear long-range plasmonic waveguides,” Phys. Rev. A82(3), 033812 (2010). [CrossRef]
  11. K. Hayata and M. Koshiba, “Full vectorial analysis of nonlinear-optical waveguides,” J. Opt. Soc. Am. B5(12), 2494–2501 (1988). [CrossRef]
  12. R. D. Ettinger, F. A. Fernandez, B. M. A. Rahman, and J. B. Davies, “Vector finite element solutions of saturable nonlinear strip-loaded optical waveguides,” IEEE Photon. Technol. Lett.3(2), 147–149 (1991). [CrossRef]
  13. X. H. Wang and G. K. Gambrell, “Full vectorial simulation of bistability phenomena in nonlinear-optical channel waveguides,” J. Opt. Soc. Am. B10(6), 1090–1095 (1993). [CrossRef]
  14. S. S. A. Obayya, B. M. A. Rahman, K. T. V. Grattan, and H. A. E. Mikati, “Full vectorial finite–element solution of nonlinear bistable optical waveguides,’,” IEEE J. Quantum Electron.38(8), 1120–1125 (2002). [CrossRef]
  15. K. Hayata and M. Koshiba, “Full vectorial analysis of nonlinear-optical waveguides,” J. Lightwave Technol.20, 1876–1884 (2002).
  16. J. P. Boyd, Chebyshev and Fourier Spectral Methods (Springer–Verlag, 2nd edition, 2001).
  17. C. C. Huang, C. C. Huang, and J. Y. Yang, “A full-vectorial pseudospectral modal analysis of dielectric optical waveguides with stepped refractive index profiles,” IEEE J. Sel. Top. Quantum Electron.11(2), 457–465 (2005). [CrossRef]
  18. P. J. Chiang, C. P. Yu, and H. C. Chang, “Full-vectorial optical waveguide mode solvers using multidomain pseudospectral frequency-domain (PSFD) formulations,” IEEE J. Quantum Electron.44(1), 56–66 (2008). [CrossRef]
  19. C. C. Huang, “Simulation of optical waveguides by novel full-vectorial pseudospectral-based imaginary-distance beam propagation method,” Opt. Express16(22), 17915–17934 (2008). [CrossRef] [PubMed]
  20. J. B. Xiao and X. H. Sun, “Full-vectorial mode solver for anisotropic optical waveguides using multidomain spectral collocation method,” Opt. Commun.283(14), 2835–2840 (2010). [CrossRef]
  21. C. C. Huang, “Numerical investigation of mode characteristics of nanoscale surface plasmon-polaritons using a pseudospectral scheme,” Opt. Express18(23), 23711–23726 (2010). [CrossRef] [PubMed]
  22. C. C. Huang, “Solving the full anisotropic liquid crystal waveguides by using an iterative pseudospectral-based eigenvalue method,” Opt. Express19(4), 3363–3378 (2011). [CrossRef] [PubMed]
  23. C. C. Huang, “Pseudospectral mode solver for analyzing nonlinear optical waveguides,” Opt. Express20(12), 13014–13029 (2012). [CrossRef] [PubMed]
  24. P. Berini, “Plasmon polariton modes guided by a metal film of finite width,” Opt. Lett.24(15), 1011–1013 (1999). [CrossRef] [PubMed]
  25. P. Berini, “Plasmon-polariton modes guided by a metal film of finite width bounded by different dielectrics,” Opt. Express7(10), 329–335 (2000). [CrossRef] [PubMed]
  26. P. Berini, “Plasmon-polariton modes guided thin lossy metal films of finite width: bound modes of symmetric structures,” Phys. Rev. B61(15), 10484–10503 (2000). [CrossRef]
  27. P. Berini, “Plasmon-polariton modes guided thin lossy metal films of finite width: bound modes of asymmetric structures,” Phys. Rev. B63(12), 125417 (2001). [CrossRef]
  28. S. J. Al-Bader, “Optical transmission on metallic wires-fundamental modes,” IEEE J. Quantum Electron.40(3), 325–329 (2004). [CrossRef]
  29. A. Degiron and D. R. Smith, “Numerical simulations of long-range plasmons,” Opt. Express14(4), 1611–1625 (2006). [CrossRef] [PubMed]
  30. P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon.1(3), 484–588 (2009). [CrossRef]
  31. G. I. Stegeman and C. T. Seaton, “Nonlinear surface plasmons guided by thin metal films,” Opt. Lett.9(6), 235–237 (1984). [CrossRef] [PubMed]
  32. J. Ariyasu, C. T. Seaton, G. I. Stegeman, A. A. Maradudin, and R. F. Wallis, “Nonlinear surface polaritons guided by meal films,” J. Appl. Phys.58(7), 2460–2466 (1985). [CrossRef]
  33. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics2(8), 496–500 (2008). [CrossRef]

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