Research Article  Open Access
Naoki Okada, James B. Cole, "FDTD Modeling of a Cloak with a Nondiagonal Permittivity Tensor", International Scholarly Research Notices, vol. 2012, Article ID 536209, 7 pages, 2012. https://doi.org/10.5402/2012/536209
FDTD Modeling of a Cloak with a Nondiagonal Permittivity Tensor
Abstract
We demonstrate a finitedifference timedomain (FDTD) modeling of a cloak with a nondiagonal permittivity tensor. Numerical instability due to material anisotropies is avoided by mapping the eigenvalues of the material parameters to a dispersion model. Our approach is implemented for an ellipticcylindrical cloak in two dimensions. Numerical simulations demonstrated the stable calculation and cloaking performance of the ellipticcylindrical cloak.
1. Introduction
An optical cloak enables objects to be concealed from electromagnetic detection. Pendry et al. developed a method to design cloaks via coordinate transformations [1]. The coordinate transformation is such that light is guided around the cloak region. Material parameters (permittivity and permeability) can be obtained in the transformed coordinate system and put into Maxwell’s equations. This approach enables one to design not only cloaks but also other metamaterials that can manipulate light flow. For example, concentrators [2], rotation coatings [3], polarization controllers [4–6], waveguides [7–11], wave shape conversion [12], object illusions [13–15], and optical black holes [16, 17] have been designed. However, not many metamaterials have been realized in the optical region [18–25], because material parameters given by coordinate transformations have complicated anisotropies.
Numerical simulations are useful to analyze complicated metamaterial structures. In this paper, we present a finitedifference timedomain (FDTD) analysis of a cloak. The FDTD method has gained popularity for several reasons: it is easy to implement, it works in the time domain, and its arbitrary shapes can be calculated [26–29]. FDTD modelings of cloaks with a diagonal (uniaxial) permittivity tensor have been demonstrated [30–38], but a cloak with a nondiagonal permittivity tensor has never been calculated by the FDTD method. The diagonal case can be stably calculated by mapping material parameters having values less than one to a dispersion model [31]. However, we found that mapping the nondiagonal elements to dispersion models causes the computation to diverge.
In this paper, we analyze the numerical stability for a cloak with a nondiagonal permittivity tensor and derive the FDTD formulation. We apply our method to simulate light propagation in the vicinity of an ellipticcylindrical cloak. To the best of authors’ knowledge, this is the first time that a cloak with a nondiagonal anisotropy has been calculated using the FDTD method.
2. Numerical Stability for Nondiagonal Permittivity Tensor
In the stability analysis, we confirm that the FDTD method for a cloak with a diagonal permittivity tensor cannot directly be extended to the nondiagonal case. Under a coordinate transformation for a cloak [39], material parameters can be expressed as where is the relative permittivity, is the relative permeability, is the metric tensor, and . Because are constructed from the symmetric metric tensor , they are symmetric. Consequently, have real eigenvalues with orthogonal eigenvectors and are thus diagonalizable. The eigenvalues, , of for an eigenvector are defined by The phase velocity of light in a material is given by ( vacuum light speed), and the CourantFriedrichsLewy (CFL) stability limit becomes where is the time step, is the grid spacing, and 1, 2, and 3 dimensions. Since the FDTD stability depends on the eigenvalues of and , to analyze nondiagonal cases, we must first find the eigenvalues and diagonalize and . After the diagonalization, the FDTD method for diagonal cases [31–38] can be applied. For diagonal and , elements having values less than one are replaced by dispersive quantities to avoid violating the causality and numerical stability [40–44].
In summary, the FDTD modeling for nondiagonal and requires three steps:(1)find the eigenvalues and eigenvectors and diagonalize the material parameters,(2)map the eigenvalues having values less than one to a dispersion model,(3)solve Maxwell’s equations using the dispersive FDTD method.
3. FDTD Formulation of the EllipticCylindrical Cloak
Two designs of ellipticcylindrical cloaks have been proposed. One has diagonal and in orthonormal ellipticcylindrical coordinates [45, 46], and in the other and are nondiagonal in Cartesian coordinates [47, 48]. We derive a FDTD formulation for the latter in the transverse magnetic (TM) polarization.
3.1. Diagonalization
Figure 1 shows an ellipticcylindrical cloak in Figure 1(a) Cartesian coordinates and Figure 1(b) transformed coordinates. The inner axis , the outer axis , and the perpendicular axes and are depicted. The ellipticcylindrical cloak is horizontal when , and vertical when . In the cloak region, , the material parameters are expressed by where where and . From (4) to (6) we can obtain three eigenvalues where Since is symmetric, it is diagonalized by the eigenvalue matrix and its orthogonal matrix as follows: where where .
(a)
(b)
3.2. Mapping Eigenvalues to a Dispersion Model
From (7) and (8), and have values less than one in the cloak region (). Thus, must be replaced by dispersive quantities by using (for example) the Drude model where is the angular frequency, is the infinitefrequency permittivity, is the plasma frequency, and is the collision frequency. For simplicity, we consider the lossless case, . Then the plasma frequencies are given by , where .
3.3. FDTD Discretization
Using the diagonalized material parameters and eigenvalues mapped to the Drude model, we derive an FDTD formulation to solve Maxwell’s equations, where is the electric flux density, is the magnetic field, is the magnetic flux density, and is the electric field. In the TM polarization, electromagnetic fields reduce to three nonzero components , , and (, , and ). The  and update equations are obtained using Yee algorithm [26–29] as follows: where we simply write ( integer) and , are the spatial difference operators defined by
To find the update equations, we consider the relation where is the vacuum permittivity. From (9), we obtain Substituting (10) in (18) and multiplying by both sides, we obtain where and . Substituting the Drude model for as shown in (12) and using the inverse Fourier transformation rule, , (19) becomes For the discretization, we use the central difference approximation and the central average operator, The central average operator improves the stability and accuracy [40, 49, 50]. Similarly, and are discretized, and we obtain the update equation where must be spatially interpolated due to the staggered Yee cell [31], and Similarly, the update equation is obtained by exchanging , , and .
To find the update equation, we consider the relation Analogously to the field, the update equation can be obtained in the form where is the vacuum permeability.
In summary, the electromagnetic fields are iteratively updated in the following sequence:(1)update the components of according to (14),(2)update the components of according to the sample given in (23),(3)update the components of according to (15),(4)update the components of according to (26).
4. Simulation of the EllipticCylindrical Cloak
We calculate electromagnetic propagation for the ellipticcylindrical cloak using the FDTD formulation shown in Section 3. Figure 2 shows the simulation setup: the computational domain is terminated with a perfectly matched layer in the direction, and a periodic boundary condition in the direction [29]; the inside of the cloak is covered with a perfect electric conductor (PEC); a plane wave source of wavelength 750 nm (400 THz) is in the TM polarization; the grid spacing is 10 nm ( 75); and the time step is given by the CFL limit, . Simulation parameters are listed in Table 1.

Figure 3 shows the FDTD results for the ellipticcylindrical cloak at the steady state (50 wave periods). Figure 3(a) shows calculated field distributions using 10 nm. The wave propagates without significant disturbance around the cloak, and the calculation is stable. The small ripples on phase planes are purely numerical errors and can be made to vanish by reducing the grid spacing. This can be confirmed by calculating the radar cross section (RCS) [29, 51]. In two dimensions, the RCS is defined by where is the scattering angle, is the scattered power in far field, and is the incident power. If there is no significant disturbance by the object, approaches zero. Figure 3(b) shows normalized RCSs on dB scale, , scattered by a PEC (without cloak) and cloak using different grid spacings, 20, 10, and 5 nm. The PEC or cloak using a coarse grid spacing scatters strongly, but the RCS of the cloak rapidly decreases as the grid spacing is reduced.
(a)
(b)
Finally, we examine the cloaking performance of the ellipticcylindrical cloak using the Drude model. Simulation parameters are the same as Table 1 and the cloak is optimized to a wavelength of 750 nm. In the wavelength band, 600 nm–900 nm, we calculate the total cross section (TCS) defined by Figure 4(a) shows the calculated TCS spectrum. The TCS rapidly increases with wavelength shifts off the optimal. Figure 4(b) shows the RCS for several wavelengths, A: 730 nm, B: 750 nm, and C: 830 nm (normalized to the RCS for 750 nm). For wavelengths A and C, the scattering is much stronger than the optimal wavelength B.
(a)
(b)
5. Conclusion
We describe a stable FDTD modeling procedure for a cloak with a nondiagonal permittivity tensor. When the eigenvalues of the material parameters are less than one, they must be mapped to a dispersion model in order to maintain numerical stability. We implement our method for an ellipticcylindrical cloak in the TM mode. Numerical calculations demonstrated stable results and the cloaking performance.
Acknowledgment
The authors deeply appreciate the financial support of GrantinAid for Japan Society for the Promotion of Science (JSPS) Fellows.
References
 J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling electromagnetic fields,” Science, vol. 312, no. 5781, pp. 1780–1782, 2006. View at: Publisher Site  Google Scholar
 M. Rahm, D. Schurig, D. A. Roberts, S. A. Cummer, D. R. Smith, and J. B. Pendry, “Design of electromagnetic cloaks and concentrators using forminvariant coordinate transformations of maxwell's equations,” Photonics and Nanostructures Fundamentals and Applications, vol. 6, no. 1, pp. 87–95, 2007. View at: Publisher Site  Google Scholar
 H. Chen and C. T. Chan, “Transformation media that rotate electromagnetic fields,” Applied Physics Letters, vol. 90, no. 24, Article ID 241105, 2007. View at: Publisher Site  Google Scholar
 D. H. Kwon and D. H. Werner, “Polarization splitter and polarization rotator designs based on transformation optics,” Optics Express, vol. 16, no. 23, pp. 18731–18738, 2008. View at: Publisher Site  Google Scholar
 Y. Luo, J. Zhang, B. I. Wu, and H. Chen, “Interaction of an electromagnetic wave with a coneshaped invisibility cloak and polarization rotator,” Physical Review B, vol. 78, no. 12, Article ID 125108, 2008. View at: Publisher Site  Google Scholar
 T. Zhai, Y. Zhou, J. Zhou, and D. Liu, “Polarization controller based on embedded optical transformation,” Optics Express, vol. 17, no. 20, pp. 17206–17213, 2009. View at: Publisher Site  Google Scholar
 M. Rahm, D. A. Roberts, J. B. Pendry, and D. R. Smith, “Transformationoptical design of adaptive beam bends and beam expanders,” Optics Express, vol. 16, no. 15, pp. 11555–11567, 2008. View at: Publisher Site  Google Scholar
 M. Rahm, S. A. Cummer, D. Schurig, J. B. Pendry, and D. R. Smith, “Optical design of reflectionless complex media by finite embedded coordinate transformations,” Physical Review Letters, vol. 100, no. 6, Article ID 063903, 2008. View at: Publisher Site  Google Scholar
 L. Lan, W. Wei, C. Jianhua, D. Chunlei, and L. Xiangang, “Design of electromagnetic refractor and phase transformer using coordinate transformation theory,” Optics Express, vol. 16, no. 10, pp. 6815–6821, 2008. View at: Publisher Site  Google Scholar
 S. Han, Y. Xiong, D. Genov, Z. Liu, G. Bartal, and X. Zhang, “Ray optics at a deepsubwavelength scale: a transformation optics approach,” Nano Letters, vol. 8, no. 12, pp. 4243–4247, 2008. View at: Publisher Site  Google Scholar
 D. H. Kwon and D. H. Werner, “Transformation optical designs for wave collimators, flat lenses and rightangle bends,” New Journal of Physics, vol. 10, Article ID 115023, 2008. View at: Publisher Site  Google Scholar
 W. X. Jiang, T. J. Cui, G. X. Yu, X. Q. Lin, Q. Cheng, and J. Y. Chin, “Cylindricaltoplanewave conver,” Physics Letters, vol. 92, no. 26, Article ID 261903, 2008. View at: Google Scholar
 Y. Lai, J. Ng, H. Chen et al., “Illusion optics: the optical transformation of an object into another object,” Physical Review Letters, vol. 102, no. 25, Article ID 253902, 2009. View at: Publisher Site  Google Scholar
 W. X. Jiang and T. J. Cui, “Moving targets virtually via composite optical transformation,” Optics Express, vol. 18, no. 5, pp. 5161–5167, 2010. View at: Publisher Site  Google Scholar
 H. Chen, C. T. Chan, and P. Sheng, “Transformation optics and metamaterials,” Nature Materials, vol. 9, no. 5, pp. 387–396, 2010. View at: Publisher Site  Google Scholar
 E. E. Narimanov and A. V. Kildishev, “Optical black hole: broadband omnidirectional light absorber,” Applied Physics Letters, vol. 95, no. 4, Article ID 041106, 2009. View at: Publisher Site  Google Scholar
 D. A. Genov, S. Zhang, and X. Zhang, “Mimicking celestial mechanics in metamaterials,” Nature Physics, vol. 5, no. 9, pp. 687–692, 2009. View at: Publisher Site  Google Scholar
 D. Schurig, J. J. Mock, B. J. Justice et al., “Metamaterial electromagnetic cloak at microwave frequencies,” Science, vol. 314, no. 5801, pp. 977–980, 2006. View at: Publisher Site  Google Scholar
 B. Kanté, D. Germain, and A. De Lustrac, “Experimental demonstration of a nonmagnetic metamaterial cloak at microwave frequencies,” Physical Review B, vol. 80, no. 20, Article ID 201104, 2009. View at: Publisher Site  Google Scholar
 R. Liu, C. Ji, J. J. Mock, J. Y. Chin, T. J. Cui, and D. R. Smith, “Broadband groundplane cloak,” Science, vol. 323, no. 5912, pp. 366–369, 2009. View at: Publisher Site  Google Scholar
 Y. G. Ma, C. K. Ong, T. Tyc, and U. Leonhardt, “An omnidirectional retroreflector based on the transmutation of dielectric singularities,” Nature Materials, vol. 8, no. 8, pp. 639–642, 2009. View at: Publisher Site  Google Scholar
 H. F. Ma and T. J. Cui, “Threedimensional broadband groundplane cloak made of metamaterials,” Nature Communications, vol. 1, article 21, 2010. View at: Publisher Site  Google Scholar
 T. Ergin, N. Stenger, P. Brenner, J. B. Pendry, and M. Wegener, “Threedimensional invisibility cloak at optical wavelengths,” Science, vol. 328, no. 5976, pp. 337–339, 2010. View at: Publisher Site  Google Scholar
 X. Chen, Y. Luo, J. Zhang, K. Jiang, J. B. Pendry, and S. Zhang, “Macroscopic invisibility cloaking of visible light,” Nature Communications, vol. 2, article 176, 2011. View at: Publisher Site  Google Scholar
 J. Zhang, L. Liu, Y. Luo, S. Zhang, and N. A. Mortensen, “Homogeneous optical cloak constructed with uniform layered structures,” Optics Express, vol. 19, no. 9, pp. 8625–8631, 2011. View at: Publisher Site  Google Scholar
 K. Yee, “Numerical solution of initial boundary value problems involving maxwell's equations in isotropic media,” IEEE Transactions on Antennas and Propagation, vol. 14, no. 3, pp. 302–307, 1966. View at: Google Scholar
 A. Taflove and M. E. Brodwin, “Numerical solution of steadystate electromagnetic scattering problems using the timedependent maxwell's equations,” IEEE Transactions on Microwave Theory and Techniques, vol. 23, no. 8, pp. 623–630, 1975. View at: Google Scholar
 A. Taflove, “Application of the finitedifference timedomain method to sinusoidal steadystate electromagneticpenetration problems,” IEEE Transactions on Electromagnetic Compatibility, vol. 22, no. 3, pp. 191–202, 1980. View at: Google Scholar
 A. Taflove and S. C. Hagness, Computational Electrodynamics: The FiniteDifference TimeDomain Method, Artech House, Norwood, Mass, USA, 3rd edition, 2005.
 E. Kallos, C. Argyropoulos, and Y. Hao, “Groundplane quasicloaking for free space,” Physical Review A, vol. 79, no. 6, Article ID 063825, 2009. View at: Publisher Site  Google Scholar
 Y. Zhao, C. Argyropoulos, and Y. Hao, “Fullwave finitedifference timedomain simulation of electromagnetic cloaking structures,” Optics Express, vol. 16, no. 9, pp. 6717–6730, 2008. View at: Publisher Site  Google Scholar
 Y. Hao and R. Mittra, FDTD Modeling of Metamaterials: Theory and Applications, Artech House, Norwood, Mass, USA, 1st edition, 2008.
 J. A. SilvaMacědo, M. A. Romero, and B. H. V. Borges, “An extended FDTD method for the analysis of electromagnetic field rotations and cloaking devices,” Progress in Electromagnetics Research, vol. 87, pp. 183–196, 2008. View at: Google Scholar
 C. Argyropoulos, Y. Zhao, and Y. Hao, “A radiallydependent dispersive finitedifference timedomain method for the evaluation of electromagnetic cloaks,” IEEE Transactions on Antennas and Propagation, vol. 57, no. 5, pp. 1432–1441, 2009. View at: Publisher Site  Google Scholar
 C. Argyropoulos, E. Kallos, Y. Zhao, and Y. Hao, “Manipulating the loss in electromagnetic cloaks for perfect wave absorption,” Optics Express, vol. 17, no. 10, pp. 8467–8475, 2009. View at: Publisher Site  Google Scholar
 C. Argyropoulos, E. Kallos, and Y. Hao, “Dispersive cylindrical cloaks under nonmonochromatic illumination,” Physical Review E, vol. 81, no. 1, Article ID 016611, 2010. View at: Publisher Site  Google Scholar
 C. Argyropoulos, E. Kallos, and Y. Hao, “FDTD analysis of the optical black hole,” Journal of the Optical Society of America B, vol. 27, no. 10, pp. 2020–2025, 2010. View at: Publisher Site  Google Scholar
 C. Argyropoulos, E. Kallos, and Y. Hao, “Bandwidth evaluation of dispersive transformation electromagnetics based devices,” Applied Physics A, vol. 103, no. 3, pp. 715–719, 2011. View at: Publisher Site  Google Scholar
 U. Leonhardt and T. G. Philbin, “Chapter 2 transformation optics and the geometry of light,” Progress in Optics, vol. 53, pp. 69–152, 2009. View at: Publisher Site  Google Scholar
 J. A. Pereda, L. A. Vielva, A. Vegas, and A. Prieto, “Analyzingthe stability of the FDTD technique by combining the von neumann method with the RouthHurwitz criterion,” IEEE Transactions on Microwave Theory and Techniques, vol. 49, no. 2, pp. 377–381, 2001. View at: Google Scholar
 G. V. Eleftheriades and K. G. Balmain, Negative Refraction Metamaterials: Fundamental Principles and Applications, WileyIEEE Press, New York, NY, USA, 1st edition, 2005.
 S. A. Tretyakov and S. I. Maslovski, “Veselago materials: what is possible and impossible about the dispersion of the constitutive parameters,” IEEE Antennas and Propagation Magazine, vol. 49, no. 1, pp. 37–43, 2007. View at: Publisher Site  Google Scholar
 P. Yao, Z. Liang, and X. Jiang, “Limitation of the electromagnetic cloak with dispersive material,” Applied Physics Letters, vol. 92, no. 3, Article ID 031111, 2008. View at: Publisher Site  Google Scholar
 Z. Lin and L. Thylén, “On the accuracy and stability of several widely used FDTD approaches for modeling lorentz dielectrics,” IEEE Transactions on Antennas and Propagation, vol. 57, no. 10, pp. 3378–3381, 2009. View at: Publisher Site  Google Scholar
 H. Ma, S. Qu, Z. Xu, J. Zhang, B. Chen, and J. Wang, “Material parameter equation for elliptical cylindrical cloaks,” Physical Review A, vol. 77, no. 1, Article ID 013825, 2008. View at: Publisher Site  Google Scholar
 E. Cojocaru, “Exact analytical approaches for elliptic cylindrical invisibility cloaks,” Journal of the Optical Society of America B, vol. 26, no. 5, pp. 1119–1128, 2009. View at: Publisher Site  Google Scholar
 W. X. Jiang, T. J. Cui, G. X. Yu, X. Q. Lin, Q. Cheng, and J. Y. Chin, “Arbitrarily ellipticalcylindrical invisible cloaking,” Journal of Physics D, vol. 41, no. 8, Article ID 085504, 2008. View at: Publisher Site  Google Scholar
 T. J. Cui, D. R. Smith, and R. Liu, Metamaterials: Theory, Design, and Applications, Springer, New York, NY, USA, 1st edition, 2009.
 F. B. Hildebrand, Introduction to Numerical Analysis, Dover Publications, New York, NY, USA, 2nd edition, 1987.
 Y. Zhao, P. A. Belov, and Y. Hao, “Modelling of wave propagation in wire media using spatially dispersive finitedifference timedomain method: numerical aspects,” IEEE Transactions on Antennas and Propagation I, vol. 55, no. 6, pp. 1506–1513, 2007. View at: Publisher Site  Google Scholar
 K. S. Kunz and R. J. Luebbers, The Finite Difference Time Domain Method for Electromagnetics, CRC Press, New York, NY, USA, 1st edition, 1993.
Copyright
Copyright © 2012 Naoki Okada and James B. Cole. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.