ISRN Optics

Volume 2012 (2012), Article ID 536209, 7 pages

http://dx.doi.org/10.5402/2012/536209

## FDTD Modeling of a Cloak with a Nondiagonal Permittivity Tensor

Graduate School of Systems and Information Engineering, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan

Received 13 February 2012; Accepted 27 March 2012

Academic Editors: A. Danner, M. Midrio, and A. Tervonen

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.

#### Abstract

We demonstrate a finite-difference time-domain (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 elliptic-cylindrical cloak in two dimensions. Numerical simulations demonstrated the stable calculation and cloaking performance of the elliptic-cylindrical 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 finite-difference time-domain (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 elliptic-cylindrical 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 Courant-Friedrichs-Lewy (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 Elliptic-Cylindrical Cloak

Two designs of elliptic-cylindrical cloaks have been proposed. One has diagonal and in orthonormal elliptic-cylindrical 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 elliptic-cylindrical 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 elliptic-cylindrical 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 .

##### 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 infinite-frequency 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 Elliptic-Cylindrical Cloak

We calculate electromagnetic propagation for the elliptic-cylindrical 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 elliptic-cylindrical 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.

Finally, we examine the cloaking performance of the elliptic-cylindrical 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.

#### 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 elliptic-cylindrical cloak in the TM mode. Numerical calculations demonstrated stable results and the cloaking performance.

#### Acknowledgment

The authors deeply appreciate the financial support of Grant-in-Aid 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 · View at Google Scholar · View at Scopus - M. Rahm, D. Schurig, D. A. Roberts, S. A. Cummer, D. R. Smith, and J. B. Pendry, “Design of electromagnetic cloaks and concentrators using form-invariant coordinate transformations of maxwell's equations,”
*Photonics and Nanostructures Fundamentals and Applications*, vol. 6, no. 1, pp. 87–95, 2007. View at Publisher · View at 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - Y. Luo, J. Zhang, B. I. Wu, and H. Chen, “Interaction of an electromagnetic wave with a cone-shaped invisibility cloak and polarization rotator,”
*Physical Review B*, vol. 78, no. 12, Article ID 125108, 2008. View at Publisher · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - M. Rahm, D. A. Roberts, J. B. Pendry, and D. R. Smith, “Transformation-optical design of adaptive beam bends and beam expanders,”
*Optics Express*, vol. 16, no. 15, pp. 11555–11567, 2008. View at Publisher · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - S. Han, Y. Xiong, D. Genov, Z. Liu, G. Bartal, and X. Zhang, “Ray optics at a deep-subwavelength scale: a transformation optics approach,”
*Nano Letters*, vol. 8, no. 12, pp. 4243–4247, 2008. View at Publisher · View at Google Scholar · View at Scopus - D. H. Kwon and D. H. Werner, “Transformation optical designs for wave collimators, flat lenses and right-angle bends,”
*New Journal of Physics*, vol. 10, Article ID 115023, 2008. View at Publisher · View at Google Scholar · View at Scopus - W. X. Jiang, T. J. Cui, G. X. Yu, X. Q. Lin, Q. Cheng, and J. Y. Chin, “Cylindrical-to-plane-wave 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - R. Liu, C. Ji, J. J. Mock, J. Y. Chin, T. J. Cui, and D. R. Smith, “Broadband ground-plane cloak,”
*Science*, vol. 323, no. 5912, pp. 366–369, 2009. View at Publisher · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - H. F. Ma and T. J. Cui, “Three-dimensional broadband ground-plane cloak made of metamaterials,”
*Nature Communications*, vol. 1, article 21, 2010. View at Publisher · View at Google Scholar · View at Scopus - T. Ergin, N. Stenger, P. Brenner, J. B. Pendry, and M. Wegener, “Three-dimensional invisibility cloak at optical wavelengths,”
*Science*, vol. 328, no. 5976, pp. 337–339, 2010. View at Publisher · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 steady-state electromagnetic scattering problems using the time-dependent maxwell's equations,”
*IEEE Transactions on Microwave Theory and Techniques*, vol. 23, no. 8, pp. 623–630, 1975. View at Google Scholar · View at Scopus - A. Taflove, “Application of the finite-difference time-domain method to sinusoidal steady-state electromagnetic-penetration problems,”
*IEEE Transactions on Electromagnetic Compatibility*, vol. 22, no. 3, pp. 191–202, 1980. View at Google Scholar · View at Scopus - A. Taflove and S. C. Hagness,
*Computational Electrodynamics: The Finite-Difference Time-Domain Method*, Artech House, Norwood, Mass, USA, 3rd edition, 2005. - E. Kallos, C. Argyropoulos, and Y. Hao, “Ground-plane quasicloaking for free space,”
*Physical Review A*, vol. 79, no. 6, Article ID 063825, 2009. View at Publisher · View at Google Scholar · View at Scopus - Y. Zhao, C. Argyropoulos, and Y. Hao, “Full-wave finite-difference time-domain simulation of electromagnetic cloaking structures,”
*Optics Express*, vol. 16, no. 9, pp. 6717–6730, 2008. View at Publisher · View at Google Scholar · View at Scopus - Y. Hao and R. Mittra,
*FDTD Modeling of Metamaterials: Theory and Applications*, Artech House, Norwood, Mass, USA, 1st edition, 2008. - J. A. Silva-Macědo, 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 · View at Scopus - C. Argyropoulos, Y. Zhao, and Y. Hao, “A radially-dependent dispersive finite-difference time-domain method for the evaluation of electromagnetic cloaks,”
*IEEE Transactions on Antennas and Propagation*, vol. 57, no. 5, pp. 1432–1441, 2009. View at Publisher · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 Routh-Hurwitz criterion,”
*IEEE Transactions on Microwave Theory and Techniques*, vol. 49, no. 2, pp. 377–381, 2001. View at Google Scholar · View at Scopus - G. V. Eleftheriades and K. G. Balmain,
*Negative Refraction Metamaterials: Fundamental Principles and Applications*, Wiley-IEEE 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - 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 · View at Google Scholar · View at Scopus - W. X. Jiang, T. J. Cui, G. X. Yu, X. Q. Lin, Q. Cheng, and J. Y. Chin, “Arbitrarily elliptical-cylindrical invisible cloaking,”
*Journal of Physics D*, vol. 41, no. 8, Article ID 085504, 2008. View at Publisher · View at Google Scholar · View at Scopus - 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 finite-difference time-domain method: numerical aspects,”
*IEEE Transactions on Antennas and Propagation I*, vol. 55, no. 6, pp. 1506–1513, 2007. View at Publisher · View at Google Scholar · View at Scopus - K. S. Kunz and R. J. Luebbers,
*The Finite Difference Time Domain Method for Electromagnetics*, CRC Press, New York, NY, USA, 1st edition, 1993.