1. Introduction

In the last decade, metamaterials have attracted much attention due to their extraordinary properties, such as negative refraction, ultra refraction, anomalous dispersion and so on [1–3]. Metamaterials are artificial materials engineered to gain desired properties that cannot be found in nature and usually consist of periodically arranged materials, which affect electromagnetic waves in an unconventional manner. More precisely, they exhibit new and unusual electromagnetic properties at the macro-scale, due to their structural features smaller than the operational wavelength of the electromagnetic wave. Metamaterials include the negative index materials (NIMs), single negative metamaterials, bi-isotropic and bianisotropic media, chiral media and so on. As an application, the physicist John Pendry proposed that NIMs enable a flat perfect lens [1], which allows deep sub-wavelength imaging. In 2006, Pendry and Leonhardt [4,5] proposed that a geometric transformation of space could distort light trajectories around a bounded region, which is thus made invisible. This powerful mathematical technique is called transformation optics, and it presents great potential as it drives the fabrication of metamaterials with specially designed properties [6–9], such as invisibility cloaks, rotators, concentrators, etc.

In NIMs, or single negative metamaterials (e.g. composites with ε < 0 or µ < 0), it is supposed that metamaterials have independent electric and magnetic responses described by ε and µ. However, magnetoelectric coupling does occur in many electromagnetic metamaterials (even in the static case [10]), wherein the electric and magnetic fields are induced reciprocally. Anisotropic (resp. isotropic) media with such kind of property are referred as bianisotropic (resp. bi-isotropic) [11]. Bianisotropic media can be achieved by variations of the split ring resonator (SRR) geometry [12], direct laser writing and silver shadow evaporation on a polymeric template [13], stack of dielectric materials [14,15], etc.

In [16,17], we have theoretically proved that the Pendry-Ramakrishna lens theorem is applicable to complementary bianisotropic media. This uses the fact that as a powerful mathematical technique - transformation optics allows for a transformation of space between two different coordinate systems, while the Maxwell-Tellegen’s equations for bianisotropic media are also proved to be form invariant under space transformation. Although it is easy to understand the principle used to design novel devices such as cloaks, concentrators, rotators and so on, a numerical model is required to illustrate their electromagnetic response.

In this paper, we would like to discuss in details the implementation of bianisotropic media in a finite element model, and further show numerical results for an invisibility cloak, concentrator and rotator, the former being even achieved with a simplified multilayered design. On the other hand, we know that, Partial Differential Equations (PDEs) are generally used to model many physical phenomena such as fluid dynamics and electromagnetism. In complex media, solutions to the governing equations can be difficult to derive in closed-form by traditional analytical routes - Fourier or Laplace transform methods, power series expansion and so on, hence one often has to resort to numerical approximation of the solutions. As a particular class of numerical techniques, Finite Element Method (FEM) is efficient to solve problems in heterogeneous anisotropic media [18,19], and it has been widely applied in engineering design and analysis in mechanics starting from the late seventies, while researchers in photonics started to use it rather in the nineties. Notably, the COMSOL Multiphysics package is a finite element analysis software much used nowadays to solve various physics and engineering applications in the metamaterials’ community, as it allows entering coupled systems of PDEs, such as in opto-mechanics, thermal elasticity, etc. Here, we make use of it to set up a coupled PDE system for a bianisotropic structure. To do this, we start from the Maxwell-Tellegen’s equations in bianisotropic media and derive two coupled PDEs with the longitudinal electric and magnetic fields as the unknowns. This PDE system has the same structure as that for (coupled pressure and shear) in-plane elastic waves. Then we discuss open boundary conditions known in the photonics literature as perfectly matched layers (PMLs). To illustrate the usefulness of our numerical algorithm, we apply transformation optics to bianisotropic media in order to design some functional devices, i.e. invisibility cloak, concentrator and rotator. Their electromagnetic (EM) properties studied thanks to our PDEs model implemented in COMSOL Multiphysics prove that these well-known devices work equally well in bianisotropic media.

2. A coupled PDE system for bianisotropic media

We first recall the time-harmonic Maxwell’s equations (assuming a time dependence exp(−iωt) with t the time variable and ω the angular wave frequency)

Let us assume the constitutive relations in a bianisotropic medium described by

Now we consider an orthogonal coordinate system (x₁, x₂, x₃), and assume that the material parameters and the electromagnetic field are invariant in one direction, say, the x₃ direction, that is, ε, µ, ξ, ζ, E, H are independent of x₃.

Let us define E = (E₁, E₂, E₃)^T, H = (H₁, H₂, H₃)^T, and further assume following the terminology used in the book by F. Zolla et al. [20], that ε, µ, ξ and ζ are z-anisotropic tensors, i.e. such that

We then plug Eq. (4) into Eq. (3), and have

We rewrite Eqs. (5) and (6) in a matrix form

Similarly, Eqs. (8) and (9) become

If we define

Furthermore, from Eq. (15), we have

Substituting it into Eq. (16), we obtain

This can be recast as

Let us now plug Eq. (19) into Eq. (10)

In the same way, from Eq. (16), we have

We plug Eq. (24) into Eq. (7)

Finally, we rewrite Eqs. (21) and (26) by multiplying by ω on both sides of the equations, and we obtain

3. Implementation of the PDE system in Comsol Multiphysics

The standard form of PDEs in COMSOL Multiphysics is described by the following equation reminiscent of a governing equation in elasticity theory

Comparing Eqs. (27) and (28), we can see that there is a neat isomorphism, i.e. we establish the correspondences between the Maxwell-Tellegen’s equations for electromagnetic waves and the Navier equations for solid mechanical waves, which generalises the correspondence between in-plane elasticity and a reduced form of the Maxwell’s equations in the paper of G. W. Milton and A. B. Movchan [21]. These correspondences can be straightforwardly implemented with a finite element package, as well as a finite difference time domain (FDTD) package written for linear elastodynamics, e.g. the freeware SIMSONIC [22], in order to solve electromagnetic wave problems in complex bianisotropic media. Although we use here the FEM based software -COMSOL to deal with the analysis of bianisotropic media in the following sections, we would like to point out that the correspondences should work equally well in time and frequency domains.

We rewrite the formulae in Eq. (27) as

Comparing with Eq. (28), if we consider the coupled longitudinal electric and magnetic fields, the unknown u in Eq. (28) is

Simply stated, when the tensors of ε, µ, ξ, ζ in Eq. (13) are represented by diagonal matrices in a Cartesian coordinate system

Note that each component of the coefficients c_ij (i, j = 1, 2) is a fraction with a denominator (ε_kk µ_kk − ξ_kk ζ_kk) (k = 1, 2). In order to avoid any infinite entries of c while solving the PDEs, it should satisfy

Remark: This condition is essential and generally is met since the magnetoelectric coupling parameters for bianisotropic media are usually small.

4. Problem setup with open boundary conditions

In the last section, we have discussed the general PDEs in bianisotropic media, which allows us to introduce the PDEs model in COMSOL Multiphysics; however, in order to mimic the scattering and propagation properties of bianisotropic media in an open space, we need to consider the boundary conditions. We usually introduce a transformation of an infinite domain into a finite one, which should enforce that the wavelength is contracted to infinitely small values as it approaches the outer boundary of the transformed domain within a perfectly matched layer (PML). At this outer boundary, Dirichlet data could be set, thereby enforcing a vanishing field [23,24].

The PML is shown to be equivalent to an analytic continuation of Maxwell’s equations to complex variables’s spatial domain [25]. The form of the electromagnetic field inside a PML region can be obtained from the solutions in real space from a mapping to a complex space through this analytic continuation. Physically speaking, an equivalent artificial medium is achieved from this map in the PML region that is described by complex, anisotropic and inhomogeneous parameters, even if the original ones are real, isotropic and homogeneous. The analytic continuation means to alter the eigenfunctions of Maxwell’s equations in such a way that the propagating modes are mapped continuously to exponentially decaying modes, which allows for reflectionless absorption of electromagnetic waves. In other words, an absorbing medium dissipating the outgoing wave can be achieved through proper complex map: an artificial medium situated in a region can be added to a finite medium in order to mimic an open space within which the field is damped to a negligible value [26]. Importantly, the impedance of the equivalent medium is the same as that of the initial medium since all the parameters undergo the same transform, what ensures non-reflecting features on the interface between the medium and the PMLs.

F. L. Teixeira et al. [24, 25] derive the Maxwellian PML’s for arbitrary bianisotropic and dispersive media. In the Cartesian coordinate system, if an analytic continuation is defined by the transformation function

Based on this, we then can expand the Maxwell’s equations in the new complex space, and derive that the PMLs with bianisotropic constitutive parameters are

Take a full wave simulation for two dimensional structures in COMSOL Multiphysics as an illustration, the computational domain is terminated by PMLs in Cartesian coordinates. The PMLs are designed to decrease the distribution of the waves along the x, y and xy directions, respectively. Specifically, assuming the parameter of the matrix ν = diag(ν_xx, ν_yy, ν_zz) (ν = ε, µ, ξ, ζ), then we choose $\overline{\overline{S}}$

5. FEM implementation

To check both our PDEs model and PMLs for bianisotropic media, some conceptual devices designed from transformation optics are numerically studied in the following subsections, these structures are all achieved by bianisotropic media.

5.1. Invisibility cloak

Electromagnetic (EM) metamaterials such as invisibility cloaks can be designed through the blowup of a point [4, 5], using transformation optics which is a versatile mathematical tool enabling a deeper analytical insight into the scattering properties of EM fields in metamaterials. In this subsection, a cloak consisting of a bianisotropic medium is investigated, and the geometric transformation proposed by J. Pendry [4] is proved to be applicable to design such kind of cloak.

We start with a free space with permittivity ε = ε₀, permeability µ = µ₀ and magnetoelectric coupling parameters ξ = ξ₀, ζ = ζ₀ in the Cartesian coordinate system, and first map it onto polar coordinates (r, θ, z) as defined by

Then we introduce a geometric transformation defined by Eq. (45) which maps the field within a disk of radius r = R₂ onto an annulus R₁ < r′ ≤ R₂, i.e. the original point r = 0 in the original disk is blowup to a disk with r′ = R₁ in coordinates (r′, θ′, z′). Based on this coordinate transformation [4, 9], the rays are distorted and guided around the cloak.

The mapping function is defined as

Finally, we go back to the Cartesian coordinates (x′, y′, z′), which are radially contracted, and the compound Jacobian matrix is

More specifically, it is

The transformation matrix [17] is correspondingly

We denote

Hence, the parameters in the annulus R₁ < r′ ≤ R₂ are

Figure 1(a) shows our designed bianisotropic cloak, the cloaking region is the innermost circle of radius R₁ = 20 mm wherein a L-shaped obstacle locates, the gray annulus represents the distorted space with radius R₁ < r′ ≤ R₂, where R₂ = 40 mm. Firstly, the EM distribution of a point source (s-polarized with the electric field along z) with frequency f = 8.7 GHz radiating in a matrix with parameters ν = ν₀I₃, (ν = ε, µ, ξ, ζ) is shown in Fig. 1(b). ν₀ = ε₀, µ₀ are the permittivity, permeability of the vacuum, respectively; while |ξ₀ζ₀| ≠ 1/c₀ with c₀ the velocity of light in vacuum should be promised to ensure convergence of the numerical algorithm as indicated in Eq. (35). If there is a L-shaped obstacle with ε = (1 + 5 × i) ε₀I₃, µ = µ₀I₃, ξ = ζ = 0.99/c₀I₃ in the matrix, it leads to an interacting with the source, the plot of Re(E_z) is indicated in panel (c), where the white regions are for values outside the color scale. However, when the L-shaped obstacle is surrounded by the designed cloak, then the obstacle seems to be invisible for the outer observer, as shown in panel (d).

 

Fig. 1 (a) Schematic diagram of bianisotropic cloak with a L-shaped obstacle inside the cloaking region, and a point source (s-polarized with the electric field along z with f = 8.7 GHz) locates outside the shell. Plots of Re(E_z) with the same arbitrary units for the three following panels: (b) A reference point source in a pure matrix with parameters ε = ε₀I₃, µ = µ₀I₃ and ξ = ζ = 0.99/c₀I₃; (c) Same point source in the matrix with a presence of L-shaped obstacle, the parameters of obstacle are ε = (1 + 5 × i) ε₀I₃, µ = µ₀I₃ and ξ = ζ = 0.99/c₀I₃; (d) Same point source radiates in the presence of an L-shaped obstacle surrounded by an invisibility cloak.

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5.2. Concentrator

A cylindrical concentrator is designed to focus the incident waves with wave vectors perpendicular to the cylinder axis, enhancing the electromagnetic energy density in a given area [6].

We start with a map from Cartesian coordinates to cylindrical ones, for a cylindrical lens with a radius r = R₃, a radial transformation is introduced to distort the space as

A space is compressed into a cylindrical region with radius R₁ (the innermost circle) with the expense of an expansion of space between R₁ and R₃, where an intermediate circle is located at R₂. Similarly, we can derive the associated Jacobian matrix J_rr′, then the compound Jacobian matrix in Eq. (47) and the transformation matrix in Eq. (49). We have

Let us expand this formula in the distorted cylindrical coordinates (r′, θ′, z′), and denote ${\mathbf{T}}_{x{x}^{\prime }}^{-1}$ as in Eq. (50), finally we have

Apply matrix ${\mathbf{T}}_{x{x}^{\prime }}^{-1}$ to Eq. (52), we can obtain the formulae of these parameters in the annulus as well as the innermost circle, while the outside space of the concentrator is bianisotropic matrix with ν = ν₀I₃.

Figure 2(a) is the schematic diagram of our concentrator, the radii of the inner and outer circles are R₁ = 20 mm, R₃ = 40 mm, while the intermediate circle has a radius R₂ = 30 mm. The parameters of each region are defined in Eq. (52) along with transfer matrix ${\mathbf{T}}_{x{x}^{\prime }}^{-1}$ in Eq. (55), here the magnetoelectric coupling parameters ξ₀ = ζ₀ = 0.99/c₀ are chosen as the same values as those of the invisibility cloak. An s-polarized (the electric field is perpendicular to the xy plane) plane wave with frequency 8.7 GHz is incident from above, the plot of Re(E_z) is depicted in panel (b): the waves are compressed in the inner circle as expected.

 

Fig. 2 (a) Schematic diagram of a bianisotropic concentrator; (b) Plot of Re(E_z) under an s-polarized incidence (amplitude normalized to unity) with frequency f = 8.7 GHz radiating from the above, the parameters in the region r′ ≤ R₁, and the annulus R₁ < r′ ≤ R₃ are given by Eq. (52) with transformation matrix ${\mathbf{T}}_{x{x}^{\prime }}^{-1}$ in Eq. (55).

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5.3. Rotator

We would like to design a rotator which allows the rotation of the incident wave, the distribution of the field inside a disk r ≤ R₁ appears as it comes from a different angle comparing with that of outside r > R₂, for example, a π/2 angle rotation is achieved here in the inner disk.

Be different from the geometric transformation introduced in the concentrator, an angular transformation is introduced in this case instead of the radial transformation. The mapping is performed in the region r ≤ R₂: the domain in r ≤ R₁ is transformed by rotating a fixed angle from the region r′ = a in virtual space, while the rotation angle is continually changed from the fixed angle to zero in region R₁ < r ≤ R₂. In other words, the transformation algorithm can be expressed as

Based on this transformation, we can derive the associated Jacobian matrix J_rr′, furthermore, the compound Jacobian matrix in Eq. (47) as well as the transformation matrix in Eq. (49) can be obtained. In annulus R₁ < r′(= r) ≤ R₂, the notations in Eq. (50) become

The FEM analysis for the rotator is shown in Fig. 3. The radii of the inner and outer circles are R₁ = 20 mm, R₂ = 40 mm, respectively. The parameters of each region are defined in Eq. (52) along with transfer matrix ${\mathbf{T}}_{x{x}^{\prime }}^{-1}$ in Eq. (58), the magnetoelectric coupling parameters ξ₀ = ζ₀ = 0.99/c₀ are chosen as the same values as those of the invisibility cloak. An s-polarized incident wave with frequency f = 8.7 GHz is assumed to radiate from above. A rotation of θ₀ = π/2 of the fields in the inner circle can be observed in panel (b) comparing with the incidence. Note that, θ₀ can be an arbitrary values in [−π, π].

 

Fig. 3 (a) Schematic diagram of the bianisotropic rotator; (b) Plot of Re(E_z) under an s-polarized incidence (amplitude normalized to unity) with frequency f = 8.7 GHz radiating from the above, the parameters in the region r′ ≤ R₁, annulus R₁ < r′ ≤ R₂ are given by Eq. (52) with transformation matrix ${\mathbf{T}}_{x{x}^{\prime }}^{-1}$ in Eq. (58).

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6. A multilayered bianisotropic cloak through homogenization

Let us now consider a layered medium [29] periodic along x₁ and invariant along x₂ and x₃. A periodic cell consists of a heterogeneous bi-isotropic medium described by piecewise constant scalar valued functions ε(x₁), µ(x₁), ξ(x₁), ζ (x₁). Using two-scale expansion techniques applied to the Maxwell-Tellegen’s equations like in [30] one gets the homogenized equations

One can use these formulae to design a concentric multilayered bianisotropic cloak of inner radius R₁ = 14 mm and outer radius R₂ = 30 mm, consisting of 22 homogeneous layers (see Table 1 for the values of the inverse relative permittivity, permeability, magnetoelectric coupling parameters) in a bianisotropic background with permittivity ε = ε₀, permeability µ = µ₀ and magnetoelectric coupling parameters ξ = ξ₀ = 0.99/c₀, ζ = ζ₀ = 0.99/c₀. Notice that almost all layers have an equal thickness of 0.75 mm and one layer in two has almost the same optical properties.

Table 1. Parameters of the layered cloak. (ε = ε_rε₀, µ = µ_rµ₀, ξ = ξ_rξ₀, ζ = ζ_rζ₀.)

View Table

We show the results of numerical simulations in Fig. 4. One can see that the backscattering by the infinite conducting obstacle (of radius 14 mm) with a concentric multilayered bianisotropic cloak is much reduced in Fig. 4(b) comparing with Fig. 4(a), and the shadow zone behind the obstacle is also much reduced with the layered cloak. The interval of cloaking frequencies from 5.7 GHz to 7.7 GHz is fairly broadband.

 

Fig. 4 (a) Scattering of an infinite conducting obstacle (of radius 14 mm) by a point source at a frequency 5.7 GHz; (b) Scattering like in (a) when the obstacle is surrounded by a layered cloak; (c) Scattering like in (a) for a point source at a frequency 7.7 GHz; (d) Scattering like in (b) for a point source at a frequency 7.7 GHz. Color scale corresponds to Re(E_z) (the same arbitrary units have been used in the four panels).

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7. Conclusion

In this paper, we have discussed the PDEs model for the analysis of the propagation properties in bianisotropic media with an invariance along one direction: two coupled PDEs with scalar solutions are derived for this electromagnetic system. The spatially varying coefficients within these two PDEs can be scalar or matrix valued. Regarding the numerical model, specially designed perfectly matched layers (PMLs) are introduced in order to account for the unbounded domain. To do this, we consider a transformation which maps an infinite domain onto a finite one, and we further add the damping. This provides a reflection-less bianisotropic layer for all incident angles, and this can be considered as an extension of the PMLs introduced by J. Berenger nearly twenty years ago [23]. We implement these PMLs in scattering problems in bianisotropic media including an invisibility cloak, a field concentrator and a field rotator designed by introducing the proper geometric transformation. Importantly, these functional structures are proved to work well when they are achieved by bianisotropic media. We finally, show that one can achieve near-cloaking with a layered cloak with piecewise constant permittivity, permeability and magnetoelectric coupling parameters. The solutions of singular systems considered in this paper represent important benchmarks for the validation of non-trivial numerical calculations for bianisotropic media.