1. Introduction

The concept of transformation optics [1,2] and its use in designing and realization of a microwave invisible cloak has attracted great interest in the studies of controlling the electromagnetic fields. Many devices with novel properties have been designed in theories and experiments, such as cloak, concentrators, superscatterers, field rotators, phase transformer and so on [3–7]. Although making these devices requires materials with specific permittivity and permeability which cannot be obtained in the nature world, the metamaterials with anisotropic and inhomogeneous behaviors normally, which are formed by different subwavelength structures, can be used for providing the solutions [3,8,9]. As the development of metamaterials, transformed media can be realized not only in microwave but also in optics now [10].

Among various kinds of present devices, “cloaking” is much attractive because of its novel capability to change the scattering cross section of a perfect electrical conductor (PEC) object. For the “cloak” device, the scattering cross section of the PEC object can be changed to a point, and for the imperfect cloak [11] it can be changed smaller than the scale of the device. On the contrary, the superscatterer [5] is corresponding to an amplified scattering cross section of the PEC object. Based on the transformation optics, H.Chen et al. proposed the concept of “reshape” with which the position for scattering cross section of the PEC object can also be shifted for the giving incident electromagnetic(EM) wave although the real PEC object is still in the original position [12].

In this paper, we proposed a method for changing the scattering cross section of PEC object to correspond to an object of a different shape based on transformation optics, and the corresponding metamaterial device is called transformer. When the PEC cylinder wears the transformer, it looks as if it was other object instead of the original one.

2. Principle

In the method, the transformation optics is employed for determining the parameters of the transformer. Based on form-invariant coordinate transformations of Maxwell’s equations which has been described in detailed in ref.12 and 13, two domains D and D’, which are called virtual domain and physical domain respectively, can be defined for the virtual PEC object and original PEC object. A given coordinate transformation $x^{i^{\text{'}}}=A_i^{i^{\text{'}}}{x}^i$ maps D to D’. If the permittivity and permeability tensors in virtual domain are ε^ij and μ^ij, the relative permittivity and permeability tensors in the physical domain can be calculated by the prescription [13]

Figure 1 shows the schematic diagram of the design, where a two-dimensional arrangement is supposed for implementing the transformation in the Cylindrical coordinate. In Fig. 1, we assume that the original PEC object is with circular shape and the contour of the destination shape is denoted by the virtual square as an example. In order to transform the scattering cross section corresponding to a circular PEC object to that of a square object, the domain between the outer circle and the virtual square will be transformed to a shell between the inner and outer circle (it is physical domain corresponding to the designed transformer) through a given coordinate transformation. For simplicity, we divide the virtual domain to four equal ones as shown in Fig. 1. The coordinate transformation equations for the example design in Region I can be expressed as

 

Fig. 1 Schematic diagram of the space transformation for the transformer. The radii of outer and inner circles are R₂ and R₁, respectively, and the sidelength of the virtual square is 2a. The transformation maps point A to point A’.

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With above equations, we can obtain the Jacobian transformation matrix in the Cylindrical coordinate to be [12]

Equation (7) provides the full expressions of permittivity and permeability tensors for the quarter of the shell transformed from Region I. By using rotation operators with rotation angles of π/2, π and 3π/2 around z axis to Eq. (3), we can obtain the transformation formulas for other three quarters of the virtual domain. In the virtual space there is a cylindrical PEC with the shell, whose permittivity and permeability tensors are restricted by Eq. (7), and the shell is the metamaterial transformer. What we expect is that a square prism PEC can be observed instead of the real cylinder PEC object if the transformer is applied to it as clothes since the shape of scattering cross section has changed.

3. Simulation and result

In order to validate our design, we make full-wave simulations based on the finite-element method. For evaluating the transformer’s properties, the near-field distributions are calculated for PEC objects with different shape of scattering cross section and the corresponding patterns of electric field are shown in Fig. 2 . The plane-wave of TE polarization incidents from lower left to upper right corner with frequency 3GHz and unit amplitude, and the inner and outer radii of the shell are R₁ = 0.2m and R₂ = 0.3m, respectively. The side length of the square in virtual space is 2a = 0.2m.

 

Fig. 2 Snapshot of the total electric field. (a) (b) are the total electric field induced by PEC cylinder with radius R₂ = 0.2m and by PEC square prism with side length 2a = 0.2m, respectively. (c) is the total electric field induced by our designed device transformer.

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Figures 2(a) and 2(b) is the snapshot of the total electric field induced by PEC cylinder and PEC square prism, respectively. Figure 2(c) is the snapshot of total electric field induced by our designed device which is composed of a PEC cylinder with the transformer outside. From patterns of electric field in (a) and (c), we can find that when the PEC cylinder wears the transformer, the electric field distribution has been changed significantly. It means that the PEC cylinder was no longer cylindrical to an observer. Comparing the patterns of electric field in (b) and (c) in the region r>R₂, one can find that they are almost equivalent. If someone observes the device in the region r>R₂, he will see a PEC square prism instead of a cylindrical one. Obviously, the simulations accord with our design very well and the shape of the scattering cross section has been changed from circular to square in the example.

4. Expanded model for the transformer

In last section, we proposed a transformer that can change the scattering cross section corresponding to a circular PEC object to that of a square one. If other destination contour is set in the virtual domain to replace the virtual square, the corresponding transformer can be designed. Suppose ρ(θ) is the scattering cross section’s boundary of the PEC in virtual space, and the virtual domain is the region between the curve ρ(θ) and the circle with radius of R₂. The general coordinate transformation equations can be expressed as

According to the shape of the PEC objects, the transformer’s shape can also be designed arbitrarily by changing R₁, R₂ in Eq. (8) to R₁(θ) R₂(θ) as long as R₁(θ) is less than R₂(θ) all over the function domain (0≤θ<2π).

The further simulations are proposed in Fig. 3 to prove our method. The plane-wave incidents from left to right in these simulations compared with simulations in Fig. 2. It is found by comparing Figs. 3(a) 3(c) and 3(b) 3(d) that the circular scattering cross sections have changed by the transformers to the shape of triangular and cardioid-like respectively in Figs. 3(c) and 3(d). Here in our examples, the curve ρ(θ) is inside the outer circle (ρ(θ)<R₂). However, the curve ρ(θ) may be outside the outer circle or has intersections with it, which means ρ(θ) may be larger or smaller than R₂ or even equivalent in one curve. The difference in those three conditions is just related to the refractive-index in transformers. From Eq. (4), $a_{11}=\frac{R_2-R_1}{R_2-\rho (\theta )}$, $a_{22}=\frac{r\text{'}}{r}$ can be obtained. When ρ(θ) is larger than R₂, there are a₁₁<0, a₂₂>0. Consequently, μ_rr, μ_θθ, ε_zz are all below zero from Eq. (7) and then the refractive-index will be negative, like the case of superscatterer [5]. When ρ(θ) is smaller than R₂, the refractive-index will be positive, just like the case of imperfect cloak [11], and when ρ(θ) is equivalent to R₂, that means the intersection will be transformed to a line, and the permittivity and permeability tensors will be in zero and infinite components, just like perfect lens [14].

 

Fig. 3 Snapshot of the total electric field. (a) (b) are the total electric field induced by PEC prisms whose cross section are triangular and cardioids-like (the boundary curve ρ(θ) = h(1 + cos(θ)), where h = 0.1m), respectively. (c) and (d) are the total electric field induced by the relative transformers.

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5. Conclusion

In this work, we have presented a new transforming device designed by the transformation optics with which we can change the scattering cross section’s shape of a PEC object arbitrarily. With the development of the metamaterials, the designed transformer with anisotropic and inhomogeneous distribution can be pushed closely to the practice and find applications in many aspects like camouflaging some objects from recognition of radar or other detecting systems.