We propose to use the concept of transformation optics for the design of novel radiating devices. By applying transformations that compress space, and then that match it to the surrounding environment, we show how the electromagnetic appearance of radiating elements can be tailored at will. Our efficient approach allows one to realize a large aperture emission from a small aperture one. We describe transformation of the metric space and the calculation of the material parameters. Full wave simulations are performed to validate the proposed approach on different space compression shapes, factors and impedance matching. The idea paves the way to interesting applications in various domains in microwave and optical regimes, but also in acoustics.
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In the telecommunications domain, there are actually growing interests in the miniaturization of devices, particularly for antennas in transport and aeronautical fields. In most cases it is the physics itself that limits the possibility of size reduction. The transformation optics (or coordinate transformation) concept [1–5], an innovative approach to design new class of electromagnetic devices, can prove its usefulness for miniaturization since it allows making a link between space, time and material. The main idea of coordinate transformation is to make an equivalence between Maxwell equations described in an initial coordinate system and these same equations described in another arbitrary transformed one. The result is a direct link between the permittivity and permeability of the material and the metric tensor of the transformed space containing the desired electromagnetic properties [6–8]. This method was first used by U. Leonhardt  and J. B. Pendry  to design an electromagnetic invisibility cloak in 2006 . Since then, the invisibility cloak has been a subject of intensive studies  and later, other systems resulting from coordinate transformation have emerged. Thus, concentrators , rotators , lenses [13–16], artificial wormholes , waveguide bends and transitions [18–23], electromagnetic cavities [24,25], illusion systems [26,27] and antennas [28–35] have emerged. In most cases, the generated materials are inhomogeneous and anisotropic since the created virtual spaces make use of arbitrary coordinates. Devices generated by transformation optics can then be fabricated through the use of metamaterials, which are subwavelength engineered artificial structures that derive their properties from their structural geometry.
In this paper, transformation optics concept is applied to transform the signature of a radiating source. We show that a linear space compression followed by a space expansion, make the radiation pattern of a small aperture antenna appear like that of a large one. The material parameters generated from the transformation are discussed and the results are validated by numerical simulations performed using sources of different shapes and lengths. We further show that the proposed transformation can also be applied to an array of miniaturized electromagnetic radiators.
2. Transformation formulations
To achieve the transformation of a small aperture source into a much larger one, we discretize the space around the latter radiating element into two different zones; a first zone which will make our source appear bigger than its real physical size and a second zone which ensures the impedance matching with the surrounding radiation environment. The operating principle is shown by the schematic in Fig. 1(a) . In a space point of view, the technique consists in compressing a circular region of space of radius R1/q1 (with q1 < 1), delimited by the red circle in Fig. 1(a)) in a region of radius R1. In the studied transformation, our space is described by polar coordinates and the angular part of these coordinates remains unchanged. The second part of the transformation consists in an impedance matching with the surrounding space through an annular expansion zone defined between circular regions with radius R1 and R2, as illustrated in Fig. 1(b). This space expansion can be performed using three different transformations: a positive exponential transformation, a negative exponential transformation, and a linear one. We denote below and in the rest of the paper the two different zones by the index i, where i = 1 corresponds to the first zone and i = 2 to the second zone. The final virtual space describing our device is represented in Fig. 1(c). Figure 1(d) summarizes the different transformations considered. To secure the impedance and metric matching, continuity of our transformations is assured at the boundary of the first region (point A in Fig. 1(d)) and at the outer boundary of the device (point B in Fig. 1(d)).
Mathematically, the transformation in the different regions can be written as:
The coefficient of our material can be written as in the cylindrical coordinates. The material parameters obtained using the transformation in Eq. (1) are:
These parameters are relatively simple for the transformation in the first zone since it leads to constant values. But the permittivity and permeability components have to be expressed in the Cartesian coordinate system so as to have a perfect equivalence in Maxwell’s equations and also to physically design our device. Using matrix relations between cylindrical and Cartesian coordinates, we have:
The angular part in the coordinate transformation described above allows obtaining more general and adjustable parameters for a possible physical realization of the device. However, in the present study, we consider fi,θ = 0 to simplify the calculations. To apply our proposed coordinate transformation, we consider a radial compression of the space in region 1. This leads to a material with high permittivity and permeability tensors. For the transformation, we choose with q1 being a coefficient lower than 1. The physical meaning of the factor q1 is the compression factor applied in the central region. This factor has a transition value which can be defined as where the material of the matching zone (region 2 in Fig. 1(c)) switch from a right-handed (positive refractive index) to a left-handed (negative refractive index) material. Indeed when q1 < q0 the material presents a negative index and the final apparent size of the source can be larger than 2R2. Now if this embedded source has a small aperture, much smaller than the wavelength, then after transformation this antenna will behave like one with a large aperture, typically q1 times larger and potentially much greater than the wavelength. A small aperture antenna is well known to radiate isotropically. The same antenna embedded in the material defined by Eq. (5) will present a directive radiation and therefore electrically appear as if its size is larger than the working wavelength. Moreover, we can obtain the radiation of a conventional array of antennas using much smaller dimensions for the latter array embedded in zone 1. To assure a good impedance matching for the radiated fields, a matching zone (region 2) is added around region 1. To design this zone, we consider three different possible transformations that match the space from R1 to R2. The first studied transformation for this matching region is a linear one that takes the form whereas the two other transformations have logarithmic forms that can be expressed as where and are constant values. In these two cases, the inverse transformation defining r from r’ has an exponential form defined by: . This exponential transformation can be characterized by the factor q2 that indicates the shape of the progressive metric matching to vacuum (), as illustrated in Fig. 1(d). A small value of q1 indicates a high compression of the space in the first region. To compensate this high compression, the transformation in the second region gives negative electromagnetic parameters due to the relative positions of points A and B (Fig. 1(d)), as presented in Fig. 2 . In such case, the wave propagates with a backward phase in this region. Figure 2 shows the variations of the different components of the permittivity and permeability tensors for the matching region 2. For the linear transformation, the minimum and maximum of the material parameters depend on the geometrical properties of the problem and thus they depend only on α and γ which are given by:
We can note that the trends of the permittivity and permeability values in the Cartesian coordinates are quite similar for both linear and exponential transformation. The values depend only on R1, R2, q1 and q2 for the linear transformation. For the case of the exponential transformation in region 2, the parameters considered are q1 = 1/16, q2 = 15, R1 = 5 mm and R2 = 45 mm and as it can be observed, the calculated components ψxx, ψyy and ψzz are always negative.
3. Numerical validation
In order to validate the proposed concept, we use the commercial software Comsol MULTIPHYSICS to perform numerical simulations of the different transformation cases presented above. All the simulations are run in the microwave domain at 10 GHz. The validation of our design is performed in a two-dimensional configuration in a transverse electric mode (TEz) (E parallel to the z-axis). Different current sources perpendicular to the xy plane are used as radiating elements in order to show that our transformation can be applied to any type of source embedded in the region 1. Continuity and matched conditions are applied respectively to the boundary of zone 1 and zone 2.
To verify our design, we fix R1 = 2 mm and R2 = 45 mm. The results obtained from linear transformations both in region 1 and 2, as defined by the continuous blue trace in Fig. 2(a), are presented in Fig. 3 . In Fig. 3(a), the electric field distribution of a current source radiating in free space is plotted. The source is supposed to have a width d = 80 mm (2.7λ at 10 GHz). For such a large size, the radiation is equivalent to that of an array of several elements and therefore, the radiated field is directive. Figure 3(b) shows a similar source but with a much smaller size d = 2 mm (λ/15 at 10 GHz) embedded in the metamaterial shell having a compression factor q1 = 1/40. In this scenario, a radiation pattern similar to the large aperture source is observed, demonstrating that small aperture antennas inserted in the proposed material shell present the same electromagnetic behavior as much larger aperture antennas in free space. However, this same miniature source will radiate in an isotropic manner in free space (Fig. 3(c)). The same observations can be made when replacing the linear current source by a crossed-type one, as illustrated in Figs. 3(d)-3(f).
In Figs. 4(a) -4(c), the linear transformation is followed by an exponential one with q2 = 15 in region 2 and the compression factor in region 1 is decreased to q1 = 1/40. The transformation is defined by the dashed blue trace in Fig. 2(a). The small size linear current source with d = λ/15 is embedded in the metamaterial shell defined by the proposed coordinate transformation. A directive emission is observed as in the previous case and as illustrated in the enlarged view of Fig. 4(b), we can clearly note the exponential form of the radiated field. We can also observe the perfect impedance matching between the regions 1 and 2 and between the region 2 and free space. This is clearly confirmed by the continuity of the electric field norm at the interface r = R1 and by the absence of stationary waves in region 2 in Fig. 4(c).
In Figs. 4(d)-4(f), the material in region 1 is defined by q1 = 1/16 and in region 2, the material is defined by an exponential transformation with q2 = 15. This transformation corresponds to the dashed red trace of Fig. 2(a) and the crossed-type source is embedded in the metamaterial shell. In this case also, a bidirectional directive beam can be observed even if the size of the source is very small compared to the working wavelength. In each case, the small aperture size of the radiating element has been transformed into a larger one: 40 times for the linear source and 16 times for the crossed-type source.
In the absence of the matching region, there is a high impedance mismatch at the boundary of the region 1 and all the energy emitted by the source is reflected at the boundary and confined in this latter region. This phenomenon is illustrated in Fig. 5 by the norm of the electric field. Stationary waves appear in the structure due to reflection at r = R1.
The above results show that we are indeed able to hide the physical appearance of radiating sources by miniaturizing their physical dimensions without altering their radiation diagrams. Furthermore, we show that our transformation still holds for an array of small antennas. We have simulated an array of three sources of length L = 12.5 mm, spaced by a distance a = 5 mm and with a 30° phase shift between each element. These sources radiate in vacuum and as illustrated in Fig. 6(a) , we observe a radiated beam pointing in an off-normal direction due to the phase shift applied between the different elements of the array. When the dimensions of these antennas are reduced by a factor of 25 (q1 = 1/25) the dimensions of the array become smaller compared to the wavelength and the radiated field becomes isotropic as shown in Fig. 6(b). By embedding the small sources in a material defined from the double linear transformation, we are able to recover the beam steering of the source array as shown in Fig. 6(c). This last example confirms the ability of our transformation to change the electromagnetic appearance of a group of radiators. Figures 6(d) and 6(e) show that in both cases, with and without transformed material, the impedance matching between the metamaterial shell and free space is perfect.
This work points out the use of transformation electromagnetics concept to design an artificial shell which allows hiding the physical appearance of electromagnetic sources by miniaturizing them. The latter concept makes use of two transformations; the first one to compress space and the second one to expand it. Numerical simulations have confirmed the operating principle of the transformations on sources of different geometries. We have shown that a very small source can emit a directive radiation comparable to an antenna with a large aperture. Furthermore, the concept has also been applied to an array of miniaturized radiating elements which is able to show an off-normal directive beam direction. The proposed idea constitutes an important step towards miniaturized devices in order to achieve performances that have been till now possible only with large physical devices. We can also imagine that such a device can be benefit in the design of headphones with a sound fidelity comparable to classical stereo speaker. High efficiency low profile field concentrators can also be imagined for energy harvesting.
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