A planar metasurface composed of electronically tunable meta-atoms incorporating voltage-controlled varactor diodes is proposed as a reconfigurable meta-mirror for wavefronts control in microwave antenna applications. The dispersion responses of the cells are individually tailored in the reconfigurable metasurface so as to overcome the bandwidth limitations of passive metasurfaces and also to control the phase characteristics. By controlling the bias voltage of the varactor diodes on the planar metasurface, the phase characteristics of reflectors can be engineered. The reconfigurable meta-mirror is utilized to implement three different types of reflectors. As such, a reflectarray, a cylindrical parabolic reflector and a dihedral reflector are numerically verified in microwave regime through finite element method. Moreover, experimental measurements are performed on a fabricated prototype to validate the proposed device. Frequency agility, beam deflection and beam focusing are the main functionalities demonstrated from the proposed reconfigurable meta-mirror.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Artificial composites formed by subwavelength microstructures known as metamaterials, can possess arbitrary values of permittivity and permeability and can thus offer the possibility to manipulate light in an unprecedented manner. Thanks to their unusual electromagnetic properties, negative refraction  sub-diffraction imaging  and invisibility cloaking  are among the most striking applications that have been made possible through the use of metamaterials. Recently, metasurfaces that are ultra-thin and planar versions of metamaterials, have been proposed to exhibit various light manipulation capabilities . Metasurfaces present the main advantage of having reduced profile and losses, and also being conformable on curved objects. The fact that metasurfaces consist generally in thin sheets materials greatly facilitates their fabrication compared to that of three-dimensional (3D) bulky metamaterials. The possibility to tailor and control the reflection and transmission characteristics of metasurfaces has led to other fascinating device applications in the microwave as well as the optical domains. By controlling the reflection and/or transmission characteristics of metasurfaces, anomalous reflection and refraction have been made possible [5–11]. Applications to flat lenses [12–18], propagating to surface waves transformers  and waveplates [20–22] have also been proposed and validated. In the field of microwave antennas, metasurfaces have been successfully implemented in planar lens antennas [23–26], cavity antennas [27–29] and leaky-wave antennas  to achieve high directivity.
In the microwave regime, reflector antennas [31–35] are among the most widely applied antennas and have extensively been used due to their high-gain properties for applications related to remote sensing, satellite communications, radio astronomy, radars, and global positioning system (GPS). The conventional reflector is generally made of a metallic surface or grid having a parabolic, dihedral or trihedral shape that forms the diameter of the antenna. Dihedral and trihedral reflectors also known as corner reflectors are generally used as radar targets, in calibrating test equipment. Such antennas present the main disadvantage of being bulky due to mainly their non-planar profile. In this letter, we propose a planar meta-mirror where the phase characteristics can be reconfigured to act as an interesting alternative to non-planar reflectors. Voltage-controlled electronic elements are inserted in the reflective metasurface to engineer desired phase profiles of reflectors for wavefronts control. Such a tunable meta-mirror also allows achieving reconfigurability mechanisms that cannot be obtained from classical continuous metallic reflectors. As such, phase profiles can be easily tailored over a broad frequency range simply by changing the spatial distribution of bias voltage of each varactor diode, in order to have a frequency agile reflector. Furthermore, deflection of the radiated beam to off-normal directions can be achieved. Moreover, the phase profile can be dynamically modified to transform the radiated beam of an antenna at will, depending on the working context.
2. Design of the reconfigurable meta-mirror
A schematic view of the proposed meta-mirror is shown in Fig. 1(a). The structure is composed of copper strips printed on a low loss (tan δ = 0.003) copper-backed ARLON AD450 dielectric substrate with relative permittivity εr = 4.5 and thickness 1.52 mm. The copper strips have a width w = 0.5 mm and are separated by a gap g = 1.9 mm. When oriented perpendicular to the incident electric field, the strips play the role of a capacitive grid. On the other face, and in order to get a higher efficiency of the reflected wave, a continuous ground plane is printed. Additionally, the ground plane plays the role of an inductive grid. Classically, the resonant frequency of the LC unit cell can be changed by varying geometrical dimensions of the structure. However, such operation mode is not really practical, particularly when we need to dynamically control the dispersion characteristics. Here, our main goal is to perform an electronic control of the meta-mirror by incorporating electronic components in the unit cells. Thus, varactor diodes are integrated between two consecutive strips in the capacitive grid. By applying a reversed bias DC voltage, the capacitance of each unit-cell can be tuned leading to a shift in the resonant frequency. In our case, Aeroflex MGV 125-08 varactor diodes that present a dynamic capacitance varying from 0.055 pF to 0.6 pF are used. A prototype of the proposed meta-mirror is realized and a photography of the physical device together with the electronic bias system is presented in Fig. 1(b). The structure is composed of 30 columns, each containing 30 resonant unit cells.
The meta-mirror is characterized experimentally in an anechoic chamber. The experimental setup shown in Fig. 2(a), consists in using two horn antennas in order to measure the reflection responses of the meta-mirror as shown in Fig. 2(a). Measurements are performed for different bias voltages that are applied similarly through the whole structure. The magnitude and phase responses for the different applied bias voltages are reported in Figs. 2(b)-2(e). The results show that the tunable meta-mirror allows to control the intrinsic dispersions of the structure for a reconfigurability mechanism. As shown in Figs. 2(b) and 2(d), the resonance frequency of the meta-mirror shifts from 8.35 GHz to 11.6 GHz when the bias voltage varies from 0.5 V to 20 V, corresponding to a capacitance of 0.6 pF and 0.055 pF respectively. The highest phase gradient which is close to 290° is achieved at 9 GHz.
In order to validate the reconfigurability mechanism, the proposed meta-mirror is used to design reflector antennas that are described in the following section.
3. Wavefronts control in microwave antenna applications
Since the response of the reflected wave on the meta-mirror can be dynamically controlled as desired, we propose to use it for the design of three different configurations of reflector antennas in order to demonstrate the various potentialities of the proposed structure.
As a first functionality, we consider the meta-mirror as a reconfigurable reflectarray antenna. Since the generalization of the reflection and/or refraction laws , we can predict accurately the wave deflection by considering local phase variation. Therefore, the meta-mirror can be a good alternative to classical reflectarrays. However, metasurfaces generally suffer from a very low bandwidth due to the intrinsic response of resonant unit cells. This point has been discussed in , where the authors have proposed to overcome this problem by incorporating lumped electronic elements in the structure so as to achieve a broadband operation. However, it should be noted that a passive phase gradient was applied in the metasurface and the implementation of lumped elements allows only to control the working frequency. Here in this study, to allow a broad operating frequency bandwidth, we propose a meta-mirror that is totally agile and that allows to control beam steering and working frequency.
For the proof of concept, the far-field radiation patterns measurement setup illustrated in Fig. 3(a) has been established in an anechoic chamber. The incident wave is generated by an X-band (8.2 GHz – 12.4 GHz) horn antenna which is positioned along the normal in front of the meta-mirror and both are fixed on a rotating plate. When the rotating plate moves, both the sample and horn antenna rotate together, in order to keep the meta-mirror illuminated by a normal incident electromagnetic wave. A second broadband horn antenna is placed in the far-field in order to measure the radiation pattern. The two horn antennas are connected to an Agilent 8722ES network analyzer.
First, we demonstrate the control of the direction of the reflected beam. For that, we select three angles of 15°, 30° and 45° at a fixed frequency of 9 GHz and the corresponding phase gradients Δϕ that should be applied are calculated by the generalized law of reflection :Fig. 3(b) and are set by applying the corresponding bias voltages for each cell presented in Fig. 3(c). The measured radiation pattern corresponding to each case is presented in Fig. 3(d). All the measured radiations patterns are normalized with respect to the reflection of a metal sheet of similar size which is illuminated by a horn antenna placed at an off-normal incidence of 10°. The results show a deflection of the main beam to the different considered angles. Simulations of the structure have been performed by using high frequency structure simulator (HFSS) finite element method (FEM) based commercial code by ANSYS . Measurements data are in good agreement with simulation results. However, we can observe that in simulation the radiation patterns are narrower than in measurements. This is due to the use of an incident plane wave in numerical simulations which is more directive than the horn antenna used as wave launcher in the experimental tests.
Furthermore, it is also possible to obtain the off-normal radiation at other frequencies between 9 and 11 GHz. In order to exhibit such frequency agility mechanism, θr is fixed to 30° and the phase gradients are calculated at 9 GHz, 9.5 GHz, 10 GHz, 10.5 GHz and 11 GHz. The corresponding phase gradients and bias voltages are presented in Figs. 4(a) and 4(b), respectively. The simulated and measured results are shown in Figs. 4(c)-4(g). The radiation patterns are also normalized to the one obtained for a metal sheet. The scattering angles are 30° deflecting from the normal of the meta-mirror, which is in good agreement with the calculations from (1). As it can be clearly observed, operation spanning from 9 GHz to 11 GHz with low level of specular reflections is obtained. However, above 11 GHz, the scanning angle will be less since the phase gradient is smaller.
On the overall, the experimental data validate the use of the meta-mirror as a reflectarray antenna. Reconfigurability mechanisms of the proposed electronically tunable meta-mirror, namely frequency agility and beam steering properties have been demonstrated. Furthermore, the meta-mirror reflector shows an efficiency of more than 55%, which is quite remarkable when considering the integration of lumped electronic components.
3.2 Parabolic reflector antenna
In the previous section, beam steering and frequency agility have been demonstrated by electronically controlling the reflection phase profile of the planar meta-mirror. In addition, the phase of the meta-mirror can be controlled to emulate a reflector with a non-planar geometry. This second functionality consists in using the designed meta-mirror as a parabolic reflector antenna. To validate such functionality, a cylindrical parabolic phase profile is applied along the meta-mirror. The planar reflector must take into account the phase shift that exists between the planar surface and a parabolic one:
Figure 5(a) presents the schematic principle of using the meta-mirror as a cylindrical parabolic reflector antenna. Two different mechanisms are studied from the planar parabolic reflector. As a first mechanism, the frequency agility performance of the reflector antenna is experimentally tested. Figure 5(b) presents the phase profile calculated using (2) for x0 = 0 at four different frequencies (9 GHz, 10 GHz, 11 GHz and 12 GHz), where we are able to secure enough reflection phase variation along the meta-mirror so as to provide the required parabolic phase profile. The profiles are symmetric with respect to the middle plane of the metasurface (x0 = 0) containing the focal point. For the experimental validation, far-field radiation patterns of the antenna system are measured in an anechoic chamber and are presented in Fig. 5(c). Compared to the antenna pattern of the feeding source alone, the reflector antenna shows a directive beam. After reflection on the meta-mirror, the cylindrical wave fronts emanating from the feeding source are flattened and transformed into quasi-planar waves, producing a highly directive radiation at the four tested frequencies.
The second tested mechanism consists in electronically steering the radiated beam of the reflector antenna system. In conventional parabolic reflector antennas, beam steering is generally achieved by moving laterally the feeding source in the focal plane [38–40]. In our case, both the meta-mirror and the feeding source are physically kept fixed. Therefore, instead of mechanically moving the reflector, only by using the reconfigurable cells, we are able to shift electronically the phase profiles such that the feeding source is virtually displaced in the focal plane. Three different phase profiles are considered at 10 GHz as presented in Fig. 6(a). The reference reflection phase is laterally shifted along the x-direction on the meta-mirror, such that φ0 lies at x0 = 6 mm, 12 mm and 48 mm. The feeding source is kept fixed at x = 0. The corresponding measured radiation patterns of the antenna system are presented in Fig. 6(b). A beam steering can be clearly observed where the deflection angle can reach 55° for a phase profile shift by 48 mm. It is worthwhile to note that in our case the phase profile shift is limited by the lateral dimensions of the meta-mirror. A larger structure will therefore allow increasing the latter shift, and also potentially achieving higher beam deflection. The proposed meta-mirror thus offers an interesting alternative to design planar parabolic reflector antennas that present the possibility to be reconfigured.
3.3 Corner reflector antenna
Finally, we propose to use the meta-mirror as a dihedral reflector antenna, commonly known as corner reflector antenna. In order to obtain such functionality, we apply the corresponding phase profile by taking into account the phase shift that exists between the planar surface and a corner reflector one, as shown in Fig. 7(a). The corresponding phase profile is given as:Fig. 7(a). Therefore, the size of the meta-mirror acting as reflector has been reduced to 82 mm × 180 mm by applying electromagnetic absorbers on the borders of the meta-mirror.
The DC bias voltage is applied to each column such that capacitance value and thus phase value can be varied only along the x-axis. In this configuration, the incident field impinging on the meta-mirror must also have its electric component oriented along x-direction. Therefore, the driven dipole element is placed horizontally as shown in Fig. 7(a).The proposed reconfigurable meta-mirror allows modifying the internal angle α electronically. To illustrate the properties of the planar meta-mirror reflector as a reconfigurable corner reflector, two configurations are tested: α = 90° and α = 120°. The bias voltages are set so as to apply the required phase profiles shown in Fig. 7(b). The far-field radiation patterns of the corner reflector antenna system in both xOy and yOz planes are measured in an anechoic chamber and presented in Figs. 7(c) and 7(d), respectively. The radiation patterns at 10.3 GHz are normalized with respect to those of the driven dipole. Compared to the antenna patterns of the driven source alone, the reflector antenna shows only a forward radiated beam where a gain increase of approximately 4 dB is observed.
In summary, we designed a varactor-loaded reconfigurable meta-mirror where the reflection phase is engineered through an electronic control of the capacitance. The designed electronically tunable meta-mirror has been characterized experimentally and used to design three different reflector antennas. First, a reconfigurable reflectarray has been designed. Beam steering functionality has been demonstrated by applying different phase gradients along the meta-mirror at a fixed frequency. Then, broadband anomalous reflection has been electronically controlled over 20% frequency bandwidth around 10 GHz. Furthermore, reflection phase is controlled to mimic non-planar reflectors. As such, in a second functionality the planar meta-mirror is used to design a parabolic profile reflector, where a broadband high directive beam has been obtained over 28% frequency bandwidth around 10.5 GHz. Beam steering mechanisms have also been demonstrated by electronically shifting the phase profile along the meta-mirror. Finally, we have shown that by engineering the geometry of the phase profile, the meta-mirror allows to mimic a dihedral corner reflector where the internal angle can be further reconfigured.
In this study, we have shown the potentials of the tunable structure, where a single meta-mirror can be judiciously reconfigured to design different reflectors for microwave antenna applications. Other reflector geometries can also be engineered by correctly applying the corresponding phase profiles. This planar structure constitutes an interesting alternative to non-planar reflectors with the additional possibility to reconfigure the operating frequency and the reflection direction. Furthermore, such reconfigurable metasurface design concept can be extended to the optical domain. Technologies such as nano-optomechanical and phase-change can be exploited for the reconfigurability mechanism. Magneto-electric changes in optical response can be tailored in a metasurface by the Lorentz force on simultaneous application of external electric and magnetic fields . Phase change medium such as chalcogenide compound constitutes an interesting technology that enables nonvolatile switching of optical properties [42-43].
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