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multi-Reconfigurable Antenna SoluTions based on REflectarray technOlogy

Final Report Summary - RASTREO (multi-Reconfigurable Antenna SoluTions based on REflectarray technOlogy)

Reflectarray antennas are comprised of an array of radiating elements, reflecting the energy that is impinged from a primary feed. These antennas are interesting hybrids between aperture antennas (reflectors) and conventional arrays. They are spatially-fed, which means that they do not require a lossy feeding network with dedicated transceivers, reducing the overall losses, the production cost and the manufacturing complexity, allowing also the introduction of electronic phase control. Reflectarrays have been studied extensively in the last years, mainly for fixed-beam. However, if dynamically control is implemented at the element level, the reflectarray allows steering the beam towards a predefined direction or even changing the shape of the beam. Reflectarrays offer a simple feeding mechanism combined with the independent control at the element level in order to provide very versatile capabilities. The Marie Curie IEF project RASTREO (multi-Reconfigurable Antenna SoluTions based on REflectarray technology, http://mnwave.epfl.ch/rastreo) has contributed to addressing new challenges in the dynamic reconfiguration, including not only spatial reconfiguration, but also polarization and frequency reconfiguration. It is worth mention that in the case of spatial reconfiguration, the use of graphene as a reconfiguration technique in reflectarrays has been proposed for the first time. Although the use of this new material was not included in the initial proposal, the experienced researcher and the former scientist in charge (Prof. Perruisseau-Carrier) together agreed to explore the interesting properties of graphene with the aim of implementing scanned-beam reflectarray antennas in Terahertz (in the second year the research was extended to the infrared band). In any case, this update cannot be considered as a deviation respect to the original gaols of the project, contrary, the obtained results have opened a very promising research line with interesting applications in different fields and allowing the proposed reconfiguration capabilities. For the case of polarization and frequency reconfiguration, other reconfiguration methods have been analyzed (solid state switches or liquid metal). In the following lines, a summary of the public final results obtained in the framework of RASTREO are presented.

Beam bending in reflectarrays by using graphene at THz and mid-Infrared
Graphene is a true 2-D material (monatomic layer of carbon atoms arranged in a honeycomb structure), which has attracted tremendous interest thanks to its unique electrical and mechanical properties. Graphene’s complex conductivity can be efficiently controlled via a perpendicular bias electric field. As a result graphene is envisioned for a variety of applications at THz and optical frequencies, including the possibility of dynamic tuning via the electric field effect. This dynamic tuning was demonstrated at 1.3 THz by using graphene patches. The patch resonance occurred when its size was around λ0/24 (into a λ0/16 unit cell). This phenomenon is due to the well-known slow-wave propagation associated with graphene plasmonic modes. The phase of reflection coefficient, at 1.3THz produced by a square graphene patch as a function of both, the patch size and the chemical potential was accurately computed. Chemical potential is varied by electronically gating the graphene. The maximum phase variation is obtained for patches of 10 m, yielding a range of around 300° in a large bandwidth, which is enough for producing a pencil-beam. It is observed that the phase shift experiences an almost constant phase variation with small errors. For instance, phase errors lower than 37° in the frequency band from 1.1 THz to 1.5 THz has been obtained. This is a 31% of bandwidth (namely, large bandwidth). The loss of the element varies between 0.5 dB and 6 dB on the whole range between 1.1 THz and 1.6 THz, which is another very promising performance at such frequencies.
As aforementioned, surface plasmons can concentrate electromagnetic energy at the subwavelength scale. These electron oscillations appear in graphene nanoribbons at much lower frequencies than in their noble metals counterpart, providing subwavelength confinement from mid-infrared down to terahertz frequencies for a vast range of applications. In a new design, the control of a light beam at nanoscale level is proposed by using an array of reflective graphene nanoribbons. The array is between a gating superstrate and a grounded substrate. The difference respect the case of a square patch is that, in the late proposed concept the switching of the reflected beam is produced using a very simple biasing structure, providing a confined beam and low losses of energy. The working principle of the proposed array of graphene nanoribbons is as follows. A mid-Infrared laser beam collimated into free space as a Gaussian beam is focused so that the waist of the Gaussian beam impinges with certain incidence angle on a 224-element array. By properly adjusting the physical width of each nanoribbon, a progressive phase-shift is introduced upon reflection along the array in the x-axis direction. This phase difference can be fixed in order to produce a constructive interference of all the reflected waves at each ribbon, as usual in microwave antenna arrays. This interference collimates a far-field beam towards certain direction. In this case, all the nanoribbons are electrically doped with a chemical potential µc1, equivalent to certain gating voltage. The physical width of the graphene nanoribbons cannot be modified. However, if the electrical doping is turned to a value µc2, capable of producing a constant phase of the reflection coefficient for all the ribbons of the array, a far-field beam is collimated towards the specular direction. The unitary element in the array can be physically modelled by an equivalent circuit. Using transmission line theory, the reflection coefficient of each element can be computed at a reference plane in both amplitude and phase. The substrate and the superstrate can be represented as a transmission line with their respective characteristic impedance Z0 and propagation constant, while the graphene nanorribbon can be modelled using a surface impedance Zg which depends on the geometry of the graphene strip and the surface conductivity obtained by the Kubo formula. The short-circuit represents the ground plane, while the line is loaded with the intrinsic impedance of free space. The produced beam can be efficiently bend from specular to boresight directions.