Metamaterials are artificially engineered materials to possess certain properties that can not be found in naturally occurring materials. Combining metamaterials with liquid crystals that can regulate their electromagnetic properties as a function of bias voltage supplied we create a spatial light modulator (SLM). SLM's ability to change the direction and shape of the radiation beam finds interesting applications in wireless communication systems such as beamforming and beam steering. Liquid crystal-loaded metamaterial applications are suitable for use at GHz and THz spectrum.
Unit cell design
Two prototypes of unit cell design were created, single layer and double layer (picture 4). The design consists simply of a patch at the bottom of the 2 mm quartz substrate. This patch is active, as a voltage is applied across the polarisation lines (bias lines) that attach to the patch. The liquid crystal is inserted into a space of 40 μm thickness, limited by the substrate of 2 mm on one side, and by a plane of mass located on one of the faces of another 7 mm quartz substrate on the other side.
For the double-layered cell design, there is an additional passive metamaterial figure at the top of the quartz substrate.
For the continuation, we consider the double-layer structure as preferred because obtained results showing the single-layer design were considered as satisfactory, however, the double-layer design shows a greater phase offset range than the single-layer patch.
Reflectarray design and prototyping
Two types of prototypes of electronically steerable reflectarray in 1D, consisting of 33x29 and 57x49 units cells respectively. A later prototype was created with more unit cells in order to achieve a narrower beam reflected from reflectarray. The 33 and 57 are the number of polarization lines for respective prototypes.
Next, the manufacturing process of the mockups is described, starting from the raw quartz wafers until mounting on the quasi-optic bench.
Thus, once the selected quartz wafers have been cut and prepared, a thin film of nickel is placed on them, using a sputtering machine. A photolithography process (with an accuracy of ± 0.1-0.2 µm) is applied to these parts, obtaining the lithographed wafers to be used in the subsequent liquid crystal injection.
A thin polyimide sheet is applied to these wafers by means of a spin coating process and then brushed in such a way that the liquid crystal molecules, at rest, are aligned in the direction of the brushing. 40μm high teflon pieces are then placed to fix the space between the two wafers, which define the space where the liquid crystal is to be injected. Finally, the edges of the set are sealed to achieve proper consolidation.
An auxiliary PCB is designed to connect the wafer polarisation lines to the voltage control device. The assembled prototype is presented in picture 7.
The control software was entirely developed by Tafco Metawireless, an affiliate company to Spatialite Antenna systems. Control software shall process two main tasks i) calculate phase to be assigned to each row in order to achieve the required deviation angle from incident wave signal ii) communicate corresponding correct values to the voltage control unit (picture cc).
As mentioned above, the actual reflectarray behavior is non-linear and different from the results of a simulation, hence the application must take into account the empirically obtained voltage-phase offset ratios.