10 October 2013
Liquid Crystals (LC) are a group of organic molecules that show a set of peculiar properties . One property is that an LC molecule's polarisability depends on its orientation with respect to the electric (or magnetic) field. This is not unusual in many crystals (also organic crystals) where you find anisotropic optical or microwave properties. For liquids, however, this is unique as they are usually isotropic. Due to strong interactions between the molecules, LCs exhibit a set of liquid phases: isotropic and nematic, to mention only two. Their properties were described by de Gennes .
We are using the nematic phase, where molecules align in a correlated way, i.e. a molecule tends to align in parallel to its neighbours thus, in macroscopic view, constituting an anisotropic, yet liquid material. The overall orientation of such a nematic LC can be influenced by an external electric field (static or, in order to avoid migration, alternating bias).
The molecules move slowly compared to an RF field. Therefore, in setting an orientation you let the RF field “feel” a fixed, anisotropic dielectric. This can be used to change polarisation, as it is commonly done in displays. But you can also impose the polarisation and change the propagation speed.
When an RF field orientation is imposed by the wave-guide, the LC has an effective permittivity that is seen by the RF field. So, in changing the LC's orientation, this permittivity can be changed and thus the electric length of the device: the wave travels faster or slower depending on your bias field.
Compared to solid state semiconductor phase-shifters, for instance, this process is incredibly slow (scale of milliseconds to minutes depending on topology) but it's losses are very low. We could show a total loss in our device of less than 4 dB, making for a phase-shift per loss – which is the figure of merit to compare phase-shifters – of 180°/dB.
So, while being slow and thus suited only for a niche market, our technology is very effective.
The objective of the project is to go from lab prototype  to space qualifiable low-weight phase shifter. We filled the cross-section entirely with polymer rather than air in the previous design to ensure robustness. Surprisingly, we could still exceed the figure of merit. Also, biasing is done electrically, which is challenging mainly because the biasing electrodes needed to be compatible with RF propagation within the wave-guide. Normally, one tries to avoid electrodes in a wave-guide, if they can.
So, for the first time we have incorporated a number of technologies to produce such a low-weight device. We are still at the stage of testing it in a rather heavy brass split-block wave-guide, but we are ready to use the exact same device and electro-plate the wave-guide around it. The main challenge is being light-weight yet robust enough to be used in an antenna array that, eventually, is intended to fly into space.
The higher the frequencies the more interesting things get for LCs. We are currently looking at an antenna to establish GEO to LEO links and track satellites to ensure permanent up and down links.
Going higher in frequency, this technique may be used to build compact arrays that optimise point-to-point connections automatically, for instance at an airport where you want to load large amounts of media and flight information to a plane while boarding. You know the approximate position of the plane. But it will never be at the exact same spot. So, an antenna that adjusts itself and optimises the link gain can be a great advantage.
One challenge certainly was the strong interdisciplinary approach of the project, named LISA ES, short for light-weight inter-satellite antenna – electric steering . The team recruits itself from various fields (space and aviation, light-weight construction, microwave technology and electronics) at two universities (TU Munich and TU Darmstadt) and a number of companies (NTP, IMST and Airbus Astrium).
RF engineers tend to make everything from massive metal blocks. Fortunately, the group had huge experience with electro-plated wave-guide structures. So our LC RF technology needed to be transferred into that.
The Institute for Microwave Engineering and Photonics (IMP) at TU Darmstadt  works on a variety of topics related to tunable materials and sensing concepts. Integrating LC technology into array concepts is an ongoing topic, but also making LC technology available in the higher mm-wave and THz range is being worked on. LCs look promising in this range as well, with constantly low loss and high tunability up to several THz. Merck , market leader in LC and located in Darmstadt, is working on this topic.
Sensing is also very important. Recently the group has received funding by the local LOEWE initiative, developing “Sensors Towards Terahertz” .
We would like to see losses in LC to go down further and tuning speed to go up. This has happened before in display technologies. There, another constant driver of switching speed was topology. We are working on that.
It seems that the German Aerospace Centre (DLR) is interested in this approach for inter-satellite links as a complementary technology to laser links, which suffer high free space losses compared to Ka band links.
We are also looking forward to new high data-rate wireless network concepts. If you look at 4k high definition video streams, wireless HDMI and 70 GHz Wi-Fi, the future looks promising for us.
 University of Gent: Tutorial on the physics of LC (new window)
 Pierre-Gilles de Gennes: http://en.wikipedia.org/wiki/Pierre-Gilles_de_Gennes (new window)
 IEEE Xplore: 'Investigation of high performance transmission line phase shifters based on liquid crystal' (new window)
 Technische Universität München LISA MS Project: http://web.lrt.mw.tum.de/index.php?id=65&L=1 (new window)
 Technische Universität Darmstadt Institut für Mikrowellentechnik und Photonik: www.imp.tu-darmstadt.de (new window)
 Merck: www.merck.com (new window)
 Technische Universität Darmstadt: www.stt.tu-darmstadt.de (new window)
This article is based on the Letter: Recent measurements of a Compact Electronically Tunable Liquid Crystal Phase Shifter in Rectangular Waveguide Topology (new window).
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