Mastering liquid crystal phase technologies for terahertz communication

Liquid crystal-based technologies could revolutionise the control of terahertz radiation — key to the future of high-bandwidth communications.

Dr Masahito Oh-e from National Tsing Hua University, Taiwan, considers how phase shifters and modulators may pave the way for faster, more efficient 6G networks.

Read the original research: doi.org/10.1002/chem.201803330

Read more in Research Outreach

Transcript

Hello and welcome to Research Pod! Thank you for listening and joining us today. 

In this episode we look at the work of Dr Masahito Oh-e at the National Tsing Hua University in Taiwan, who has been exploring how liquid crystal-based technologies might be used to make new devices to harness the power of terahertz radiation. Terahertz radiation is very useful in a number of fields, including communications and biological imaging, but it has historically been difficult to control for such applications. In recent work, he has demonstrated how liquid crystals that switch their orientation in response to an applied voltage can shape terahertz radiation with rapid response times.  

Many digital technologies, from televisions to communication, rely on the idea of control – where it is possible to change the properties of a material or the device by changing an external factor, such as a voltage or current. One of the most promising future technologies for wireless communication is the use of terahertz radiation because it offers an excellent way of performing high-bandwidth communication.

As we share more and more information digitally, networks and communications technologies have to become faster with more bandwidth to cope. Any image or text can be thought of as bits – small packets of information. The larger and more complex the file, the larger the number of bits that need to be transferred when you send your message.

Bandwidth refers to how much information, or how many bits, can be transferred per second. Suppose you want to be doing information-heavy activities like streaming movies online or playing video games. In that case, you will want a large bandwidth to ensure the game or movie runs smoothly.   

There is some discussion of whether terahertz radiation will be the backbone of future 6G communication networks, but one of the issues is that it is very difficult to control. Dr Masahito Oh-e at the National Tsing Hua University, Taiwan, has been working on this challenge and developed a new generation of liquid crystal-based technologies to help modulate terahertz signals.

Dr Oh-e has extensive experience in the field of liquid crystal displays and, in particular, controlling their switching for applications. Probably the most common place you will encounter liquid crystals is in display technologies. Liquid crystal display screens, or better known as LCD screens, are often found in devices like computer monitors, digital cameras, and smartphones, and used to be a popular technology for televisions. Some of Dr Oh-e’s liquid crystal technologies have enabled the development of ultra-broad angle LCD screens, which are now an industry standard and used in portable devices such as iPhones. 

Liquid crystals are somewhat unusual materials as they are between a solid and liquid in their behaviour, and, in some liquid crystal materials, the orientations of the molecules that make up the liquid crystals can be controlled using an electric field.  

The pixels in an LCD have a backlight, and liquid crystals are switched electronically to change the polarisation of the light passing through. By using this switching device in combination with a polarisation filter, the light can either pass through the filter or be blocked, and the whole process is controlled electronically.

The key to Dr Oh-e’s previous developments in the field of wide ultra-broad viewing angle displays was finding a way to control the liquid crystal switching behaviour so that vibrant colours could be achieved from all angles, which is called in-plane switching technology. Now, he has found a way to control some liquid crystal behaviour not with visible light, as you would for a television, but with terahertz radiation. 

While such technologies were known previously, a fundamental problem in making terahertz phase shifters to control the properties of the terahertz radiation was that the switching cells needed a thick cell gap, which caused a very slow liquid crystal response and made fast switching unfeasible. 

By realising that combining in-plane with out-of-plane switching of liquid crystals could maximise the orientation range of liquid crystals, Dr Oh-e and his collaborators have found a novel way to control the terahertz radiation fields. Adopting both in-plane and out-of-plane switching in a display was previously not thought possible because it degrades the viewing angle characteristics in a display. Dr Oh-e aimed to change the conventional way of thinking when challenging an application of liquid crystals to the new frequency domain. Further, Dr Oh-e’s team found a way to overcome the issue of slow switching times with previous terahertz liquid crystal technologies by making special electrodes to allow for faster switching between three distinctive orientation states when an electric field was reversibly applied to the liquid crystal.

Through this research on liquid crystal devices, Dr Oh-e aimed to develop terahertz modulation devices, including a phase shifter. Electromagnetic waves, like terahertz radiation, have a ‘phase’ – a measure of how the wave has ‘shifted’ from a given reference point. For example, if you send a wave down a rope and measure the heights of the wave at different points along the rope, the displacement of the rope will be different in different sections, which is what the phase of a wave measures.

The phase of a wave is crucial. If the wave interacts with another wave, the relative phases of the two waves will determine whether the waves constructively interfere to amplify a signal or destructively interfere and cancel out, destroying the signal.

Generally, phase shifters could be used to apply different phases to the terahertz radiation, which is a way of encoding information into the wave that could then be transmitted and used for applications such as telecommunications. One of the significant challenges of exploiting the potential of terahertz in communications is developing new devices that can control terahertz radiation and be used to manipulate properties like the phase.

For the visible or infrared light that is more commonly found in fibre optic cables, there are many devices that can be used to encode signals into the radiation, but this is not so common for terahertz radiation. Dr Oh-e’s novel type of liquid crystal switching for controlling terahertz radiation through tailoring the electrodes is therefore an important step in helping make terahertz radiation a more viable option for more efficient communications. 

The key to Dr Oh-e’s invention of an electrode layout and structure for effectively switching liquid crystals in building a phase shifter was a previous analytical investigation on ‘terahertz in-plane and terahertz out-of-plane’ switching. Alongside this, Dr Oh-e realised that the switching behaviour of the liquid crystals was not just dependent on the liquid crystal material itself but incredibly sensitive to the overall electrode geometries.

Dr Oh-e found that varying the electrode dimensions influences a number of parameters, such as the phase shift and response time of liquid crystal switching. From this finding, he proposed a novel type of liquid crystal switching, which enables reversible switching between the three orientation states of liquid crystals: two in-plane states and one out-of-plane state, just by changing the electrode layouts and geometries.

Now, Dr Oh-e has virtually demonstrated this idea in principle with liquid crystals capable of switching between the three states with a rapid response. This is part of the developments which have enabled the creation of more sophisticated terahertz phase shifters or modulators; however, addressing more challenges, including compatibility of tunability with rapid responses, low voltage operation, and broadening the phase range, is indispensable in addition to conducting feasibility studies.

Dr Oh-e’s current attempts to develop new phase shifters and modulators involve looking at some of the fundamentals of liquid crystal science and whether a rapid response is possible with a thick cell gap that is indispensable for terahertz modulation. If this attempt is successful, liquid crystal technology can be applied to terahertz modulation technology, which will be significant as an application beyond displays as well. Dr Oh-e believes properly unravelling basic physical problems in our attempt is eventually the shortest path to practical use. 

Dr Oh-e and his team are looking into electrode designs and the geometries and configurations of the electrodes to combine rapid responses with tunability and help reduce the thick cell gap, which causes slower responses. The team is also researching designs of substrates that do not require such strong alignment of the bottom and top parts, as they have found that changes in the alignment can have a big effect on the switching characteristics. 

Further investigations are underway to pursue better potential performance in reversibly switching liquid crystals between the three orthogonal orientational states of liquid crystals while elucidating the operational principles of the proposed electrode structures. Having found that one of the electrodes in the device that was previously thought to have little effect on the switching behaviour actually has a very strong influence, Dr Oh-e has not only been able to reveal more about exactly how the switching mechanisms of liquid crystals work but also provided suggestions of how electrodes might be tailored to achieve optimal designs. 

6G testing is estimated for commercial release in 2030. Dr Oh-e’s developments might mean that 6G technologies end up being based on terahertz radiation, which has great potential for transferring vast amounts of data. The quest for liquid crystal-based terahertz devices continues.

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