Faster switching in liquid crystals
&ball; Physics 14, 101
Researchers have demonstrated a faster way to turn the transmission of light on and off in a liquid crystal.
Liquid crystals, used in visual displays for over 50 years, can be electrically switched between two molecular arrangements: one transparent and the other opaque. Newly developed liquid crystal can change state faster than those currently in use . If the same acceleration can be achieved in materials that operate at lower, more convenient temperatures, new types of fast-responding electro-optical devices could result, such as virtual reality displays.
Conventional liquid crystal displays (LCDs) use so-called nematic materials, in which the molecules typically have rod shapes. The molecules are free to move like in a traditional liquid, but their rod axes all point in the same general direction, much like a crowd of people moving, all with their heads facing up. This direction, characterized by a parameter called director, determines whether or not polarized light passes through the material and therefore whether an LCD pixel is bright or dark.
Control of the director is possible with electric fields. In an LCD pixel, a liquid crystal layer is sandwiched between two electrodes. A voltage applied to the electrodes generates an electric field that can realign the director, producing a so-called Freedericksz transition which causes a change in light transmission.
Some liquid crystals can also form another ordered phase, in which the molecules, in addition to being aligned, are stacked in layers. In this so-called smectic phase, molecules move freely within a layer but rarely jump between layers. The Fréedericksz transition had not been observed before in smectic liquid crystals because reorienting the director would change the thickness of the layer, which would require too much energy.
However, the possibility of obtaining a Fréedericksz transition in a smectic phase seems attractive, explains Ivan Dozov, of the University of Picardy Jules Verne (UPJV) in France. The layering order in smectics causes molecules to respond faster than in nematics, and thus light transmission could potentially be turned on and off more abruptly. “Fast response times are extremely important for applications and are in demand,” he says.
Dozov, as well as UPJV colleague Claire Meyer and their colleagues, thought that such a switching could be achieved if the liquid crystals did not have one but two directors pointing in different directions. Then maybe one director could be switched – with a corresponding change in optical properties for polarized light aligned with that director – while keeping the other director, which determines the layer spacing, unchanged.
This strategy requires a molecule shaped like a flat board, with two axes that can be oriented independently. Switching is then like rotating the orientation of the faces of the boards in a stack, while their end-to-end length remains unchanged, comparable to the way venetian blinds are turned to turn off the light.
To make this idea a reality, the researchers used organic molecules called BNA-76, which are shaped much like bananas. These molecules form a smectic phase in which the molecules extend over adjacent layers so as to create two plank-shaped axes in each layer. To prevent the smectic phase from forming a solid crystal, the BNA-76 molecules must be mixed with a second compound which on its own forms a nematic phase.
The researchers found that the mixture undergoes a Freedericksz transition at around 100 ° C with 4 volts applied across it. This switch causes a change in the transmission of polarized light through the material. As the team hoped, the switching is fast: it occurs in less than a millisecond, about 30 times faster than in typical nematic phases.
The idea of faster switching in such “biaxial” smectics has been around for some time, says liquid crystal expert Geoffrey Luckhurst of the University of Southampton in the UK, but achieving it has turned out to be “A difficult task”. This demonstration that it is doable is “very exciting,” he says.
Materials physicist Oleg Lavrentovich of Kent State University in Ohio agrees. The work is an “example of how subtle changes in the structure of organic molecules can give a material spectacular physical properties,” he says. These new materials, he says, “could one day find applications in emerging technologies such as augmented reality or 3D information screens.”
But for that to happen, he adds, the operating temperature will have to be lowered. Dozov agrees and thinks lower temperatures should be possible with other molecules (or mixtures) that have similar plank-like structures.
Philip Ball is a freelance science writer in London. His latest book is Modern myths (University of Chicago Press, 2021).
- C. Meyer et al., “Fredericksz-type transition in a biaxial smecticA phase,” Phys. Rev. X 11, 031012 (2021).