&Bullet; physics 14, 101

Researchers have shown a faster way to turn light transmission on and off in a liquid crystal.

Liquid crystals, which have been used in visual displays for more than 50 years, can be electrically switched between two molecular arrangements: one transparent and one opaque. A newly developed liquid crystal can change states more quickly than currently used [1] . If the same speed can be achieved with materials that operate at lower, more practical temperatures, new types of fast-responding electro-optical devices, such as virtual reality displays, could emerge.

Conventional liquid crystal displays (LCDs) use so-called nematic materials, in which the molecules typically have rod-shaped shapes. The molecules are free to move like in a conventional liquid, but their rod axes all point in the same general direction, much like a crowd of people walking around with their heads turned up. This direction, characterized by a parameter called a director, influences whether polarized light penetrates the material or not and thus whether an LCD pixel is light or dark.

The director can be controlled with electric fields. In an LCD pixel there is a liquid crystal layer between two electrodes. A voltage applied to the electrodes creates an electric field that can realign the director, creating what is known as a Fréedericksz junction, which causes a change in light transmission.

Some liquid crystals can also form another ordered phase in which the molecules are stacked in layers in addition to being aligned. In this so-called smectic phase, the molecules move freely within a layer, but rarely hop between the layers. The Fréedericksz transition has not yet been observed in smectic liquid crystals, as realigning the director would change the layer thickness, which would require too much energy.

However, the possibility of getting a Fréedericksz transition in a smectic phase looks attractive, says Ivan Dozov of the University of Picardy Jules Verne (UPJV) in France. The layered order in smectics makes the molecules react faster than in nematics, and so the light transmission could possibly be switched on and off more abruptly. “Fast response times are extremely important in applications and are in great demand,” he says.

Dozov, along with UPJV colleague Claire Meyer and her staff, envisioned that such switching could be achieved if the liquid crystals had not one, but two directors pointing in different directions. Then perhaps one director could be switched – with a corresponding change in the optical properties for polarized light aimed at that director – while the other director, which determines the layer spacing, remains unchanged.

This strategy requires a molecule whose shape resembles a flat board, with two axes that can be oriented independently. Switching is then like rotating the orientation of the panel sides in a stack while keeping their length unchanged from end to end – much like the way blinds are rotated to turn off the lights.

To make this idea a reality, the researchers used organic molecules called BNA-76, which are shaped a bit like bananas. These molecules form a smectic phase in which the molecules span adjacent layers in such a way that two board-like axes are created 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 itself forms a nematic phase.

The researchers found that the mixture goes through a Fréedericksz junction at around 100 ° C with 4 volts applied to it. This switch causes a change in the transmission of polarized light through the material. As the team had hoped, the change is quick: It happens in less than a millisecond, around 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 from the University of Southampton in Great Britain, but it has proven to be “a challenge”. This demonstration of the feasibility is “extremely exciting,” he says.

Materials physicist Oleg Lavrenovich of Kent State University in Ohio agrees. The work is an “example of how subtle modifications in the structure of organic molecules can give a material spectacular physical properties,” he says. These new materials, he says, “could one day find application in new technologies such as augmented reality or 3D information displays.”

To do this, however, the operating temperature must be reduced. Dozov agrees, and thinks that lower temperatures should be possible with other molecules (or mixtures) that have similar plate-like structures.

–Philip Ball

Philip Ball is a freelance science writer based in London. His latest book is The modern myths (University of Chicago Press, 2021).

References

1. C. Meyer et al., “Fréedericksz-like transition in a biaxial smecticA Phase,” Phys. Rev. X11, 031012 (2021).