A Faster Liquid Crystal
The brightness of a pixel in a typical flat screen display is regulated by an electric field that controls the orientations of molecules of a liquid crystal. In Physical Review Letters, researchers report a much faster way of using the field to change the state of the molecules and alter the light transmission. Although the measured effect is small, it is thousands of times faster than the conventional technique and might be increased enough to allow new design options for displays.
Most liquid crystals consist of rodlike molecules that naturally line up in the same direction as their neighbors, even as the molecules flow past each other as in an ordinary liquid. This alignment has a strong influence on light travelling through the material. A typical display sandwiches the liquid crystal between two conductive, transparent plates in a stack that includes other thin optical elements. Applying a voltage between the two plates aligns the molecules in the direction perpendicular to the plates, which, in combination with the rest of the stack, prevents light from passing through. When the field is removed, the molecules align parallel to tiny grooves in the surfaces of the plates, which allows light through again.
The initial alignment with the field is very fast, but when the field is removed, it takes much longer—many milliseconds—for the original orientation to spread from the surface molecules back through the rest of the fluid. For thin enough cells, this is still fast enough for full-motion video. But a team at Kent State University in Ohio, led by Oleg Lavrentovich, has now demonstrated a field-induced effect that returns to the field-free state much faster, because it doesn’t require the molecules to reorient their neighbors. “Higher speed will open new opportunities,” says team member Sergij Shiyanovskii. Extra speed could give engineers more flexibility to improve other aspects of display designs and should also help devices that rapidly reroute light beams in telecommunications and research.
The alignment of molecules with the field is never because individual molecules deviate from their neighbors to varying degrees. But their average orientation is aligned with the field in a typical LCD display. To demonstrate the new effect, the researchers chose a molecule called CCN-47 that naturally aligns perpendicular to the field. Since the molecules start out parallel to the plates, a perpendicular field doesn’t reorient them. Instead, the field increases the uniformity of alignment—it reduces the extent of molecule-to-molecule variations. When the field is turned off, CCN-47 returns to its original state much more rapidly than conventional liquid crystals would, says Shiyanovskii, because the variations in alignment begin increasing immediately among all of the molecules. There is no delay during which the “signal” must propagate from the fluid surface.
To test the new scheme, the team shined a laser through a stack similar to a liquid-crystal display. They changed the applied voltage across a pair of plates surrounding the liquid crystal and detected the resulting change in transmitted light, which indicates the degree of molecular alignment. The researchers found that the field-induced increase in alignment caused a measurable change in light transmission that went away just tens of nanoseconds after the field was turned off.
CCN-47 is an off-the-shelf liquid crystal, and the effect was probably too small to be very useful. “We are working now to increase the amplitude,” says Shiyanovskii, for example by using liquid crystals that produce a larger effect on light transmission for the same degree of alignment. Another possibility is to use molecules shaped more like boards than like rods, which may allow them to form a so-called biaxial arrangement, in which they line up not just lengthwise but also face to face. An electric field could enhance this additional ordering, dramatically increasing the optical effect.
As for the observed changes, “it was rather clear that this kind of effect must occur,” says Florenta Costache, of the Fraunhofer Institute for Photonic Microsystems in Dresden, Germany, but until now the concept was only theoretical. Still, she cautions that the materials changes needed to enhance the effect could also make other, slow changes too big to ignore in real devices.
Don Monroe is a freelance science writer in Murray Hill, New Jersey.
A New Direction for Liquid Crystals (Physics Focus story from 2004)