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Lights Outs: Physicists Find a Faster Way to Switch LCDs
6 September 2013 5:30 pm
Physicists have invented a much faster way to switch off liquid crystals, the materials that control light in many computer screens and televisions. The new technique probably won't end up in liquid crystal displays (LCDs), as the switching is far faster than needed in those devices. But it puts a new twist on the concept of an LCD.
"This is something new and very fresh," says Tigran Galstian, an engineering physicist at Laval University in Quebec, Canada. "People must think about this to see if there is some practical application."
Liquid crystals resemble both of their namesakes. As in a liquid, molecules in a liquid crystal jumble about freely and flow. But as in a crystal, the rodlike molecules orient themselves in the same direction. The alignment defines an optical axis and gives the liquid crystal unusual properties. A key one is the way it affects polarized light—light whose electromagnetic waves ripples in a single direction. As it passes through a liquid crystal, light polarized parallel to the optical axis travels at a different speed than light polarized perpendicular to it. And because of that speed difference, or birefringence, light polarized at an angle to the material’s optical axis can have its polarization rotated.
That rotation makes an LCD work. The display consists of a layer of liquid crystal between two plates of glass, which sit between two more plates of polarizing glass. The polarizers are set at a 90° angle, so that light that enters the display from behind and passes through the first polarizer is blocked by the second. In the "off" state, the liquid crystal is aligned so that it does nothing to the light and leaves the screen dark (see figure). When flipped "on," however, an electric field reorients the molecules so that collectively they rotate the polarization of the light, allowing it to pass through the second polarizer and out of the screen. To form a picture, bits or "pixels" of the screen are controlled individually.
The scheme has a basic limitation, says Oleg Lavrentovich, a physicist at Kent State University in Ohio. The electric field wrenches the molecules into the "on" orientation in nanoseconds. When the power goes off, the molecules relax back into their original orientation, which is set by a pattern etched into the confining glass—but they do so 1000 times more slowly, in milliseconds. "That's the Achilles' heel of liquid crystals," Lavrentovich says.
Now, he and Kent colleagues Volodymyr Borshch and Sergij Shiyanovskii have demonstrated a faster way to switch a liquid crystal, as they report today in Physical Review Letters. They begin with the usual crossed polarizers and a liquid crystal called CCN-47. In the experiment, in the off state the molecules start out in an orientation that lets light through. Lavrentovich and colleagues then apply an electric field. But they do it in a way that does not rotate the molecules but instead changes the amount of birefringence.
Here's how that happens. The molecules in CCN-47 aren't cylindrical, but are shaped like planks. Normally, the planks all point in the same direction lengthwise, but neighboring molecules twist randomly in all directions, as thermal energy keeps the individual molecules jiggling. The electric field overcomes the twisting and stacks the planks like lumber. In that more orderly state, the liquid crystal has a slightly different birefringence, which changes the angle by which the light's polarization rotates and the amount of light allowed through the cell. When the electric field vanishes, thermal jiggling of the individual molecules restores the liquid crystal to its initial condition in just 30 nanoseconds.
The change in birefringence doesn’t shut off light completely, so the display is only dimmed rather than darkened. But the contrast could be heightened by adjusting the geometry and materials, Lavrentovich says. He says the technique might find uses in steering laser beams like the ones that can carry signals between satellites or in creating ultrafast shutters.
The real value in the work may be the new approach, which relies on the collective behavior of the molecules to turn the polarization of light and their individual jiggling to flip between on and off configurations, Galstian says: "It's a clever idea."