Liquid crystals, used in many devices such as laptop computers and TV screens, are popular because they modulate light in response to switching by an electric current, but the millisecond speed at which they switch can prove to be quite sluggish. However, physicists in the US now report observing a more subtle kind of switching that takes place on nanosecond timescales, a phenomenon that might be exploited in displays of the future, they say.
Liquid crystals owe their light-manipulating abilities to the fact that they are neither wholly liquid nor wholly solid but a cross between the two. They consist of rod-shaped molecules that are free to move, as in a liquid, but oriented in particular directions, as in a solid. The kinds of liquid crystals that are exploited in displays, called nematics, consist of layers of molecules with an average orientation, or director, that changes very slightly from one layer to the next. This is done by exposing the material to an electric field that, in turn, affects the material’s optical properties.
Twist and turn
A typical liquid-crystal display (LCD) consists of a slab of liquid crystal that is sandwiched between a pair of electrodes and two flat pieces of glass. Each piece of glass has a series of tiny grooves etched into its inner surface that align the molecules of the liquid crystal and a light-polarizing filter attached to its outer surface. With the two sets of grooves at right angles to one another and the electric current off, the orientation of the molecular layers twists through 90° across the thickness of the slab. And if the filters are aligned with their respective grooves, then any light entering the display will pass through unimpeded.
However, with the current switched on, the layers untwist and the polarization axis of the light reaching the lower piece of glass is perpendicular to that of the second filter. So, the light is blocked and the display now appears dark. There are different ways to then exploit this principle in displays but in the simplest devices images are built up through a suitable patterning of the electrodes, which breaks the display up into discrete units.
Unfortunately, existing LCDs are slow. The time needed for the molecules to untwist can be made very short since it is proportional to the square of the electric field. But their re-twisting is slow because it is determined by material properties of the liquid crystal, such as its elasticity, rather than the size of the electric field.
Oleg Lavrentovich and colleagues at Kent State University in Ohio have now demonstrated a smaller but quicker effect. The molecules in a nematic liquid crystal do not line up perfectly with one another, resulting in a finite distribution of orientations around that of the director. The magnitude of this variation affects the phase of light passing through the liquid crystal and as a result its intensity. Since an applied electric field changes that magnitude, it also changes the amount of light passing through. Physicists have known for decades that such an effect ought to exist. What Lavrentovich and co-workers have done is to prove experimentally that it does exist and that it takes place over much shorter timescales than the relaxation of molecular reorientation in conventional LCDs.
The researchers shone a helium–neon laser beam at a liquid crystal placed between two polarizing filters and subject it to a series of sharp voltage pulses. They found that the voltage pulses moved in step with changes in the intensity of the light reaching a detector on the far side of the liquid crystal. They observed the delay between the two to be minuscule – of no more than about 30 nanoseconds – both when the voltage was switched on and when it was switched off.
Team member Sergij Shiyanovskii explains that the lightning-quick response time even when the voltage is switched off is down to the fact that changes to the variation in molecular orientation do not depend on macroscopic properties of the liquid crystal, as is the case in conventional LCDs, but on an effect that (very slightly) changes the orientation of each molecule simultaneously.
According to Shiyanovskii, this effect might lead to improved LCD displays. He points out that current top-of-the-range LCD TV screens have refresh rates of 240 Hz, which is high enough for most kinds of viewing (although slower than competing plasma technology). However, to reach these speeds the red, green and blue components of each pixel must be switched at the same time and therefore laid out separately on the screen. Much faster switching times, he says, would allow colours to be switched one after another, using the same pixel, so tripling the screen’s resolution.
Another potential benefit of this work could be improved fibre-optic and free-space communications. The electro-optic properties of liquid crystals are exploited to both split and steer beams of light running along fibres, so being able to switch them more quickly would allow higher data rates along such fibres, says Shiyanovskii.
However, while the newly demonstrated effect is very quick, it is also very small – having led to fractional changes in intensity during the experiment of just a few per cent. The researchers are therefore trying to enhance it. Shiyanovskii points out that in their experiment, he and his colleagues used an off-the-shelf liquid crystal – CCN-47 – because it was simple and cheap to use. More tailor-made materials, he believes, should lead to a larger and therefore more exploitable effect.
The research is published in Physical Review Letters.