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LCDs enter the fast laneSep 20, 2013

Quick and easy: LCDs with nanosecond switching

Liquid crystals, used in many devices such as laptop computers and TV screens, arepopular because they modulate light in response to switching by an electric current, butthe 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 takesplace on nanosecond timescales, a phenomenon that might be exploited in displays ofthe future, they say.Liquid crystals owe their light-manipulating abilities to the fact that they are neitherwholly liquid nor wholly solid but a cross between the two. They consist of rod-shapedmolecules that are free to move, as in a liquid, but oriented in particular directions, as ina 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 veryslightly from one layer to the next. This is done by exposing the material to an electricfield that, in turn, affects the material's optical properties.

Twist and turnA typical liquid-crystal display (LCD) consists of a slab of liquid crystal that issandwiched between a pair of electrodes and two flat pieces of glass. Each piece ofglass has a series of tiny grooves etched into its inner surface that align the moleculesof the liquid crystal and a light-polarizing filter attached to its outer surface. With the twosets of grooves at right angles to one another and the electric current off, the orientationof the molecular layers twists through 90 across the thickness of the slab. And if thefilters are aligned with their respective grooves, then any light entering the display willpass through unimpeded.

However, with the current switched on, the layers untwist and the polarization axis ofthe 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 tothen exploit this principle in displays but in the simplest devices images are built upthrough a suitable patterning of the electrodes, which breaks the display up into discreteunits.Unfortunately, existing LCDs are slow. The time needed for the molecules to untwistcan be made very short since it is proportional to the square of the electric field. Buttheir re-twisting is slow because it is determined by material properties of the liquidcrystal, such as its elasticity, rather than the size of the electric field.

Quick switch

Oleg Lavrentovichand colleagues at Kent State University in Ohio have nowdemonstrated a smaller but quicker effect. The molecules in a nematic liquid crystal donot line up perfectly with one another, resulting in a finite distribution of orientationsaround that of the director. The magnitude of this variation affects the phase of lightpassing through the liquid crystal and as a result its intensity. Since an applied electricfield changes that magnitude, it also changes the amount of light passing through.Physicists have known for decades that such an effect ought to exist. WhatLavrentovich and co-workers have done is to prove experimentally that it does exist and

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that it takes place over much shorter timescales than the relaxation of molecularreorientation in conventional LCDs.The researchers shone a heliumneon laser beam at a liquid crystal placed betweentwo polarizing filters and subject it to a series of sharp voltage pulses. They found thatthe 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 tobe minuscule of no more than about 30 nanoseconds both when the voltage wasswitched on and when it was switched off.Team memberSergij Shiyanovskiiexplains that the lightning-quick response time evenwhen the voltage is switched off is down to the fact that changes to the variation inmolecular orientation do not depend on macroscopic properties of the liquid crystal, asis the case in conventional LCDs, but on an effect that (very slightly) changes theorientation of each molecule simultaneously.

Speedy displayAccording to Shiyanovskii, this effect might lead to improved LCD displays. He pointsout 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 plasmatechnology). However, to reach these speeds the red, green and blue components ofeach pixel must be switched at the same time and therefore laid out separately on thescreen. Much faster switching times, he says, would allow colours to be switched oneafter another, using the same pixel, so tripling the screen's resolution.

Another potential benefit of this work could be improved fibre-optic and free-spacecommunications. The electro-optic properties of liquid crystals are exploited to both splitand steer beams of light running along fibres, so being able to switch them more quicklywould 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 theirexperiment, 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 inPhysical Review Letters.

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