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New Products and Materials

New light on liquid crystals

   
 
Two industrial manufacturers and two research institutions have devised a range of new liquid crystal materials for the opto-electronics industry. Four new applications are being investigated, two of which - a less power-hungry flat screen for portable computers and a faster solid-state lens shutter for video cameras - are expected to appear on the market in 1997.

Liquid crystal technology has found its way into every modern household. Small, grey liquid crystal displays (LCDs) are commonplace on calculators, watches, telephones, radios, car dashboards and elsewhere. Larger, full-colour displays can be found in laptop computer screens and miniature TV sets. Yet this is only the beginning of the range of possibilities offered by these versatile materials.
Liquid crystals are organic molecules, in the shape of either rods or plates, which have properties of both solids and liquids. The molecules tend to line up with each other, as in a crystal, but they can still flow like a liquid. The direction of alignment can be controlled by a variety of techniques - this is the key to the ability of liquid crystals to modify light passing through them.
In this project, partners from the Netherlands, UK, France and Spain have cooperated to produce and exploit a range of new liquid crystal materials with novel properties. They focused on a technique called 'photopolymerisation' to construct liquid crystal materials in which the orientation of the molecules is fixed.
Relatively small molecules are first aligned by surface treatment. When exposed to ultraviolet light the molecules join up to form densely cross-linked networks - they are effectively frozen in position. Although the technique was discovered by Philips as long ago as 1985, APOCALIPS was the first concerted effort to exploit the process to discover new materials.

Search for new molecules

More than 200 possible new liquid crystal materials have been investigated. The University of Zaragoza was mainly concerned with improving their optical properties. They also looked for ways of incorporating metals into the molecules to make materials that would better refract or reflect light. The Commissariat à l'Energie Atomique (CEA) concentrated on the plate-like molecules, with a potential to align without special surface treatment and which yield film with special properties towards optical retardation.
As for the industrial partners, Merck investigated the properties of liquid crystal mixtures. A single type of molecule may not have all the properties desired, so a mixture of different types is often used. A complex balance has to be struck to achieve the optimum result. Merck also studied the problems of producing materials in sufficient quantities for industrial manufacture.
The lead partner, Philips Research, coordinated the work and focused on devising new applications of the materials to the opto-electronics industry.
One benefit is the discovery of liquid crystals which can be manufactured at a temperature of around 40C, which is much more convenient than the more usual 100C, and reduces the likelihood of the molecules polymerising too soon.

Reflective polariser

The most promising outcome of the APOCALIPS project is a new method for polarising light. Ordinary light is unpolarised; the electromagnetic waves of which it is composed vibrate in many different directions. In polarised light the waves all vibrate in the same direction.
The usual way of producing polarised light is to pass ordinary light through a special filter which, for example, may only transmit waves vibrating up and down, absorbing all waves which vibrate from side to side. As at least half of the original light is absorbed, these filters can never be more than 50% efficient.
In APOCALIPS, the partners devised a filter that can be tuned to any range of wavelength and which polarises by reflection rather than by absorption. The filter is a thin film of rod-like molecules arranged like the steps of a spiral staircase going down 'into' the film. Light will be reflected where the pitch of the staircase is similar to the wavelength of light. Since the pitch increases with depth, the shortest (bluest) waves are reflected at the top of the 'staircase' and the longest (reddest) at the bottom. By carefully adjusting the range of pitch between the top and bottom of the film, the filter can be tuned to reflect light over any desired range of wavelengths.

Up to 80% brighter screens

What makes the device so interesting is that because of the direction of twist of the molecules, only one polarisation is reflected while the others pass through. This has an immediate application in the flat-screen LCDs used in portable computers. These are illuminated by polarised light, which is produced by an absorbing polariser behind the display. If a reflective polariser were used instead, the unwanted polarisation could be reflected back, depolarised (by bouncing it off a diffusing screen) and then sent back out through the display. Experiments at Philips and Merck have resulted in screens which are up to 80% brighter for the same power consumption. Computers using the new screen will be able to run longer on a full battery charge because less energy is wasted.
A similar application is in the type of TV projection system which works like a slide projector, except that the slide is replaced by a small LCD similar to those used for computer screens. Again, half of the light energy is absorbed in creating the polarised beam necessary for the LCDs to work. Projector bulbs have to be very powerful, resulting in a lot of unwanted heat. With a reflective polariser it should be possible to generate a polarised beam with much less wasted heat.
Another application, this time based on refraction rather than reflection, has appeared in magneto-optical recording, where information is stored on a disc in binary code as a pattern of tiny magnetised dots. The code is read by reflecting a laser beam from the dots - the polarisation of the reflected beam is altered depending on whether a dot is magnetised or not. At present the beam goes through a special quartz 'beam splitter' (a Wollaston prism) which sends the different polarisations to different detectors. The new liquid crystal beam splitter will do the same job, but more cheaply and in a compact component less than 0.5 mm thick.
The fourth application, a non-mechanical lens shutter for video cameras, is an 'active' device which exploits the ability of liquid crystals to be switched between different states of transparency. It is a layer of liquid crystals which becomes opaque when a voltage is applied to it. The new device works much faster than the solid-state shutters currently used on video cameras and eliminates the smearing effect often seen in brightly lit scenes.
Philips expect that the reflective polarisers will start to appear in computer screens in 1997 and that video cameras incorporating the new shutters will be on the market about the same time.

 

Project Title:  
Active and passive optical components based on in-situ formed anisotropic liquid-crystalline polymeric systems

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-5363

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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