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
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
One benefit is the discovery of liquid crystals which can be manufactured
at a temperature of around 40°C, which is much more convenient than
the more usual 100°C, and reduces the likelihood of the molecules
polymerising too soon.
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
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.