liquid crystal displays
liquid crystals, including Smectic A and Ferroelectric, have two possible
stable molecular orientations, which do not require an electrical field
to be applied to maintain the liquid crystal orientation state. However,
if a suitable electrical signal pulse is applied to the cell, the liquid
crystal can be switched from one stable state to another. Such materials
are extremely useful if one wishes to make a binary (on or off) display.
Furthermore, the lack of requirement for a maintaining electric field
offers the potential of very low power consumption, making bistable
displays a highly attractive technology for mobile devices, such as
laptops, mobile phones, etc.
devices using the smectic A liquid crystal phase are low cost,
simple and highly efficient. This liquid crystal phase has orientational
order, as in nematic liquid crystals, but also has a degree
of positional ordering. The positional order gives rise to unusual
properties, such as negative conductivity anisotropy, that form
the basis of useful electro-optic effects.
devices work in the following manner: for low frequency electric
fields (e.g. < 100 Hz), the flow of ionic dopant in the liquid
crystal induces a highly scattering opaque state – ‘write’.
As the frequency is increased (e.g > 1 kHz), the ionic motion
is damped out and the device becomes transparent by dielectric
reorientation of the liquid crystal molecules – ‘erase’.
The resultant electro-optic effect is similar to the PDLC device
described above; in the smectic A case, however, both the write
and erase modes are stable at zero field. This is termed bistability
and allows production of highly efficient devices since power
is only required when the image updates.
of the key differences between these materials and previous
Smectic A liquid crystals is due to their unprecedented conductivity
properties, which allow for low-voltage driving; in addition
to the wide temperature range performance and ease of fabrication.
In addition, these materials allow for much increased flexibility
in the driving schemes, since frequency or voltage (or both)
can be varied to induce the two modes. This is achieved by careful
design of the material properties. For example, it would be
possible to drive a ‘smart window’ e.g. for architectural
use, that can be driven off mains voltage at 50 Hz. This would
allow considerable simplification of driving circuitry compared
to the standard designs of electrochromic and Polymer dispersed
liquid crystal (PDLC) smart windows, for example. Power consumption
estimates suggest that, for one hour’s use, the amount
used in the Smectic A window is 0.8% that of the PDLC type.
Further work is in progress to optimise material performance
and extend the technology to full motion frame rates to open
up new application areas.
information about the phenomenon of bistable switching can be
Coates, W. A. Crossland, J. H. Morrissy, B. Needham
J. Phys. D Appl. Phys. 11, 2025 (1978)
LCs for Fast-Switching Bistable Scattering Devices
Gardiner D.J. and Coles H.J.
Phys. D: Appl. Phys. 39(23), 4948-4955, (2006)
Anisotropic Conductivity in Organosiloxane LCs
Gardiner D.J. and Coles H.J.
Appl. Phys. 100, 124903, (2006)
smectic A devices used standard cyanobiphenyl liquid crystal
materials. Recently, however, a novel class of liquid crystals
containing siloxane units have been shown to offer much potential
for use in smectic A based devices. In these compounds, although
the siloxane group is bonded to the rest of the molecule, the
three components of the liquid crystal molecule - siloxane,
alkyl chain and aromatic core- are chemically distinct. This
distinction results in self-assembly of the components on a
molecular level – a phenomenon termed microphase separation.
By virtue of this structure, organosiloxane smectic A materials
Highly anisotropic conductivity – up to 3 orders of magnitude
higher than standard smectic A materials.
2. Inherent wide-range temperature operation
– the neat materials possess smA phases in excess of 100
3. Solubility. It is possible to significantly increase
solubility of additives e.g. dyes, which ordinarily possess limited
solubility in the host, by chemically bonding siloxane units to
such additives the solubility can be significantly increased.
In summary the advantages of smectic A technology are:
Simplicity. No polarisers, alignment layers or colour
filters are required. Dyes can be readily incorporated into these
systems leading to colourful displays with wide viewing angle.
2. Bistability. The electrically induced textures can
be stored long-term; images have been maintained for longer than
10 years. Additionally, any intermediate, greyscale value can
be selected and stored in the same manner.
3. Readable. Devices using this electro-optic effect
show excellent daylight readability and 180° viewing angle
– unlike standard nematic liquid crystal displays, for example.
4. Formulation. Using organosiloxane materials as the
host material, it is possible to generate smectic A mixtures with
wide temperature ranges, low driving voltages and ruggedness.
liquid crystal displays
to be added on devices using flexo effect]
information about the phenomenon of the flexoelectro-optic effect can
be found here.
crystal laser displays
colour displays commonly utilise pixels that are independantly
capable of displaying three colours: red, green and blue. By
combining red, green and blue in different combinations and
ratios, it is possible to create any desirable colour output
at any position on the screen. In LCDs, this is achieved using
a white back-light, which is then filtered by electrically driven
LC elements to leave only red, green and blue light remaining.
Other technologies, such as LED (including OLED and PLED technologies)
are more light efficient, and instead use separate red, green
and blue light emitters, which can be rapidly switched and their
displays are soon set to reach the market, which utilise three
independant red, green and blue lasers as light sources. Like
LED-based technology, this offers improved light efficiency
over back-light filtered systems. Laser displays can also be
far brighter than other display technologies, which is of particular
interest for the projector TV/monitor markets. More importantly
however, the narrow spectral linewidth (single wavelength) characteristic
of lasers, gives the opportunity to manufacture displays with
far higher colour resolutions than any other competing technolgies
(including LED, OLED and PLED). The coherent nature of lasers
also opens the door to new opportunities in diffractive or holographic
of the problems with laser displays is the complexity and high
cost of utilising three independant red, green and blue lasers.
Furthermore, if an extremely high colour resolution display
is desired, then the emission wavelengths of these three lasers
must be at very specific values, lying at the extreme corners
of the visible colour gamut. Sourcing cheap lasers that emit
efficiently at all these specific wavelengths is very difficult.
research into photonic
band-gap materials has led to the development of the liquid
crystal laser. CMMPE has carried out extensive research
in this field. It is hoped that LC lasers might be suitable
for use in future laser display systems, as they offer the significant
advantages of being mechanically simple, cheap, self-assembly
construction, low-speckle, highly efficient and have the ability
to be be tuned to virtually any visible wavelength that is desired.
here for more information about CMMPE's research on liquid crystal
liquid crystal laser arrays towards display applications
S.M. Morris, P.J.W. Hands, S. Findeisen-Tandel, R.H. Cole, T.D.
Wilkinson, H.J. Coles
Express, 16 (23), 18827-18837, (2008)
phase liquid crystal displays
liquid crystal (PDLC) displays
liquid crystal switching mode that we are interested in is the
polymer dispersed liquid crystal (PDLC). In this case, the LC
is contained within polymer vesicles. Due to the mismatch of
the refractive indices between the LC and the polymer, light
is scattered when no electric field is applied across the cell
and the sample appears opaque. Specifically, the average refractive
index of the LC and that of the polymer are different. With
the application of an electric field the vesicles reorient so
that the ordinary refractive is aligned with the polarisation
of the propagating electromagnetic wave. If the refractive index
of the polymer matches that of the ordinary refractive of the
LC then light is transmitted and the sample appears transparent.
The principle of the LC mode is shown in Figure 1. Our research
includes dye-doped PDLCs, photopolymerized PDLCs and emulsion-based
a more detailed introduction to PDLC materials, please click
to be added here soon.