Research at CMMPE — Electrical/optical phenomena — Bistable switching

 

Most liquid crystals, including nematics, display an analogue response to applied electric field. There is only one stable configuration for the molecules whilst there is no applied electric field. As the voltage to the cell is increased, the average molecular orientation then gradually rotates to align with the applied electric field. However, certain classes of liquid crystals are bistable (or multi-stable), posessing two (or more) stable molecular configurations. When electric fields are applied to such materials, the liquid crystal molecules rapidly switch in binary fashion between these two states. Furthermore, once applied electric fields are removed, the liquid crystal retains in its switched state, with no need for a retaining field. This bistable phenomena is of particular use for the displays industry, where fast-switching, low power consumption displays are highly desirable.

Bistability is often commonly associated with the class of liquid crystals known as smectics (Sm). The following text describes bistability within these types of materials. The text focusses particularly upon SmA and also unwound chiral SmC* (ferroelectric) materials. These materials are of particular interest to us, because recent research at CMMPE has uncovered new siloxane and organosiloxane liquid crystal materials, some of which have Sm phases and have excellent potential for new bistable devices.

Jump directly to the following sections: Smectic A switching - Ferroelectric switching

Smectic A switching

The Smectic A phase of liquid crystals exhibits orientational order, like nematic liquid crystals, but also possesses a further degree of positional ordering in that the molecules arrange themselves into a layer like structure. (For more background into liquid crystal structures, please see CMMPE's Introduction to Liquid Crystals).

The layered structure allows observation of unusual electro-optic effects not observed in other mesophases. We have exploited some of these effects to make devices which are bistable (or even multi-stable) and possess indefinite memory. In this section, we will describe some of these effects.

Scattering

The layered structure of the Smectic A phase permits ions (which are present naturally or as an additive) to travel more easily along the layers rather than across them. This is expressed as the conductivity anisotropy i.e. the ratio of the parallel to perpendicular conductivities. Conventionally, ‘parallel’ means in the direction parallel to the preferred orientational direction (or director) and vice versa (see figure 1 for definition of terms). Such conductivity anisotropy, which is less than 1 in the Smectic A phase, means that for voltages applied at low frequencies (e.g. 100 Hz), the liquid crystal molecules respond in a chaotic fashion and induce a highly scattering texture that, macroscopically, appears opaque. Figure 2 shows the induction of this texture in a test cell.

Theoretically, the driving voltage of the effect (which is a key device parameter), is controlled by the degree of conductivity anisotropy and the parallel dielectric constant. Our recent work has investigated organosiloxane liquid crystals and we have discovered that these materials possess an unprecedented degree of anisotropy. In some cases, this can be up to 1000 times greater than other Smectic A materials (e.g. 8CB). Figure 3 shows the molecular structure of these materials. They comprise three, chemically distinct, parts: the alkyl chain, aromatic core and siloxane unit.

 

 

 

Dielectric reorientation

Like nematic liquid crystals, the molecules present in the Smectic A phase will align parallel to an external electric field. Unlike nematics, however, the presence of the layered structure, and greater viscosity, allows the induced texture to be stored.

The special case of the Smectic A requires the use of electric fields with a frequency greater than a certain value (called the critical frequency) otherwise the motion of charged impurities causes scattering to occur. Typical values are greater than around 500 Hz. Figure 4 shows the clearance of a previously induced scattering texture, via dielectric reorientation, into a clear state. The driving voltage for dielectric reorientation is proportional to the difference in the parallel and perpendicular dielectric constants, known as the dielectric anisotropy.

 

Using both the effects described above, scattering and dielectric reorientation, it is possible to have highly efficient electro-optic and display devices which possess indefinite storage and do not require polarisers (link to devices).

Ferroelectric switching

Ferroelectric liquid crystal (FLC) devices are also bistable, and are made using chiral smectic C (SmC*) materials, which have their helix unwound. This unwinding is made possible by surface stabilisation techniques, whereby substrate alignment layers (combined with very thin cell separations) force liquid crystal directors to lie in a planar configuration throughout the cell. Smectic layering therefore occurs in planes that lie perpendicular to the substrates (as opposed to non-chiral SmC cells, where smectic planes lie parallel to the substrates). The resulting structure is therefore similar to the non-chiral SmC phase, but rotated through 90 degrees.

A conventional (non-chiral) SmC liquid crystal has molecular orientations restricted to anywhere on the surface of an associated surrounding cone. In the forced unwound chiral smectic structure of an FLC cell, liquid crystal directors are restricted to one of only two possible in-plane orientations on opposite sides of this cone. This is because surface stabilisation makes it energetically unfavourable for the director to swing out of plane of the cell. The result is a bistable liquid crystal (FLC) device, with two in-plane states. Furthermore, because these liquid crystal molecules rotate only at one of their ends, and with little resistance to motion, FLC devices also are capable of very rapid switching speeds.

The rapid switching properties of FLC devices make them very useful in binary phase spatial light modulators and in switchable waveplate applications. Their bistability also makes them attractive for low power consumption and fast frame-rate displays. Furthermore, research is being carried out to use FLC devices in optical telecommunication devices, for high-speed modulation of optical signals.

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Selected publications for further reading

High-efficiency multistable switchable glazing using smectic A liquid crystals
D.J. Gardiner, S.M. Morris, H.J. Coles
Solar Energy Materials and Solar Cells, 93 (3), 310-306, (2009)

Enhancing lifetime in a bistable smectic A liquid crystal device
Gardiner, D.J. and Coles H.J.
J. Phys. D: Appl. Phys. 40(4), 977-981, (2007)

Electro-Optic Bistability in Organosiloxane Bimesogenic Liquid Crystals
Gardiner D.J., Davenport C.J., Newton J. and Coles H.J.
J. Appl. Phys. 99, 113517, (2006)

Organosiloxane Liquid Crystals for Fast-Switching Bistable Scattering Devices
Gardiner D.J. and Coles H.J.
J. Phys. D: Appl. Phys. 39(23), 4948-4955, (2006)

Highly Anisotropic Conductivity in Organosiloxane Liquid Crystals
Gardiner D.J. and Coles H.J.
J. Appl. Phys. 100, 124903, (2006)

Electro-optic effects in novel siloxane containing oligomeric liquid crystals I: Smectic A materials
Coles H.J., Butler I., Raina K., Newton J., Hannington J. and Thomas D.
SPIE, 2408, 14-21, (1995)

Electro-optic effects in novel siloxane containing oligomeric liquid crystals: II Smectic C materials
Coles H.J., Owen H., Newton J., and Hodge P.
SPIE, 2408, 22-29, (1995)

Synthesis and Properties of Low-Molar Mass Liquid Crystalline Siloxane Derivatives
Newton J., Coles H.J., Hodge P. and Hannington J.
J. Mater. Chem., 4(6), 869-874 (1994)

Investigations of Smectic Polysiloxanes, 1- Electric Field Induced Turbulence
Simon R. and Coles H.J.
Mol.Cryst.Liq.Cryst., 102(2) , 43-48, (1984)

Investigations of Smectic Polysiloxanes, 2- AC Field Induced Director Reorientation
Coles H.J. and Simon R.
Mol.Cryst.Liq.Cryst., 102(3), 75-80, (1984)

Other references

Electrically Induced Scattering Textures in Smectic A Phases and Their Electrical Reversal
D. Coates, W. A. Crossland, J. H. Morrissy, B. Needham
J.Phys. D Appl. Phys. 11, pp. 2025, (1978)

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