Research at CMMPE — Materials — Introduction to liquid crystals (page 4 of 4)

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5. Optical properties of liquid crystals

5.1 Birefringence

The speed that light propagates through isotropically structured transparent materials is determined by the refractive index, n. Light travels more slowly through high refractive index materials, whilst lower index materials permit light to travel more quickly. However, materials that consist of an anisotropic molecular structure will give rise to an correspondingly anisotropic refractive index. Light therefore travels at a different speed in one direction through these materials than it will in another. We call such materials 'birefringent'. Birefringence (or 'double refraction' as it is also known) occurs naturally in materials such as calcite, but is also a property of liquid crystals.

More specifically, birefringent materials will have different refractive indices not only in different directions, but also for light of different polarisations. This is illustrated in the opposite image, where unpolarised light is used to view some text through a birefringent crystal of calcite. The component of the light that is polarised in one direction is refracted through a different angle to the component of the light that is polarised in an orthogonal direction. This results in a double image of the text. These two rays of light are referred to as the 'ordinary ray' (which has been modulated by the ordinary refractive index, no) and the 'extraordinary ray' (which has been modulated by the extraordinary refractive index, ne). The difference between ordinary and extraordinary refractive index (ne-no) is called the birefringence.

Birefringence in a crystal of calcite. Different polarisations
of light are refracted through the crystal by different amounts,
giving rise to a double image.

Furthermore, there exists a single orientation of the (uniaxial) birefringent crystal for which there is no difference in refractive index. We refer to this direction as the 'optic axis' of the crystal.

5.2 Birefringence in nematic liquid crystals

Two identical nematic liquid crystal cells viewed between
crossed polarisers. When the director is aligned parallel or
perpendicular to the incident polarisation (left), a dark
state results. At other angles, light is transmitted, with
maximum transmission at alignment of 45 degrees (right).

The anisotropic molecular shape and alignment structure of nematic liquid crystals give them birefringence. The optic axis is determined by the director of the nematic (ie: in the direction of the long axis of the molecules).

Consider two crossed polarisers. When an isotropic medium (like air) is between the polarisers, no light is permitted to pass through. However, consider now insering a nematic liquid crystal cell between the crossed polarisers, its director aligned at an arbitrary angle to the 1st polariser. Linearly polarised light emerging from the first polariser can in general be considered to have 2 components of linear polarisation: one aligned with the LC director, the other perpendicular to it. The birefringence of the LC will cause these two components of light to travel at different velocities, and therefore fall out of phase with each other. The result is that light emerging from the liquid crystal is elliptically polarised. This elliptically polarised light will be aligned with the 2nd polariser twice every wave-cycle, and therefore some light is permitted to pass through, despite the two polarisers being crossed relative to each other. Liquid crystals therefore appear bright when viewed between crossed polarisers, except when the director of the LC is aligned either parallel or perpendicular to the incident polarisation.

6. Electro-optic switching of liquid crystals

The electrically polar nature of liquid crystalline molecules means that the lowest energy configuration state for them to be in, is one in which their electric dipole (usually parallel to the liquid crystal director for positive dielectric anisotropy materials) aligns with any externally applied electric field. The liquid-like state of LCs therefore allows them to physically rotate when a voltage is applied to a liquid crystal cell. Due to the anisotropic optical properties of liquid crystals, as they rotate, the refractive indices (for different incident polarisations) also change. The total amount of phase shift between ordinary and extraordinary rays is therefore controllable electrically. If one first polarises the incoming light, one can therefore use liquid crystals in variable retarder applications, and this also forms the basis of the majority of electro-optical switching applications of liquid crystals.

Intensity modulation of transmitted light using nematic liquid crystals is achieved by simply placing a planar aligned liquid crystal cell between crossed polarisers, and modulating the applied voltage. At different voltages, different phase modulations are achieved. At phase differences equalling pi radians (or odd integer multiples thereof), the resulting light is not polarised in a direction to pass through the second polariser, and a dark state results. At phase differences equalling 2pi (or even integers of pi), the resulting light does pass through the second polariser, and a bright state results. Intermediate brightnesses are achieved by driving at intermeidate voltages.

A nematic LC cell aligned to a bright state with no applied field
(left), and with an applied field (right), illustrating a darkened
state in the centre where the electrodes are positioned.

Diagram representing a liquid crystal cell acting as an intensity modulator when switching between a bright state (left)
and a dark state (right). Note that the director needs only to rotate enough to induce a 2pi phase modulation,
but for clarity is illustrated in the fully on and off states.

Nematics however switch slowly. In display applications, LCDs therefore use alternative switching mechanisms, including super-twisted nematics, which are capable of switching at must faster speeds and with lower applied voltages. This however goes beyond this simple introduction to the electro-optical switching effect. For more information on CMMPEs research into new materials and new switching mechanisms for display applications, please visit our LC displays research pages.

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