Liquid Crystal Polarization

Liquid crystal polarization of light allows rapid and low power control of user interfaces in automotive, medical and consumer devices.

In a Nutshell

To understand liquid crystals, we must first understand the polarization of light.

In this article, we explain what light is, how it is filtered by polarizers and how liquid crystal polarization operates at such low power consumption.

Applications of liquid crystals include human-machine interfaces for automotive, medical and industrial devices as well as augmented reality near-eye displays.

What is Light?

Light is an electromagnetic wave and can be visualised as electric and magnetic fields perpendicular to each other, and both perpendicular to the direction of travel.

In the diagram below, the electric field is shown on the y-axis (pointing upwards), the magnetic field on the z-axis (coming out of the page), and the direction of travel is on the x-axis (pointing to the right).

Both the electric and magnetic fields are sinusoidal waves, with peaks that may coincide (meaning they are in phase), or peaks that do not coincide (they are out-of-phase), or anything in between.

Electromagnetic wave with electric and magnetic fields perpendicular to each other, shown here in-phase

Electromagnetic wave with electric and magnetic fields perpendicular to each other, shown here in-phase

If you ‘look into’ the direction of travel of the wave, the electric field vector would appear as a vertical line. Sometimes it would point upwards (at 0º from the vertical), like the minute hand of a clock at noon, or downwards (180º from the vertical), when the clock shows half past the hour.

Liquid crystal polarization showing electric field vectors

We also see in the above diagram an electric field vector at 10º from the vertical and one at 90º from the vertical.

Daylight is random and spontaneous

On Earth, daylight is produced by a random and spontaneous emission of electromagnetic waves from our closest star, the Sun.

So we can expect the orientation of the electric field in daylight to also be random.

And it is.

In fact, the polarization of light changes direction every 10-8 seconds, as if the minute hand of the clock were randomly and rapidly changing position.

It is the position of the electric field vector that defines the polarization of light.

If the tip of the electric field vector does not change over time (e.g. it is constantly pointing to 10 minutes past the hour) then the light is said to be linearly (or plane-) polarized.

Let’s not forget that a changing electric field induces a perpendicular magnetic field, and likewise, a changing magnetic field induces a perpendicular electric field. 

We cannot simply ‘remove one of them’ by polarizing light.

In this article, we focus on the electric field since it is of more relevance to liquid crystals.

What is a Liquid Crystal Polarizer?

A polarizing filter only allows through light which is ‘pointing’ in a particular orientation.

It is a filter. 

It stops stuff.

More technically, it absorbs certain light polarizations and allows other polarizations to pass through.

A liquid crystal polarization filter is composed of a network of infinitesimally small parallel lines, constructed at the molecular level by transparent chemical compounds.

If we apply a vertical polarization filter to daylight, light emerges vertically polarized. 

No surprises there.

If we have a sequence of polarizing filters which are ‘crossed’ (i.e. perpendicular with respect to each other), no light will emerge from the second filter! 

This is because the vertically-polarized light from the first filter cannot pass through the second (horizontal) filter.

Cool, eh?

Liquid crystal polarization filters applied to light producing vertically polarised light or no light

Polarization filters applied to light producing (a) vertically polarized light or (b) no light

The top portion of the diagram shows how the first (vertical) filter polarizes light vertically. This light can pass through the second (also vertical) filter since it has the same orientation. From the right side, we would simply see light emerging.

In the lower part of the diagram, the horizontally-polarized light exiting the first filter cannot pass through the second (vertical) filter, and no light emerges from this arrangement. The output looks opaque to an observer positioned on the right, looking in.

This arrangement was first investigated by Étienne-Louis Malus (1775-1812), a French physicist, who created a mathematical formula to calculate how much light emerges through polarizing filters depending on their orientation.

I won’t bore you with the maths.

Elliptically and Circularly Polarized Light

If the tip of the electric field vector is not constant over time, but rather traces the outline of an elliptically shaped clock, then the light is said to be elliptically polarized.

A special case of an ellipse is a circle, and light filtered by a circular polarizer emerges as circularly polarized light (where the electric field vector traces the outline of a circular clock).

If we were to ‘look into’ the direction of propagation of a circularly-polarized light wave (from the right side), we would merely see a circle, with the electric field vector moving like the minute hand around the clock.

However when we take time into account, you can see from the diagram below that the circularly polarized light now traces out a helix.

Liquid crystal polarization (circular) where light traces out a helix over time

Circularly polarized light traces out a helix over time

Furthermore, if the electric field vector is rotating clockwise, we call this ‘right circularly polarized’ light; if the vector is rotating anticlockwise, it is called ‘left circularly polarized’ light.

What is the ‘Order’ of a Liquid Crystal?

Liquid crystals sit somewhere between solids and liquids, that is, they have no positional order but they do have some orientational order:

  • Positional order refers to molecules which are in certain positions with respect to each other. Think of pieces on a chessboard and how they move on an 8 x 8 matrix, always respecting their relative positions (according to the rules of chess).
  • Orientational order is when molecules are constrained to point in certain directions. In our analogy, the chess pieces must always sit upright on the board. That is their ‘preferred’ orientation.

Solids exhibit both positional and orientational order, whereas liquids do not have positional or orientational order. 

Liquid molecules are randomly distributed.

In a liquid crystal the molecules, on average, point in a particular direction. This ‘average direction’ is called the ‘director’ of the liquid crystal, which we can think of as a vector (shown as an arrow).

The ‘order parameter’ of the liquid crystal takes on a number between 0 and 1. The image below shows how the order parameter varies with temperature for a typical liquid crystal. 

We can see that it is highly temperature dependent.

On the x-axis, the value ‘Tc’ is the transition temperature when the liquid crystal turns into a liquid, at which point the order parameter goes to zero, confirming that liquids have no order.

Order Parameter vs Temperature for a typical liquid crystal

Order Parameter vs Temperature for a typical liquid crystal

Liquid Crystals and Electric Fields

A dipole is a charged molecule with a ‘positive end’ and a ‘negative end’ (much like a magnet has a North pole and a South pole).

The image below shows a ‘point dipole’, a theoretical object of infinitesimally small size with no separation between the two equal and opposite charges. 

We also see a ‘physical dipole’ (a real world object) with two equal and opposite charges, separated by some distance.

Point dipole vs. physical dipole

Point dipole vs. physical dipole, image attribution

When an electric field is applied, the dipole will align with it. If the molecule did not originally have a dipole, then one is induced when the field is applied, caused by the charges separating.

The electric field needed to accomplish this is pretty minimal in liquid crystals, whereas the electric field has little effect on solids because the molecules are held by very strong bonds.

In liquids, electric fields have no influence over the molecules, since they possess a lot of kinetic energy.

By the way, some liquid crystals require an alternating (AC) voltage, and some require a direct (DC) voltage. 

This depends on the type of liquid crystal used.

Liquid Crystals and Birefringence

Liquid crystals are ‘anisotropic’, meaning that any measured parameter depends on the direction of measurement.

So, light entering the surface of the crystal will ‘see’ a different refractive index, depending on the density of molecules in the direction of travel of the light wave.

When the refractive index depends on the direction of travel and on the polarization of the light wave, the material is considered to be ‘birefringent’.

One value of refractive index corresponds to light polarized parallel to the director, and the other to light polarized perpendicular to the director.

Birefringence in crystalline materials

Birefringence in liquid crystals, leading to multiple wave paths through the crystal

In a birefringent material, these two light waves will propagate through the liquid crystal at different speeds and may arrive out of phase when they exit the crystal, sometimes producing colours and other visual effects.

Liquid Crystals and Polarization

A liquid crystal molecule is a dipole and hence will align itself with an applied electric field, which is exactly what happens in a liquid crystal display (LCD).

LCDs can be found in handheld calculators, digital clocks, computer monitors, aircraft cockpit displays and petrol station fuel indicators.

The elements of a LCD are shown below:

Liquid crystal polarization in a Liquid Crystal Display

Liquid Crystal Display with polarizers, Image attribution

This LCD is based on twisted nematic (TN) liquid crystals, with the following features:

  1. Vertical liquid crystal polarization filter to polarize incoming daylight.
  2. Glass substrate with indium tin oxide (ITO) electrodes. The electrodes form the dark shapes that will appear when the LCD is activated. There are vertical ridges etched on the surface so the liquid crystals are aligned with the polarized light.
  3. Twisted nematic liquid crystals.
  4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.
  5. Horizontal filter to block/allow through light.
  6. Reflective surface to send light back to the viewer.

Each pixel of an LCD can be switched on or off, depending on whether a voltage is applied to it.

The liquid crystals are sandwiched between two transparent electrodes of indium tin oxide (ITO) and have two crossed liquid crystal polarization filters.

The third ‘polarizing filter’ is of course the liquid crystal itself, controlled by our applied voltage. In a twisted nematic liquid crystal, the molecules arrange themselves in a helix structure, like a spiral staircase. It is the helix that rotates the incident light.

When not activated, twisted nematic liquid crystals feature a naturally occurring 90º twist, which polarizes light. 

Incoming light is first vertically polarized by the outer filter before being horizontally polarized by the liquid crystal layer (when unactivated), whereupon this horizontally polarized light is passed by the second horizontal filter unperturbed, and reflected by the mirror back out to the user.

When the liquid crystal is activated, it will not polarize the light, which will then not pass through the second (horizontal) filter. That pixel will then appear dark.

By controlling the voltage across each liquid crystal pixel, light can thus be controlled to switch an LCD user interface.

It must be remembered that liquid crystals are light modifiers, not light producers.

This factor allows for a very low power consumption, typically 1 to 300 microwatts of power per square centimetre.

Outlook

There is plenty of life left in liquid crystal technologies, despite their existence since the late 19th century.

Current and active research is leading us into territories new and bold, with applications in pharmaceuticals, medical, smart windows and augmented reality displays.

Rest assured you can expect more from us on this topic in future articles.

References

1. Liquid Crystal, MIT Media Lab, URL

2. Colour and the Optical Properties of Materials, Richard Tilley, ISBN: 978-1-119-55468-4, URL

3. Computer Peripherals, Nanyang Technological University of Singapore, URL

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