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Guiding Light

A new kind of glass pane containing liquid crystal droplets can guide and manipulate beams of light.

by Patrick L. Barry

The Information Age rides on beams of carefully controlled light. Because lasers form the arteries of modern communications networks, dexterous manipulation of light underpins the two definitive technologies of our times: telecommunications and the Internet.

Now researchers at Harvard University have developed a new way of steering and manipulating light beams.

Using droplets of liquid crystals - the same substance in laptop displays - the scientists can make a pane of glass that quickly switches from transparent to diffracting and back again. When the pane is transparent a laser beam passes straight through, but when the pane is diffracting, it splits the beam, bending it in several new directions.

The change is triggered by applying an electric field, so the pane could easily be controlled by the electric signals of a computer, offering a powerful new way to steer beams of light.

"Telecommunications could be one application, but at this point we're still looking at the basic properties of these droplets. Their potential is great, and it's hard to imagine all the ways engineers might use them," says David Weitz, Gordon McKay Professor of Applied Physics at Harvard University and lead scientist for the NASA-supported research.

Beyond telecommunications, one could imagine this lightt-steering ability being useful in astronomy. For example, these liquid-crystal panes could be used in reverse to combine (rather than split) beams of light from multiple telescopes. Combining light from many telescopes, a technique called interferometery, is a good way to search for distant planets around other stars.

Another application: a liquid crystal pane held in front of the mirror of a telescope could be used to "unwrinkle" light that has passed through Earth's turbulent atmosphere. Such adaptive optics telescopes could gain a crystal-clear view of the heavens from Earth's surface.

The many uses of steering light are part of the reason that NASA recently decided to award Weitz and colleagues a grant for this research. In addition, NASA can provide a unique environment for experimenting with liquid crystals: low gravity.

"We've already seen several exciting results from fluid physics experiments done in Earth orbit," says Brad Carpenter, lead scientist for NASA's Physical Sciences Research Division. "This latest project of Dr. Weitz, who has already completed some successful experiments on the International Space Station (ISS), was selected for funding with the vision of aiding advances in optical information technologies."

Image courtesy Harvard University.

The droplets of liquid crystal in the Harvard group's experiments, like those shown here, are of equal size and arranged in a regular pattern.

Liquid crystals are a class of liquids whose molecules are more orderly than molecules in regular fluids. Because of this orderliness, when these liquids interact with light, they can affect the light like crystals do.

A technique invented by Weitz and his colleagues produces equal-sized droplets of liquid crystal, each about a dozen microns across (a micron is one thousandth of a millimetre). Because they're all the same size, packing the droplets together on a glass plate causes them to arrange themselves into a honeycomb pattern.

It's this regular pattern that gives sheets of liquid crystal droplets their light-steering ability.

Making droplets of liquid crystals is nothing new; the basic technology has been around since the mid-1980s. Today you can find such droplets in the window-walls of some executives' offices. With the flip of a switch, the office's transparent windows magically change to opaque walls somewhat like frosted glass.

"The big difference between what we do and what has been done before is that older-style glass panes contain a random distribution of drops and drop sizes - tiny ones and big ones. They're not ordered at all," explains Darren Link, one of the scientists on the research team.

Without any order in the drop size and spacing, these older liquid crystal systems simply scatter light in all directions - hence the frosted-glass effect.

Image courtesy Harvard University.

Light striking a surface after passing through the flat sheet of liquid crystal droplets doesn't appear as a single dot, but is split and bent to produce a patterns of dots.

"In our case, because we make all the drops the same size, we're able to steer light in specific directions," Link says.

The molecules in a liquid crystal droplet are long and rod-shaped. An electric field can steer these rods (much as a magnetic field moves a compass needle) and so control how they guide rays of light passing through them.

Steering light at will is useful enough, but Link suspects that the most interesting results will come from the next phase of their research.

"Where I think the interesting new physics is going to be is in getting away from having a 2-D sheet and going into 3-dimensional ordered structures," Link says. "From now on our research will focus on doing this with real 3-D ordered structures using smaller particles."

Link and colleagues aren't sure what they're going to find when they shine light through several stacked-up layers of these ordered droplets... that's what's so exciting about doing it! It might split the light up into a rainbow, like a prism, or it might affect the light in a totally unexpected way.

Image courtesy NASA.

 3-dimensional stacked layers of liquid crystal droplets could have some novel and useful effects on light that passes through them.

But first they need to find reliable ways to arrange the droplets into various 3-dimensional patterns. This is where low gravity comes in handy.

Weightlessness greatly simplifies making 3-D structures from fluid droplets. The tiny droplets have a different density than the liquid in which they are suspended. On Earth they'll either float or sink, which greatly complicates arranging them into a pattern. In orbit, the lack of buoyancy allows droplets to remain nicely suspended, allowing researchers to explore many configurations that would be difficult or impossible to create on the ground.

Weitz says they intend to design a space-experiment and eventually fly it on the ISS. First, though, more research on the ground is needed to understand the basic physics of these droplets - how they respond to the applied electric field, and exactly how those responses affect the passing light. It's details such as these that could so on give researchers a new tool to use in their ever-expanding mastery of light.


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First Science 2014