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.
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.
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.
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:
"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
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.
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
"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.
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.