Knots of light and invisible darkness

The circle represents the hologram, out of which the knotted light emerges. Or, if that's all too much, look at the pretty colours... Image: University of Bristol

The image above is of an optical vortex loop –a knot tied in a beam of light. It was created by UK researchers using a sophisticated computer-generated hologram to wrap the light from a laser around itself.

In an optical vortex, light doesn’t travel in a straight line. Instead it is twisted tightly around itself like a corkscrew. At the very centre of the corkscrew the light waves cancel each other out – leaving a thin core of complete darkness. Bizarrely, light is doing this all around us; we just can’t see it!

The researchers have managed to take this twisted light and tie it into knots in the laboratory using holograms based on an obscure branch of mathematics called knot theory. It was originally developed to study real-life knots in rope but quickly grew to encompass theoretical knots and now encompasses more than six billion different knots!

Light-knotting is more than just a physics party-trick. It could be used to improve anything that requires very precise control over a beam of light. In particular, persuading light to twist like this will lead to better ‘optical tweezers’: focussed lasers that can be used to gently measure or nudge tiny particles, such as bacteria, viruses or individual molecules, with pinpoint accuracy.

Paper Reference: Dennis, M.R., King, R.P., Jack, B., O’Holleran, K., and Padgett, M.J. (2009) Isolated optical vortex knots Nature Physics, published online 17 January 2010. Doi:10.1038/nphys1504

Butterfly colours copied

Researchers have made an accurate copy of a butterfly’s wing, right down to the microscopic structures that give a butterfly its dazzling colours. The team of researchers from America and Spain have come up with a new technique to copy the intricate detail of the wing without distorting it in the process. The work could result in new materials with unique light-bending properties.

Butterflies - better engineers than people? Image: Hypothesis Now

The beautiful iridescent sheen of a beetle carapace or butterfly wing is caused by microscopic structures that bend and reflect light in a certain way, rather than relying on a normal colour pigment such as we find in the skin of other animals. The structures are made of thin layers of a material called chitin, which all insects use to make their protective outer shell, and are built on such a small scale that we can’t copy their construction with our existing technology.

So the researchers did the next best thing – they made a near-perfect replica. They coated the surface of the wing with a substance called chalcogenide glass, before burning away the wing with an acid which dissolves chitin. When the wing had been destroyed, the researchers found the remaining glass had accurately copied its structure and, most importantly, its optical properties.

This area of research is known as biomimetics, which aim to copy nature’s designs and adapt them to improve human technology. The chitin structures of a butterefly wing bend light in fantastic ways to produce a range of colours and, if we can learn how to produce accurate copies, the researchers think it could lead future gadgets like more efficient solar panels or tiny fly-eye lenses for mobile phones, endoscopes or surveillance cameras.

And in case you were wondering, the researchers didn’t kill any butterflies to test their new process. The research paper clearly states they used a dead butterfly they found in a park in Madrid!

Paper reference: doi: 10.1088/1748-3182/4/3/034001

WIMPs to be found by scintillating bolometer!

Scientists from France and Spain have built a scintillating bolometer. The device is now running at close to -273.15^oC in an underground laboratory in Spain.

Look, it's a shiny blue crystal thingy! Image: IAS/SINC

Which is great, but does raise a few questions: what is a scintillating bolometer, and why is it kept at extremely low temperatures and buried beneath a mountain?

The answer is about as strange as you’d expect for a machine called a scintillating bolometer. It is a prototype device which has been created to detect something that barely interacts with the world we know, yet makes up about 22% of our universe – dark matter. Dark matter is one of the most enduring mysteries of modern physics: our best models of the universe predict its existence, and we can infer its presence from the effect its gravity has on normal matter, yet we have never actually seen it!

The difficulty is that the predicted particles of dark matter – aptly known as WIMPs, or Weakly Interacting Massive Particles – usually shoot straight through normal matter without leaving any sign of their passing. The scintillating bolometer picks up the tiny bursts of heat and light released when, very rarely, a WIMP slams into the nucleus of an atom within the unique detector crystal at the heart of the machine.

The heat released from such a collision raises the temperature of the crystal by a tiny, but measurable, fraction of a degree. At the same time, the tiny flash of light is absorbed by a separate metallic disc, which also heats up slightly. The combination of certain amounts of heat and light will show that a WIMP has failed to escape the scintillating bolometer, proving the existence of one of physics’ most elusive particles.

So what about the extremely low temperature and underground lab? The rock walls of the underground lab effectively shield the detector from any stray radiation, such as cosmic rays, which might trigger the detector and give a false reading; the frigid temperature ensures the tiny temperature change can actually be detected.

Paper Reference: doi:10.1016/j.optmat.2008.09.016

How to hide from an earthquake

Earthquakes are one of the most destructive forces known to man. When one strikes, it twists the very ground we walk on, sending great rippling shockwaves across a massive area. It is these waves that cause the most damage, literally shaking buildings apart.

Yet scientists now believe it is possible to protect our most important buildings, such as hospitals, from the devastating shockwaves caused by earthquakes. By using metamaterials – materials made from many different substances layered together – the waves can be redirected around the protected structure, keeping it safe from harm.

The proposed earthquake defence works in exactly the same as an “invisibility cloak”. Light also travels in waves, although on a miniscule scale, and scientists have been able to use metamaterials to redirect light waves around objects in the laboratory, hiding the objects from sight. It works because the metamaterials are carefully engineered to bend light in a very specific pattern. Because the light neatly avoids the object it appears to an observer that whatever is hidden by the “cloak” just isn’t there.

By building metamaterials on a much larger scale, the researchers hope to employ the same technique to redirect the waves caused by earthquakes.

This isn’t the first time the search for an invisibility cloak has had unexpected benefits. Waves are everywhere in nature – in the air, the sea and the sky. By using the metamaterials approach, the same team of scientists also hope to deflect tsunamis around vulnerable sea-based structures, such as oilrigs or fragile coastlands. The scientists hope a ring of posts, sunk into the seabed, could act as a metamaterial, redirecting even the power of a tsunami and protecting whatever is sheltered inside.

An illustration of the devastation caused by the 1755 Lisbon Earthquake and Tsunami. Could metamaterials stop this happening in future?

Picking up good vibrations

How do you weigh the basic building block of matter, the atom? You can make an educated guess by weighing a whole clump of identical atoms, use A-Level chemistry to estimate how many atoms you should have in your clump, and divide one by the other to give you your answer. This imprecise method wasn’t good enough for one group of European researchers, however. Instead they’ve came up with a clever way of weighing one atom at a time.

To do so they use carbon nanotubes – sheets of carbon rolled into tubes thousands of times narrower than a human hair. Carbon nanotubes vibrate, like a guitar string, at a characteristic frequency. The frequency changes if the weight of the nanotube changes – for example if another atom is stuck to it. By running an electrical current along the nanotubes the researchers could measure the vibrations of the nanotube by looking at changes to the electrical current. From there they could compare the vibrations of the nanotube with and without the atom attached, and work out its weight. Simple, yes?

The researchers believe the work will allow molecular biologists and chemists to follow, in incredible detail, how molecules (which are made up of many different atoms) change as they interact in real-time. This will tell us much more about how our cells function at the smallest scale: something we currently know very little about.

Atoms don't really look like this, but it'll do...