Threshold broken for tiny lasers

Scientists have shown off the smallest-ever laser that works at the colours of light used in telecommunications and at room temperature.

The tiny light sources switch on with no "threshold", meaning they operate much more efficiently than earlier, small laser attempts.

They are just one-fifteenth the size of the light waves that they produce.

The advance, described in Nature, may help bring about faster computers or in future "optical computing" approaches.

Lasers are ubiquitous in daily life, from supermarket checkouts to CD players, but the quest for a smaller laser has been underway for years.

The principal application for the tiny lights would be in computing and telecommunications. Laser beams can, in principle, carry vast amounts of information faster than traditional semiconductor electronics.

But as the laser "cavities" in which light waves are amplified have shrunk to near the size of the light waves themselves, new effects have come into play.
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Almost all lasers require that a certain threshold of energy is put in, after which the light waves can line up in sync to form a laser beam.

Much research in recent years has focused on confining lasers to tiny boxes made from metals.

However, at such minuscule scales, much of the energy that is put in to create a laser beam - and the light that comes out along with it - is wasted.

That raises the threshold energy so high as to make the resulting lasers impractical.
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The trick in the new work, said lead author Mercedeh Khajavikhan of the University of California, San Diego, was to use not a box but a cylinder, in a so-called co-axial arrangement.

"Most people are familiar with co-axial cables that bring TV signals to their homes," Dr Khajavikhan told BBC News.

"What they may not be very familiar with is that co-axial structures can support a [laser beam], no matter how much they are shrunk in size."

The team fabricated a number of the lasers - just 200 millionths of a millimetre high - with a metal rod at their centres, surrounded by a mix of semiconductor materials. They put energy into the tiny lasers using a much larger laboratory laser.

They found that the tiny lasers were able to harness all that energy, focusing it into laser beams of colours that are used in the telecommunications industry - all switched on with no threshold.

While they are not the smallest lasers ever made, the ease of fabrication of the team's tiny lights - and the fact that they work at room temperature - makes them attractive for future applications.

The first step will be to adjust the approach to work using not another laser but with electricity - similar to more familiar lasers in CD players and laser pointers.

With that, Dr Khajavikhan said the team "expect this work to have major impacts in several areas" - namely the ferrying of optical information on chips and, eventually, to all-optical computing technology.

"We feel this is just a beginning of a new family of light emitters with superior characteristics, and many advances in this new area are yet to come," she said.