Lasers scan future possibilities

Lasers have already had a profound impact on our daily lives but the potential of the technology has only just been tapped, scientists believe.

Sunday marks the 50th anniversary of the first demonstration of a ruby laser at the Hughes Research Labs in the US.

The light beams have since found myriad uses, from scanning shop prices to trying to sense the ripples in space-time made by colliding black holes.

Researchers say the coming decades will see even more remarkable progress.

"The laser gave a capability over previous light sources that was just so immense that you simply can't digest and exhaust all that in the matter of a few decades," said David Hanna, emeritus professor at the Optoelectronics Research Centre, University of Southampton, UK.

People have to use their imagination about what amazing and crazy things can be done with all that capability," he told BBC News.

There is debate over which key technological step made the laser possible, but Theodore Maiman's success in stimulating a ruby rod to produce an intense narrow beam of light by shining a flash lamp on it was an undoubted landmark.

There had been a race to demonstrate "light amplification by stimulated emission of radiation" (Laser), and Maiman beat everyone to it on 16 May, 1960.

At the time, the technology was said to be the classic "solution looking for a problem", but its ability to direct a powerful stream of energy from one location to another soon opened up a world of possibilities.

Anyone who scans a tin of beans at the checkout is using a laser. Anyone who listens to music on a CD is using a laser to read data stored on a disc. Anyone who sends an e-mail is reliant on the lasers that drive the world's fibre optic communications networks. Anyone who gets into a car is sitting in a box that owes its construction to lasers.

"There is a phenomenal amount of laser processing on a car; you wouldn't believe how much - laser cutting, marking, measurement, drilling, hardening, laser brazing, laser deposition, and laser welding," explained Tim Holt, the chief executive of the Institute of Photonics, University of Strathclyde.

"Modern cars today would not be possible without lasers."

THEODORE MAIMAN'S LASER (1) A powerful lamp is wrapped around a ruby rod, depositing energy in the form of undirected light of many colours, or wavelengths (2) Atoms in the rod absorb and store energy from the light (3) Reflecting mirrors then allow a small amount of light to bounce back and forth in the rod, collecting some of the atoms' stored energy on each pass (4) Some of the light escapes through one of the mirrors; it is a directed, intense beam of synchronised light waves of a specific colour

The worldwide market in lasers is worth some $5-7bn annually. Most of that value is in lasers sold to manufacturing outlets for use in material processing, but the two other key markets are lasers for use in communications systems and in data storage.

But as much as lasers have infiltrated our everyday world, there is still much more they could do, scientists believe.

There is hope lasers could provide us with a near-limitless supply of clean power.

Laser cutters deliver a lot of energy to a very specific spot

In the US, the National Ignition Facility will soon train 192 giant laser beams on a tiny pellet of hydrogen fuel in an attempt to fuse the element to make helium, and so release a colossal amount of energy - much like the Sun does at its core.

In Europe, researchers want to take this approach forward in a project known as HiPER that would be the prototype power station of the future.

"It's possible to use lasers to crush and heat material to temperatures that are 10 times hotter than at the centre of the Sun," says Dr Kate Lancaster from the UK's Central Laser Facility in Didcot.

"A standard laser pointer is about a milliwatt; we'll be approaching a petawatt - 10 million million times more powerful than a standard lightbulb.

"Fusion would be immensely efficient but it's extremely hard to do. Ever since Maiman demonstrated his laser, however, people have realised that this technique should be possible."

In astronomy, lasers are already used to sharpen the images of the world's very best telescopes. By projecting a "reference star" on the sky, scientists can work out how to correct their observations for the distortions introduced by atmospheric turbulence.

Fibre lasers allow far more data to be transferred much faster

But lasers are also pioneering a completely new form of astronomy, one that attempts to probe the Universe without the need to detect light. These laser interferometers would measure the disturbances in the very fabric of space-time generated every time massive stars imploded.

Such gravitational waves are extremely weak, however, and only lasers have the precision to measure their passing. If the technique works, it should be possible to see remnant gravitational radiation from the very moment of creation itself.

And in medicine, too, the possibilities seem boundless. Lasers can be used to manipulate atoms and molecules, "to unfold proteins and tickle DNA", says Dr Lancaster.

"Lasers can accelerate particles to high energy and we can use that to treat cancer," she adds.

"At the moment, with the way we treat cancer with radiation, photons travel into the body and they deposit energy in healthy tissue as well as at the cancer site.

"Whereas when protons and ions travel into the body, they will deposit most of their energy only at the very end of their range. So we can use lasers to tune protons to deposit their energy just within the tumour site."

The world's biggest telescopes use lasers to sharpen their images

As lasers have got ever more powerful, their pulse rate has also pushed new boundaries.

We already have lasers that trace time at the femtosecond level - a thousandth of a millionth of a millionth of a second. Researchers now are also working on attosecond lasers, which count time in divisions of a millionth of a millionth of a millionth of a second.

Working at these scales, it is possible to see how matter works, to record for example the moment chemical reactions occur.

"On the femtosecond timescale, the atoms in a molecule will vibrate. So that is the relevant timescale for taking a detailed look at what is going on - as it happens - in a molecule," Professor Hanna told BBC News.

Divide that tiny time by 10 - a reduction of an "order of magnitude", as it is known - three times over, and science can now speak of what happens in attoseconds. This, Professor Hanna explained, is the timescale of processes inside the very atoms that make up molecules.

"Every time we pick up an extra order of magnitude - and there are many more to be got - we need to go back to the drawing board and think 'what on Earth do we do with that?'."

Laser could be used to make rain on demand

Being able to create enough droplets is a key challenge

Ultra-fast pulses from a powerful laser can create droplets of water out of thin air, according to a new study. With the right conditions and large enough droplets, the researchers say, the technique could be used to make rain on demand.

Rain forms when water condenses around tiny particles in the atmosphere. Most of the time, dust or pollen do the job, but humans have long attempted to speed the process by seeding clouds with chemicals like silver iodide. Those chemicals provide the so-called "condensation nuclei" that trigger the consolidation of water into raindrops.

Unfortunately, such methods are difficult and could have environmental side effects, said Jérôme Kasparian, an optical physicist at the University of Geneva, Switzerland who was on the team that demonstrated the laser-triggered condensation. The study was published online May 2 in the journal Nature Photonics. 

"The potential advantage of laser is that it can work continuously," Kasparian said. If lasers can trigger rain on a large scale, he said, it would also be more efficient and cheaper than spraying silver iodide out of airplanes or shooting it into the sky from rockets.

Researches have long known that short, strong laser pulses can ionize air molecules, creating pathways of ionized gas called plasma channels. Kasparian and his team wanted to find out whether those plasma channels could be of use to wannabe rainmakers.

"Our idea is to use the laser to ionize the air, and the ions that are produced can then serve as the condensation nuclei," Kasparian said.

To test the idea, the researchers first used an atmospheric cloud chamber, a box that enabled them to vary temperature and humidity. After saturating the air in the chamber, the team flipped on a several-terawatt laser (one terawatt is a trillion watts) and watched with surprise as visible water droplets formed. Three seconds after the laser pulsed, the droplets swelled to diameters of 80 micrometers, smaller than a raindrop but larger than expected.

"What was very amazing was the fact that the cloud was very well visible to the [naked] eye, so the effect was quite strong," Kasparian said.

The next step was to take the laser outside. Using a weaker laser to monitor the formation of foggy air, the team blasted their multiple-terawatt laser into the sky of Berlin in Autumn 2008. Again, they saw heartening evidence: Particles coalescing in the atmosphere.

"This means that the laser can trigger the formation of droplets inside a cloud chamber, [but also] in the real atmosphere," Kasparian said. "Now the challenge is to find conditions that will allow the droplets to grow further into the size where they will fall and get turned into rain."

 

Laser fusion test results raise energy hopes

A major hurdle to producing fusion energy using lasers has been swept aside, results in a new report show.

The controlled fusion of atoms - creating conditions like those in our Sun - has long been touted as a possible revolutionary energy source.

However, there have been doubts about the use of powerful lasers for fusion energy because the "plasma" they create could interrupt the fusion.

An article in Science showed the plasma is far less of a problem than expected.

The report is based on the first experiments from the National Ignition Facility (Nif) in the US that used all 192 of its laser beams.

Along the way, the experiments smashed the record for the highest energy from a laser - by a factor of 20.

Star power

Construction of the National Ignition Facility began at Lawrence Livermore National Laboratory in 1997, and was formally completed in May 2008.

The goal, as its name implies, is to harness the power of the largest laser ever built to start "ignition" - effectively a carefully controlled thermonuclear explosion.

INERTIAL CONFINEMENT FUSION 192 laser beams are focused through holes in a target container called a hohlraum Inside the hohlraum is a tiny pellet containing an extremely cold, solid mixture of hydrogen isotopes Lasers strike the hohlraum's walls, which in turn radiate X-rays X-rays strip material from the outer shell of the fuel pellet, heating it up to millions of degrees If the compression of the fuel is high enough and uniform enough, nuclear fusion can result Giant laser experiment powers up

It is markedly different from current nuclear power, which operates through splitting atoms - fission - rather than squashing them together in fusion.

Proving that such a lab-based fusion reaction can release more energy than is required to start it - rising above the so-called breakeven point - could herald a new era in large-scale energy production.

In the approach Nif takes, called inertial confinement fusion, the target is a centimetre-scale cylinder of gold called a hohlraum.

It contains a tiny pellet of fuel made from an isotope of hydrogen called deuterium.

During 30 years of the laser fusion debate, one significant potential hurdle to the process has been the "plasma" that the lasers will create in the hohlraum.

The fear has been that the plasma, a roiling soup of charged particles, would interrupt the target's ability to absorb the lasers' energy and funnel it uniformly into the fuel, compressing it and causing ignition.

Siegfried Glenzer, the Nif plasma scientist, led a team to test that theory, smashing records along the way.

"We hit it with 669 kiloJoules - 20 times more than any previous laser facility," Nif's Siegfried Glenzer told BBC News.

That isn't that much total energy; it's about enough to boil a one-litre kettle twice over.

However, the beams delivered their energy in pulses lasting a little more than 10 billionths of a second.

By way of comparison, if that power could be maintained, it would boil the contents of more than 50 Olympic-sized swimming pools in a second.

'Dramatic step'

Crucially, the recent experiments provided proof that the plasma did not reduce the hohlraum's ability to absorb the incident laser light; it absorbed about 95%.

But more than that, Dr Glenzer's team discovered that the plasma can actually be carefully manipulated to increase the uniformity of the compression.

The 130-tonne target chamber is kept under vacuum for the experiments

"For the first time ever in the 50-year journey of laser fusion, these laser-plasma interactions have been shown to be less of a problem than predicted, not more," said Mike Dunne, director of the UK's Central Laser Facility and leader of the European laser fusion effort known as HiPER.

"I can't overstate how dramatic a step that is," he told BBC News. "Many people a year ago were saying the project would be dead by now."

Adding momentum to the ignition quest, Lawrence Livermore National Laboratory announced on Wednesday that, since the Science results were first obtained, the pulse energy record had been smashed again.

They now report an energy of one megaJoule on target - 50% higher than the amount reported in Science.

The current calculations show that about 1.2 megaJoules of energy will be enough for ignition, and currently Nif can run as high as 1.8 megaJoules.

Dr Glenzer said that experiments using slightly larger hohlraums with fusion-ready fuel pellets - including a mix of the hydrogen isotopes deuterium as well as tritium - should begin before May, slowly ramping up to the 1.2 megaJoule mark.

"The bottom line is that we can extrapolate those data to the experiments we are planning this year the results show that we will be able to drive the capsule towards ignition," said Dr Glenzer.

Before those experiments can even begin, however, the target chamber must be prepared with shields that can block the copious neutrons that a fusion reaction would produce.

But Dr Glenzer is confident that with everything in place, ignition is on the horizon.

He added, quite simply, "It's going to happen this year."