23 September 2010

Bright future

Cosmos Magazine
Next generation lasers may not light up, but they will snap atoms in action and detect ripples in space-time.
photon lasers

An array of photon lasers at the California Institute of Technology, which will produce beams of sound waves by driving drumhead-like devices to release identical vibrations. Credit: Jiangang Zhu, Lan Yang, LCLS, University of Texas

LASERS AND LIGHT seem as inseparable as snow and cold: if you have one, you have to have the other. From presentation pointers to Darth Vader’s lightsabre, lasers are synonymous with bright beams of visible light.

But it wasn’t always that way. Lasers began as a special variety of the maser – short for microwave amplification by stimulated emission of radiation – that swapped ‘light’ for ‘microwave’.

Soon after the invention of these devices, scientists proposed other ‘-asers’ for waves across the electromagnetic spectrum, such as ‘uvasers’ for ultraviolet light or ‘grasers’ for gamma rays. These acronyms never caught on. But laser became a household name.

And now, at age 50, the laser has extended its dominion far beyond the realm of light. Physicists have built lasers that emit different kinds of waves. Laser-like ‘hard’ – high energy – X-ray pulses, for example, can freeze atoms in their tracks, providing a ringside view of chemical reactions.

And phonon lasers vault the technology out of the electromagnetic spectrum altogether, creating coherent beams of sound.

Light-based lasers themselves play prominent roles in the exploration of other wave types. Laser-induced plasma ripples can accelerate particles to breakneck speeds in the space of a metre. And a proposed space telescope will use lasers to look for subtle shudders in space-time invisible to conventional telescopes.

EVERYWHERE THEY GO, lasers show that they’re about more than just light. A torrent of proposals for other-wavelength devices followed the first laser flash. But it took a while for some ideas to mature. Lasers that emit the shortest type of X-rays, for instance, have been built only in the past few years.

These hard X-rays, electromagnetic waves with energies up to 10,000 times that of visible light, have proved their mettle as powerhouses of diagnostic medical imaging. Because they have wavelengths close to the width of an atom, the rays have the potential to capture the motions behind basic chemistry.

“If you want to look at small things, the nanoworld, what do you need?” asks Keith Hodgson of the SLAC National Accelerator Laboratory in California. “You need a wavelength that is roughly the same as the objects you want to study,” he replied in a February 2010 talk at a meeting of the American Association for the Advancement of Science (AAAS).

“If you want to study atoms, and the distances between atoms, that means hard X-rays.”

But old-school X-ray sources take blurry pictures because the radiation produced is not uniform. These sources are “more like a flashlight than a laser,” said physicist Margaret Murnane of the University of Colorado at Boulder in another talk at the AAAS meeting.

By generating X-rays that march in lockstep, as the light waves in a laser do, scientists should be able to get rid of that blur. Pulses of such X-rays could serve as a strobe light to take snapshots of atoms and molecules in motion.

SCIENTISTS JUST GOT such a strobe last year in the form of SLAC’s Linac Coherent Light Source, which revved up in 10 April 2009.

The light source uses the last third of SLAC’s 3.2 km long accelerator to speed up electrons and then send them wiggling through a tooth-like series of undulating magnets. As the electrons pass through, they toss off exceptionally bright X-rays. Although those X-rays are still spread out, they create electromagnetic fields that force the electrons into small, compact bunches. Those bunches emit bursts of bright, unified X-rays.

With beats less than 100 femtoseconds (a tenth of a trillionth of a second) apart, this strobe can expose proteins unfolding and bonds breaking – atomic scale chemistry in action. “We really have the ability to capture any motion, electron or atom, that is relevant to our natural world,” Murnane said.

Like the hard X-ray laser, the phonon laser – which doesn’t produce electromagnetic waves at all – was a long time coming. In April 1961, Charles Kittel of the University of California, Berkeley proposed lasers that shoot phonons – quantised ‘particles’ of sound.

An optical laser builds a beam of light by making electrons release identical photons; a phonon laser would build a beam of sound by driving a drum to release identical vibrations.

Since phonons and photons are both a type of particle called a boson, the translation from light to sound should be easy, Kittel argued. One of the defining qualities of bosons is that several with the same quantum properties can pile together at the same energy. The coherent beam of light streaming out of a laser is a physical manifestation of bosons being bosons.

It turns out that making a pile-up happen with phonons in the lab wasn’t so easy, and Kittel’s dream wasn’t realised until 2008. The breakthrough came when a group at the Max Planck Institute for Quantum Optics in Garching, Germany, produced a laser-like stream of coherent phonons from a vibrating magnesium atom trapped in a laser field. A paper on the results appeared in Nature Physics in August 2009 with a simple title, “A phonon laser.”

“That was a big spur for me,” says California Institute of Technology physicist Kerry Vahala, who was visiting the Max Planck Institute when the device was built. Within a year of returning to California, he had built his own version of a phonon laser.

Though the German group’s laser resembled a type that uses atomic vibrations to produce photons, Vahala says his team’s is “nearly identical to the way the first optical lasers were realised.”

Vahala’s set-up includes two glass drumheads, called whispering-gallery–mode resonators, about 63 micrometres in diameter. When a traditional light-based laser shines on the drumheads, they hum at a tuneable frequency, an effect Vahala exploited to create the phonon laser.

The resonators are named after whispering galleries, the spaces under domes where words softly spoken at one wall can be heard clearly at another, such as in St Paul’s Cathedral in London. Just as a domed ceiling guides sound waves with almost no loss of volume, the whispering-gallery–mode resonators guide the laser light in a circle without losing any brightness.

When there are two resonators, the circles touch to form a figure eight. As the light glides around, it exerts a force on the resonators, making them vibrate to produce phonons. That emission, Vahala says, is analogous to a flashbulb making electrons eject photons in a traditional laser. He and his colleagues sent more and more laser light whizzing around the resonators to make more phonons of the right frequency, amplifying the signal to create a coherent beam of sound.

Vahala’s phonon laser, reported in Physical Review Letters in February 2010, produces sound waves with a frequency over 20 to 400 megahertz – too high for humans to hear, but not high enough for medical imaging, etching or other proposed uses. “In terms of where phonon lasers can go,” he says, “it’s just the beginning.”

Light-based lasers aren’t helping just to make sound waves. The devices can produce another type of wave that may usher in the next generation of accelerators: plasma waves.

Particle physicists, driven to decipher the fundamental nature of matter, have built bigger and bigger accelerators to smash particles together at higher and higher energies. These efforts have culminated in the Large Hadron Collider, a subterranean monster straddling the border between Switzerland and France. But some worry that the LHC may be pushing the limit on what resources and real estate can support.

“Accelerators have made the incredible transition from something that’s handheld to something that’s the size of a small European country,” Wim Leemans of Lawrence Berkeley National Laboratory in California said in a talk at the AAAS meeting. “What do we do for an encore?” To explore new realms, accelerators have to reach ever higher energies. “How do we build this thing?” Leemans asked.

A new idea that could both shrink accelerator size and boost energy relies on lasers. Shining a laser into plasma, a gas-like state of matter where electrons float freely away from their atoms, could make a wave for electrons to surf.

Although no one has built such an accelerator yet, plasmas have been made many times in the lab and the laser-plasma acceleration concept has been making waves in the physics community. In October 2009 in Washington DC, at the Accelerators for America’s Future symposium, six physicists were asked what they would do if they had $10 million a year for the next 10 years. Four mentioned laser-driven acceleration.

Here’s how it works: a pulse of laser light crashes through the plasma, pushing free-floating electrons out of the way, like the bow of a boat pushing through the ocean and scattering seagulls.

Positive ions are left behind and electrons congregate in a negatively charged clump behind the laser pulse. The resulting pattern of charges forms a plasma wave, which contains a strong electric field.

“What’s special about the electric field is it’s just like the wake behind the motorboat – it’s following the laser pulse,” Leemans said. “As this laser pulse barrels through the plasma, behind it is this nice accelerating structure.”

Like surfers catching a wave at just the right moment, electrons can ride down the plasma wave and pick up energy as they go. In this way, the particles can reach energies similar to those attained in conventional accelerators in a fraction of the distance.

A cascade of advances in laser-plasma accelerators has come in the past decade. Leemans’ group holds the record for the most energy in the least distance. In 2006, with the help of Simon Hooker of the University of Oxford in England, the team pushed electrons to a billion electron volts in just 3.3 cm – a fiftieth the energy of SLAC’s linear accelerator in a hundred-thousandth the distance.

And theoretically, there’s no limit on how energetic the accelerators can get. Right now Leemans’ team is shooting for 10 billion electron volts – more than a thousandth the maximum energy of a proton beam at the LHC – in less than a metre. The researchers are also looking at chaining several plasma wave accelerators together to combine their energies. Instead of spanning two countries, a future collider could fit in a backyard.

JUST AS LASER-DRIVEN plasma waves can carry particle physics to new territories, lasers can propel astrophysics out of the electromagnetic spectrum to catch a new kind of wave altogether.

Telescopes, like lasers, are best known for their close relationship with light. By observing all kinds of electromagnetic waves, detectors can discover the structure of galaxies and the birthplaces of stars. But scientists are building a new telescope. LISA, launching in 2020 at the earliest, will listen for a different vibration. And its ‘L’ stands for laser.

Short for Laser Interferometer Space Antenna, LISA will look for gravitational waves, ripples in space-time that Einstein predicted but haven’t yet been found. These waves would be shaken up when a massive body accelerates, and they would have a major advantage over light: “they don’t scatter and are not absorbed as they traverse space-time,” says Stanford University physicist Robert Byer.

“They give you a really close look at objects that radiate them,” objects like waltzing pairs of superdense stars and merging black holes.

LISA will consist of three spacecraft arranged in a triangle, five million kilometres on each side, which will cartwheel around the Sun just behind Earth. Each spacecraft will carry a two kilogram cube of gold-platinum alloy and will shield the cubes from all external influences – except ripples in space-time.

Here’s where the lasers come in. To tell if the cubes have been nudged, each spacecraft aims an infrared laser at each other spacecraft. The laser beams reflect off the test cubes and a signature returns to the original spacecraft, which counts wavelengths to see if the cubes have shifted. Any movement, however slight, could be a sign of gravitational waves.

And the movement would be slight indeed. “If you think of space as like a fabric, or a rubber sheet, what happens when a gravitational wave goes by is it stretches it in one direction,” says physicist Jeffrey Livas of NASA’s Goddard Space Flight Centre in Greenbelt, Maryland, pulling a rubber sheet to demonstrate. “This doesn’t stretch very much.”

“Neither does space,” adds his Goddard colleague James Ira Thorpe. For the cosmic bodies LISA is tuned to, the lasers will have to detect movements of about a picometre (one trillionth of a metre) from five million kilometers away. That’s like trying to measure the radius of a helium atom that’s as far away as the Sun.

In its first months, LISA will observe a few known systems that are expected to make gravitational waves. This detection alone would be cause for celebration, proving that gravitational waves are real and allowing physicists to study them in detail.

But the unexpected sources will be the exciting part – just as the unimagined applications of the laser are the ones that revolutionised society.

“It’s the stuff you don’t know that’s out there that makes it an interesting experiment,” Thorpe says.

The same goes for X-ray lasers, phonon lasers and plasma acceleration. Scientists have their wish list of applications, but even more transformative, unpredicted uses may emerge, Vahala says. “The applications will be stimulated – no pun intended – by the device itself.”

Lisa Grossman is a freelance science writer for Science News, where this article first appeared.

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