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.
