Lasers fire beams of light that can cut through steel or etch microchip patterns, depending on the power and wavelength. Now one team of scientists at the SLAC National Accelerator Laboratory in Menlo Park, Calif., led by Nina Rohringer, has created an X-ray laser that fires more energy, with a more precise wavelength, than any previous model. The results are being published in the journal Nature.
X-ray lasers are already used in spectroscopy, as a way to look into the depths of molecules like DNA. Back in the 1980s the “Star Wars” missile defense program even floated the idea of X-ray lasers as weapons, powered by nuclear bombs. (The idea was never implemented.) This laser will let scientists see things smaller than ever before.
A laser works by exciting atoms in a crystal, gas or liquid, pushing the electrons nearest to the nucleus into higher energy levels — a process called “pumping.” When the electrons return to their ground states, neighboring atoms stimulate each other and they emit light of a specific wavelength — ultimately the laser.
Most lasers, like the ones in CD players and laser pointers, use visible light or electricity to excite the atoms.
But getting a laser beam to work in the X-ray part of the spectrum requires tons of energy. Scientists use particle accelerators to push electrons to near the speed of light and then send them through a set of magnets. The electrons emit laser light in the X-ray spectrum.
But Rohringer’s team at SLAC wanted an even more powerful laser than that, so they used the first laser to pump atoms of neon. Hitting the neon with the beam pushed the electrons closest to the nuclei of the atoms into a high-energy state, and generated a beam with a shorter and more precise wavelength. To get an idea, a visible light laser (like a laser pointer) has a wavelength of a few hundred nanometers. The laser beam in Rohringer's experiment had a wavelength of 1.5 nanometers.
Laser energies are sometimes measured in electron volts, and this one has a range of 1 eV. Previous X-ray lasers had a range of 8-15 eV. That's important when you want to see what atoms are doing, since the smallest object one can see is limited by the wavelength of the light you use to see it. To look at anything smaller than a few nanometers requires a light beam (a laser in this case) with a wavelength that size, and to get a sharp picture, you also need a well-defined wavelength. Previous X-ray lasers had too much “spread.” The neon laser gets past that limitation, allowing for pictures of smaller and faster phenomena.
Images: Greg Stewart, SLAC (top); SLAC National Accelerator Laboratory (bottom)