Interest in tabletop x-ray lasers broadens out

AAAS Meeting : Margaret Murnane and Henry Kapteyn group at JILA, a joint institute of the University of Colorado at Boulder and NIST, has made some breakthroughs on how to build a tabletop x-ray laser. The laser could be used for super high-resolution imaging, while also giving scientists a new way to at the nanoscale at objects such as a single cell.Murnane and Kapteyn presented highlights of their research at the American Association for the Advancement of Science annual meeting in San Diego."Our goal is to create a laser beam that contains a broad range of x-ray wavelengths all at once that can be focused both in time and space,” Murnane said. “If we have this source of coherent light that spans a huge region of the electromagnetic spectrum, we would be able to make the highest resolution light-based tabletop microscope in existence that could capture images in 3-D and tell us exactly what we are looking at. We’re very close."Most of today’s x-ray lasers require so much power that they rely on fusion laser facilities the size of football stadiums or larger, making their use impractical. Murnane and Kapteyn generate coherent laser-like x-ray beams by using an intense femtosecond laser and combining hundreds or thousands of visible photons together with a desktop-size system.They can already generate laser-like x-ray beams in the soft x-ray region and believe they have discovered how to extend the process all the way into the hard x-ray region of the electromagnetic spectrum."If we can do this, it could lead to all kinds of possibilities,” Kapteyn said. “It might make it possible to improve x-ray imaging resolution at your doctor’s office by a thousand times. The x-rays we get in the hospital now are limited. For example, they can’t detect really small cancers because the x-ray source in your doctor’s office is more like a light bulb, not a laser. If you had a bright, focused laser-like x-ray beam, you could image with far higher resolution."Their method can be thought of as a coherent version of the x-ray tube, according to Murnane. In an x-ray tube, an electron is boiled off a filament, then it is accelerated in an electric field before hitting a solid target, where the kinetic energy of the electron is converted into incoherent x-rays. These incoherent x-rays are like the incoherent light from a light bulb or flashlight—they aren’t very focused.In the tabletop setup, instead of boiling an electron from a filament, they pluck part of the quantum wave function of an electron from an atom using a very intense laser pulse. The electron is then accelerated and slammed back into the ion, releasing its energy as an x-ray photon. Since the laser field controls the motion of the electron, the x-rays emitted can retain the coherence properties of a laser, Murnane said.Being able to build a tabletop x-ray laser is just the beginning, said Kapteyn."An analogy that is pretty close to what is going on in this field is the MRI, which started as just a fundamental investigation,” said Kapteyn. “People then started using it for microscopy, and then it progressed into a medical diagnostic technique.”