X-Ray Visionaries

The Kapteyn/Murnane group had the idea that it might be possible to produce bright, laser-like beams of x-rays using an ultrafast laser that fits on a small optics table. It was one of those “it probably can’t be done, but we have to try” moments that motivated them to put together a team that includes the Becker theory group, and 16 collaborators in New York, Austria, and Spain. The lead scientist on this effort, Dr. Tenio Popmintchev, was most concerned about the possibility of an explosion, because to generate x-rays at high photon energies, the laser needed to be focused into a fiber containing high-density helium gas at pressures as high as 80 atmospheres. Eighty atmospheres is 80 times the normal air pressure at sea level.

The process the team used to convert laser light into x-rays is called high harmonic generation, or HHG. It produces x-rays when electrons are first plucked from atoms by the laser, and then smashed back into their parent ions when the oscillating field of the laser reverses, like the motion of a boomerang. The atoms emit any excess energy gained by the electrons as higher-energy photons that are high harmonics of the original laser light. Harmonics of light are like the higher-frequency overtones heard when a piano key or guitar string is struck violently.

In a recent experiment reported in Science, the researchers showed that by using a laser with relatively long wavelengths (~4  microns), they could produce x-rays with short wavelengths (corresponding to high photon energies) that span from the ultraviolet into the soft x-ray region.

This is a surprising result. Scientists previously used visible laser wavelengths to produce laser-like beams of x-rays using HHG. However, this method limited the bright x-ray emission to lower energies in the x-ray spectrum. However, these x-ray waves could not add constructively and keep in step with the laser wave above a certain limit. In the new study, the x-rays emerged with a range of energies and wavelengths (or colors if our eyes could see x-rays). In theory, when the different x-ray wavelengths are added together, they could produce the fastest strobe light in existence - as short as 2.5 attoseconds. One attosecond is one quintillionth of a second, or 10^-18 s.

Such short bursts of light will make it possible to capture the fast dance of electrons and atoms inside molecules or materials with nanometer resolution in thick samples in three dimensions. This capability should allow scientists to understand the limiting speeds of electronics, energy harvesting, catalysis, or data storage. Until now, such questions could only be explored by large and expensive x-ray facilities.

In the future, it may be possible to generate higher energy x-rays from lasers, which would improve the crispness of medical x-rays, making it possible to use a pencil-thin x-ray beam instead of a broad beam (like those from light-bulbs). The technology may even allow scientists to produce even faster x-ray bursts — measured in zeptoseconds. A zeptosecond is one sextillionth of a second, or 10^-21 s. This is the time scale of things that happen inside the nucleus of an atom! For an idea how fast this is, it takes light 350 zs to travel the width of a hydrogen atom. Zeptosecond x-ray laser pulses will allow researchers to take an almost leisurely stroll through the quantum states of matter.

The 20 researchers involved in this complex project included senior research associates Tenio Popmintchev and Agnieszka Jaron-Becker, graduate students Ming-Chang Chen, Dimitar Popmintchev, and Susannah Brown, former research associate Paul Arpin, Fellows Andreas Becker, Margaret Murnane, and Henry Kapteyn as well as their colleagues from the Vienna University of Technology, Cornell University, and Universidad de Salamanca.