Lasers approach X-ray limits

17 Jun 2002

A worldwide team of researchers has produced a laser that could light the way to unparalleled X-ray brightness.

Researchers from University of Lund, Sweden, US-based Argonne National Laboratory and Russia's Budker Institute of Nuclear Physics have made a free-electron laser that exhibits self-amplified spontaneous emission, growth and saturation at visible and ultraviolet wavelengths. Their calculations show that this feat is also possible at lower X-ray wavelengths, which means that very bright X-ray lasers could soon be used to study structures at an atomic scale and the dynamics of chemical reactions.

Self-amplified spontaneous emission (SASE) in a free-electron laser is widely held as being key to generating bright coherent X-rays. Past studies have demonstrated SASE at visible and ultraviolet wavelengths, but achieving this at X-ray wavelengths has baffled scientists. However, by directing an electron beam through a line of high-quality magnets, S Milton and his team believe that they have moved a step closer (Sciencexpress 17 May 2001).

"We passed a high-energy electron beam through the periodic magnetic field of a long series of undulator magnets," explained Milton. "The radiation produced [by the interaction of the beam and the magnetic field] grows exponentially in intensity until it reaches a saturation point."

The researchers were also able to "tune" the wavelength of the laser beam from visible to ultraviolet by scaling it with the electron beam energy.

"Confirming the theory is an essential step towards assuring ourselves that SASE to saturation is possible at short X-ray lengths," said Milton. "It is through experiments such as these that the possibility of building a high-brightness tunable coherent X-ray source will shortly become a reality."