Pure as the Driven X-Ray

Seeding x-ray free electron lasers with customized electron beams produces incredibly stable laser pulses that could enable new scientific discoveries.

lasers
A SLAC-led research team manipulated a beam of electrons (from top left to bottom right) with conventional laser light (red) in a way that could produce purer, more stable pulses in x-ray lasers. Image courtesy of SLAC National Accelerator Laboratory

The Science

New electronic devices and energy solutions depend on scientists clearly seeing atoms and molecules at work. Modern x-ray free-electron lasers do just that using electrons to produce intense x-ray pulses that provide atomic-resolution snapshots. To produce stable, organized pulses, scientists demonstrated a new scheme called echo-enabled harmonic generation (EEHG) at the SLAC National Accelerator Laboratory in California. They used conventional lasers to precisely manipulate certain electron beams to create brighter, purer, more stable pulses of light.

The Impact

The team’s new technique, EEHG, is a promising method to probe nature at the nanometer scale with femtosecond resolution. This method could allow scientists to obtain an atomic-resolution view of incredibly fast reactions that occur in everything from your laptop computer, to fuel production, to the heart of a star.

Summary

Modern x-ray free-electron lasers use high-energy electrons to produce intense x-ray pulses to study the structure and behavior of matter. However, typically the individual x-ray pulses, while lasting only a few millionths of a billionth of a second, are composed of many still smaller and random sub-pulses that are temporally uncorrelated. To produce stable and fully coherent single x-ray pulses, scientists demonstrated a new scheme, EEHG, in proof-of-principle experiments at the SLAC National Accelerator Laboratory. The EEHG uses the celebrated echo effect in physics, wherein the laser perturbations remain deeply encoded in the electron beam distribution, only to reemerge at high harmonics through a recoherence effect. The well-defined spectra of fully coherent pulses is required for precision probing techniques such as high-resolution x-ray spectroscopy. Proposed in 2009 at SLAC by Gennady Stupakov, EEHG harnesses the highly nonlinear phase mixing of the celebrated echo phenomenon to generate coherent harmonic density modulations in the electron beam with conventional lasers. The researchers performed experiments at the Next Linear Collider Test Accelerator facility at SLAC. They demonstrated EEHG up to the 75th harmonic, producing 32-nanometer light from a 2400-nanometer laser. At this high harmonic order, EEHG looks to be a promising way to produce even brighter soft x-rays with ultraviolet lasers. Results show the potential of laser-based manipulations to achieve precise control over the coherent spectrum in future x-ray free-electron lasers, like the upcoming Linac Coherent Light Source II, enabling new opportunities in science.