Rainbows of Soft X-Rays

The vision of a tabletop x-ray laser has taken a giant step into reality, thanks to Tenio Popmintchev, Ming-Chang Chen and their colleagues in the Kapteyn/Murnane group. By focusing a femtosecond laser into a gas, Popmintchev and Chen generated many colors of x-rays at once, in a band that stretched from the extreme ultraviolet into the soft x-ray region of the electromagnetic spectrum, spanning wavelengths of ranging from about 6 to 2.5 nm. This broad x-ray band has so many different colors that all the waves can be added together to form the shortest strobe light in existence.

This light may be as short as 10 attoseconds [or 10 quintillionths of a second (10^-17 s)]. Proof of the laserlike nature of the soft x-ray beams was featured on the cover of the October 22, 2010, issue of Physical Review Letters. 

The new rainbow of x-rays spans the “water window” region of the spectrum, where biological molecules rich in carbon, hydrogen, and nitrogen can be clearly imaged without being obscured by absorption due to water. Researchers can use this new technology to probe the internal structures of cells or nanostructures with x-rays produced by a tabletop setup. The new x-ray laser is, in essence, a coherent version of the Roentgen x-ray tube.

Remarkably, the Kapteyn/Murnane group accomplished more than just opening up the water window region to tabletop laser x-rays. Popmintchev has already predicted that by using longer driving laser wavelengths, high-harmonic generation (HHG) can reach the hard x-ray region of the spectrum. [HHG was described “Exotic Probes” in the Spring 2008 issue of JILA Light & Matter (http://jila.colorado.edu/content/exotic-probes)]. And, incredibly, the even-broader x-ray rainbows that will be generated there have the potential to produce zeptosecond (10^-21 s) laser pulses!

The invention of the ultrafast tabletop source of laserlike x-rays is opening up whole new worlds in research, including capturing electron dynamics in molecules and materials, developing high-resolution nanoscale microscopes, understanding the limiting switching speeds in magnets, following energy and charge transport at the nanoscale, imaging chemical reactions at the level of atoms and electrons, and the imaging of subcellular structures.

Nature Photonics recognized the importance of x-ray laser research in its December 2010 issue highlighting the generation and use of ultrafast, coherent x-rays. The issue included cover art by JILAns Brad Baxley and Popmintchev, who was also the first author of an invited article on bright coherent x-ray generation by the Kapteyn/Murnane group. - Julie Phillips