Popmintchev and JILA graduate student Ming-Chang Chen worked out conditions that enable X-ray waves from many atoms in the gas to interfere constructively. The key was to use a relatively long-wavelength, mid-infrared laser and a high pressure gas cell that also guides the laser light. The resulting bright, X-ray beams maintain the coherent, directed beam qualities of the laser that drives the process.
The HHG process is effective only when the atoms are hit "hard and fast" by the laser pulses, with durations nearing 10-14 seconds--a fundamental limit representing just a few oscillations of the electromagnetic fields. Murnane and Kapteyn pioneered the technology for generating such light pulses in the 1990s, and used those lasers to develop and utilize HHG-based light sources in the extreme-ultraviolet (EUV) region of the spectrum in the 2000s. However, while researchers were using those lasers and the HHG technique to measure ever-shorter duration light pulses, they were stymied in how to make coherent light at shorter wavelengths in the more penetrating X-ray region of the spectrum.
The new paper in Science, under lead author and senior research associate Popmintchev, demonstrates that breakthrough, showing that the understanding of the HHG process the researchers developed is broadly valid.
"We would have never found this if we hadn't sat down and thought about what happens overall during HHG, when we change the wavelength of the laser driving it, what parameters have to be changed to make it work," added Kapteyn. "The amazing thing is that the physics seem to be panning out even over a very broad range of parameters. Usually in science you find a scaling rule that prevents you from making a dramatic jump, but in this case, we
|Contact: Josh Chamot|
National Science Foundation