ARLINGTON, Va. – For the first time, a device small enough to fit on a single laboratory table has converted a coherent, directed light beam generated from more than 5000 low-energy photons into one high-energy x-ray photon.
“This device can be a valuable tool for nanoscience and nanotechnology, with the capability to image materials in 3-D and capture the fastest process relevant to function with very high space and time resolution,” JILA professor Margaret Murnane told Photonics Spectra
. She led the study with engineering and physics professor Henry Kapteyn in collaboration with scientists from Cornell University in Ithaca, N.Y., the University of Salamanca in Spain and Vienna University of Technology.
For quite some time, scientists have understood how to use nonlinear optics to combine low-energy laser photons to generate higher energy photons, but it was not until recently that they understood that this process could be pushed to an extreme limit – where 5000 mid-IR laser photons could be combined to generate a 1.5-keV photon, Murnane said.
The team, led by engineers from the National Science Foundation’s Engineering Research Center for EUV Science and Technology, focused intense pulses of IR light from a 4-µm tabletop laser into a high-pressure gas cell (up to 80 atmospheres of helium gas) and converted part of the original laser energy into a coherent supercontinuum of light that extends well into the x-ray region.
The emerging x-ray burst had much shorter wavelengths than the original laser pulse, making it possible to follow the tiniest, fastest physical processes in nature, including the coupling of electrons and ions in molecules as they undergo chemical reactions or the flow of charges and spins in materials.
“There are many practical applications for broad-bandwidth coherent x-rays,” Murnane said. “We are using similar light sources in the extreme-ultraviolet region (where they are already quite bright) to capture the fastest processes in materials and molecules. This understanding is needed to design and optimize next-generation electronics, data- and energy-storage devices, and medical diagnostics.”
The scientists used high-harmonic generation (HHG) in their experiment – a method discovered in the late 1980s when scientists focused a powerful, ultrashort laser beam into a spray of gas – and discovered that the output beam contained not only the original laser wavelength, but also a small amount of many different wavelengths in the ultraviolet region. Gas atoms ionized by the laser created the new UV wavelengths.
“Just as a violin or guitar string will emit harmonics of its fundamental sound tone when plucked strongly, an atom can also emit harmonics of light when plucked violently by a laser pulse,” Murnane said. “The laser pulse first plucks electrons from the atoms, before driving them back again where they can collide with the atoms from which they came. Any excess energy is emitted as high-energy ultraviolet photons.”
When HHG was first discovered, there was little science to explain it, but after many years of work, scientists eventually understood how very high harmonics are emitted. However, one significant challenge remained: The output HHG beams were extremely weak for most wavelengths in the x-ray region.
To turn the phenomenon into a useful x-ray source, Murnane, Kapteyn and their students developed a tabletop-scale instrument that had been a goal for laser science since shortly after the first laser was demonstrated by Theodore Maiman in 1960.
The task proved difficult because – unlike lasers, which get more intense as more energy is pumped into the system – in HHG, if the atoms are hit too hard by the laser, too many of the electrons are freed from the gas atoms, causing the laser light to speed up, Murnane said. If the laser and x-ray speeds are not matched, the x-ray waves cannot be combined to generate a bright output beam because the x-ray waves from different gas atoms destructively interfere with one another.
An actual coherent (laserlike) x-ray beam. In contrast to the
incoherent light emitted in all directions from a roentgen x-ray tube,
the x-rays produced by high-harmonic generation emerge as well-directed
laserlike beams. Courtesy of Tenio Popmintchev, JILA, University of
Colorado at Boulder.
To make x-ray waves from multiple gas atoms interfere constructively, the team used a relatively long-wavelength mid-IR laser and a high-pressure gas cell that also guides the laser light. The result was a bright x-ray beam that maintained the coherent, directed beam qualities of the laser that drives the process.
The HHG process works only when the atoms are hit “hard and fast” by the laser pulses, with durations nearing 10 to 14 s – a fundamental limit that represents just a few oscillations of the electromagnetic fields. This technology was developed in 1990 by Murnane and Kapteyn, using lasers that developed HHG-based light sources in the extreme-UV region in the 2000s.
Although scientists have used this technology to measure ever-shorter-duration light pulses, the two JILA professors were stuck on how to make coherent light at shorter wavelengths in the more penetrating x-ray region.
The experimental setup used to create a coherent version of the roentgen
tube in the soft x-ray region of the spectrum. When a long-wavelength
femtosecond laser is focused into this hollow waveguide filled with
high-pressure helium gas, part of the laser is converted into an
ultrafast, laserlike x-ray beam. Courtesy of Tenio Popmintchev, JILA,
University of Colorado at Boulder.
“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,” Kapteyn said. “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 were able to generate 1.6 keV – each x-ray photon was generated from more than 5000 infrared photons.”
The findings appeared in Science
In the future, the team hopes to break into the hard x-ray regime.
“Our best guess is that we will understand the underlying physics in five years, and then we will know better if we can generate hard x-ray beams from tabletop femtosecond lasers,” Murnane said. “The exciting thing is that we currently see no roadblock to achieving hard x-rays, and there are several routes that we can use. We need femtosecond lasers with millijoules of energy in the five- to twenty-micron-wavelength region, or other advanced pulse-shaping technologies, to test our understanding experimentally.”
For now, she said, her colleagues want to use soft x-rays for nanoscience and nanotechnology. They also intend to explore the limits of how far they can push the science of extreme nonlinear optics.
“By having multicolor x-rays all perfectly synchronized with respect to one another, we can probe many different molecules and materials and generate the shortest pulses to date,” Murnane said. “Even shorter zeptosecond pulses will be possible if we use longer laser wavelengths.”