As part of a new $4.5 million initiative, Wendell T. Hill III will help develop instrumentation and measurement procedures to measure a laser's peak intensity and focal spot quality at full power, on every shot.
For a fraction of a trillionth of a second, the world’s most powerful lasers can concentrate light into a tiny spot a few millionths of a meter across. In that instant, focused laser pulses recreate conditions that can only otherwise be observed in astrophysical settings—the crushing interiors of stars and giant planets and the extreme electromagnetic fields of the early universe. Consequently, the focal volume of these lasers provides a unique opportunity for physicists to investigate a host of fundamental properties of nature as well as mysteries of the universe.
However, such studies are inhibited today because physicists have no reliable way to measure the intensity of these focused pulses each time the laser fires. Traditional measurement tools that rely on placing a meter in the focus will not work because all known materials will be instantly destroyed at the anticipated intensities. Without precise intensity measurements, these lasers cannot be used to compare theory against experiment, which is essential to understanding the underlying physics.
Wendell T. Hill III, professor and director of the Institute for Physical Science and Technology at the University of Maryland, is working to tackle that problem.
Hill will lead a team in the Diagnostics for Extreme-LIGHT (DELIGHT) program, a new $4.5 million, six-institution initiative funded by the U.S. National Science Foundation (NSF) and led by University of Nevada, Reno Physics Chair Thomas White. The program will build precision instruments and computer models needed to turn extremely powerful laser shots into science that researchers can quantify.
“These machines are extraordinary, but a laser you cannot measure is a laser you cannot do quantitative physics with,” White said. “DELIGHT is about building a standardized toolkit so researchers can turn these massive flashes of energy into reliable data. For my team, that means using these intense lasers to generate specialized X-ray sources we need to diagnose and see inside extreme states of matter in real time.”
DELIGHT comprises six complementary efforts, including real-time feedback systems that diagnose and correct distortions in a laser's focus at full power, detectors fast enough to keep pace with lasers that fire many times a second and simulation tools that predict what a given experiment should see.
“The cross-fertilization inherent in working together gives us a chance to design and build a larger collection of interesting and useful tools than would be possible if we were working individually,” Hill said.
Hill will lead the effort on pulse optimization and focal spot assessment, which targets the measurement problem directly. The most common approach to determine the focal-spot intensity is to make a series of measurements at lower powers and calculate what the intensity would be when the power is tuned to full strength.
“Such an approach requires one to hope that things at full power work the same as they do at low power; there is strong evidence that this is not always the case,” Hill said. “At full power, beam distortions and other effects can cause the laser to work differently than it does at low powers.”
Hill will work with chemical physics Ph.D. student Rohan Mahnot and physics major Lionel Ngassiki on a method that turns the focus’ destructive power into part of the measurement procedure. The team will inject a small amount of a rare gas—e.g., argon, krypton or xenon—where the laser beam comes to a focus. The laser tears electrons off the gas atoms and hurls them outward. The most tightly bound electrons break free only at the most intense part of the pulse. The energy those electrons carry away serves as a measuring stick for just how intense the laser pulse was.
The shape of the spray, or how the electrons were hurled, also matters. A symmetric pattern indicates a uniform, tightly focused spot. A lopsided pattern generally means the beam is flawed; some of the energy is wasted and does not contribute to the intensity in the focal spot. Hill’s team will develop techniques that will allow laser operators to visualize exactly what needs to be corrected to achieve the most intense field possible in the focal spot.
“Because every laser of this class has the same problem, the techniques we’re developing should serve as a universal way to measure and maximize their intensities,” Hill said.
The funding for the DELIGHT program comes as a new generation of facilities takes shape. The NSF Zettawatt-Equivalent Ultrashort pulse Laser System (ZEUS) is now operating at the University of Michigan, and the proposed NSF Optical Parametric Amplifier Line (OPAL) facility at the University of Rochester would be among the most intense light sources ever built.
"User facilities with high-power lasers will enable tremendous scientific and technological advances over the next decade,” said NSF Plasma Physics Program Director Vyacheslav "Slava" Lukin. “Team efforts like DELIGHT maximize the scientific value of these facilities by developing systematic measurement capabilities for lasers and plasma, while providing unique training opportunities for students and early-career researchers."
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This article was adapted in part from text provided by the University of Nevada, Reno.
