Los Alamos, NM--A new type of plasma-based optical switch could potentially be used to tailor high-energy laser pulses for physics research.
The Trident laser at Los Alamos National Laboratory (LANL) delivers a 200 TW power pulse in just 0.5 ps. When the laser intensity reaches a certain threshold, relativistic transparency in plasma can turn the initially opaque plasma transparent in less than a tenth of a picosecond.
High-energy lasers are used to drive plasmas in next-generation particle accelerators and x-ray beams. One shortcoming of these beams is that their pulses don't have an instant turn-on time, but are limited by the gradual rise of laser power from zero to its maximum level. Using an optical switch, this ramp-up time can be reduced to less than a tenth of a picosecond, delivering peak laser power to the plasma on a faster time scale.
Relativistic transparency
When a laser beam is incident on plasma, electrons in the plasma react to the laser light to cause absorption in the plasma. But when the laser is powerful enough to accelerate electrons close to the speed of light, the mass of the electrons increases, making them "heavier." These heavier electrons cannot react quickly enough; hence the laser beam propagates through the plasma.
Now, for the first time, scientists at LANL and Ludwig-Maximilian Universität (Munich, Germany) have been able to make a direct observation of relativistic transparency in thin plasmas using a frequency-resolved optical gating (FROG) device. The discovery was made possible by two key capabilities: the ability to fabricate carbon foils a few nanometers thick to produce thin plasma, and the elimination of optical noise preceding the Trident laser pulse on a few-picosecond timescale.
Initially, the researchers observed pulse-shortening due to relativistic transparency and consistent spectral broadening. Later, they also measured the shape of the laser pulses incident on and transmitted through the plasma to directly observe the transparency. The transmitted laser pulse is roughly half the duration of the incident laser pulse, with a transparency turn-on time around a fifth of a picosecond. The experimental results are consistent with that of computer simulation, except for the loss of fast turn-on time due to propagation effects arising from diffraction. Efforts are currently underway to eliminate diffraction limitations to observe the true turn-on time.
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John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.