Using a plasma medium, scientists have amplified short laser pulses of picojoule-level (pJ-level) energy up to 100 millijoules (mJ), a gain of more than eight orders of magnitude over existing capabilities. The Vulcan (TAW) laser target area at the Central Laser Facility, with the Raman amplification setup. Courtesy of University of Strathclyde. A large Raman backscattered energy of up to 170 mJ was measured for a monochromatic 70 joule (J) pump pulse incident at an angle of 175 degrees. Injected pJ seed pulses were observed to grow at a rate corresponding to a gain coefficient of 180 cm-1. Seventy mJ of the backscattered pump was attributed to amplification of noise, which was proportional to the pump intensity and inversely proportional to the square root of the plasma density. Measurements of amplified noise Raman scattering (N-RS) along the pump axis indicated that more than 10 percent of the pump energy could be backscattered, showing the potential of the amplifying medium for high-efficiency amplification. To avoid the deleterious effects of N-RS, researchers said that tailoring of the plasma density and/or a frequency chirped pump may be required. The team also noted that short laser pulse amplification starting in the Raman regime was limited by chaotic nonlinear behavior for a beatwave above a certain amplitude. The team also observed that the seed profile was dominated by the pump profile, which it mimics. To create the high-gain, high-power laser amplifier, a relatively long, high-energy laser pulse was made to collide in plasma with a short, very low-energy pulse. At the point of collision, a beatwave was formed. The light pressure of the beat pattern drove plasma electrons into a regular pattern that mimicked the beatwave. This multilayer pattern acted as a very high-reflectivity, time-varying mirror that swept up the energy of the high-energy pulse, reflecting it into the low-energy pulse, thus amplifying the low-energy pulse and compressing its energy into an ultra-short duration pulse of light. For the experiments, the research team, from the University of Strathclyde, used 150 J pulses from the Vulcan laser system located at the Science and Technology Facilities Council's Central Laser Facility (CLF) in Oxfordshire, England. Over the course of the experiments, the Strathclyde team worked closely with CLF staff to adapt the Vulcan laser so that two different color lasers could exchange energy in the plasma medium. The measured gain coefficient of 180 cm-1 is more than 100 times larger than what can be achieved from existing high power laser system amplifiers based on solid-state media. “Our results are very significant in that they demonstrate the flexibility of the plasma medium as a very high gain amplifier medium,” said professor Dino Jaroszynski. "We also show that the efficiency of the amplifier can be quite large, at least 10 percent, which is unprecedented and can be increased further. However, it also shows what still needs to be understood and controlled in order to achieve a single-stage high-gain, high-efficiency amplifier module. “One example of the challenges that we still face is how to deal with amplification of ‘noise’ produced by random plasma fluctuations, which is exacerbated by the extremely high gain,” said Jaroszynski. “This leads to undesirable channels for the energy to go. We are making excellent progress and believe that we are in an excellent position to solve these problems in our next experimental campaigns.” Gregory Vieux, who led the CLF research team, said, “Plasma is a very attractive medium to work with. It has no damage threshold since it is already a fully broken-down medium, therefore we can use it to amplify short laser pulses without the need for stretching and recompressing. Another advantage is that further compression during the amplification is theoretically possible. This could pave the way for the development of the next generation of laser systems delivering ultra-intense and ultra-short pulses and at a fraction of the cost of existing lasers. “Still, we are not quite there yet," Vieux continued. "The scheme relies on controlling the Raman instability. It has such a large growth factor that it can develop and grow from small plasma fluctuations.” By harnessing waves in plasma, the size of laser amplifiers could be greatly reduced, while providing a route to much higher peak powers than possible now, exceeding the petawatt range to possibly reach exawatts. The team believes this to be a worthy goal, because very intense laser pulses can be used for fundamental studies, such as accelerating particles, helping drive nuclear fusion or even extracting particles from vacuum and recreating the conditions inside stars or the primordial condition of the universe in the laboratory. The research was published in Scientific Reports (doi:10.1038/s41598-017-01783-4).