Superradiant Amplification of Ultra-Short Laser Pulses
Superradiant Amplification (SRA) is a novel amplification scheme for ultra-short laser pulses. The pulse is amplified in plasma while it is counter-propagating to a pump pulse that delivers the energy. In other words: a portion of the long pump is compressed into the short signal pulse. The two pulses act on the plasma electrons in the interaction region such that they get arranged to form a density grid with a period of half the laser wavelength. The pump pulse is reflected partly from this structure by coherent Compton scattering. The reflection is similar to Bragg reflection. The coherence originates from the almost perfect bunching of the electrons at a single position within a period of the grid. Therefore all electrons see the same phase of the pump wave and radiate in phase. The density grid decays again as the signal pulse passes on and the backscattering ceases. Its transient nature prevents the signal from becoming longer. Instead the bunching process implies even a shortening while getting amplified. An input signal (seed pulse) with a duration of 25 fs and an intensity of about 1015 W/cm² will end with less than 10 fs duration and more than 1018 W/cm². Assuming that the cross section grows as well an energy gain of more than 1000 is possible. The necessary intensities of signal and pump and the plasma density scale with the laser wavelength. For a Ti:sapphire pump the initial intensities are about 1015..1016 W/cm2 and the plasma density is 1018-1019 W/cm2.
Stretching and recompression of the signal, as well as any broadband laser medium for the amplification is not necessary in the SRA scheme. Furthermore, high laser intensities will not cause any damage, since the plasma is already fully ionized and can withstand arbitrary electric field strengths. These advantages may prove SRA to be a superior alternative to chirped pulse amplification on the way to highest powers for ultrashort laser pulses.
SRA experiment at MPQ
Our object is to implement SRA in our lab using ATLAS-2 as pump. This way we get pulses of several ps duration with an energy of up to 200 mJ. A major effort is the generation of the seed pulse. The SRA scheme requires its duration to be less than 30 fs, its intensity to be higher than 1015 W/cm² and a slight red shift in frequency with respect to the pump pulse. We consider different approaches to generate the seed pulse:
Having generated pump and seed pulse they are focused from opposite directions into a gas jet, where a plasma channel is created by the leading low intensity part of the pump. For our first experiments the seed pulse has been created with the hollow fiber technique, this means the frequencies of pump and signal are equal. The leading part of the pump pulse forms a completely ionized plasma along a channel in a H2 gas jet. We have set up diagnostics to analyze the signal output after the experiment. We measure its energy with an Ulbricht sphere and take the spectrum. To get information about the temporal profile its autocorrelation trace is measured in a single shot device.
Fig. 2 Setup of the experiment using a hollow fiber to generate the seed pulse