A novel amplifications scheme for 20-30-fs laser pulses has been demonstrated. The signal pulses are amplified in a plasma by few picosecond long, counterpropagating pump pulses. The signal and pump pulses arrange the plasma electrons to a density grating reflecting the pump light back into the signal pulse. In the superradiant regime, the plasma electrons scatter the pump pulse coherently. By an intrinsic mechanism a further shortening of the signal pulse to less than 10 fs is possible.The experiment was set up at the "Max-Planck-Institut für Quantenoptik", using the ATLAS laser system as source for the pump and input signal pulses. Both a regime with pump and signal pulse at the same initial wavelength and with a input signal shifted towards longer wavelengths with respect to the pump pulse were investigated. The unshifted signal pulse showed some initial amplification but further amplification was frustrated by the onset of Brillouin scattering. The superradiant regime was observed for the red-shifted input signal pulse. It is proven by 20 times energy amplification, the spectral broadening indicating the pulse shortening, and the breakup of the 80-fs long signal pulse caused by the very mechanism responsible also for the pulse shortening. An outlook is given for improved amplification in non-planar geometries.
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