Experimental Demonstration of Superradiant Amplification of Ultra-Short Laser Pulses in a Plasma

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.