Signatures of the Self-Similar Regime of Strongly Coupled Stimulated Brillouin Scattering for Efficient Short Laser Pulse Amplification

Plasma-based laser amplification is considered as a possible way to overcome the technological limits of\ud present day laser systems and achieve exawatt laser pulses. Efficient amplification of a picosecond laser\ud pulse by stimulated Brillouin scattering (SBS) of a pump pulse in a plasma requires to reach the self-similar\ud regime of the strongly coupled (SC) SBS. In this Letter, we report on the first observation of the signatures\ud of the transition from linear to self-similar regimes of SC-SBS, so far only predicted by theory and\ud simulations. With a new fully head-on collision geometry, subpicosecond pulses are amplified by a factor\ud of 5 with energy transfers of few tens of mJ. We observe pulse shortening, frequency spectrum broadening,\ud and down-shifting for increasing gain, signatures of SC-SBS amplification entering the self-similar regime.\ud This is also confirmed by the power law dependence of the gain on the amplification length: doubling the\ud interaction length increases the gain by a factor 1.4. Pump backward Raman scattering (BRS) on SC-SBS\ud amplification has been measured for the first time, showing a strong decrease of the BRS amplitude and\ud frequency bandwidth when SBS seed amplification occurs.\ud