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