Demonstration of ultrashort laser pulse amplification in plasmas by a counterpropagating pumping beam

We experimentally demonstrated amplification of ultrashort laser pulses (200 fs pulses) in microcapillary plasmas by a counterpropagating pumping beam. Energy amplification as large as a factor of 5 was observed. Importantly, a nonlinear pump depletion regime must have been accessed, since the parameters support not only the amplification of the pulse, but an absolutely growing instability if the pump is not depleted. Further indication of accessing the nonlinear depletion is indicated by the relative insensitivity to the initial pulse energy.