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

We experimentally demonstrated amplification of ultrashort laser pulses (20 0 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 param eters support not only the amplification of the pulse, but an absolutely gr owing instability if the pump is not depleted. Further indication of access ing the nonlinear depletion is indicated by the relative insensitivity to t he initial pulse energy.