Backward Raman amplification of a short laser pulse in a plasma waveguide is demonstrated. With a guided seed pulse of 0.8-microJ energy and a pump pulse of 345-mJ energy in a 9-mm-long optically preformed plasma waveguide, 910-fold energy amplification is achieved. Heating of the plasma by the long pump pulse is identified to be a key issue for plasma-waveguide-based backward Raman amplifiers.

As an intense laser pulse propagates through an underdense plasma, the strong ponderomotive force pushes away the electrons and produces a trailing plasma bubble. In the meantime the pulse itself undergoes extreme nonlinear evolution that results in strong spectral broadening toward the long-wavelength side. By experiment we demonstrate that this process can be utilized to generate ultrashort midinfrared pulses with an energy three orders of magnitude larger than that produced by crystal-based nonlinear optics. The infrared pulse is encapsulated in the bubble before exiting the plasma, hence is not absorbed by the plasma. The process is analyzed experimentally with laser-plasma tomographic measurements and numerically with three-dimensional particle-in-cell simulation. Good agreement is found between theoretical estimation, numerical simulation, and experimental results.