Unveiling nonlinear regimes of light amplification in fused silica with femtosecond imaging spectroscopy

We observe and characterize two nonlinear light-amplification regimes with unique spectral properties and fluence dependencies in laser-excited fused silica using ultrafast pump-probe imaging spectroscopy. The two underlying mechanisms are switchable within 150 fs via the time delay between an intense near-infrared femtosecond pump and an ultraviolet femtosecond probe pulse. While the earlier amplification regime is linked to the effect of laser amplification in excited dielectrics, and thus to the presence of conduction-band electrons, the latter can be directly related to the ultrafast formation of extremely long-lived self-trapped excitons. It is shown that, locally, the amplification can reach one order of magnitude. We discuss the temporal and spectral characteristics of the amplification as well as its dependencies on pump and probe fluences and form a complete picture within a tentative model.