Spectrum- A nd time-resolved investigation of pre-excited argon atoms

It has been observed that in a pre-excited medium produced by femtosecond laser pulses, nonlinear interaction can be dramatically enhanced. The underlying mechanism is still under debate because of the lack of detailed information on the species, energy states, and temporal dynamics of the excited medium. Here, we report a dramatic enhancement of seventh-harmonic generation in pre-excited argon atoms. By measuring the evolution of the enhanced signal with the propagation distance, we are able to separate the contributions from the improved phase matching and the enhanced nonlinear susceptibility. The time- A nd spectrum-resolved measurements clearly show that generation of excited states is responsible for the enhanced nonlinear susceptibility. The maximum population of excited states on the picosecond and nanosecond timescales can be attributed to two possible pathways.