We experimentally demonstrate ultrafast dynamic of generation of a strong
337-nm nitrogen laser by injecting an external seed pulse into a femtosecond
laser filament pumped by a circularly polarized laser pulse. In the pump-probe
scheme, it is revealed that the population inversion between the excited and
ground states of N2 for the free-space 337-nm laser is firstly built up on the
timescale of several picoseconds, followed by a relatively slow decay on the
timescale of tens of picoseconds, depending on the nitrogen gas pressure. By
measuring the intensities of 337-nm signal from nitrogen gas mixed with
different concentrations of oxygen gas, it is also found that oxygen molecules
have a significant quenching effect on the nitrogen laser signal. Our
experimental observations agree with the picture of electron-impact excitation.Comment: 9 pages, 5 figure
