Dynamic wavelength switching of a remote nitrogen or air laser with chirped femtosecond laser pulses

We experimentally investigate the influence of the chirp of 800 nm Ti:sapphire pump laser pulses on the lasing behaviors of N₂⁺ ions for the transitions between the excited electronic B²∑_u⁺ state (v' = 0) and the ground X²∑_g⁺ state (v = 0,1) at wavelengths of 391 and 428 nm. We found that as the chirp of the pump laser pulses varies from negative to positive, the intensities of the lasing signals at 391 and 428 nm show different chirp-dependent behaviors. Namely, the coherent emission at 391 nm reaches the maximum when the chirp of the pump pulse is transform-limited; whereas the 428 nm emission becomes strongest when the pump pulse is negatively chirped. This observation is ascribed to different self-generated seed sources produced in the plasma channel, which is verified by introducing an external seed for a pump-probe measurement. Our finding enables switching of wavelengths of remote lasing emissions of N₂⁺ by manipulating the chirp of the pump laser pulses.