Lasing and fluorescence of air plasma in presence of an external electric field

Cavity-free lasing of nitrogen molecules pumped by intense femtosecond laser pulses holds the potential for remote sensing of electric fields. Here we compared the influence of an external direct current (DC) electric field on both the directional lasing radiation and omnidirectional fluorescence emission of neutral nitrogen molecules. It was found that the forward lasing radiation in both pure nitrogen gas and ambient air shows a sensitive dependence on the direction and strength of the DC field, while the fluorescence is not influenced. The effect of pump laser polarization was also investigated. The distinct behavior of the lasing and fluorescence in response to the DC field was attributed to their different dependences on the population distribution of excited nitrogen molecules. This study consolidates the method for standoff detection of electric field with an air lasing effect in the atmosphere.