Influence of laser polarization on plasma fluorescence emission during the femtosecond filamentation in air

•The evolution of plasma fluorescence emission under different laser polarizations in different focusing condition along the filament axis is investigated.•The influence of laser polarization state on the fluorescence from excited nitrogen molecules and molecular nitrogen ions during femtosecond filamentation is quite different as the focal length of external focusing lens varies.•The mechanism of plasma fluorescence emission during femtosecond filamentation are discussed based on the analysis of fluorescence under different laser polarizations in different focusing conditions, which will be helpful to the remote sensing and spectrum analysis.

The laser polarization state has a great influence on the plasma fluorescence emission during femtoseond filamentation in air. For the spectral lines from N2, in the case of focusing lens with longer focal length (f=100 cm), due to the impact excitation, circular polarization leads to stronger fluorescence emission when the laser energy is higher than the ‘energy threshold’ (2.0 mJ). As a lens with shorter focal length (f=40 cm) is used, a similar phenomenon can be observed, however, the ‘energy threshold’ is much lower, which is lower than 0.8 mJ. For the lines from N2+, especially for the 391 nm one, their emission is stronger in the linear polarization state. The mechanism of plasma fluorescence emission during femtosecond filamentation is discussed based on the analysis of these phenomena, which will be helpful to the remote sensing and spectrum analysis.