Experimental study on counter-propagating filaments in air

We experimentally investigate the interaction of two collinearly counter-propagating filaments in air. The fluorescence is enhanced by 4 times due to the increase of the clamped intensity and electron (or plasma) density. The output energy at the end of a filament, the spectra of the excitation beams, and the fluorescent intensity are found to be dependent on the relative pulse delays between the counter-propagating pulses. The results indicate that the modulation of the filamentation-induced fluorescence intensity with another filament launched from the opposite direction is feasible, which provides a new perspective for studying the interaction of filaments and may improve the detection sensitivity for fluorescence sensing.