When ultrashort intense laser pulses propagate in optically transparent media, such as in air, intensity-dependent nonlinear effects, for example, Kerr focusing, absorption, and gas ionization, lead to pulse reshaping and the formation of filaments, which can sustain nonlinear effects over several tens of meters. [1,2] Laser filamentation in ambient air has attracted considerable attention in recent years owing to its central role in several applications, such as the formation of stationary localized waves, [3] electric-discharge steering, [4][5][6] machining, [7] control and guiding of lightning, [8][9][10][11] atmospheric control, [12][13][14] remote sensing, [15][16][17] pulse compression, [18] optical communication, [19][20][21][22] and the generation of coherent radiation in hardly accessible spectral regions such as the deep UV and THz. [23][24][25][26][27][28][29] Filaments modify the gas density in which they propagate through different nonlinear light-matter interaction pathways, such as by excitation of molecular
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