Fig. 1. Lateral spatial distribution of fluorescence induced by femtosecond laser filamentation. (a) Measuring length of filament by backward-fluorescence time-of-flight method^[37]; (b) multiple self-focusing phenomenon^[41] and (c)-(e)competitive processes of optical filaments^[40] observed in methanol solution dissolved with dyes

Fig. 2. Effects of laser parameters on lateral fluorescence spatial distribution. Lateral nitrogen fluorescence spectra induced by linearly and circularly polarized pumped laser filamentation at laser energies of (a) 250 μJ and (b) 8.3 mJ^[48]; (c) lateral spatial distribution of nitrogen fluorescence under different laser repetition rates^[51]; (d) fitting curve of fluorescence signal intensity versus pulse period^[51]

Fig. 3. Effects of molecular alignment on fluorescence induced by filament. (a) Calculated molecular alignment versus pump-probe delay in N₂, O₂, and air^[54]; (b) lateral distributions of fluorescence induced by filament under different molecular alignments^[54]; (c) relative intensity of nitrogen fluorescence versus polarization direction of pump light^[39]; (d) spatial angular distribution of 391 nm fluorescence signal when N₂ molecule orientation is parallel or perpendicular to laser polarization direction^[39]; (e) Time-averaged azimuthal distribution of 391 nm fluorescence signal under different laser intensities^[39]

Fig. 4. Anti-correlated plasma density and THz intensity during two-color field filamentation^[55]. (a) THz production efficiency versus time delay of two-color field obtained by experiment and simulation; (b) lateral plasma density distribution versus time delay of two-color field

Fig. 5. Laser intensity inside filament deduced by ratio of 391 nm to 337 nm nitrogen fluorescence signal. (a) Energy level diagram of N₂ and N2+^[57]; (b) typical nitrogen fluorescence spectrum^[57]; (c) peak light intensity in filament versus laser energy under different focusing conditions^[59]; (d) ratio of 391 nm to 337 nm nitrogen fluorescence signal and laser intensity in filament versus laser transmission distance^[38]

Fig. 6. Characterizing plasma density and temperature by lateral fluorescence signal. (a) Oxygen atomic line^[60]; (b) plasma density and temperature inside filament^[61]

Fig. 7. Regulating filament by spatiotemporal focusing^[34]. (a) Schematic of spatiotemporal focusing experimental device; (b) lateral spatial distributions of 337 nm fluorescence signal with (solid line) and without (dotted line) chirp

Fig. 8. Lengthening filament by spatiotemporal phase shaping. (a) Schematic of experimental setup for generating ring beam^[73]; (b) comparison of filament induced by different beams^[73]; (c) diagram of experimental setup for coupling spatial dispersion and time chirp^[75]; (d) schematic of experimental setup for generating curved filament^[76]; (e) lateral spatial distribution of curved filament^[76]

Fig. 9. Controlling multiple filaments by different methods. (a) Using auxiliary beam^[90]; (b) using deformable mirror^[86]; (c) using phase plates and generated multiple filaments^[85]; (d) changing beam ellipticity^[88]

Fig. 10. Gain curves of backward ASE fluorescence. (a) N₂ fluorescence at 357 nm^[74]; (b) CN fluorescence at 388 nm^[93]; (c) OH fluorescence at 308.9 nm^[94]; (d)NH fluorescence at 336 nm^[95]; (e) oxygen atom fluorescence at 845 nm^[91]

Fig. 11. Spatial distributions of backward fluorescence. (a) Spatial distribution of backward oxygen atom laser at 845 nm^[91]; (b) spatial distribution of nitrogen laser generated by argon collision excitation^[97]; (c) backward angular distributions of 357 nm nitrogen fluorescence signal with input energies of 5, 10, and 15 mJ^[100]; (d) backward angular distributions of 337 nm nitrogen fluorescence induced by filament^[101]; (e) backward angular distribution of sodium fluorescence at 589 nm in NaCl aerosols with different mass fractions^[101]; (f) comparison of backward angular distribution of NaCl aerosol fluorescence and angular distribution of Mie scattering signal intensity^[101]

Fig. 12. Polarization-dependence backward fluorescence signal at 337 nm. (a) Change of fluorescence signal at 337 nm with rotation angle of quarter-wave plate^[49]; (b) backward fluorescence at 337 nm versus incident laser energy under different laser polarization conditions^[49]; (c) spatial distribution of seed pulse^[102]; (d) spatial distribution of backward ASE fluorescence signal at 337 nm^[102]; (e) spatial distribution of 337 nm optical signal generated by seed optical amplification^[102]

Fig. 13. Forward air laser and its polarization characteristics^[57]. (a) Tunable multi-wavelength air laser generated by pumping light with different wavelengths; (b) gain curve of 391 nm air laser

Fig. 14. Far-field spatial distributions of forward air laser. (a) Far-field distribution of 337 nm air laser generated by third harmonic excitation of picosecond laser^[108]; (b) far-field distribution of 428 nm air laser self-excited by filament-induced white light^[104]; (c) far-field spatial distribution of 337 nm air laser excited by 3.9 μm mid-infrared laser^[109]; (d) spatial distributions of 391 nm air laser self-excited by filament under different atmospheric pressures^[105]; (e) far-field spatial distributions of 428 nm air laser self-excited by filament under different atmospheric pressures^[105]; (f) simulated far-field angular distribution of air laser at 391 nm^[110]; (g) divergence angle of air laser versus external focusing condition ^[110]