Fig. 1. Ultrafast laser filamentation formation observed in laboratory. (a) In air; (b) in quartz glass

Fig. 2. Schematic of the moving focus model

Fig. 3. Spatial distributions of laser intensity inside the filament recorded by different methods.(a) Ablation depth of glass plate^[44]; (b) gray scale of thermal-sensitive paper^[46]

Fig. 4. Typical fluorescence spectra of nitrogen molecules induced by filament^[49]

Fig. 5. Multiple self-focusing phenomena observed by three photon fluorescence excited by filament in the methanol solution with dissolved dye^[50]

Fig. 6. Three typical dynamic competition situations of multifilament observed by three photon fluorescence excited by filament in the methanol solution with dissolved dye^[51]

Fig. 7. Filament length measurement results. (a) Time-of-flight measurment method of backward nitrogen fluorescence^[55]; (b) measurement method of the lateral signals of ultrasound and microwave^[56]

Fig. 8. TF-FROG experiment^[58]. (a) Schematic of experimental setup; (b) measurement results of the time-domain envelope, spectra, and phase of the laser in filament

Fig. 9. Experiment of free electron density measured by atomic fluorescence spectroscopy method^[64]. (a) Experimental setup; (b) fluorescence spectrum of oxygen atom excited in filament at 777 nm and its Voigt line fitting

Fig. 10. Numerical simulation results of the spatial and temporalvariation of laser pulse in the process of air filament^[2]

Fig. 11. Numerical simulation result of the laser angular spectrum distribution in filament^[70]

Fig. 12. Numerical simulation results of the interaction between diffraction and light Kerr self-focusing during the laser-filament process^[73](laser mode at z=41 cm is near ideal Gaussian)

Fig. 13. Laser intensity inside air filament based on self-guiding model^[22]

Fig. 14. Numerical simulation results of different diameter apertures inserted in the middle of filament (when the diameter of aperture is larger than 2 mm, it hardly affects the generation of filament; when the diameter is smaller than 2 mm, the filament will be cut off)

Fig. 15. Numerical simulation results of different diameter obstacles inserted in the middle of filament (the filament can pass through submillimeter obstacle)

Fig. 16. Supercontinuum spectra induced by the laser filament in different media^[90]

Fig. 17. Energy fluctuation of signal light during the four-wave maxing process (FWM) in filament^[33]. (a) Output energy fluctuation of the pumped Ti∶sapphire femtosecond lasers; (b) energy fluctuation of input infrared signal light; (c) output energy fluctuation of visible light generated by the FWM below the critical power for self-focusing in air; (d) output energy fluctuation of visible light generated by the FWM above the critical power for self-focu

Fig. 18. Light spot distributions in the air captured by ordinary digital camera^[100]. (a) Spot distribution at transmission distance of 18 m (multifilaments are forming); (b) spot distribution at transmission distance of 60 m (supercontinuum spectra of multifilament radiation interfere)

Fig. 19. New “hot spot” generated by interference of the angular radiation of two filaments^[99]. (a)(b) Experimental results; (c)(d) numerical simulation results

Fig. 20. Independent multifilaments generated by the π phase plate^[107]

Fig. 21. Stimulated amplification of fluorescence and signal lasers in filaments. (a) Stimulated amplification of backscatteredfluorescence intensity of N₂ in filament^[108]; (b) amplification of signal light with different wavelengths in filament^[111]

Fig. 22. Experimental setup for adjusting the spatial position of filament by optical telescope system (the setup includes a lidar device that collects backward fluorescence signal)^[113]

Fig. 23. Influence of the competition relationship between optical filament and breakdown on the direct writing waveguide in glass with filament(only the experimental parameters corresponding to region 4 can be used to obtain high quality waveguides)^[118]

Fig. 24. Filament intensity control method based on spatiotemporal focusing method^[129].(a) Experimental setup; (b) spectral numerical simulation results

Fig. 25. Schematic of spatiotemporal phase control of filaments based on spatial dispersion and temporal chirp coupling^[131]

Fig. 26. Experimental results of using spatial light modulator to generate phase-nested beam to extend the length of filaments^[137]. (a) Experimental setup; (b) phase distribution of phase-nested beam; (c) simulation result of interference pattern; (d) experiment result of interference pattern; (e)-(h) experiment results of the filament extended by phase-nested beam

Fig. 27. Nanosecond laser assisted pulse technology^[148]. (a) Experimental setup; (b) curve of fluorescence signal of filament radiation with the energy of the nanosecond laser

Fig. 28. Deep ultraviolet ultrafast laser produced by FWM in filaments^[152]. (a) Experimental setup; (b) output deep ultraviolet ultrafast laser spectrum

Fig. 29. Theoretical numerical calculation results of the TW femtosecond laser propagation from the orbit at an altitude of 400 km toward earth's surface^[30]. (a')-(a'?) Beam diameter as a function of altitude; (b)(c) maximum intensity and plasma density versus altitude

Fig. 30. Characteristic fingerprints of simulation sample of air pollution sources induced by filament. (a) Three main components of Freon^[35]; (b) solid protein powder^[168]; (c) metal sample^[169](the inset is the 590 nm fluorescence spectra of depleted and highly enriched uraniums induced by laser filament^[170

Fig. 31. Phenomena induced by filament in air. (a) Artificial rainfall^[166] ; (b) snowfall^[39]

Fig. 32. Application of THz wave generated by filaments in atmospheric remote sensing^[174]