Fig. 1. Schematic of dynamic compensation model^[34]

Fig. 2. Time evolution of degree of alignment inN2 at 50 K with different pulse durations and peak intensities^[38]. (a) τ=50fs,I0=2.5×1013W/cm2; (b) τ=1ps,I0=2.5×1012W/cm2; (c) τ=50ps,I0=2.5×1012W/cm2

Fig. 3. Optimization of alignment of O2 at 60 K by Fourier transform limited pulses with different peak intensities^[49]

Fig. 4. Detecting evolution process of molecular rotational wave packet in real time by air laser spectroscopy^[56]. (a) Typical spectrum of 391 nm nitrogen laser emission; (b) total emission intensity of P-branch band versus time delay between pump pulse and detection pulse and its Fourier transform; (c) signal intensities of R-branch lines versus time delay between pump pulse and detection pulse with details for time delay in 5-10 ps window shown in inset

Fig. 5. Three different single electron ionization mechanisms^[66]. (a) Multiphoton ionization; (b) tunneling ionization; (c) over-barrier ionization

Fig. 6. Experimental results and theoretical model of double ionization process. (a) Electron momentum distribution of sequential double ionization^［67］; (b) He ion yields under action of laser with pulse width of 100 fs and wavelength of 780 nm in which solid line is theoretical result and dotted line is experimental result^[71]; (c) schematic of three-step model^[72]

Fig. 7. Double ionization probability versus laser intensity driven by linear polarized light and circular polarized light^[81].(a) Mg atom; (b) Ar atom; (c) He atom

Fig. 8. Ionization angular distributions of three molecules. (a) Ionization angle distribution of oxygen molecule^[97]; (b) ionization angular distribution of carbon dioxide molecule^[97]; (c) ionization angular distribution of oxygen carbon sulfur (OCS) molecule^[98]

Fig. 9. Relevant experimental results of influence of intermediate state energy level on high field ionization process^[99]. (a) Relation between filament plasma density and two-color field time delay; (b) electronic excited state energy level of nitrogen molecule calculated by density functional theory

Fig. 10. Fluorescence spectra generated by femtosecond laser filamentation in air and electronic energy level diagram of nitrogen molecule. (a) Long pulse-induced breakdown and short pulse-induced fluorescence spectra of air obtained in atmosphere in which energy of each pulse is 5 mJ, and durations of short and long laser pulses are 42 fs and 200 ps, respectively^[112]; (b) energy level diagram of N2 and N2+^[125]

Fig. 11. Angular distributions of fluorescence radiations of 337 nm and 357 nm nitrogen molecules and 391 nm and 428 nm nitrogen positive ions under action of lasers with different wavelengths and energy values^[130]. (a) Fluorescence radiation angle distributions under action of 800 nm pulse; (b) fluorescence radiation angle distributions under action of 400 nm pulse

Fig. 12. Experimental results of angular distribution of fluorescence radiation measured by using laser filament with wavelength of 800 nm and pulse width of 35 fs^[132]. (a) Angular distributions of fluorescence radiation at different filament positions; (b) energy level transition diagram of 391 nm N2+ fluorescence; (c) N2 molecular orientation versus time when air plasma is ionized by pulsed laser with pulse width of 35 fs and intensity of 0.5×1014W/cm2 at room temperature; (d) spatial angular distributions of 391 nm fluorescence when N2 molecular orientation is horizontal and vertical, respectively; (e) spatial distributions of 391 nm fluorescence under different laser field intensities

Fig. 13. Time-domain spectrum and frequency-domain spectrum of higher harmonics^[135]. (a) Time-domain spectrum; (b) frequency-domain spectrum

Fig. 14. Schematic of “three-step model” for higher harmonic generation^[139]

Fig. 15. Generation mechanisms and emission spectra of two kinds of atomic air lasers. (a) Generation mechanism of oxygen atom air laser^[155]; (b) back-collected oxygen atom laser spectrum^[155]; (c) generation mechanism of nitrogen atom air laser^[156]; (d) emission spectrum of nitrogen atom laser^[156]

Fig. 16. Excitation and radiation characteristics of nitrogen molecule air laser. (a) Schematic of laser energy level of nitrogen molecule excited by argon atom collision^[162]; (b) laser output spectrum of nitrogen molecule^[162]; (c) electron energy distributions under different polarized light ^[131]; (d) laser radiation intensity distribution excited by different polarized light ^[131]

Fig. 17. Schematics of particle number inversion in nitrogen molecular ions established by 800 nm femtosecond laser pulse^[170]. (a) Molecules are aggregated into various electronic states of N2+ by tunnel ionization; (b) population redistribution of three electronic states of N2+ by coupling between ground state and excited state