Fig. 1. Schematic of four-wave mixing

Fig. 2. Schematic of geometric phase matching of non-collinear four-wave mixing

Fig. 3. Schematic of the generation of femtosecond VUV pulses by two-photon near-resonant four-wave difference-frequency mixing. (a) Schematic of two-photon near-resonant four-wave difference-frequency mixing in argon; (b) schematic of a femtosecond laser system with a center wavelength of 774 nm; (c) schematic of tunable femtosecond VUV laser source system and femtosecond light detection system^[40]

Fig. 4. Schematic of cascaded four-wave mixing in hollow-fiber^[26]

Fig. 5. Schematic of femtosecond VUV laser generated by direct four-wave mixing process in hollow-fiber^[42]

Fig. 6. Femtosecond VUV laser source. (a) Femtosecond VUV laser source and detection system^[25] using the direct four-wave mixing process 3ω+3ω-ω®5ω in argon; (b) femtosecond VUV laser source system^[45] using the direct four-wave mixing process 3ω+3ω-ω®5ω in krypton

Fig. 7. Femtosecond VUV laser source and detection system using the noncollinear direct four-wave mixing process 3ω+3ω-ω®5ω in filament in argon^[47]

Fig. 8. Schematic of femtosecond UV laser generation using collinear four-wave mixing in filament^[27]

Fig. 9. Photoelectron imaging apparatus based on a tabletop femtosecond VUV laser source. (a) Instrument diagram; (b) schematic of differential pumping system between neon filament tube and reflection/collimation chamber^[51]

Fig. 10. Schematic of the generation of femtosecond VUV laser (133 nm, 6ω)

Fig. 11. Schematic of femtosecond VUV laser source based on infrared light and 3ω laser as driving lasers for four-wave mixing in filament^[56]