Fig. 1. Schematic diagram of the femtosecond pump-probe system^[31]. (a) Optical path diagram of a typical pump-probe scheme; (b) spatial profiles of the pump pulse obtained by monitoring the fluorescence intensity of a fluorescent bead scanned over the pump spot and the corresponding Gaussian fitting result; (c) temporal characteristics of the pump (above) and probe (below) pulses in the sample; (d) transmissive pump structure; (e) reflective pump structure

Fig. 2. Relaxation time of the two-step mechanism for pump at 800 nm, 825 nm, and 865 nm^[12]

Fig. 3. Schematic diagrams of two-dimensional spectroscopy^[39]. (a) Schematic diagram of two-dimensional infrared spectroscopy based on pulse shaping; (b) timing diagram of three pulses in two-dimensional infrared spectroscopy

Fig. 4. Two-dimensional spectra of N3- in negatively charged reverse micelles at different waiting time^[43]

Fig. 5. Near-field ultrafast pump-probe imaging^[60]. (a) Near-field imaging of carrier concentration at different relaxation time; (b) structural diagram of the scanning section of the near-field pump-probe imaging system and continuous relaxation time slices of carrier concentration near-field imaging

Fig. 6. Images show representative time points from a live-cell 4D movie of kinetochores (magenta) and chromosomes (cyan) ^[65]

Fig. 7. Schematic diagram of 4D electron microscope^[70]. (a) Structural diagram of 4D electron microscope; (b) ultra-thin liquid sample cell device; (c) single pulse electron imaging in liquid environment

Fig. 8. Energy level diagram of multiphoton technology^[87]

Fig. 9. Schematic diagram of multimodal microscope^[94]. (a) Schematic diagram of microscope structure; (b) energy level spectrum of multimodal microscope

Fig. 10. Multimodal images of EV metabolism in normal and cancerous breast tissues^[95]

Fig. 11. Imaging of THG (blue) and 3PM (green) in the visual cortex and hippocampus of Thy1-eGFP mice^[93]. (a) Merged imaging of THG and 3PM; (b) separate imaging displays of THG (left) and 3PM (right)

Fig. 12. Multimodal imaging of SECARS (red), SHG (green), and 2PF (blue) at different depths of thick tissue sample^[99]. (a) 0 μm;