Fig. 1. The excitation of single-photon and two-photon. (a) Single-photon excitation of fluorescein by focused 488 nm (NA is 0.16)^[41]; (b) two-photon excitation fluorescein by focused femtosecond pulses of 960 nm (NA is 0.16)^[41]; (c) the depth and resolution of MPM imaging^[42]

Fig. 2. Attenuation length and signal background ratio. (a) The theoretical model of effective attenuation lengths based on water absorption and Mie scattering^[44], the black circle indicates the effective attenuation lengths in mouse brain in vivo; (b) two-photon and three-photon signal background ratio of 1300 nm and 1700 nm wavebands^[24]

Fig. 3. Effective attenuation lengths of 800 nm, 1300 nm, and 1700 nm. (a) 2PF effective attenuation length of 775 nm(130 μm) and 2PF effective attenuation length of 1300 nm(285 μm)^[28]; (b) 2PF effective attenuation length of 920 nm(150 μm) and 3PF effective attenuation length of 1300 nm(300 μm)^[45]; (c) 3PF effective attenuation length of 1700 nm(400 μm)^[24]; (d) 2PF effective attenuation length (170 μm) and 3PF effective attenuation length of 1300 nm (230 μm,trough skull)^[6]; (e) 3PF effective attenuation length of 1700 nm(240 μm,trough skull)^[6]

Fig. 4. Energy diagrams of MPM^[43]

Fig. 5. Soliton energies and model field area of various optical fibers^[43]

Fig. 6. SSFS spectra of LMA fiber^[43]. (a) SSFS spectra of LMA fiber (experiment); (b) SSFS spectra of LMA fiber (simulation)

Fig. 7. Construction of femtosecond pulse light source for MPM imaging at 1700 nm waveband. (a) Experimental setup for linearly-polarization soliton and circularly-polarization soliton generation^[76]; (b) in vivo mouse white matter layer with 1617 nm circularly-polarization soliton and linearly-polarization soliton THG imaging^[77]; (c) experimental setup for polarized soliton synthesis^[78]; (d) THG imaging (green) and blood vessel 3PF imaging (red) of mouse white matter layer with 1613 nm soliton (horizontal, vertical, and synthesis)^[78]; (e) experimental setup of the polarization multiplexing technology with no polarization-maintaining PC rod fiber^[79]; (f) 3PF imaging of mouse brain blood vessels with polarized soliton (horizontal, vertical, and both)^[79]; (g) experimental setup of the polarization multiplexing technology with polarization-maintaining LMA fiber^[79]; (h) SHG imaging of mouse tail tendon excited with polarized soliton (horizontal, vertical, and both)^[79]; (i) experimental setup of SSFS of hollow-core fiber and PC rod fiber^[71]; (j) SR101 fluorescent dye of 3PF imaging and skull cells of THG imaging with 1600 nm soliton (hollow-core fiber and PC rod fiber)^[71]

Fig. 8. 3PF of mouse deep-brain vasculature imaging with 1700 nm waveband in vivo. (a) 3D reconstruction of 3PF images of the mouse brain vasculature with Texas red-labeled and the signal background ratio of different depth vessels^[24]; (b) 3D reconstruction of 2PF images of the mouse brain vasculature with ICG-labeled and the different depths of two-dimensional of blood vessels^[25]; (c) 3D reconstruction of 3PF images of the mouse brain vasculature with Qtracker655-labeled and the signal background ratio of different depth vessels^[26]

Fig. 9. In vivo mouse deep-brain vasculature imaging with AIE-labeled. (a) 3D reconstruction of 3PF images of mouse deep-brain vasculature with AIE-BONAPs labeled and the different depths of two-dimensional of blood vessels, green represents THG imaging^[84]; (b) 3D reconstruction of 3PF images of mouse deep-brain vasculature with AIE- DPNA-NZ labeled and the signal background ratio of different depth brain blood vessels in vivo^[85]; (c) 3PF imaging of mouse deep-brain blood vessels with MTTCM NP labeled and the signal background ratio of different depth brain blood vessels in vivo^[87]

Fig. 10. In vivo measurement of mouse brain blood flow speed at 1700 nm waveband. (a) The measurement of mouse brain blood flow speed with 3PF imaging in vivo (Qtracker655)^[26]; (b) the measurement of mouse brain blood flow speed with 3PF imaging in vivo (AIE-DPNA-NZ)^[85]; (c) the label-free measurement of mouse brain blood flow speed with THG imaging in vivo^[92], red represents 3PF imaging, green represents THG imaging; (d) the measurement of mouse brain blood vessels through the intact skull for blood flow speed with 3PF imaging in vivo (MTTCM NP)^[87]

Fig. 11. In vivo mouse deep-brain cell of 3PF imaging at 1700 nm waveband. (a) 3D reconstruction of deep-brain astrocytes of 3PF imaging and the different depths of 2D of astrocytes^[83]; (b) 3D reconstruction of transgenic mouse of deep-brain neurons of 3PF imaging^[24]; (c) 3D reconstruction of deep-brain microglia of 3PF imaging^[94]

Fig. 12. In vivo through-skull mouse brain vasculature of 3PF imaging at 1700 nm window. (a) THG and 3PF imaging of skull cells^[59]; (b) 3D reconstruction of through-skull mouse brain vasculature of 3PF imaging^[6]; (c) 3D reconstruction of transparent-skull imaging of brain vasculature for 3PF imaging^[99]; (d) the 3PF blood vessels through the intact skull and the signal background ratio of blood vessels images at different depths^[87]

Fig. 13. In vivo mouse skin imaging of MPM at 1700 nm waveband. (a) Mouse skin for SHG imaging (red) and THG imaging (green)^[104]; (b) mouse hindlimb lymphatic vessel for 2PF imaging and SHG (THG) imaging, red represents THG imaging, green represents 2PF imaging and SHG imaging^[105]; (c) mouse digital skin myelin for 3PF imaging^[106]; (d) mouse skin elastic fibers for 3PF imaging^[107]