Fig. 1. Principles of single-photon excited fluorescence (SPEF), TPEF and SHG. (a) Single-photon excited fluorescence, two-photon excited fluorescence; (b) second harmonic generation^[4]

Fig. 2. SHG imaging schematic and example image of mouse skin^[13]

Fig. 3. SHG signaling generation principle for collagen fibers. (a)(b) Collagen fibers acting as dipoles; (c) the amount of SHG signal produced for different polarization orientations^[13]

Fig. 4. BaTiO₃ nanocrystals were injected into zebrafish embryos at the single-cell stage for continuous SHG imaging during development^[7]. (a) SHG nanoprobes provide strong signal with superior signal-to-noise when zebrafish body is detected in epidirection; (b) SHG nanoprobe targeting specificity

Fig. 5. Imaging and tracking mMSCs internalized with B-GQDs in wound healing in vivo over time^[36]

Fig. 6. Visualization results of microtubule network by SHG microscope. (a) SHG signal intensity of microtubule is sensitive to laser polarization; (b) SHG signal is narrow peak; (c)(d) SHG signal comes from microtubule polarization structure^[59]

Fig. 7. SHG recording of action potentials. (a) Line-scan recording of membrane potential with SHG in cultured neurons; (b) SHG recording of action potentials along neuron neurite^[60]

Fig. 8. Multimodal two-photon imaging using a second harmonic generation-specific dye. (a) Chemical structures of Ap3 and FM4-64; (b) SHG signal changes on action potential in neurons; (c) SHG and TPF signals obtained from cultured CHO cells loaded with FM4-64 and Ap3; (d)(e) multimodal two-photon imaging of cellular structures; (d) intensity profiles of TPF signals before and after the application of FM4-64; (e) merged images after the application of FM4-64^[35