Fig. 1. CARS. (a) Principle of CARS^[10] (Reproduced under the terms of the CC-BY license, Copyright 2014, Potcoava M C et al.); (b) schematic of CARS microscopy^[11](Reproduced with permission, Copyright 2018, Society of Photo‑Optical Instrumentation Engineers)

Fig. 2. Principle of SRS. (a) Energy diagram of SRS; (b) input and output spectra of SRS; (c) detection scheme of SRL; (d) setup of SRS microscopy^[12] (Reproduced with permission, Copyright 2008, American Association for the Advancement of Science)

Fig. 3. CARS images of 3T3-L1 cell culture at different time after adding induction media^[15]. (a) 0 h; (b) 24 h; (c) 48 h; (d) 60 h; (e) 96 h; (f) 192 h (Reproduced under the terms of the CC-BY license, Copyright 2003, American Society for Biochemistry and Molecular Biology )

Fig. 4. Comparison between SRS images and HES images of pancreatic tissue^[28]. (a) SRS image of healthy pancreatic tissue; (b) HES image of healthy pancreatic tissue; (c) SRS image of cancerous pancreatic tissue; (d) HES image of cancerous pancreatic tissue; (e) SRS image of the tumor area of pancreatic tissue; (f) HES image of the tumor area of pancreatic tissue (Reproduced under the terms of the CC-BY license, Copyright 2019, Sarri B et al.)

Fig. 5. Schematic diagram of photothermal microscopy^[31](Reproduced with permission, Copyright 2004, American Physical Society)

Fig. 6. Photothermal microscopy images. (a) COS-7 cells before digitonin addition^[32]; (b)(c) COS-7 cells after digitonin addition^[32](Reproduced under the terms of OSA Open Access Publishing Agreement, Copyright 2007, Optical Society of America); (d) single mitochondria^[33] (Reproduced with permission, Copyright 2010, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim); (e) HeLa cells^[34] (Reproduced under the terms of OSA Open Access Publishing Agreement, Copyright 2019, Optical Society of America); (f) melanoma cells^[35] (Reproduced under the terms of OSA Open Access Publishing Agreement, Copyright 2015, Optical Society of America); (g) oligodendrocytes^[36] (Reproduced with permission, Copyright 2019, American Chemical Society); (h) vesicular stomatitis virus^[37]; (i) poxvirus^[37] (Reproduced with permission, Copyright 2021, American Chemical Society)

Fig. 7. 3D images of photothermal microscopy. (a) Zebrafish gill larvae blood vessels^[38] (Reproduced with permission, Copyright 2010,American Institute of Physics); (b) melanoma^[39] (Reproduced under the terms of the CC-BY license, Copyright 2016,He J P et al.); (c) lipid droplets stored in PC-3 prostate cancer cells^[40] (Reproduced under the terms of the CC-BY license, Copyright 2016,Zhang Delong et al.)

Fig. 8. Schematic diagrams of SPRM. (a) Schematic diagram of wide-field SPRM; (b) schematic diagram of scanning SPRM^[43](Reproduced under the terms of OSA Open Access Publishing Agreement, Copyright 2012, Optical Society of America)

Fig. 9. SPRM imaging. (a) Single H1N1 influenza virus^[46]; (b) single T4 bacteriophage^[47] (Reproduced with permission, Copyright 2017, Society of Photo‑Optical Instrumentation Engineers); (c) single DNA molecular chain^[48] (Reproduced with permission, Copyright 2014, American Chemical Society)

Fig. 10. Study on molecular dynamic using SPRM. (a) 3D structure of microtubule^[51] (Reproduced with permission, Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim); (b) z-axis motility profile of single bacteria^[52] (Reproduced with permission, Copyright 2016, American Chemical Society); (c) 2D tracking of single exosome^[53]

Fig. 11. Study on cell-substrate interaction using SPRM. (a) Distribution of adhesion intensity of cell-substrate^[55]; (b) distribution of cell-substrate distance^[56]; (c) 3D reconstruction process of cell-substrate^[57] (Reproduced with permission, Copyright 2012,2014,2018, American Chemical Society)

Fig. 12. Schematic diagram of iSCAT^[62]. (a) Wide-field iSCAT; (b) fast beam-scanning iSCAT (Reproduced under the terms of the CC-BY-NC-ND license, Copyright 2021, Priest L et al.)

Fig. 13. iSCAT imaging of single virus and biomolecule. (a) Single SV40^[63]; (b) single streptavidin^[66] (Reproduced with permission, Copyright 2007,2017, American Chemical Society)

Fig. 14. Study on biomolecular dynamics using iSCAT. (a) Vesicle changes^[67](Reproduced with permission, Copyright 2013, American Chemical Society); (b) nanodomain formation^[68]; (c) myosin filament dynamics^[70](Reproduced under the terms of the CC-BY license, Copyright 2020, Biophysical Society)

Fig. 15. Study on structure of living cells using iSCAT^[71]. (a) Cell focaladhesions and filopodium structure;(b) cell edge structure (Reproduced under the terms of the CC-BY-NC license, Copyright 2018, The Royal Society of Chemistry)

Table 1. Performance comparison of four label-free optical microscopic imaging techniques