[1] Rajadhyaksha M, Grossman M, Esterowitz D et al. In vivo confocal scanning laser microscopy of human skin: melanin provides strong contrast[J]. Journal of Investigative Dermatology, 104, 946-952(1995).

[2] Betzig E, Patterson G H, Sougrat R et al. Imaging intracellular fluorescent proteins at nanometer resolution[J]. Science, 313, 1642-1645(2006).

[3] Rust M J, Bates M, Zhuang X W. Sub-diffraction-limit imaging by stochastic optical reconstruction microscopy (STORM)[J]. Nature Methods, 3, 793-796(2006).

[4] Dempsey W P, Fraser S E, Pantazis P. SHG nanoprobes: advancing harmonic imaging in biology[J]. BioEssays, 34, 351-360(2012).

[5] Staedler D, Magouroux T, Hadji R et al. Harmonic nanocrystals for biolabeling: a survey of optical properties and biocompatibility[J]. ACS Nano, 6, 2542-2549(2012).

[6] Sun C L, Li J, Wang X Z et al. Rational design of organic probes for turn-on two-photon excited fluorescence imaging and photodynamic therapy[J]. Chem, 5, 600-616(2019).

[7] Pantazis P, Maloney J, Wu D et al. Second harmonic generating (SHG) nanoprobes for in vivo imaging[J]. Proceedings of the National Academy of Sciences of the United States of America, 107, 14535-14540(2010).

[8] Denk W, Strickler J, Webb W. Two-photon laser scanning fluorescence microscopy[J]. Science, 248, 73-76(1990).

[9] Croissant J G, Zink J I, Raehm L et al. Two-photon-excited silica and organosilica nanoparticles for spatiotemporal cancer treatment[J]. Advanced Healthcare Materials, 7, 1701248(2018).

[10] Evans C L, Xie X S. Coherent anti-stokes Raman scattering microscopy: chemical imaging for biology and medicine[J]. Annual Review of Analytical Chemistry, 1, 883-909(2008).

[11] Zumbusch A, Holtom G R, Xie X S. Three-dimensional vibrational imaging by coherent anti-stokes Raman scattering[J]. Physical Review Letters, 82, 4142(1999).

[12] Franken P A, Hill A E, Peters C W et al. Generation of optical harmonics[J]. Physical Review Letters, 7, 118(1961).

[13] Mostaço-Guidolin L, Rosin N, Hackett T L. Imaging collagen in scar tissue: developments in second harmonic generation microscopy for biomedical applications[J]. International Journal of Molecular Sciences, 18, 1772(2017).

[14] Chen X Y, Nadiarynkh O, Plotnikov S et al. Second harmonic generation microscopy for quantitative analysis of collagen fibrillar structure[J]. Nature Protocols, 7, 654-669(2012).

[15] Nakayama Y, Pauzauskie P J, Radenovic A et al. Tunable nanowire nonlinear optical probe[J]. Nature, 447, 1098-1101(2007).

[16] Ma C R, Yan J H, Wei Y M et al. Enhanced second harmonic generation in individual Barium titanate nanoparticles driven by Mie resonances[J]. Journal of Materials Chemistry C, 5, 4810-4819(2017).

[17] Hsieh C L, Grange R, Pu Y et al. Three-dimensional harmonic holographic microcopy using nanoparticles as probes for cell imaging[J]. Optics Express, 17, 2880(2009).

[18] Kim E, Steinbrück A, Buscaglia M T et al. Second-harmonic generation of single BaTiO3 nanoparticles down to 22 nm diameter[J]. ACS Nano, 7, 5343-5349(2013).

[19] Schwung S, Rogov A, Clarke G et al. Nonlinear optical and magnetic properties of BiFeO3 harmonic nanoparticles[J]. Journal of Applied Physics, 116, 114306(2014).

[20] Duan Y L, Ju C G, Yang G et al. Aggregation induced enhancement of linear and nonlinear optical emission from a hexaphenylene derivative[J]. Advanced Functional Materials, 26, 8968-8977(2016).

[21] Chervy T, Xu J L, Duan Y L et al. High-efficiency second-harmonic generation from hybrid light-matter states[J]. Nano Letters, 16, 7352-7356(2016).

[22] Long N J. Organometallic compounds for nonlinear optics: the search for en-light-enment![J]. Angewandte Chemie International Edition in English, 34, 21-38(1995).

[23] Verbiest T, Houbrechts S, Kauranen M et al. Second-order nonlinear optical materials: recent advances in chromophore design[J]. Journal of Materials Chemistry, 7, 2175-2189(1997).

[24] Xu J L, Semin S, Niedzialek D et al. Self-assembled organic microfibers for nonlinear optics[J]. Advanced Materials, 25, 2084-2089(2013).

[25] Li J H, Qiu J C, Guo W B et al. Cellular internalization of LiNbO3 nanocrystals for second harmonic imaging and the effects on stem cell differentiation[J]. Nanoscale, 8, 7416-7422(2016).

[26] Knabe B, Buse K, Assenmacher W et al. Spontaneous polarization in ultrasmall lithium niobate nanocrystals revealed by second harmonic generation[J]. Physical Review B, 86, 195428(2012).

[27] Kachynski A V, Kuzmin A N, Nyk M et al. Zinc oxide nanocrystals for nonresonant nonlinear optical microscopy in biology and medicine[J]. The Journal of Physical Chemistry C, 112, 10721-10724(2008).

[28] Bonacina L, Mugnier Y, Courvoisier F et al. Polar Fe(IO3)3 nanocrystals as local probes for nonlinear microscopy[J]. Applied Physics B, 87, 399-403(2007).

[29] Ladj R, El Kass M, Mugnier Y et al. SHG active Fe(IO3)3 particles: from spherical nanocrystals to urchin-like microstructures through the additive-mediated microemulsion route[J]. Crystal Growth & Design, 12, 5387-5395(2012).

[30] Marder S R, Beratan D N, Cheng L T. Approaches for optimizing the first electronic hyperpolarizability of conjugated organic molecules[J]. Science, 252, 103-106(1991).

[31] Grinvald A, Hildesheim R, Farber I C et al. Improved fluorescent probes for the measurement of rapid changes in membrane potential[J]. Biophysical Journal, 39, 301-308(1982).

[32] Dombeck D A, Sacconi L, Blanchard-Desce M et al. Optical recording of fast neuronal membrane potential transients in acute mammalian brain slices by second-harmonic generation microscopy[J]. Journal of Neurophysiology, 94, 3628-3636(2005).

[33] Eisenthal K B, Yuste R. Second harmonic generation in neurons: electro-optic mechanism of membrane potential sensitivity[J]. Biophysical Journal, 93, L26-L28(2007).

[34] Millard A C, Jin L, Wei M D et al. Sensitivity of second harmonic generation from styryl dyes to transmembrane potential[J]. Biophysical Journal, 86, 1169-1176(2004).

[35] Nuriya M, Fukushima S, Momotake A et al. Multimodal two-photon imaging using a second harmonic generation-specific dye[J]. Nature Communications, 7, 11557(2016).

[36] Qi X Y, Liu H P, Guo W J et al. New opportunities: second harmonic generation of boron-doped graphene quantum dots for stem cells imaging and ultraprecise tracking in wound healing[J]. Advanced Functional Materials, 29, 1902235(2019).

[37] Burnette D T, Sengupta P, Dai Y et al. Bleaching/blinking assisted localization microscopy for superresolution imaging using standard fluorescent molecules[J]. Proceedings of the National Academy of Sciences of the United States of America, 108, 21081-21086(2011).

[38] Salthouse C, Hildebrand S, Weissleder R et al. Design and demonstration of a small-animal upconversion imager[J]. Optics Express, 16, 21731-21737(2008).

[39] Corstjens P, Li S, Zuiderwijk M et al. Infrared up-converting phosphors for bioassays. [C]∥IEE Proceedings-Nanobiotechnology. IET Digital Library, 152, 64-72(2005).

[40] Sivakumar S. Diamente P R, van Veggel F C. Silica-coated Ln 3+-doped LaF3 nanoparticles as robust down- and upconverting biolabels[J]. Chemistry: A European Journal, 12, 5878-5884(2006).

[41] Qian X M, Peng X H, Ansari D O et al. In vivo tumor targeting and spectroscopic detection with surface-enhanced Raman nanoparticle tags[J]. Nature Biotechnology, 26, 83-90(2008).

[42] Zavaleta C L, Smith B R, Walton I et al. Multiplexed imaging of surface enhanced Raman scattering nanotags in living mice using noninvasive Raman spectroscopy[J]. Proceedings of the National Academy of Sciences of the United States of America, 106, 13511-13516(2009).

[43] Nie S. Probing single molecules and single nanoparticles by surface-enhanced Raman scattering[J]. Science, 275, 1102-1106(1997).

[44] Michaels A M, Nirmal M, Brus L E. Surface enhanced Raman spectroscopy of individual rhodamine 6G molecules on large Ag nanocrystals[J]. Journal of the American Chemical Society, 121, 9932-9939(1999).

[45] Wu S, Han G, Milliron D J et al. Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals[J]. Proceedings of the National Academy of Sciences of the United States of America, 106, 10917-10921(2009).

[46] Shaner N C, Steinbach P A, Tsien R Y. A guide to choosing fluorescent proteins[J]. Nature Methods, 2, 905-909(2005).

[47] Freund I. Deutsch M. Second-harmonic microscopy of biological tissue[J]. Optics Letters, 11, 94-96(1986).

[48] Huang J Y, Lewis A, Loew L. Nonlinear optical properties of potential sensitive styryl dyes[J]. Biophysical Journal, 53, 665-670(1988).

[49] Sugiyama N, Sonay A Y, Tussiwand R et al. Effective labeling of primary somatic stem cells with BaTiO3 nanocrystals for second harmonic generation imaging[J]. Small, 14, 1870036(2018).

[50] Betzer O, Meir R, Dreifuss T et al. In-vitro optimization of nanoparticle-cell labeling protocols for in-vivo cell tracking applications[J]. Scientific Reports, 5, 15400(2015).

[51] Grange R, Lanvin T, Hsieh C L et al. Imaging with second-harmonic radiation probes in living tissue[J]. Biomedical Optics Express, 2, 2532-2539(2011).

[52] Dubreil L, Leroux I, Ledevin M et al. Multi-harmonic imaging in the second near-infrared window of nanoparticle-labeled stem cells as a monitoring tool in tissue depth[J]. ACS Nano, 11, 6672-6681(2017).

[53] Ramos-Gomes F, Möbius W, Bonacina L et al. Bismuth ferrite second harmonic nanoparticles for pulmonary macrophage tracking[J]. Small, 15, 1803776(2019).

[54] Pu Y, Grange R, Hsieh C L et al. Nonlinear optical properties of core-shell nanocavities for enhanced second-harmonic generation[J]. Physical Review Letters, 104, 207402(2010).

[55] Wang Y F, Barhoumi A, Tong R et al. BaTiO3-core Au-shell nanoparticles for photothermal therapy and bimodal imaging[J]. Acta Biomaterialia, 72, 287-294(2018).

[56] Kharin A Y, Lysenko V V, Rogov A et al. Bi-modal nonlinear optical contrast from Si nanoparticles for cancer theranostics[J]. Advanced Optical Materials, 1801728(2019).

[57] Liu Y, Zhu X Q, Huang Z F et al. Texture analysis of collagen second-harmonic generation images based on local difference local binary pattern and wavelets differentiates human skin abnormal scars from normal scars[J]. Journal of Biomedical Optics, 20, 016021(2015).

[58] Mercatelli R, Ratto F, Rossi F et al. Three-dimensional mapping of the orientation of collagen corneal lamellae in healthy and keratoconic human corneas using SHG microscopy[J]. Journal of Biophotonics, 10, 75-83(2017).

[59] Van Steenbergen V, Boesmans W, Li Z et al. Molecular understanding of label-free second harmonic imaging of microtubules[J]. Nature Communications, 10, 3530(2019).

[60] Sacconi L, Dombeck D A, Webb W W. Overcoming photodamage in second-harmonic generation microscopy: Real-time optical recording of neuronal action potentials[J]. Proceedings of the National Academy of Sciences of the United States of America, 103, 3124-3129(2006).

[61] Gayen A, Kumar D, Matheshwaran S et al. Unveiling the modulating role of extracellular pH in permeation and accumulation of small molecules in subcellular compartments of gram-negative escherichia coli using nonlinear spectroscopy[J]. Analytical Chemistry, 91, 7662-7671(2019).

[62] Reeve J E, Anderson H L, Clays K. Dyes for biological second harmonic generation imaging[J]. Physical Chemistry Chemical Physics, 12, 13484-13498(2010).

[63] Campagnola P J, Loew L M. Second-harmonic imaging microscopy for visualizing biomolecular arrays in cells, tissues and organisms[J]. Nature Biotechnology, 21, 1356-1360(2003).

[64] Kachynski A V, Pliss A, Kuzmin A N et al. Photodynamic therapy by in situ nonlinear photon conversion[J]. Nature Photonics, 8, 455-461(2014).

[65] Bi H T, Dai Y L, Yang P P et al. Glutathione mediated size-tunable UCNPs-Pt(IV)-ZnFe2O4 nanocomposite for multiple bioimaging guided synergetic therapy[J]. Small, 14, 1703809(2018).

[66] Gu B B, Pliss A, Kuzmin A N et al. In-situ second harmonic generation by cancer cell targeting ZnO nanocrystals to effect photodynamic action in subcellular space[J]. Biomaterials, 104, 78-86(2016).

[67] Zhou X Y, Chen Y, Su J et al. In situ second-harmonic generation mediated photodynamic therapy by micelles co-encapsulating coordination nanoparticle and photosensitizer[J]. RSC Advances, 7, 52125-52132(2017).

[68] Barhoumi A. NIR-triggered drug delivery by collagen-mediated second harmonic generation[J]. Advanced Healthcare Materials, 4, 1159-1163(2015).