Fig. 1. Different types of SNOM tips. (a) Aperture type; (b) scattering type; (c) plasmonic type

Fig. 2. Conceptual diagram of near-field optical imaging

Fig. 3. Near-field scanning tips. (a) Grating conical metallic tip prepared by FIB etching^[51]; (b) conic plasmonic lens^[52]; (c) metal pyramid shaped three-dimensional plasma nanofocusing^[53]; (d) grating groove shaped quartz tip prepared by FIB etching^[54]; (e) optical fiber-silver nanostructure composite tip^[56]

Fig. 4. Hollow spiral taper SNOM tips^[24]. (a) Process of direct laser writing for preparing tips; (b) working principle of tip; (c) electric field intensity distribution of gold spiral taper in x-z plane under excitation of different polarized lights

Fig. 5. “Three high”p-SNOM tips^[25]. (a) Working principle of tip; (b) spiral-grating solid tip; (c) light spot from tip apex; (d) scanning results of one-dimensional grating and resolution of tip with a grating linewidth of 80 nm and a tip resolution of 5.7 nm

Fig. 6. Applications of near-field detection technology in biomacromolecules. (a) Infrared spectroscopy, near-field imaging, and structural model of phospholipid bilayers^[65]; (b) distribution of nuclear components in lymphocytes^[66]; (c) near-field images of influenza X31 virus in an environment with a pH of 7^[67]; (d) comparison of near-field imaging effects of tobacco mosaic virus before and after spectral signal processing^[68]

Fig. 7. Applications of near-field detection technology in cells and microorganisms. (a) SNOM morphology, reflection, and transmission images of sperm^[69]; (b) nano-FTIR spectra detection of single sEV^[70]; (c) tomography reconstruction of Chlamydomonas reinhardtii^[71]

Fig. 8. Detecting various types of polaritons by SNOM imaging. (a) Near-field imaging of graphene plasmons^[81]; (b) phonon polariton images detected at different frequencies^[84]; (c) nano-optical imaging of a MoSe₂ planar waveguide^[98]; (d) time-domain interferometry nanoimaging of hyperbolic polariton pulse propagation^[100]

Fig. 9. Near-field optics and nonlinear effects of low-dimensional materials. (a) Generating FWM in graphene^[101]; (b) anticorrelation behavior between decoherence time and FWM intensity of single-layer WSe₂^[102]; (c) near-field enhanced SHG signal of ZnO NW^[103]

Fig. 10. Near-field optical data storage technology. (a) Schematic diagram of data storage concept“Millipede”based on AFM^[105]; (b) utilizing a tip to create bit indentations on PMMA for achieving high-capacity data recording^[105]; (c) controlled deformation of silk protein induced by enhanced electric field of tip^[106]; (d) adjusting point height of writing by changing incident light frequency and exposure time^[106]; (e) repeated erasing and rewriting process^[106]