Fig. 1. Schematic diagrams of microsphere-mediated light field modulation and the development process of super-resolution imaging and fluorescence enhancement through microsphere. (a) Schematic diagram of microsphere-mediated light field modulation; (b) the development process of super-resolution imaging and fluorescence enhancement through microsphere

Fig. 2. Microsphere brightfield super-resolution imaging device and non-immersion, semi-immersion, and complete immersion results. (a) Schematic diagram of the structure of brightfield super-resolution imaging based on microspheres^[19]; (b) scanning electron microscope (SEM) of the surface structure of Blu-ray disc (top) and anodized aluminum sample (bottom) without immersion SiO₂ microspheres (left) and super-resolution imaging of microspheres (right)^[19]; (c) the comparison results of the surface structure of Blu-ray disc under the condition of non-immersion SiO₂ microspheres (left) and ethanol semi-immersed SiO₂ microspheres (right)^[24]; (d) super-resolution imaging of the surface structure of a Blu-ray disc under complete immersion of BaTiO₃ microspheres (left: n=1.9, d∈[5 μm, 20 μm], right: n=2.1, d=53 μm) in isopropanol^［25］

Fig. 3. Factors affecting microsphere imaging. (a) SEM image of the surface structure of a Blu-ray disc, and the image results of the surface structure of the Blu-ray disc when BaTiO₃ microspheres are completely immersed in water, 40% sucrose solution, and microscope oil respectively^[26]; (b) the relationship between the diameter of the microsphere and the imaging magnification, the imaging field of view^[25]; (c) comparison of imaging results of surface structure of the Blu-ray disc by low refractive index microspheres in air and high refractive index microspheres immersed in liquid when the ratio of refractive index of microspheres to environmental refractive index is the same as 1.5^[28]; (d) the comparison of the imaging results of the nanoplasma by the microsphere through the objective lens with different parameters^[29]

Fig. 4. Fluorescence super-resolution imaging based on microspheres. (a) BaTiO₃ microspheres with the diameter of 60 μm can magnify the fluorescence image by a factor of 5.4 and resolve the fluorescent particles of 100 nm^[35]; (b) non-contact and large-scale fluorescence super-resolution scanning imaging of mouse myoblasts (C2C12) by BaTiO₃ microspheres with the diameter of 56 μm^[36]; (c) fluorescence imaging comparison with and without BaTiO₃ microspheres under widefield microscope and fluorescence imaging comparison of 20×, 0.4 objective combined with 150 μm BaTiO₃ microspheres and 100×, 1.4 objective^[22]; (d) the imaging results and the resolution of quantitative analysis of 40 nm fluorescent beads under traditional microscope, microspheres, LPSIM, and microspheres combined with LPSIM, respectively^[37]

Fig. 5. Principle of super-resolution imaging based on microspheres. (a) Virtual imaging theory^[39]; (b) microsphere coupled evanescent wave transmission^[42]; (c) schematic diagram of photonic nanojet formation^[44]

Fig. 6. Single-photon fluorescence enhancement based on microspheres/microsphere arrays. (a) Microspheres for single-molecule detection enhancement^[47]; (b) the focusing difference of Gaussian beam with/without microspheres^[47]; (c) ultraviolet fluorescence enhancement of single layer fused silica microsphere array placed on the surface of ZnO film^[49]; (d) fluorescence enhancement results based on single-layer high-refractive-index microsphere arrays composited with semiconductor quantum dot flexible film structure^[51]

Fig. 7. Multiphoton fluorescence enhancement based on microspheres. (a) Schematic diagram of 30% luminescence enhancement of Rhodamine B by SiO₂ microspheres^[53]; (b) schematic diagram of 110 times luminescence of two-dimensional perovskite enhanced by SiO₂ microspheres^[54]; (c) schematic diagram of 3 times of upconversion luminescence enhanced by SiO₂ microspheres^[55]; (d) schematic diagram of experimental device for microsphere arrays enhancing upconversion luminescence^[56]; (e) upconversion luminescence comparison diagram with/without microsphere arrays under different intensities of excitation light^[56]; (f) simulation diagram of the intensity distribution of the excitation light after passing through the half-microsphere arrays^[57]; (g) the intensity distribution of excitation light along the selected line before (blue line) and after (red line) passing through the half-microsphere arrays^[57]; (h) the upconversion luminescence spectra with/without half-microsphere arrays^[57］

Fig. 8. Schematic diagrams of enhanced fluorescence of microsphere combined with surface plasmon resonance. (a)(b) Schematic diagrams of luminescence enhancement of quantum dots by hybrid structure of microspheres/gold nanorods^[61]; (c)(d) schematic diagrams of enhanced upconversion luminescence of semi microsphere array/gold nanorod composite structure^[62]

Fig. 9. Examples of biological microspheres enhanced fluorescence. (a) Schematic diagram of bacteria labeled with upconversion nanoparticles and biological microsphere enhance upconversion luminescence^[63]; (b) schematic diagram of enhanced upconversion luminescence by spherical yeast cells^[63]; (c) schematic illustration of spherical endogenous lipid droplets in mature adipocytes focusing excitation light and collecting fluorescence signals to form magnified fluorescence images^[64]; (d) (e) diagram of lipid droplets enhancing fluorescence in living cells and fluorescence imaging of cytoskeleton^[64]; (f) schematic diagram of focal length adjustment by stretching chloroplasts in cells with optical tweezers^[65]; (g) experimental and simulation results of light field distribution of spherical or ellipsoidal chloroplast^[65]

Fig. 10. Principle model of fluorescence enhancement based on microspheres. (a) Focusing incident light intensity diagram of photon nanojets simulated by the author; (b) schematic diagram of improving quantum efficiency by whispering gallery mode^[66]; (c) schematic diagram of microspheres improving fluorescence signal collection efficiency^[54]

Fig. 11. 2D simulation results of the influence of microsphere diameter on light field enhancement. (a)-(f) Focusing effect diagrams; (g)-(i) the relationship between the maximum focused light intensity and the diameter of the microsphere, the relationship between the maximum power density of the focused light and the diameter of the microsphere, and the relationship between full width at half maxima and the diameter of the microsphere in the horizontal and vertical directions of the focused beam

Fig. 12. Factors of fluorescence enhancement based on microsphere. (a) Example of the fluorescence enhancement factor first increasing and then decreasing with the increase of excitation light intensity^[54]; (b) example of the fluorescence enhancement factor decreasing with the increase of excitation light intensity^[57]; (c) the fluorescence excitation mechanism^[56]; (d) the difference between the refractive index of the microsphere and the environment^[35]

Fig. 13. 2D simulation results of the influence of the ratio of microsphere refractive index to environmental refractive index on the optical field enhancement. (a)-(e) Focusing effect diagrams; (f) relation between the focusing light intensity and the refractive index inside and outside the microspheres

Fig. 14. 2D simulation diagrams of focusing distance of microspheres. (a) Schematic diagram of the focal distance of the microsphere; (b) the effect of the diameter of the microsphere on the focal position of the microsphere; (c) the influence of the ratio of the refractive indices of the inner and outer environments of the microsphere on the focal position of the microsphere, the red dotted line is the boundary of the microsphere