Research On Improving The Imaging Resolution Of High Refractive Index Microspheres

The resolution of a conventional optical microscope is limited to about half of the wavelength of illumination due to the diffraction limit.In recent years,many new approaches have been put forth to overcome the diffraction limit.Among these techniques,microsphere-assisted super-resolution microscopy is a complementary imaging technique to achieve lens-based super resolution beyond the diffraction limit.In microsphere-assisted super-resolution microscopy,the immersion medium affects the position of the focus point of the microsphere,which in turn affects the resolution and magnification.It has been found that low refractive index media have the best imaging performance under semi-immersion conditions,while high refractive index microspheres require super-resolution imaging under full immersion conditions.The main research of the thesis is as follows:First,we introduce a method to improve the resolution of a microsphere lens by increasing its numerical aperture.In our proposed structure,BaTiO3 glass(BTG)microsphere lenses are semi-immersed in a S1805 layer with a refractive index of 1.65,and then,the semi-immersed microspheres are fully embedded in an elastomer with an index of 1.4.We experimentally demonstrate that this structure,in combination with a conventional optical microscope,can clearly resolve a two-dimensional 200-nm-diameter hexagonally close-packed(hep)silica microsphere array.On the contrary,the widely used structure where BTG microsphere lenses are fully immersed in a liquid or elastomer cannot even resolve a 250-nm-diameter hcp silica microsphere array.The improvement in resolution through the proposed structure is due to an increase in the effective numerical aperture by semi-immersing BTG microsphere lenses in a high-refractive index S1805 layer.Our results will inform on the design of microsphere-based high-resolution imaging systems.Then,we add a 20-nm-thick SiO film between a fully immersed BaTiO3 glass(BTG)microsphere and a sample with sub-diffraction features,and study its imaging properties.Compared with a fully immersed BTG microsphere without the SiO film,the magnification becomes larger,and the initial image position is farther away from the sample.The imaging rules no longer satisfy the imaging rules used in geometrical optics.We propose that the thin SiO film can enhance evanescent waves and the enhanced evanescent waves affect the magnification and the image position in BTG microsphere imaging.Our studies will help to understand the imaging rules of microspheres more comprehensively.