Realization of noncontact confocal optical microsphere imaging microscope

Microsphere-assisted dark-field microscopy based on a fully immersed low refractive index microsphere

Here we find that a fully immersed low refractive index SiO₂ microsphere (or a microcylinder, a yeast cell) can clearly distinguish a sample with sub-diffraction features in dark-field illumination mode. The resolvable area of the sample by microsphere-assisted microscopy (MAM) is composed of two regions. One region locates below the microsphere, and a virtual image of this part of the sample is formed by the microsphere first and then the virtual image is received by the microscope. The other region is around the edge of the microsphere, and this part of the sample is directly imaged by the microscope. The simulated region of the enhanced electric field on the sample surface formed by the microsphere is consistent with the resolvable region in the experiment. Our studies show that the enhanced electric field on the sample surface generated by the fully immersed microsphere plays an important role in dark-field MAM imaging, and this finding will have a positive effect on exploring novel mechanisms in resolution improvement of MAM.

Imaging properties of microsphere superlenses with varying background refractive indices under inclined illumination

Microsphere lenses can overcome the optical diffraction limit and can be used to observe features smaller than 200 nm under white light. Inclined illumination benefits from the second refraction of evanescent waves in the microsphere cavity, prohibiting the influence of background noise and improving the imaging resolution and quality of the microsphere superlens. Currently, there is a consensus that microspheres immersed in a liquid environment can improve imaging quality. Microsphere imaging under inclined illumination is performed using barium titanate microspheres immersed in an aqueous environment. However, the background medium of a microlens varies depending on its diverse applications. In this study, the effects of continuously changing background media on the imaging properties of microsphere lens under inclined illumination are investigated. The experimental results demonstrate that the axial position of the microsphere photonic nanojet changes with respect to the background medium. Consequently, owing to the refractive index of the background medium, the imaging magnification and the position of the virtual image change. Using a sucrose solution and polydimethylsiloxane with the same refractive index, we demonstrate that the imaging performance of microspheres is related to the refractive index rather than the background medium type. This study helps associate microsphere superlenses with a more universal application spectrum.

Bilayer-film-decorated microsphere with suppressed interface reflection for enhanced nano-imaging

Microspheres as special optical lenses have extensive applications due to their super-focusing ability and outstanding resolving power on imaging. The interface reflection between the microsphere and sample surface significantly affects nano-imaging as exhibited in the form of the Newton's rings pattern in virtual images. In this work, a new scheme of decorating the microsphere with a dielectric bilayer thin film is proposed to suppress the interface reflection and thus enhance the imaging performance. The particle swarm optimization algorithm is performed with a full-wave simulation to refine the bilayer thin film decorated microsphere design, which is successfully realized via a novel fabrication strategy. Experimental imaging results demonstrate that the Newton's rings pattern in virtual images is substantially diminished. Both the imaging contrast and effective field-of-view of the microsphere nano-imaging are improved via this effective light manipulation scheme, which is also applicable to promoting the performance of the microsphere in other optical applications.