It is experimentally shown that barium titanate glass microspheres with diameters (D) in the range 2–220 μm and with high refractive index (n ∼ 1.9–2.1) can be used for super-resolution imaging of liquid-immersed nanostructures. Using micron-scale microspheres, we demonstrate an ability to discern the shape of a pattern with a minimum feature size of ∼λ/7, where λ is the illumination wavelength. For spheres with D > 50 μm, the discernible feature sizes were found to increase to ∼λ/4. Detailed data on the resolution, magnification, and field-of-view are presented. This imaging technique can be used in biomedical microscopy, microfluidics, and nanophotonics applications.

We demonstrate a series of advantages of microsphere-assisted imaging over confocal and solid immersion lens microscopies including intrinsic flexibility, better resolution, higher magnification, and longer working distances. We discerned minimal feature sizes of ∼50-60 nm in nanoplasmonic arrays at the illumination wavelength λ = 405 nm. It is demonstrated that liquid-immersed, high-index (n ∼ 1.9-2.1) spheres provide a superior image quality compared to that obtained by spheres with the same index contrast in an air environment. We estimate that using transparent microspheres at deep UV wavelengths of ∼200 nm might make possible imaging of various nanostructures with extraordinary high ∼30 nm resolution.

We demonstrated feasibility of super-resolution imaging through high-index microspheres embedded in transparent elastomers. We performed imaging, with resolution improvement by a factor of two, by using implanted barium titanate glass microspheres (diameters $\sim 30–150\mathrm{\mu m}$ and refractive index $\sim 1.9–2.1$) in a thin film of polydimethylsiloxane elastomer placed over the specimen. Microsphere-assisted imaging technique is a promising candidate for applications in cancer research. As a proof-of-principle, we used microsphere-assisted imaging technique for the observation of radiation-induced $\gamma$-H2AX foci formation in U87 human glioblastoma cells irradiated by clinical proton beams.

Dielectric microelements with circular symmetry have shown interesting optical properties: photonic nanojets (PNJs) and whispering gallery modes (WGMs). They can confine light inside the cavity, forming WGMs, or focus the light in their proximity, forming PNJs. Both WGMs and PNJs have found numerous applications, including sensing and imaging. In this work, a review of PNJs and their applications in contemporary literature is provided.