Converting Evanescent Waves into Propagating Waves: The Super-Resolution Mechanism in Microsphere-Assisted Microscopy

Abstract

The lateral resolution beyond the classical diffraction limit can be achieved by the combination of a dielectric microsphere and an optical microscope, and this paper utilizes microsphere-assisted microscopy as a platform to study the mechanism of using optical microstructure to realize super-resolution imaging. A barium titanate glass (BTG) microsphere was immersed into ethanol, and the imaging resolution of a BTG microsphere-assisted microscope was recorded in situ during evaporation of ethanol, while a numerical simulation was done to study how the ethanol's immersion depth affected the BTG microsphere's capability to convert evanescent waves in the near field to propagating waves in the far field. The experimental trend of the imaging resolution versus the immersion depth was consistent with that of the conversion capability. Our results indicate that superresolution may come from the conversion of evanescent waves into propagating waves by the optical microstructure.

Near- to Far-Field Coupling of Evanescent Waves by Glass Microspheres

“…The role of coherence in the imaging process has been pointed out where two point objects need to be out-ofphase in order to be resolved [20]. The role of evanescent waves was first experimentally suggested through a decrease in resolution with an increase in the distance microspheresample [21] and more-recently confirmed using semi-immersed BaTiO 3 microspheres [22]. To our knowledge, the most advanced explanation has been proposed by Zhou et al [23] where whispering-gallery modes excited inside the microsphere by high-spatial frequencies from the object make it possible to contribute to a virtual image resolution.…”

Chalcogenide Microsphere‐Assisted Optical Super‐Resolution Imaging

“…The greatly improved spatial resolution can be attributed to enlarged effective numerical aperture after semiimmersion of As 2 S 3 microspheres in PMMA. [33][34][35][36] The increase in refractive index of the vicinity of nanostructures weakens the attenuation of near-field evanescent waves, therefore, more high spatial frequency components can be coupled into the microsphere and converted into propagation waves, resulting in an enhanced spatial resolution. It is worth mentioning that unlike the decrease in the magnification of virtual imaging with a SiO 2 microsphere after semi-immersion in ethanol, [33] the magnification increases for a high-refractive-index As 2 S 3 microsphere after semi-immersion in PMMA, benefitting from its real imaging property.…”

Microlens‐assisted microscopy for biology and medicine

“…Figure 1 shows an illustration of the evolution of a PNJ with a decrease in the RI of a circular dielectric cylinder illuminated by a plane wave. The RI of immersion liquids also influences the magnification factor [30,31]. PNJs can be created with small spheres with R ≈ λ, as well as with large spheres R > 20λ [32].…”