Performance of microsphere-assisted imaging in bright-field and dark-field microscopy

In this work, we study the imaging performance of microsphere-assisted microscopy (MAM) using microspheres with different refractive indices and immersion conditions under both bright-field illumination (BFI) and dark-field illumination (DFI). The experimental results show that the position of the photonic nanojet of the microsphere plays an important role in MAM imaging. The contrast in imaging is affected by the reflection from the microsphere, the background signal without the microsphere, and the electric field on the substrate surface. In MAM, BaTiO₃ glass microspheres achieve better imaging results under BFI when immersed in a polydimethylsiloxane (PDMS) film but are challenging to image under DFI. However, SiO₂ and polystyrene microspheres exhibit improved imaging performance under both BFI and DFI with PDMS-covered semi-immersion, and the imaging contrast in DFI is superior to that in BFI under the same conditions. Besides, the Talbot effect is observed by MAM under DFI when imaging 300-nm-diameter hexagonally close-packed nanoparticle arrays. This work reveals the advantage of MAM under DFI in improving the contrast.