Contrast-enhanced optical microscopy using a planar photonic substrate and dark-field illumination.

Visualization of low-index dielectric nanoparticles and biological nanostructures is challenging under a traditional optical microscope. In this work, we propose a planar photonic substrate to enhance the extremely weak scattering signal and improve the contrast in imaging low-index samples under a dark-field illumination (DFI) optical microscope. Specifically, a planar photonic substrate is prepared by depositing a multilayer optical film on a silica substrate with the maximum electric field intensity distributed on the substrate surface. The scattering signal of a sample placed on the planar photonic substrate is enhanced due to the strong substrate-nanoparticle interaction. The experimental results show that the scattering intensity of a 150-nm-diameter SiO₂ nanoparticle placed on a planar photonic substrate is about 4.8 times of that on a silica substrate. In addition, individual SiO₂ nanoparticles with 50 nm diameter and bacterial flagella about 50 nm thick can be clearly observed, which indicates that the proposed method can improve the contrast in imaging low-index samples with subwavelength features under a traditional optical microscope.