Contrast-enhanced optical microscopy using 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.