242 / 2024-08-20 08:33:29

A planar darkfield illuminator for wide-field high-contrast imaging of specimens in microfluidic chips

Darkfield imaging,one-dimensional photonic crystal,microfluidics,biological cells

Darkfield microscopy is an effective technique and well suited for uses involving live and unstained biological samples. However, traditional darkfield microscopy needs a bulky condenser and requires precise alignment in the optical path to produce a cone of illumination light, which increases the operation complexity. Here, we invented a compact darkfield illuminator that is of a planar shape and can work in a broad band wavelength. It can be easily incorporated into a conventional microscope as a substrate to produce hollow cones of lights for large-area darkfield illumination, resulting in a high-contrast high-sensitive widefield microscopy image. The darkfield illumination is composed of four components. The first one is an all-dielectric 1DPC chip that is made of alternative layers of SiO₂ and Si₃N₄. The role of this 1DPC chip is to tailor the angular-dependent transmission of the light emitted below this chip so that hollow cones of light can be formed on every spot above the top surface of the 1DPC chip. The second component is the multicore fiber that is placed below the 1DPC chip. Its role is to inject illumination light into a darkfield illuminator from an outside light source. The third component is a scatter layer between the 1DPC and the multicore fiber. It was used to scatter the light from the multicore fiber in all directions. The fourth component is a thick gold film. Its role is to prevent the leakage of illumination light into the bottom space and to reflect the scattered light to the top space. To demonstrate the high-contrast imaging capability of our proposed on-chip darkfield technique, we employ the principles of partially coherent quasimonochromatic illumination imaging to theoretically analyze the quality of the images enabled by the darkfield illuminator. Taking advantages of its planar and compact structure, we successfully integrated this darkfield illuminator with a microfluidic chip, thus actions and reactions of biological and chemical specimens flowing inside the micrometer-sized channels, can be real-time imaged with high contrast and sensitivity in a large field-of-view (FoV).