A new light source for dark-field microscopy
Objects with low refractive-index contrast are tricky to image under a microscope. They barely alter light passing through them, which makes them nearly invisible in bright-field images. A better approach for such objects is dark-field microscopy. In the technique, a hollow cone of light illuminates a sample. The cone’s inner angle is so large that photons miss the objective lens unless they’re refracted by the object. The result is a bright shape on a dark background—the opposite of a typical microscope image.
Dark-field microscopes need specialized condensers or objectives to shape their light cones. Those elements can be bulky and expensive, and they sometimes compromise the resolution of the final image. Now Cécile Chazot and her coworkers at MIT have developed a substrate that can do the same job with far less equipment. The substrate supports the sample being imaged and generates the illuminating light cone by itself. A normal bright-field microscope collects the light scattered by the sample and forms an image.
Adapted from C. A. C. Chazot et al., Nat. Photonics, 2020, doi:10.1038/s41566-020-0593-1
A schematic of the substrate design is shown in the first image. The emitting layer comprises core–shell quantum dots embedded in a polymer matrix. The quantum dots produce light when they’re excited by a laser. Above the layer sits a 1-μm-thick Bragg mirror made of 13 layers of alternating titanium dioxide and silicon dioxide. The mirror reflects photons incident below a certain angle. By tuning the mirror’s layers, the researchers shaped a hollow light cone whose inner angle was wider than the collection cone of a standard microscope objective.
With just those elements, most of the light produced by the quantum dots would be lost because the mirror rejects light at many incident angles. To improve the device’s yield, the researchers patterned the bottom surface of the emitting layer with concave reflectors. Because the photons bounced back and forth a few times between the curved surface and the Bragg mirror, their incident angles changed and allowed many more to eventually join the light cone.
Adapted from C. A. C. Chazot et al., Nat. Photonics, 2020, doi:10.1038/s41566-020-0593-1
The second figure shows side-by-side bright-field (left) and dark-field (right) images of a microalgae in seawater; the dark-field image’s contrast is nearly 20 times that of the bright-field image. In addition to expanding the capabilities of laboratory microscopes, the MIT researchers’ device could make dark-field microscopy more accessible and affordable to researchers working in marine environments. (C. A. C. Chazot et al., Nat. Photonics, 2020, doi:10.1038/s41566-020-0593-1 .)
