A light-powered microscope has a resolution limit of around 200 nanometers—which makes observing specimens smaller or closer together than that all but impossible. Engineers at the University of California San Diego have found a clever way to improve the resolution of a conventional microscope, but surprisingly it involves no upgrades to the lenses or optics inside it.
According to the Rayleigh Criterion theory, proposed by John William Strutt, 3rd Baron Rayleigh, back in 1896, a traditional light-based microscope’s resolution is limited by not only the optics capabilities of glass lenses but the nature of light itself, as a result of diffraction that occurs when light rays are bent. The limitation means that an observer looking through the microscope at two objects that are closer than 200 nanometers apart will perceive them as a single object.
Electron microscopes, by comparison, blast a sample with a highly focused beam of electrons instead of visible light, and can instead achieve resolutions of less than a single nanometer. There’s a trade-off, however, as samples being observed through an electron microscope need to be placed inside a vacuum chamber which has the unfortunate downside of killing living things, so observing cells and other living phenomena in action isn’t possible. To date, there hasn’t been an in-between option, but it sounds like that’s exactly what these engineers have created.
“Artistic rendering of the new super resolution microscopy technology. Animal cells (red) are mounted on a slide coated with the multilayer hyperbolic metamaterial. Nanoscale structured light (blue) is generated by the metamaterial and then illuminates the animal cells.”
To create what’s known as a “super-resolution microscope” the engineers didn’t actually upgrade the microscope at all. Instead, they developed a hyperbolic metamaterial—materials with unique structures that manipulate light, originally developed to improve optical imaging—that’s applied to a microscope slide, onto which the sample is placed. This particular hyperbolic metamaterial is made from “nanometers-thin alternating layers of silver and silica glass” which have the effect of shortening and scattering the wavelengths of visible light that pass through it, resulting in a series of random speckled patterns.
Those speckled light patterns end up illuminating the sample sitting on the microscope slide from different angles, allowing a series of low-resolution images to be captured, each highlighting a different part. Those images are then fed into a reconstruction algorithm which intelligently combines them and spits out a high-resolution image.
Comparison of images taken by a light microscope without the hyperbolic metamaterial (left) and with the hyperbolic metamaterial (right): quantum dots.
Image: University of California San Diego
It’s not unlike the sensor-shift approach used in some digital cameras to produce super-resolution photos where the image sensor is moved ever so slightly in various directions while multiple images are captured and then combined to merge all of the extra details captured. This technology—detailed in a paper recently published in the Nature Communications journal—can boost a conventional light microscope’s resolution to 40 nanometers, while still allowing living organisms to be observed. It still can’t compete with what electron microscopes are capable of, but it’s no less remarkable given how easily it can improve the capabilities of more affordable and safer hardware already in use in labs all around the world.
