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Doughnut-shaped laser beams help physicists see tiny objects

05 Dec 2023

Colorado researchers use new method to view items not visible by regular microscopy.

researchers at Colorado University Boulder have used doughnut-shaped beams of light to take detailed images of objects too tiny to view with traditional microscopes.

The researchers say that the new technique could help scientists improve the inner workings of a range of nanoelectronics including in computer chips. The development is described in the journal Optica.

The research is the latest advance in the field of ptychography, which is described as “a powerful technique for viewing very small things”. Unlike traditional microscopes, ptychography tools do not directly view small objects of interest. Instead, they direct lasers at a target, then measure how it scatters.

So far, the approach has worked remarkably well, with one major exception, said study senior author and Distinguished Professor of physics Margaret Murnane. “Until recently, it has completely failed for highly periodic samples, or objects with a regularly repeating pattern,” said Prof. Murnane, fellow at JILA, a joint research institute of CU Boulder and the U.S. National Institute of Standards and Technology (NIST). “It’s a problem because that includes a lot of nanoelectronics.”

She said many important technologies like some semiconductors are made up of atoms like silicon or carbon joined together in regular patterns. To date, those structures have proved difficult for scientists to view up close using ptychography.

In the new study, however, Prof. Murnane and her colleagues came up with a solution. Instead of using traditional lasers in their microscopes, they produced beams of extreme ultraviolet light in the shape of doughnuts. The team’s novel approach can collect accurate images of tiny and delicate structures that are roughly 10 to 100 nm in size.

In the future, the researchers expect to zoom in to view even smaller structures. The doughnut, or optical angular momentum, beams also will not disturb sensitive electronics in the process — as some existing imaging tools, like electron microscopes, sometimes can. “This method could be used to inspect the polymers used to make and print semiconductors for defects, without damaging those structures in the process,” Prof. Murnane said. Bin Wang and Nathan Brooks, who earned their doctoral degrees from JILA in 2023, were first authors of the new study.

The research, Prof. Murnane said, pushes the fundamental limits of microscopes; because of the physics of light, imaging tools using lenses can only see the world down to a resolution of about 200 nm. Scientists can freeze and kill viruses to view them with powerful cryo-electron microscopes, but cannot yet capture these pathogens in vivo and in real time.

Ptychography, which was pioneered in the mid-2000s, could help researchers push past that limit.

‘Doughnut microscopy’

In the new study, Prof. Murnane and her colleagues generated beams of extreme ultraviolet light, then employed a spiral phase plate to twist those beams into the shape of a vortex; when such a vortex of light shines on a flat surface, it forms a shape like a doughnut.

The team discovered that when these types of beams bounced off repeating structures, they created much more complex “shadow puppets” than regular lasers. To test out the new approach, the researchers created a mesh of carbon atoms with a tiny snap in one of the links. The group was able to spot that defect with precision not seen in other ptychographic tools.

“If you tried to image the same thing in a scanning electron microscope, you would damage it even further,” Prof. Murnane said. Moving forward, her team wants to make their doughnut strategy even more accurate, allowing them to view smaller and even more fragile objects – including, one day, the workings of living, biological cells.

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