Tiny Miracles Revealed: How Donut Beams Solve Microscopic Mysteries

In a new study, researchers at the University of Colorado Boulder used a donut-shaped beam to take detailed images of objects too small to see with traditional microscopes.

Progress in Nanoelectronic Imaging

The new technology could help scientists improve the inner workings of a range of “nanoelectronics,” including microelectronics semiconductor in computer chips. The findings are highlighted in a special issue on December 1 Optics and Photonics News is called Optics in 2023.

Overprinting: a lens for observing the microscopic world

The research is the latest advance in the field of superimposition, a hard-to-pronounce (the “p” is silent) but powerful technique for seeing very small things. Unlike traditional microscopes, overprint tools cannot directly observe small objects. Instead, they shine a laser on a target and measure how the light scatters—sort of like creating the microscopic effect of a shadow puppet show on a wall.

Overcome Overprinting Challenges

Margaret Murnane, the study’s senior author and Distinguished Professor of Physics, said this approach has worked very well so far, with one major exception.

“Until recently, it failed completely for highly periodic samples or objects with regularly repeating patterns,” said Murnane, a researcher at JILA, a joint institute of the University of Colorado Boulder and the National Institute of Standards and Technology (NIST). . “That’s a problem because there’s a lot of nanoelectronics involved.”

She noted that many important technologies, such as some semiconductors, are made of atoms such as silicon or carbon linked together in regular patterns, such as small grids or grids. Until now, it has proven difficult for scientists to use lamelligraphy to see these structures up close.

Ring light breakthrough

In the new study, however, Munan and her colleagues came up with a solution. Instead of using a traditional laser in the microscope, they generated a donut-shaped beam of extreme ultraviolet light.

The team’s novel method can collect precise images of tiny and delicate structures with dimensions on the order of 10 to 100 nanometers, or many times smaller than a millionth of an inch. In the future, the researchers hope to zoom in to observe smaller structures. Ring beams or optical angular momentum beams also do not damage microelectronics in the process, as some existing imaging tools, such as electron microscopes, sometimes do.

“In the future, this method could be used to inspect polymers used to make and print semiconductors for defects without damaging these structures in the process,” Moonan said.

Bin Wang and Nathan Brooks, first authors of the new study, received their PhDs from JILA in 2023.

Pushing the limits of microscopy

Murnane said the research pushes a fundamental limit of microscopy: Due to the physics of light, imaging tools using lenses can only see the world at a resolution of about 200 nanometers, which is not precise enough to capture many viruses that, for example, infect Humanity. Scientists can freeze and kill viruses so they can be viewed with powerful cryo-electron microscopy, but there is no way to instantly capture the activity of these pathogens.

Overlay techniques pioneered in the mid-2000s could help researchers overcome this limitation.

The mechanics of overprinting

To understand how this works, go back to those shadow puppet shows. Imagine that a scientist wants to collect an overprinted image of a very small structure, perhaps the letters that spell out “CU.” To do this, they first illuminated the letters with a laser beam and scanned them multiple times. When light hits “C” and “U” (in this case, the puppets), the beams split and scatter, creating complex patterns (shadows). Scientists record these patterns using sensitive detectors and then analyze them using a series of mathematical equations. Murnane explains that given enough time, they can reshape the puppet entirely based on the shadow it casts.

“We don’t use lenses to retrieve images, we use algorithms,” Murnane said.

She and her colleagues have previously used this method to look at submicroscopic shapes such as letters or stars.

But this method is not suitable for repeating structures such as silicon or carbon grids. For example, if you shine a regular laser beam on a semiconductor with such regularity, it will usually produce an extremely uniform scattering pattern, and it will be difficult for an overprinting algorithm to understand a pattern without much variation.

This question has puzzled physicists for nearly a decade.

donut microscope

In this new study, however, Munan and her colleagues decided to try something different. They didn’t use ordinary lasers to make the shadow puppets. Instead, they generate beams of extreme ultraviolet light and then use a device called a helical phase plate to twist these beams into a spiral or vortex shape. (When such a vortex of light hits a flat surface, it forms a donut-like shape.)

The donut crossbows don’t have a pink glaze or powdered sugar, but they get the job done. The team found that when these types of beams bounce off repeating structures, they create more complex shadow puppets than ordinary lasers.

To test the new method, the researchers created a network of carbon atoms with a tiny snap in one of the links. The team was able to spot the defect with a precision not seen with other overprinting tools.

“If you try to image the same object in a scanning electron microscope, you’ll damage it further,” Moonan said.

Moving towards finer details

Going forward, her team hopes to make their donut strategy more accurate, allowing them to observe the workings of smaller, more fragile objects—including, one day, living biological cells.

Reference: “High-fidelity stack imaging of highly periodic structures by vortex harmonic beams” Author: Michael Tanksalvala, Henry C. Kapteyn, Bin Wang, Peter Johnsen, Yuka Esashi, Iona Binnie, Margaret M. Murnane , Nicholas W. Jenkins and Nathan Brooks, September 19, 2023 Optics.
DOI: doi:10.1364/OPTICA.498619

Other co-authors of the new study include physics professor and JILA researcher Henry Kapteyn, as well as current and former JILA graduate students Peter Johnsen, Nicholas Jenkins, Yuka Esashi, Iona Binnie and Michael Tanksalvala.