‘Donut’ beam helps physicists observe incredibly small objects

In a new study published in opticaresearchers at the University of Colorado Boulder used donut-shaped rays to Detailed images of objects too small to see using a conventional microscope.

The new technology could help scientists improve the inner workings of a variety of “nanoelectronics,” including the tiny semiconductors inside computer chips. The discovery also highlighted Published in a special issue of Optics & Photonics News.

This research is the latest advance in the field of ptychography, a difficult to pronounce (the ‘p’ is silent) but powerful technique for seeing very small things. Unlike traditional microscopes, ptychography tools do not directly observe small objects. Instead, it shines a laser at a target and measures how the light is scattered. This is a bit like making shadow puppets on a wall when viewed through a microscope.

So far, this approach has worked surprisingly well, with one major exception, said Margaret Mahne, the study’s lead author and distinguished professor of physics.

“Until recently, it had completely failed for very periodic samples or objects with regularly repeating patterns,” says the UW-Boulder/National Institute of Standards and Technology (NIST) joint research institute. Mr. Marne, a JILA researcher, says: “This is problematic because it has a lot of nanoelectronics in it.”

She pointed out that many important technologies, such as some semiconductors, are composed of atoms such as silicon and carbon bonded in regular patterns, like lattices or meshes. So far, it has proven difficult for scientists to observe these structures up close using ptychography.

But in a new study, Murunet and colleagues have devised a solution. Instead of using a traditional laser in a microscope, they generated a donut-shaped beam of extreme ultraviolet light.

The researchers’ new approach can collect precise images of small, delicate structures that are around 10 to 100 nanometers in size, or many times smaller than a millionth of an inch. In the future, researchers expect to be able to zoom in and observe even smaller structures. The donut beam, or angular momentum beam of light, also does not damage small electronic equipment during the process, as existing imaging tools such as electron microscopes do.

“In the future, this method could be used to inspect polymers used in semiconductor manufacturing and printing for defects without damaging the semiconductor structure during the process.” Marne said.

Pushing the limits of microscopy

Mahne said this research pushes the fundamental limits of microscopy. Because of the physics of light, lens-based imaging tools can only see the world to a resolution of about 200 nanometers, which is not precise enough to capture many viruses. For example, those that infect humans. Although scientists can freeze viruses to death and view them with powerful cryo-electron microscopes, they still cannot capture the activity of these pathogens in real time.

Ptychography, developed in the mid-2000s, could help researchers break through that limit.

To understand how, go back to shadow puppets. Imagine a scientist wants to collect stylized images of very small structures, perhaps the letters that spell out “CU.” To do this, they first shine a laser beam on the letters and scan them multiple times. When light hits “C” and “U” (in this case, the dolls), the light rays break and scatter, creating a complex pattern (shadow). Scientists record those patterns using sensitive detectors and analyze them using a series of mathematical formulas. Given enough time, they will perfectly recreate the shape of the doll from the shadow it casts, Mahne explained.

“Instead of using a lens to capture an image, we use an algorithm,” Mahne says.

She and her colleagues have previously used such approaches to observe submicroscopic shapes such as letters and stars.

However, this approach does not work for repeating structures like silicon or carbon grids. For example, shining a regular laser beam onto a semiconductor with such regularity often produces an incredibly uniform scattering pattern. Ptychographic algorithms have a hard time understanding patterns that don’t vary much.

Physicists have been puzzling over this question for nearly a decade.

donut microscopy

But in the new study, Murunet and colleagues decided to try something different. They didn’t use regular lasers to make shadow puppets. Instead, they generated beams of extreme ultraviolet light and used a device called a spiral phase plate to twist those beams into a spiral, or vortex. (When such a vortex of light hits a flat surface, it becomes shaped like a donut.)

There was no pink glaze or sprinkles on the donut beams, but it worked. The researchers found that when this type of beam repeatedly bounces off a structure, it creates a much more complex shadow puppet than a regular laser.

To test the new approach, the researchers created a mesh of carbon atoms with a small snap in one of the links. The group was able to find the flaws with an accuracy not seen with other ptychographic tools.

“If you try to image the same thing with a scanning electron microscope, you’re going to do even more damage,” Mulneh says.

Looking ahead, her team hopes to make the donut strategy even more precise, allowing them to observe smaller and more fragile objects, including the workings of living biological cells, in the future.