In a new study, CU Boulder researchers used donut-shaped beams of light to capture detailed images of objects that are too small to see with traditional microscopes.
Advances in Nanoelectronics Imaging
The new technique will help scientists develop the inner workings of a variety of “nanoelectronics,” including miniature ones. semiconductors of computer chips. The discovery was highlighted on December 1 in a special issue of Optics & Photonics News called Optics in 2023.
Ptychography: A Lens on the Microscopic World
The research is the latest development in the field of ptychography, a difficult to pronounce (the “p” is silent) but powerful technique of seeing small objects. Unlike traditional microscopes, ptychography tools do not look directly at small objects. Instead, they shine lasers on a target, then measure how the light scatters—the microscopic equivalent of making a shadow puppet on a wall.
Overcoming the Ptychography Challenge
So far, the method has worked very well, with one major exception, said senior study author and Distinguished Professor of physics Margaret Murnane.
“Until recently, this has completely failed for constant samples, or objects with a constant repeating pattern,” said Murnane, a fellow at JILA, a joint research institute. at CU Boulder and the National Institute of Standards and Technology (NIST). “It’s a problem because that involves a lot of nanoelectronics.”
He noted that many important technologies such as some semiconductors are made up of atoms such as silicon or carbon joined together in regular patterns such as a small grid or mesh. Until now, those structures have proven difficult for scientists to visualize closely using ptychography.
Enlightenment With Donut-Shaped Light
In the new study, however, Murnane and his colleagues have come up with a solution. Instead of using traditional lasers in their microscopes, they produce beams of extreme ultraviolet light in the shape of doughnuts.
The team’s new method can collect precise images of small and delicate structures roughly 10 to 100 nanometers in size, or many times smaller than one millionth of an inch. In the future, researchers hope to zoom in to see even smaller structures. Donut, or optical angular momentum, beams also don’t damage small electronics in the process—as some imaging devices, such as electron microscopes, sometimes can.
“In the future, this method can be used to inspect the polymers used to make and print semiconductors for defects, without damaging the structures in the process,” said Murnane.
Bin Wang and Nathan Brooks, who earned their doctoral degrees from JILA in 2023, are the first authors of the new study.
Pushing the Limits of the Microscope
The research, Murnane says, 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 nanometers — which isn’t enough. accurate to remove many viruses. , for example, which harms people. Scientists can freeze and kill viruses to see them with the powerful cryo-electron microscope, but cannot yet capture these pathogens in action and in real time.
Ptychography, pioneered in the mid-2000s, helps researchers overcome that limitation.
The Mechanics of Ptychography
To understand how, go back to shadow puppets. Imagine that scientists want to collect a ptychographic image of a very small structure, perhaps letters that spell “CU.” To do that, they first zap a laser beam at the letters, scanning them multiple times. When the light hits the “C” and the “U” (in this case, the puppets), the beam breaks and scatters, creating a complex pattern (the shadows). Using sensitive detectors, scientists record the patterns, then analyze them with a series of mathematical equations. With enough time, Murnane explained, they recreate the shape of their puppets completely from the shadows they cast.
“Instead of using a lens to capture the image, we use algorithms,” Murnane said.
He and his colleagues had previously used such a method to visualize submicroscopic shapes such as letters or stars.
But the method doesn’t work on repeating structures like silicon or carbon grids. If you shine a regular laser beam on a semiconductor with such regularity, for example, it will often create a scatter pattern that is incredibly uniform—ptychographic algorithms struggle to understand patterns without There are many differences in this.
The problem has left physicists scratching their heads for nearly a decade.
In the new study, however, Murnane and his colleagues decided to try something different. They don’t make their shadow puppets with regular lasers. Instead, they produce beams of extreme ultraviolet light, then use a device called a spiral phase plate to twist those beams into the shape of a corkscrew, or vortex. (When a vortex of light shines on a flat surface, it creates a shape like a donut.)
Donut beams don’t have pink glaze or sprinkles, but they do the trick. The team discovered that when these types of beams bounced off repeating structures, they created more complex shadow puppets than regular lasers.
To test the new method, the researchers created a mesh of carbon atoms with a small snap in one of the links. The team found such a flaw in accuracy that other ptychographic tools have not seen.
“If you try to visualize the same thing with a scanning electron microscope, you’re going to damage it even more,” Murnane said.
Progress To Finer Details
Going forward, his team wants to make their donut strategy more precise, allowing them to look at smaller and more fragile objects—including, one day, the workings of living, biological organisms. cell.
Reference: “High-fidelity ptychographic imaging of very periodic structures enabled by vortex high harmonic beams” by Michael Tanksalvala, Henry C. Kapteyn, Bin Wang, Peter Johnsen, Yuka Esashi, Iona Binnie, Margaret M. Murnane, Nicholas W. Jenkins and Nathan J. Brooks, 19 September 2023, Optica.
Other co-authors of the new study include Henry Kapteyn, professor of physics and JILA fellow, and current and former JILA graduate students Peter Johnsen, Nicholas Jenkins, Yuka Esashi, Iona Binnie and Michael Tanksalvala. .
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