In a recent publication in Optica, researchers from the University of Colorado Boulder have harnessed doughnut-shaped beams of light to capture detailed images of minuscule objects that traditional microscopes struggle to visualize.
This innovative technique holds promise for enhancing the functionality of various “nanoelectronics,” including the miniature semiconductors embedded in computer chips. The groundbreaking study was also featured in a special edition of Optics & Photonics News.
The research represents a significant stride in the realm of ptychography, a potent technique for scrutinizing extremely small entities. In contrast to conventional microscopes, ptychography tools don't directly observe diminutive objects. Instead, they project lasers onto a target and then measure the scattered light—a microscopic analogy to creating shadow puppets on a wall.
While ptychography has proven effective, it faced a notable limitation until now, as outlined by the study's 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,” explained Murnane, who is also a fellow at JILA, a collaborative research institute of CU Boulder and the National Institute of Standards and Technology (NIST). “It's a problem because that includes a lot of nanoelectronics.”
Many crucial technologies, such as certain semiconductors, consist of atoms like silicon or carbon arranged in regular patterns like a grid or mesh. Viewing these structures up close with ptychography has posed challenges.
In response, the researchers devised an ingenious solution in their latest study. Instead of employing traditional lasers, they generated beams of extreme ultraviolet light in the distinctive shape of doughnuts.
This novel approach enables the accurate imaging of delicate structures measuring approximately 10 to 100 nanometers—many times smaller than a millionth of an inch. The researchers anticipate further refinement to observe even smaller structures in the future. Notably, the doughnut-shaped beams, also known as optical angular momentum beams, are gentle on tiny electronics, avoiding potential harm that some existing imaging tools, such as electron microscopes, may inflict.
Looking ahead, this method holds the potential to inspect polymers used in manufacturing and printing semiconductors for defects without causing damage to these intricate structures. Margaret Murnane envisions a future where this groundbreaking technique transforms the landscape of nanoelectronics and offers unprecedented insights into the microscopic world.
Pushing the limits of microscopes
The forefront of microscopy is challenged by fundamental limits, shaped by the physics of light. Traditional imaging tools with lenses, due to these constraints, can only discern details down to a resolution of approximately 200 nanometers. This limitation proves inadequate for capturing the intricacies of many viruses that afflict humans, as an example. While cryo-electron microscopes offer a powerful means to observe viruses by freezing and immobilizing them, real-time monitoring of these pathogens in action remains elusive.
Ptychography, a technique pioneered in the mid-2000s, emerges as a potential solution to surpass this resolution limit. To comprehend its application, consider the analogy of shadow puppets. In this scenario, scientists aim to create a ptychographic image of a minute structure, say letters spelling “CU.” By directing a laser beam at the letters and scanning them multiple times, the interaction causes the beam to disperse, creating a intricate pattern analogous to shadows. Employing sensitive detectors, scientists capture and record these patterns, subsequently subjecting them to a series of mathematical equations. Over time, they reconstruct the complete shape of the puppets based on the shadows they cast.
“Instead of using a lens to retrieve the image, we use algorithms,” explains Margaret Murnane, the senior author of the study.
While this approach has proven effective for viewing submicroscopic shapes such as letters or stars, its applicability faces challenges with repeating structures like silicon or carbon grids. When subjected to a regular laser beam, these structures often produce a highly uniform scatter pattern. Ptychographic algorithms encounter difficulties interpreting patterns lacking sufficient variation.
This enigma has perplexed physicists for nearly a decade, prompting the exploration of innovative solutions to unlock the full potential of ptychography. Margaret Murnane and her colleagues aim to overcome these challenges, pushing the boundaries of microscopy and opening new avenues for real-time observation of dynamic microscopic processes, including those involving viruses, which could revolutionize our understanding of these elusive entities.
In their innovative study, Murnane and her team opted for a departure from the conventional. Rather than employing regular lasers to create their shadow puppets, they harnessed beams of extreme ultraviolet light. Taking it a step further, they utilized a spiral phase plate to impart a corkscrew or vortex shape to these beams. When these vortex-shaped beams, akin to doughnuts, interacted with a surface, they produced intricate shadow patterns.
Although devoid of pink glaze or sprinkles, these doughnut-shaped beams proved remarkably effective. The team observed that when these beams interacted with repeating structures, they generated significantly more complex shadow puppets compared to regular lasers.
To validate this novel approach, the researchers fabricated a mesh of carbon atoms with a minute break in one of the links. With this method, the team could pinpoint the defect with a level of precision unparalleled by other ptychographic tools.
Murnane emphasized the advantages of this approach, stating, “If you tried to image the same thing in a scanning electron microscope, you would damage it even further.”
Looking ahead, the team aims to refine their doughnut strategy, enhancing its accuracy to visualize even smaller and more delicate objects. The ultimate goal is to extend this technique to explore the intricate workings of living biological cells, opening unprecedented avenues for real-time observation at the microscopic scale.