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Researchers from Regensburg and Birmingham overcome a fundamental limitation of optical microscopy. With the help of quantum mechanical effects, they succeed for the first time in performing optical measurements with atomic resolution.
From smartphone cameras to space telescopes, the desire to see ever finer detail has driven technological progress. Yet as we probe smaller and smaller length scales, we encounter a fundamental boundary set by light itself. Because light behaves as a wave, it cannot be focused arbitrarily sharply due to an effect called diffraction. As a result, conventional optical microscopes are unable to resolve structures much smaller than the wavelength of light, placing the very building blocks of matter beyond direct optical observation.
Now, researchers at the Regensburg Center for Ultrafast Nanoscopy, together with colleagues at the University of Birmingham, have found a novel way to overcome this limitation. Using standard continuous-wave lasers, they have achieved optical measurements at distances comparable to the spacing between individual atoms.
They achieve this incredible resolution by bringing a sharp metal tip extraordinarily close to the surface of a material under study – separated by a gap smaller than the size of a single atom. A continuous-wave laser illuminates the system, “squeezing” infrared light into the tiny gap and concentrating it at the tip’s apex. Confining light in this manner circumvents the diffraction limit and enables a spatial resolution on the order of the radius of curvature of the tip apex – typically about 10 nanometers.
While this already represents a dramatic improvement over conventional far-field techniques, it is still about a factor of 30 too coarse to resolve atomic-scale features. Determined to find the absolute limit of spatial resolution, the team kept moving the tip closer to the surface. What followed took them all by surprise. “At very small distances, the signal shot up dramatically,” says Felix Schiegl from the University of Regensburg. “We didn’t immediately understand what was happening. The real surprise came when we realized we were resolving atomic-scale features down to 0.1 nanometers.”
The explanation lies in quantum mechanics. Although the tip and the surface do not physically touch in the classical sense, electrons can still tunnel between them. The continuously oscillating electric field of the infrared light forces electrons to move back and forth between tip and surface. Much like electrons oscillating in a radio antenna, this motion produces a faint electromagnetic signal – and the researchers have been able to detect this near-field optical tunneling emission (NOTE).
“It is remarkable that just one electron moving over a distance smaller than the size of an atom every hundred cycles of the light can already produce light that is strong enough for us to detect,” says Dr. Tom Siday from the University of Birmingham. From this emitted light, the motion of electrons between tip and sample – and thereby material properties such as conductivity – can be measured with atomic-scale precision. “The decisive step is that we are no longer limited by how tightly light can be confined,” says Valentin Bergbauer from the University of Regensburg. “Instead, we directly control and measure quantum electron motion confined to atomic dimensions – a quantum leap that pushes optical microscopy to length scales nearly one hundred thousand times smaller than what conventional light-based microscopes can resolve.” Importantly, this effect can be driven using a standard continuous-wave laser, rather than more powerful but costly ultrafast lasers that were previously thought to be necessary. This simplicity could help make the technique more widely accessible and accelerate its adoption in laboratories around the world.
The work shows that optical measurements can now reach distances once thought inaccessible, made possible by precise control of atomically sharp tips. In the future, this novel approach could allow scientists to study how materials interact with light at the scale of individual atoms, providing insight into how microscopic processes at these tiny scales fundamentally determine the macroscopic properties of materials.
caption:
Artistic representation of the microscopic mechanism behind near-field optical tunneling emission: Laser light drives electrons (bright spheres) to leap back and forth between the apex atom of a sharp metallic tip (top) and a sample (bottom), giving rise to electromagnetic emission which enables all-optical microscopy at the atomic scale.
© Brad Baxley, PtW,
Dr Markus Huber
Faculty of Physics
University of Regensburg
Phone: +49 941 943 2064
Mail: markus.huber@ur.de
Felix Schiegl, Valentin Bergbauer, Svenja Nerreter, Valentin Giessibl, Fabian Sandner, Franz J. Giessibl, Yaroslav. A. Gerasimenko, Thomas Siday, Markus A. Huber & Rupert Huber, Atomic-Scale Optical Microscopy with Continuous-Wave Mid-Infrared Radiation.
In: Nano Letters. DOI: 10.1021/acs.nanolett.5c05319
Artistic representation of the microscopic mechanism behind near-field optical tunneling emission
Source: Brad Baxley, PtW
Copyright: Brad Baxley, PtW
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