For generations, scientists have chased sharper vision. Every improvement in cameras and microscopes has brought the world into slightly better focus, revealing patterns that were once invisible. But deep down, light has always guarded a hard boundary. Because it spreads like a wave, it refuses to be squeezed beyond a certain point. No matter how clever the lenses, optical microscopes have been unable to directly see the very atoms that make up matter. Those building blocks have lived in a realm beyond light.
That long-standing limit has now been challenged. Researchers from the University of Regensburg and the University of Birmingham have demonstrated something that once seemed impossible: optical measurements with atomic resolution. Their results, published in Nano Letters, show that light can be used to probe distances comparable to the spacing between individual atoms. It is not a trick of better lenses or stronger lasers. It is a quiet collaboration between light and quantum mechanics, unfolding in a space smaller than an atom itself.
A Tiny Gap Where Light Is Forced to Behave
The story begins with a familiar frustration. Light has a wavelength, and because of diffraction, it resists being focused into spots much smaller than that wavelength. This is why conventional optical microscopes blur out when asked to see extremely small structures. The rules are not about technology; they are written into the wave nature of light.
To bend those rules, the researchers did not fight light head-on. Instead, they guided it into an unusual situation. They brought a sharp metal tip extraordinarily close to the surface of a material, leaving a gap smaller than the size of a single atom. Into this almost impossibly narrow space, they shone infrared light from a continuous-wave laser.
The light did not spread out as usual. It was squeezed into the gap, concentrated at the very apex of the metal tip. In this confined near field, the usual diffraction limit no longer applied. The achievable spatial resolution was no longer tied to the wavelength of light but to the shape of the tip itself, typically around 10 nanometers.
That alone was a dramatic leap beyond traditional optics. Yet atoms were still out of reach. Ten nanometers is vast on an atomic scale. The team wanted to know how far this approach could truly go.
The Moment the Signal Refused to Stay Quiet
So they pushed further. Slowly, carefully, they moved the tip closer and closer to the surface, shrinking the gap beyond what seemed reasonable. Then something unexpected happened.
“At very small distances, the signal shot up dramatically,” recalls Felix Schiegl from the University of Regensburg. At first, the behavior made no sense. The light should not have changed so suddenly. But as the data accumulated, a startling pattern emerged. The measurements were resolving features as small as 0.1 nanometers, well within the scale of individual atoms.
This was not just better resolution. It was a complete shift in what was being measured.
The explanation did not lie in optics alone. It lay in the strange rules of the quantum world.
Electrons Crossing Without Touching
In everyday experience, two objects separated by a gap do not interact. In quantum mechanics, that intuition breaks down. Even when the metal tip and the surface never physically touch, electrons can still move between them through a process known as quantum tunneling.
The continuously oscillating electric field of the infrared light forced electrons to shift back and forth across this atom-sized gap. Each movement was tiny, smaller than the size of an atom, and rare, happening only every hundred cycles of the light. Yet this motion was enough.
Just like electrons oscillating in a radio antenna, the tunneling electrons produced a faint electromagnetic signal. The researchers were able to detect this emission, known as near-field optical tunneling emission, and use it as a direct probe of electron motion.
“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.
In this regime, the microscope was no longer limited by how tightly light could be focused. Instead, it was reading out the motion of individual electrons confined to atomic dimensions.
When Optical Microscopy Becomes Quantum Measurement
This shift marks a profound change in perspective. Traditional optical microscopy tries to see smaller objects by shaping light more precisely. In this new approach, light becomes a tool to drive and read quantum electron motion.
“The decisive step is that we are no longer limited by how tightly light can be confined,” explains Valentin Bergbauer from the University of Regensburg. “Instead, we directly control and measure quantum electron motion confined to atomic dimensions.”
The scale of this leap is difficult to overstate. Conventional light-based microscopes are restricted to features thousands of times larger than atoms. By tapping into quantum tunneling, the researchers pushed optical measurements to length scales nearly 100,000 times smaller than what those microscopes can resolve.
From the detected light, they could infer how electrons moved between the tip and the sample. That motion, in turn, reveals material properties such as conductivity, now measurable with atomic-scale precision.
The Power of Keeping It Simple
One of the most surprising aspects of this breakthrough is what it does not require. For years, it was assumed that such extreme measurements would demand powerful, expensive ultrafast lasers. Instead, the effect emerges using a standard continuous-wave laser, a far simpler and more accessible tool.
This simplicity matters. It means the technique is not locked away in a handful of specialized facilities. With precise control of atomically sharp tips, laboratories around the world could adopt this method without completely rethinking their equipment.
The discovery shows that the barrier to atomic-scale optical measurements was not the weakness of lasers, but a missing understanding of how light and electrons can work together at the smallest scales.
Why This Research Changes the Way We Look at Matter
This work matters because it redraws a boundary that once seemed fundamental. For the first time, optical measurements have reached distances that belong to the atomic world. That opens a new window onto how materials interact with light at their most basic level.
At these scales, the behavior of just a few electrons can determine the properties of an entire material. Conductivity, optical response, and other macroscopic traits all emerge from processes happening between individual atoms. By directly probing those processes with light, scientists gain a powerful way to connect the quantum world to the materials we use every day.
More broadly, this research shows that limits in science are often invitations rather than walls. The diffraction limit of light stood firm for centuries, not because it could never be crossed, but because the path around it required thinking in quantum terms. By listening to the faint glow of a single tunneling electron, researchers have shown that light can reach places it was never supposed to go.
Study Details
Felix Schiegl et al, Atomic-Scale Optical Microscopy with Continuous-Wave Mid-Infrared Radiation, Nano Letters (2026). DOI: 10.1021/acs.nanolett.5c05319
