New optical microscope «sees» atoms — scientists bypass diffraction

An international team of researchers has developed a new visualization method ULA-SNOM, which allows optical microscopes to distinguish parts as small as 1 nanometer.

Modern optical microscopes are widely used in medicine and materials science, allowing researchers to observe cells, viruses, and the behavior of particles in nanomaterials. However, even the most powerful microscopes have a fundamental limitation known as the diffraction limit. This prevents them from seeing objects smaller than 200 nanometers clearly.

Because of this limitation, scientists have been unable to observe how light interacts with individual atoms and molecules. This is critical for the further development of materials science, electronics, and quantum physics.

The new visualization method ULA-SNOM (scanning near-field optical microscopy with ultra-low amplitude of probe oscillations) allows researchers to observe how light behaves at the level of individual atoms. Prior to this, the following observations could be made only with the help of electron microscopes. 

To overcome the limitations of traditional optics, the researchers used a method called scanning near-field optical microscopy of the scattering type (s-SNOM). A sharp metal tip is illuminated by a laser and scans the surface of the material. 

Light is scattered across the surface to form patterns that detect objects at the nanoscale. However, traditional s-SNOM setups only achieve a resolution of 10-100 nanometers. Using the new approach, the researchers were able to bring the oscillations of the sharp point to an incredibly low level. The tip oscillated with an amplitude of from 0.5 to 1 nanometer, which is approximately the width of three atoms. 

This precise motion is large enough to register optical signals, but small enough to detect fine structural details. A larger amplitude would degrade the optical resolution, while a smaller amplitude would overload the signal with noise. 

The point was made of polished silver, which was given a precise shape using a focused ion beam to ensure a smooth and stable surface. A red laser beam was directed at the tip with a wavelength of 633 nanometers and a power of six milliwatts, creating a phenomenon called a plasmonic cavity, — a tiny confined pocket of light that forms between the tip and the surface of the sample.

This cavity was compressed to 1 cubic nanometer, allowing it to interact with the material at the level of individual atoms. To maintain the stability of this complex setup, the entire experiment was carried out in an ultra-high vacuum and at an ultra-low temperature of -265°C.

The low temperatures eliminated unnecessary vibrations and contamination, helping the scanning device to stay exactly 1 nanometer away from the surface. To filter out background light and enhance the actual signal, the researchers used a specialized method called — the most fashionable detection. This allowed us to obtain clearer and more reliable data.

The researchers used the ULA-SNOM facility and method to image 1-atom-thick silicon islands placed on the surface of silver. Despite the fact that these silicon layers were only one atom high, the microscope was able to clearly show where the silicon ends and the silver begins, not only in terms of shape, but also in terms of how each material reacts to light.

This confirmed that the new imaging method can capture true optical contrast with resolution at the level of individual atoms. In addition, the device simultaneously collected various types of information, measuring electrical conformity, mechanical properties, using the built-in capabilities of scanning tunneling microscopy and atomic force microscopy.

By analyzing the probe’s response at different vibration frequencies (harmonics), the scientists were able to separate the signals from the sources. In particular, the fourth harmonic revealed the clearest differences in the optical behavior of different materials.

For the first time, researchers were able to clearly see how a single atom or defect affects the optical properties of a material. This development could potentially lead to the precise design of nanostructures in electronics, the discovery of new photonic materials, or even the creation of more advanced solar cells that absorb light more efficiently.

The results of the study are published in Science Advances

Source: Interesting Engineering