Viruses, proteins, and biomolecules are not only some of the most important elements of biological study, but also some of the smallest. These nanoscale objects are smaller than the diffraction limit of most optical imaging techniques, making their observation challenging. Fluorescence microscopy is one technique to overcome this obstacle, using nanoscale markers to identify objects of interest, but the application of labels can itself create challenges and limitations.

In recent years, a label-free imaging method using contact microspheres emerged as a solution. However, its ability to observe nanoscale features with a resolution apparently exceeding the classical diffraction limit defied explanation. Maslov and Astratov have employed numerical modeling to develop a theoretical understanding of this phenomenon.

According to their theory, a key part of achieving sub-diffraction resolution with microsphere-assisted imaging involves illuminating the object with multiple plane waves from many different angles.

“In our model, each partial image formed by a particular plane wave has a coherent nature,” said author Alexey Maslov. “We show how incoherent summation of such partial images builds the sub-diffraction features of the object step-by-step by increasing the illumination cone. This subtle interplay of coherent and incoherent properties is a key to achieving sub-diffraction imaging.”

The authors plan to expand their model to three dimensions and improve the computational efficiency, while defining ways to verify their model experimentally.

“We would also like to turn our results in a direction of predicting resolution values, which would be determined by the experimental physicists for different microspheres and different geometrical shapes of the objects,” said author Vasily Astratov.

Source: “Origin of the super-resolution of microsphere-assisted imaging,” by Alexey V. Maslov and Vasily N. Astratov, Applied Physics Letters (2024). The article can be accessed at