Overcoming the diffraction limita­tion is a topic of great interest in recent research, and several approaches have been published in this area. Now, a team of researchers from the University of Kassel in Germany present an approach that uses micro­spheres placed directly on the surface of the object to extend the limits of inter­ferometric topography measurements for optical resolution of small structures.

Imaging below the resolution limit is often achieved with systems that use probe labeling, such as fluorescence microscopy, which requires prepara­tion of the sample. Other systems, such as atomic force micro­scopes, can provide 20 times better lateral resolution than diffraction-limited optical systems. However, they rely on tactile measurement principles that may be unsuitable for certain appli­cations, especially in bio-imaging. Therefore, micro­sphere assistance can provide a solution for fast and label-free imaging below the diffraction limit. 

A Linnik inter­ferometer setup comprising two highly resolving microscope objectives provides fast and non-contact topography measure­ments of fine structures. Performing a depth scan enables the acquisition of phase information that can be used to reconstruct the surface topography. With an addi­tional microsphere in the imaging path, the physical diffraction limit of this system is extended. Although experimental studies showed promising results, theoretical explana­tions considering the relevant imaging mechanisms enabling the improved reso­lution remained unclear till now.

The relevant mechanisms were examined by means of analysis in the 3D spatial frequency domain as well as by comparison with rigorous simula­tions and ray tracing computations. Inves­tigations in the Fourier domain give the spatial frequencies transmitted by the microsphere into the far field and obtained by the microscope objective. In combination with the rigorous simulations of the resulting near field, this allows a complete simulation of the imaging process with micro­spheres, and thus, extensive inves­tigations can be performed. In addition, ray tracing enables the inves­tigation of the propa­gation of individual light rays inside the microsphere and, therefore, contributes to a better under­standing of the major physical effects.

“In recent research as well as in industrial appli­cations, there is a need for fast measurement systems below the physical resolution limit that do not require extensive sample preparation. Microsphere-assisted inter­ference microscopy enables such optical topographic surface measure­ments, and this work contributes to a deeper understanding of the underlying physical mechanisms,” said Lucie Hüser. The researchers’ findings provide helpful tools for a deeper understanding of micro­sphere-assisted inter­ferometry, which can be used to enlarge the knowledge about physical mechanisms in microsphere-assisted inter­ferometry. Further­more, the effective enlargement of the numerical aperture of the system including the micro­sphere and the rather small field of view under the micro­sphere is likely the most relevant mechanism enabling topo­graphical measure­ments below the resolution limit. (Source: SPIE)

Reference: L. Hüser et al.: Microsphere assistance in interference microscopy with high numerical aperture objective lenses, J. Opt. Microsyst. 2, 044501 (2022); DOI: 10.1117/1.JOM.2.4.044501

Link: Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Kassel, Germany