J. Eur. Opt. Society-Rapid Publ. 2025, 21, 16 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025011 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
EOSAM 2024 Guest editors: Luca De Stefano and Raffaele Velotta
RESEARCH ARTICLE
Microsphere-assistance in conventional, interference and confocal microscopy – modeling and experimental results
Lucie Hüser *, Tobias Pahl, Sebastian Hagemeier, Tim Eckhardt, Felix Rosenthal, Michael Diehl, and Peter Lehmann Measurement Technology Group, Faculty of Electrical Engineering and Computer Science, University of Kassel, Wilhelmshöher Allee 71, 34121 Kassel, Germany
Received 5 December 2024 / Accepted 5 March 2025
Abstract. Topographical as well as microscopic imaging of nanoscale surfaces plays a pivotal role across various disciplines. Nevertheless, achieving fast, label-free, and accurate characterization of laterally expanded structures below the diffraction limit remains challenging. Recent studies highlight the use of microsphere assistance for resolution improvement. Confocal, conventional, and interference microscopy, augmented by microspheres, enable the imaging of small structures that were previously inaccessible. In this contribution, results of microsphere-assisted confocal and conventional bright-field microscopy( MAM) are compared to underline the decisive role of the confocal effect. Furthermore, an extensive comparison of simulated confocal results is presented to highlight the experimental results.
Keywords: Resolution, Microsphere, Confocal microscopy, Simulation, Microsphere-assisted microscopy.
1 Introduction
In the characterization process of nanostructured surfaces, topographical measurements play a crucial role throughout different disciplines from quality control in fabrication processes to biological tissue observation. In order to further analyze surface features, the characterization process must be accurate, fast and, ideally, label-free. In microscopic and interferometric measurements, however, the system is fundamentally diffraction-limited in its lateral resolution capabilities.
With the help of microspheres, the lateral resolution of an imaging system can be extended so that surface measurements are even possible below the system’ s diffraction limit [ 1, 2 ]. This is demonstrated in detail for both, microscopic and interferometric applications [ 3 – 11 ]. The theoretical background has also been highlighted in recent studies [ 12 – 16 ], including a complete simulation model that incorporates a rigorous simulation of the crucial near-field wave propagation involving the microsphere [ 17 ]. Furthermore, advances have been made in mounting and application techniques of microspheres [ 18 – 21 ].
Confocal microscopy is a powerful tool that enhances the resolution capabilities of a conventional microscope significantly [ 22 ]. The additional confocal influence when imaging with microsphere assistance is discussed in [ 23 – 25 ] and also shown in [ 26 ]. Utilizing the unique optical
* Corresponding author: lucie. hueser @ uni-kassel. de properties of microspheres in combination with confocal microscopy for topographic measurements further increases resolving power while allowing fast processing and easy sample preparation. Measurement results show how the use of microspheres enables imaging topographical structures otherwise not accessible.
Further, we demonstrated that conventional, confocal and interference microscopes can be simply modeled based on the same theory with only slight adaptations to the respective measurement technique [ 27 ]. Based on these studies, we briefly introduce how to simulate microcylinderassisted microscopy with focus on surface topography measurement.
Finally, studies obtained by using an finite element method( FEM) model are shown underlining the demonstrated resolution capabilities of the microsphere-assisted confocal microscope.
Parts of the outcomes presented here have already been shown beforehand [ 28, 29 ] and are extended in the context of this topical issue.
2 Methodology and instrumentation
The presented measurement results are obtained using a commercial confocal microscope( Nanofocus lsurf custom). With a numerical aperture( NA) of 0.95 and a central illumination wavelength of k = 505 nm( LUXEON Rebel, spectral half-width = 30 nm) a high-resolution topographical measurement is already possible using the confocal effect
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