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Coherence Scanning Interferometry (CSI) is a non-contact technique for three-dimensional (3D) surface characterization with angstrom-scale vertical resolution, but whose lateral resolution is diffraction limited. To enhance the lateral resolution, micro-optical elements like microspheres and fibres can be used [1,2]. This combination allows the design of an apparatus for 3D imaging with nanometre vertical resolution and a lateral resolution better than 100 nm.
We demonstrate the use of two-photon polymerization (2PP) for 3D-printing microspheres as a photonic nanojet (PNJ) generating structure. Since 2PP enables true 3D processing of photoresists at the microscale with sub-100 nm resolution, highly complex structures can be printed without compromises to their geometry [3]. We modelled imaging properties of different PNJ generating structures in open-source software using Monte Carlo path tracing and manufactured three versions of them using photoresists with different refractive indices.
The components were tested in a custom-made coherence scanning interferometer by imaging a sinusoidal structure with a period of 280 nm. Initial results show that the new PNJ generating structures, in combination with the CSI device, are capable of imaging structures with a lateral resolution better than 100 nm.
3D printing by 2PP allows mass-production of PNJ generating structures with complex shape without requiring any vacuum, harsh chemicals, or cleanroom environment. It also allows the PNJ generating structures to be manufactured straight onto a glass substrate, providing an integrated mounting solution for easy use with conventional optics. This will allow increased working distance and field of view as well as easier handling for the next generation 3D super-resolution imaging systems.
[1] I. Kassamakov, S. Lecler, A. Nolvi, A. Leong-Hoi, P. Montgomery, E. Hæggström, Sci. Rep. 7, 3683 (2017).
[2] A. Nolvi, I. Laidmäe, G. Maconi, J. Heinämäki, E. Hæggström, I. Kassamakov, Proc. SPIE 10539, 1053912-1 (2018).
[3] M. Farsari, B. Chichkov, Nature Photon. 3, 450 (2009).
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