KEYWORDS: Super resolution, Optical spheres, Objectives, Monte Carlo methods, Diffraction, Interferometry, Polymers, Glasses, Semiconducting wafers, 3D modeling
Scanning White Light Interferometry is a non-contacting method for three-dimensional (3D) surface characterization that provides Angstrom level vertical resolution and diffraction limited lateral resolution. This lateral resolution can be improved by implementing a photonic nanojet (PNJ) generating structure. The new method - Photonic Nanojet Interferometry (PNI) allows nanometer vertical resolution and lateral resolution better than 100 nm. In this work, a new design of a PNI system is proposed. The PNJ generating structure is a high refractive index microsphere embedded in a polymer material. We model the entire PNI objective in commercial software (Rsoft FullWAVE) and choose optimal parameters for the construct in such a way, that the working distance (FoV) is maximized while the width of the PNJ is kept below the diffraction limit. To test the new system, we imaged the data layer of a recordable Blu-ray Disc (BD). The results show that the proposed interferometer has two times higher magnification and two times larger field of view compared to the previous design featuring a 11 μm melamine formaldehyde micro-sphere. The new design also increases the fringe contrast by 1.5 times and provides easier handling of big samples by allowing them to be scanned.
Photonic nanojet interferometry (PNI) permits three dimensional (3D) label-free and super-resolution surface characterization. PNI is based on coherence scanning interferometry (CSI), featuring Ångstrom level vertical resolution. Being an optical far-field technique, CSI is diffraction limited and according to the Abbe criteria, can laterally resolve, points that are separated by a few hundred nanometers. We overcame this limitation by using dielectric microspheres that generate photonic nanojets. Now sub 100 nm features can laterally be resolved while preserving the vertical resolution of the CSI system. The microsphere material could be polymer or glass with a diameter between 8 and 12 μm, which limits the field of view (FoV) of the PNI system to ~10 μm2. Here we present a method to increase the FoV of a PNI based device by stitching a sequence of adjacent 3D images. We imaged a recordable Blu-ray Disc (BR-D) using a custom built Mirau type scanning white light interferometer with enhanced lateral resolution. Four 3D super-resolution images with constant 80% overlap, were stitched together using inhouse software. The resulting high fidelity image shows that 45% overlap and the above described procedure could be used to enlarge the FoV of label-free 3D super-resolution imaging systems.
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