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Resolution and signal enhancement of Raman mapping by photonic nanojet of a microsphere

The field of Raman spectroscopy is constantly under improvement by development of methods of enhancement. One of the latest methods, based on photonic nanojet of a microsphere, has already been proven as simple, reliable and promising method for enhancement of single point micro Raman acquisition, but so far not for imaging. To extend the photonic nanojet method to Raman mapping, we have developed microsphere attached to two perpendicularly connected tapered optical fibers. When the sample is mapped, our two stemmed microsphere stays under the laser beam of the objective, providing enhancement on all points of the image. In addition to the demonstration of the feasibility of such enhancement, the first results reported here with 5-µm two stemmed microsphere on polycrystalline silicon substrate show ~4× signal enhancement of every point, and estimated ~3× resolution enhancement compared to the same setup without the microsphere. Detailed description of fabrication and characteristics of two stemmed microspheres is provided. Mapping images are analyzed and compared to atomic-force and scanning electron microscope images. Also, theoretical calculations of the photonic nanojet are provided. Successful implementation of two stemmed microsphere opens up a gateway and could serve as foundation for future improvements and applications in Raman imaging.

Raman imaging ; photonic nanojet ; optical microsphere ; Generalized Lorenz-Mie theory ; two stemmed microsphere ; optical fiber

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