  Fixing a diamond containing a high density of Nitrogen-Vacancy (NV) center
ensembles on the apex of a multimode optical fiber (MMF) extends the
applications of NV-based endoscope sensors. Replacing the normal MMF with a
tapered MMF (MMF-taper) has enhanced the fluorescence (FL) collection
efficiency from the diamond and achieved a high spatial resolution NV-based
endoscope. The MMF-taper's high FL collection efficiency is the direct result
of multiple internal reflections in the tapered region caused by silica, which
has a higher refractive index (RI) than the surrounding air. However, for
applications involving fluidic environments whose RI is close to or higher than
that of the silica, the MMF-taper loses its FL collection significantly. Here,
to overcome this challenge, we replaced the MMF-taper with an ultra-high
numerical aperture (NA) microstructured optical fiber (MOF) which is tapered
and sealed its air capillaries at the tapered end. Since the end-sealed air
capillaries along the tapered MOF (MOF-taper) have isolated the MOF core from
the surrounding medium, the core retains its high FL collection and NV
excitation efficiency in liquids regardless of their RI values. Such a
versatile NV-based endoscope could potentially find broad applications in
fluidic environments where many biological processes and chemical reactions
occur.
