  Optically-transparent immersion liquids with refractive index (n ~ 1.77) to
match sapphire-based aplanatic numerical aperture increasing lens (aNAIL) are
necessary for achieving deep 3D imaging with high spatial resolution. We report
that antimony tribromide (SbBr$_{3}$) salt dissolved in liquid diiodomethane
(CH$_{2}$I$_{2}$) provides a new high refractive index immersion liquid for
optics applications. The refractive index is tunable from n = 1.74 (pure) to n
= 1.873 (saturated), by adjusting either salt concentration or temperature;
this allows it to match (or even exceed) the refractive index of sapphire.
Importantly, the solution gives excellent light transmittance in the
ultraviolet to near-infrared range, an improvement over commercially-available
immersion liquids. This refractive index matched immersion liquid formulation
has enabled us to develop a sapphire-based aNAIL objective that has both high
numerical aperture (NA = 1.17) and long working distance (WD = 12 mm). This
opens up new possibilities for deep 3D imaging with high spatial resolution.
