  A promising approach of designing mesostructured materials with novel
physical behavior is to combine unique optical and electronic properties of
solid nanoparticles with long-range ordering and facile response of soft matter
to weak external stimuli. Here we design, practically realize, and characterize
orientationally ordered nematic liquid crystalline dispersions of rod-like
upconversion nanoparticles. Boundary conditions on particle surfaces, defined
through surface functionalization, promote spontaneous unidirectional
self-alignment of the dispersed rod-like nanoparticles, mechanically coupled to
the molecular ordering direction of the thermotropic nematic liquid crystal
host. As host is electrically switched at low voltages~ 1V, nanorods rotate,
yielding tunable upconversion and polarized luminescence properties of the
composite. We characterize spectral and polarization dependencies, explain them
through invoking models of electrical switching and upconversion dependence on
crystalline matrices of nanorods, and discuss potential practical uses.
