Rare-earth phosphors exhibit unique luminescence polarization features originating from the anisotropic symmetry of the emitter ion's chemical environment. However, to take advantage of this peculiar property, it is necessary to control and measure the ensemble orientation of the host particles with a high degree of precision. Here, we show a methodology to obtain the photoluminescence polarization of Eu-doped LaPO nanorods assembled in an electrically modulated liquid-crystalline phase. We measure Eu emission spectra for the three main optical configurations (σ, π and α, depending on the direction of observation and the polarization axes) and use them as a reference for the nanorod orientation analysis. Based on the fact that flowing nanorods tend to orient along the shear strain profile, we use this orientation analysis to measure the local shear rate in a flowing liquid. The potential of this approach is then demonstrated through tomographic imaging of the shear rate distribution in a microfluidic system.

Lanthanide elements exhibit highly appealing spectroscopic properties that are extensively used for phosphor applications. Their luminescence contains precise information on the internal structure of the host materials. Especially, the polarization behavior of the transition sublevel peaks is a fingerprint of the crystal phase, symmetry, and defects. However, this unique feature is poorly explored in current research on lanthanide nanophosphors. We here report on a detailed investigation of the evolution of Eu luminescence during the thermally induced phase transition of LaPO nanocrystal hosts. By means of c-axis-aligned nanocrystal assemblies, we demonstrate a dramatic change of the emission polarization feature corresponding to the distinct Eu site symmetries in different LaPO polymorphs. We also show that changes of the nanocrystal structure can be identified by this spectroscopic method, with a much higher sensitivity than the X-ray diffraction analysis. This new insight into the nanostructure-luminescence relationship, associated with the unprecedented polarization characterizations, provides a new methodology to investigate phase transitions in nanomaterials. It also suggests a novel function of lanthanide emitters as orientation-sensing nanoprobes for innovative applications such as in bioimaging or microfluidics.