Spectral and spatial characterization of upconversion luminescent nanocrystals as nanowaveguides

Lanthanide upconversion (UC) luminescent nanocrystals exhibit a uniquely sharp multiband emission over a broad spectral bandwidth covering the ultraviolet region to the near-infrared (NIR) region when subjected to NIR excitation, which is vital for multichannel optical communication using wavelength-division multiplexing to achieve high transmission rates. In this study, we experimentally and theoretically investigated the spectral and spatial characterization of a single NaYF₄:Yb³⁺,Tm³⁺(Yb³⁺,Er³⁺) UC nanocrystal as a nanowaveguide. We suggest that a UC nanocrystal can be used as a nanowaveguide because it produces a range of output colors simultaneously and provides unaltered emission bands during propagation. Via the observation of single NaYF₄:Yb³⁺,Tm³⁺(Yb³⁺,Er³⁺) UC nanocrystals, we found, for the first time, that a single UC nanocrystal exhibited wavelength- and position-dependent UC emissions. In addition, by adding Ag coating to the UC nanocrystal to act as a plasmonic waveguide and introducing a photonic crystal, the scattering loss of the UC emissions was significantly suppressed in the middle of the NaYF₄ nanocrystal, indicating efficient light guiding through the UC nanocrystal. Our discovery provides a basic understanding of the use of UC nanocrystals as nanowaveguides at the single-nanoparticle level, expanding our knowledge of the performance optimization of UC nanomaterials.