Simultaneous spectra and dynamics processes tuning of a single upconversion microtube through Yb³⁺ doping...

Doping and varying pump laser parameters are the widely applied technological processes for tuning spectra to yield desirable luminescence properties and functions. For micro/nanocrystalline materials, doping is of fundamental importance in modifying electronic properties, modulating magnetism, as well as tuning the red-to-green luminescence ratio. Here we describe a tunable upconversion (UC) emission process in single NaYF₄:Yb/Er microtubes excitated with a focused laser. We show that the emission colours from single NaYF₄:Yb/Er microtubes can be rationally tuned in the red-to-green luminescence ratio and dynamics process by elevating Yb³⁺ ion concentration or pump power density. The underlying mechanism of spectral tuning is explored by using the power dependent UC luminescence, downconversion spectra and the temporal evolutions of UC emission from a series of single NaYF₄:Yb/Er microtubes. A mechanism of the red luminescence enhancement based on mediating electronic energy transfer channels by inducing three-photon processes is proposed for single microtubes. The natural decays of the luminescence levels are modified in the UC process relative to downconversion, which could be interpreted by using rate equations. Here, an insight into UC processes by use of unconventional focused experimental and theoretical techniques indicates the bidirectional feature of the electron transition of the interexcited-state by linear decays and UC processes via controlling external experimental parameters.