Upconversion spectroscopy and properties of NaYF4 doped with Er3+Er3+, Tm3+Tm3+ and/or Yb3+Yb3+

A spectroscopic investigation of NaYF4 powders doped with several different concentrations of Er3+Er3+, Tm3+Tm3+ and/or Yb3+Yb3+ is described. Rare earth-doped NaYF4 is known to be a very efficient near-infrared to visible upconverter. The overview emission spectra for all samples are presented and from these the upconversion efficiency is calculated. Raman spectroscopy of undoped NaYF4 is presented here for the first time, demonstrating that the dominant phonon modes in NaYF4 lie between 300 and 400cm-1. The fact that these phonon modes are also the optically active ones is further demonstrated by temperature-dependent excitation spectroscopy. These surprisingly low-energy phonon modes explain the extraordinarily high upconversion efficiency of the rare earth-doped NaYF4 samples. Excitation spectroscopy up to ∼70000cm-1 in an NaErF4 sample reveals a multitude of Er3+Er3+ 4f excitations, including the illustrious 2F(2)5/2 one that has not been observed in excitation spectroscopy before. From the low-temperature power-dependence of the emission intensities for an Er3+Er3+, Yb3+Yb3+ codoped NaYF4 sample, it is concluded that the dominant upconversion mechanism at low temperature is a different one than at room temperature. From direct excitation, the lifetimes of the Yb3+F5/22→F7/22, Er3+F9/24→I15/24 and Er3+S3/24→I15/24 emissions are determined as a function of temperature for all samples. At elevated temperatures, a significant decrease in the green lifetime is observed, which is correlated to a simultaneous quenching in the luminescence intensity. This quenching is ascribed to cross-relaxation between two nearby Er3+Er3+ ions.