Cooperative Energy Transfer Upconversion Research Articles

Fluorescence study of Ho3+/Yb3+ ions codoped Bismuth-boro-tellurite glasses

Ho/Yb co-doped Bismuth boro-tellurite glasses were synthesized by conventional melt-quench methods. Thermal properties of the prepared samples were carried by differential scanning calorimetry. UV–VIS-IR absorption spectra were studied for the calculation of forbidden energy gap and refractive index of prepared glasses. For the study of photoluminescence samples were excited by 980 nm laser diode, intense green and red emissions were detected easily even by naked eyes. The observed intense red emission is thought to be caused by a combination of excited state absorptions between different levels of Ho ions and cooperative energy transfer upconversion from a pair of Yb donor ions to one Ho acceptor ion, in addition to the usual direct energy transfer process from Yb3+ to Ho3+.

Universal Emission Characteristics of Upconverting Nanoparticles Revealed by Single-Particle Spectroscopy

Upconverting nanoparticles (UCNPs) have been extensively investigated for nanophotonics and biomedical applications. However, establishing a unified view of their emission characteristics to elucidate the underlying photophysics and expand the application fields of these materials is a great challenge due to their sophisticated internal energy transfer and lack of standardized single-particle spectroscopy (SPS) platform. Here, we present an SPS technique called multiband single-particle irradiance-dependent imaging (multiband SPIDI). We demonstrate that the emission characteristics of Yb3+,Tm3+-doped UCNPs are universal for three emission bands over a wide range of irradiance and dependent on the Tm3+ doping concentration, indicating that the number of emitted photons of each band is proportional to the number of activator ions and is dependent on the number of absorbed photons and the activator interionic distance. We also suggest a cooperative energy transfer upconversion (CETU) mechanism for transition to a higher-energy state through photon accumulation. For a single UCNP, the emission at 800 nm is detectable at an ultralow irradiance of 4.9 W cm-2; moreover, that at 450 nm is measurable at 98 W cm-2, based on the optimal concentration. These findings based on the multiband SPIDI platform can provide insights into the interionic energy transfer by studying irradiance-dependent steady-state dynamics to achieve brighter UCNPs and their broader applications.

Self-sensitization induced upconversion of Er3+ in core-shell nanoparticles.

A mechanistic study of upconversion from lanthanides is of great importance for the fundamental research of upconversion materials and their diverse frontier applications. However, the most efficient upconversion of lanthanides is still obtained in a commonly used sensitizer-activator coupled system. Here we report a mechanistic investigation on the upconversion of Er3+ through self-sensitization which is applicable for 808, 980 and 1530 nm excitations. It is found that the cooperative energy transfer upconversion followed by cross-relaxation occurring among Er3+ ions plays a critical role in producing and enhancing the red upconversion for the samples with high dopant concentrations (e.g., >20 mol%). The red upconversion color can be further purified and enhanced by mediating the upconversion dynamics through introducing the lanthanides of Ho3+, Tm3+ and Yb3+, which can effectively contribute to the population in the red emitting state. Moreover, the energy migration in the Er-sublattice was also found to be a possible origin for quenching upconversion, which was proved and effectively suppressed by designing a tri-layered nanostructure where the distribution of Er3+ is spatially controllable. Our results gain access into the insight of upconversion dynamics in self-sensitization induced upconversion which would help the search for other new kinds of upconversion materials.

Intense red fluorescence from Ho/Yb codoped tellurite glasses

By optimizing the concentration ratio of Ho/Yb and using conventional melting and quenching methods, a group of Ho/Yb co-doped tellurite glasses with host composition of 78TeO2–12ZnO–10Na2O (mol.%, TZN) were made for red fiber laser development. Excited by 976nm laser diode, intense green and red emissions were detected easily even by naked eyes. TZN glass codoped with 0.5wt.% Ho2O3 and 3wt.% Yb2O3 was found to present the strongest red emission with the intensity about three times higher than that of the green one, presenting a yellowish-orange color as observed. Besides direct energy transfer process from Yb to Ho as usual, it is believed that cooperative energy transfer upconversion from a pair of Yb donor ions to one Ho acceptor ion and excited state absorptions among different levels of Ho ion are commonly responsible for the observed intense red emission. © 2012 Elsevier B.V. All rights reserved.

Intense green upconversion emission in Tb^3+/Yb^3+ codoped alumino-germano-silicate optical fibers

We observed intense green upconversion emission from Tb(3+) centered at 546 nm due to transition D(4)5-->F(5)7 in Tb(3+)/Yb(3+) codoped alumino-germano-silicate optical fibers by direct excitation of Yb(3+) ions with a laser diode at 976 nm. A two-photon cooperative energy transfer upconversion among a pair of Yb(3+) donor ions and a Tb(3+) acceptor ion are responsible for the observed green emission. Appropriately increasing the concentration of Tb(3+) relative to Yb(3+) was found to facilitate the energy transfer upconversion process from Yb(3+) to Tb(3+), while an excess of Tb(3+) ions inversely introduced more of the concentration quenching effect and phonon-assistant backward energy transfer from Tb(3+) to Yb(3+) and, therefore, damaged the observed green emission. The intensity of the observed green emission was found optimum for a concentration ratio of Tb(3+) to Yb(3+) of 2:1 in alumino-germano-silicate optical glass fibers.

Analysis of energy transfers between Tb^3+ and Yb^3+ codoped in borosilicate glasses

Energy transfer analyses are performed on Tb3+ and Yb3+ codoped in borosilicate glasses to assess their potential as gain media for green lasers and amplifiers under the 0.98 μm band pumping. Based on experimental observations using the samples highly codoped with Tb3+ and Yb3+, a rate equation model of energy transfers between Tb3+ and Yb3+ is constructed considering the four types of energy transfers processes; (i) the cooperative energy transfer upconversion, (ii) the phonon-assisted energy transfer from Tb3+ to Yb3+, (iii) the cooperative cross relaxation, and (iv) the phonon-assisted energy transfer from Yb3+ to Tb3+. The rate equation model can simulate the experimental emission dynamics of Tb3+ and Yb3+ codoped samples well. It is clarified that a practical signal gain at 0.54 μm can be obtained in Tb3+-Yb3+ codoped fiber under the 0.98 μm band pumping.