By embedding NaYF 4 :Yb 3+ ,Tm 3+ nanocrystals into the top cladding layer of a resonant waveguide grating structure, we demonstrate that the upconversion fluorescence of Tm 3+ ions can be greatly enhanced, by a factor of up to 10 4 . The resonant waveguide grating structure consists of an SU8 bottom layer with sinusoidal grating morphology coated with a thin TiO 2 waveguide layer and then covered with a poly(methyl methacrylate) cladding layer doped with NaYF 4 :Yb 3+ ,Tm 3+ nanocrystals. The giant enhancement of the upconversion fluorescence is achieved first by coupling the excitation light with a guided mode of the resonant waveguide grating structure and then the fluorescent light with a second guided mode. Our numerical simulation results obtained by rigorous coupled-wave analysis indicate that the electric field of the incident light is strongly enhanced near the interface of the TiO 2 layer and the poly(methyl methacrylate) layer at guided mode resonance, and this is the major effect of the observed enhancement of the upconversion fluorescence of the nanocrystals. The resonance between the fluorescent emission and the waveguide structure further enhances the intensities of the fluorescent signal. We also find that the lifetime of upconversion fluorescence at 480 nm wavelength from the rare-earth nanocrystals is reduced about 1.34-fold when both excitation and extraction resonance occurs in the waveguide structure.

Role of Yb 3 + and Er 3 + concentration on the tunability of green-yellow-red upconversion emission of codopedZrO 2 : Yb 3 + -Er 3 + nanocrystals Influence of rhodamine 6G doping on the optical properties of TiO 2 sol-gel films J. Appl. Phys. 98, 073516 (2005); 10.1063/1.2073972 Infrared and visible luminescence properties of Er 3+ and Yb 3+ ions codoped Ca 3 Al 2 Ge 3 O 12 glass under 978 nm diode laser excitationEr 3ϩ -Yb 3ϩ codoped TiO 2 films were prepared on fused silica by sol-gel processes. The Yb 3ϩ codoping effect on the physical characteristics and ϳ1.54 m photoluminescence ͑PL͒ properties of Er 3ϩ -doped TiO 2 films was investigated. Maximum ϳ1.54 m PL intensity occurs in Er 3ϩ (5 mol %) -Yb 3ϩ (30 mol %) codoped TiO 2 samples annealed at 700°C. However, when the concentration of Yb 3ϩ ions is more than 30 mol %, the back energy transfer effect from Er 3ϩ to Yb 3ϩ will deteriorate the ϳ1.54 m PL efficiency. Extended x-ray absorption fine structure measurements show that the average spatial distance between Er 3ϩ ions is slightly decreased due to the partial substitution of Yb 3ϩ for Er 3ϩ ions in the local structure. The Yb 3ϩ ion in the Er 3ϩ -Yb 3ϩ codoped TiO 2 samples not only plays the role of disperser but is also a sensitizer of the Er 3ϩ ion. This dual effect leads to larger PL intensity in the Er 3ϩ -Yb 3ϩ codoped TiO 2 system in comparison with Er 3ϩ -Y 3ϩ codoped TiO 2 samples. Compared with SiO 2 films with Er 3ϩ (5 mol %) -Yb 3ϩ ͑30 mol %͒ codoped and annealed at optimal temperature of 985°C, the Er 3ϩ -Yb 3ϩ codoped TiO 2 film obtains better PL properties at lower annealing temperature.

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Physical characteristics and infrared fluorescence properties of sol-gel derived Er 3+ -Yb 3+ codoped TiO 2 Er 3ϩ -Y 3ϩ codoped TiO 2 films were prepared on a fused silica substrate by the sol-gel process. The effect of Y 3ϩ codoping on the ϳ1.54 m photoluminescence ͑PL͒ properties of Er 3ϩ -doped TiO 2 films are investigated. Enhancement of PL properties due to Y 3ϩ codoping by a factor of 10 for intensity and of 1.5 for the full width at half maximum in comparison with the Er 3ϩ -Al 3ϩ codoped SiO 2 system has been observed in the film annealed at Er 3ϩ :Y 3ϩ :Ti 4ϩ ϭ5:30(ϳ50):100. Extended x-ray absorption fine structure measurements show that the local chemical environment of Er 3ϩ ions in the Er 3ϩ -Y 3ϩ codoped TiO 2 films is similar to that in Er 2 O 3 . The average spatial distance between Er 3ϩ ions is enlarged due to the partial substitution of Y 3ϩ for Er 3ϩ ions in the Er 2 O 3 -like local structure. It is believed that the more intense PL emission of the Er 3ϩ -Y 3ϩ codoped TiO 2 films can be attributed to the better dispersion and distorted local structure of Er 3ϩ ions in the TiO 2 host matrix by yttrium codoping.