Under 980 nm excitation, unusual ³P₂→³H₆(~264 nm) and ³P₂→³F₄(~309 nm) emissions from Tm³⁺ ions were observed in hexagonal NaYF₄:Yb³⁺(20%)/Tm³⁺(1.5%) microcrystals. In comparison with the strong emissions from ¹D₂ and ¹I₆, the emissions from ¹G₄ and ³H₄ almost vanished due to the efficient cross-relaxation of ¹G₄+³H₄→³F₄+¹D₂(Tm³⁺). Double logarithmic plots of the upconversion emission intensity versus the excitation power are neither straight lines nor typical saturation curves. Theoretical analysis indicated that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation.

The improvement of light extraction efficiency of InGaN light-emitting diodes (LEDs) using microsphere arrays with various refractive indices was analyzed. Finite-difference time-domain (FDTD) simulations show that the use of microsphere (d_microsphere = 500 nm) arrays with refractive indices of 1.8 and 2.5 led to increase in light extraction efficiency of InGaN LEDs by 1.9 times and 2.2 times, respectively. The enhancement in light extraction efficiency is attributed to the decrease in the Fresnel reflection and increase in effective photon escape cone due to graded refractive index and curvature formed between microsphere and free space. The maximum enhancement of light extraction efficiency of InGaN quantum well LEDs was achieved by employing the refractive index matched anatase-TiO₂ microsphere arrays. The effects of microsphere diameters on the light extraction efficiency were also investigated and 2.4 times enhancement was achieved by employing 400-nm refractive index matched TiO₂ sphere arrays.