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Changing the sample's temperature from 200 K to 535 K, we observed 670-fold enhancement of a phonon-assisted upconversion emission at ≈754 nm obtained from a Nd3+-doped tellurite glass excited by 5 ns laser pulses at 805 nm. A rate-equation model, including the relevant energy levels and temperature dependent transition rates, is proposed to describe the process. The results fit well with the data when one considers the nonradiative transitions contributing for the 754 nm luminescence are promoted by an effective phonon mode with energy of 700 cm−1.

Photoluminescence (PL) and degenerate four-wave-mixing (DFWM) experiments were performed in a silica–niobic composite containing NaNbO3 nanocrystals. The PL results indicate the presence of in-gap states attributed to excitons in the nanocrystals and defect centers. The luminescence of the samples becomes more intense at low temperatures, indicating that nonradiative relaxations dominate the dynamics of the in-gap states. The DFWM experiments allowed for measurements of the homogeneous relaxation time, (20 ± 3) fs, of the third-order polarization at room temperature. The main contributions to the dynamics of the electronic response are attributed to the trapping of electrons in the in-gap states and to carrier and phonon scattering.

Frequency upconversion (UC) luminescence in nanocrystalline zirconia (ZrO2) and hafnia (HfO2) doped with Er3+ and Yb3+ was studied under continuous-wave excitation at 980 nm. Samples of ZrO2:Er3+, ZrO2:Er3+/Yb3+, and HfO2:Er3+/Yb3+ were prepared by the sol-gel technique and characterized using x-ray diffraction and electron microscopy. A study of the infrared-to-green and infrared-to-red UC processes was performed including the analysis of the spectral and the temporal behavior. The mechanisms contributing to the UC luminescence were identified as excited state absorption and energy transfer among rare-earth ions.

A frequency upconversion process in Pr3+ doped TeO2–ZnO glasses containing silver nanoparticles is studied under excitation with a nanosecond laser operating at 590 nm, in resonance with the H34→D12 transition. The excited Pr3+ ions exchange energy in the presence of the nanoparticles, originating efficient conversion from orange to blue. The enhancement in the intensity of the luminescence at ∼482 nm, corresponding to the P30→H34 transition, is due to the influence of the large local field on the Pr3+ ions, which are located near the metallic nanoparticles.

The infrared-to-visible upconversion (UC) in Tm3+-doped TeO2–PbO glass was investigated by exciting the samples with a Q-switched neodymium doped yttrium aluminum garnet laser operating at 1064nm. UC emission bands at ∼454, ∼482, and ∼518nm corresponding, respectively, to the transitions D21→F43, G41→H63, and D21→H53 were observed. Based on the dependence of the UC signals as a function of the Tm3+ concentration and excitation intensity, we concluded that the generation of the UC emissions involves excited state absorption and cross relaxation among Tm3+ ions.

Absorption and luminescence experiments were performed in thulium (Tm3+)-doped TeO2-PbO glass samples. Spectroscopic quantities such as transition probabilities, branching ratios, and radiative lifetimes related to the Tm3+ ions were determined using the Judd-Ofelt theory. Luminescence properties were studied by exciting the samples using a pulsed laser operating at 590 nm. Stokes luminescence centered at 792 nm and frequency upconversion (UC) to the blue (≈457 nm) were observed. The intensity and the time evolution of both emissions were studied for different concentrations of Tm3+ ions. The results indicate that the dipole-dipole interaction provides the most important contribution for energy transfer among the Tm3+ ions. The origin of the UC process is attributed to energy transfer among Tm3+ ions in pairs.

Optical properties of Pr3+ doped TeO2–PbO glass were investigated by resonant excitation of H43→D21 transition at ∼586nm. The decay time of D21 level, the energy transfer (ET) rates from the D21 level to other Pr3+ states, and the frequency upconversion process which leads to emission at 484nm (P03→H43 transition) were studied. The ET rates between Pr3+ ions were determined and their interaction was identified as due to dipole-dipole potential. A model used to describe the emission P03→H43 provided information on the dynamics of Pr3+ pair states. The upconversion ET rate was also determined.