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In this work, we report a systematic investigation of upconversion losses and their effects on fluorescence quantum efficiency and fractional thermal loading in Nd3+-doped fluoride glasses. The energy transfer upconversion (γup) parameter, which describes upconversion losses, was experimentally determined using different methods: thermal lens (TL) technique and steady state luminescence (SSL) measurements. Additionally, the upconversion parameter was also obtained from energy transfer models and excited state absorption measurements. The results reveal that the microscopic treatment provided by the energy transfer models is similar to the macroscopic ones achieved from the TL and SSL measurements because similar γup parameters were obtained. Besides, the achieved results also point out the migration-assisted energy transfer according to diffusion-limited regime rather than hopping regime as responsible for the upconversion losses in Nd-doped glasses. [ABSTRACT FROM AUTHOR]

In this work we present a study of energy transfer processes in Yb3+-doped phosphate glasses with different OH- contents and Yb3+ concentrations. Fluorescence and thermal lens (TL) techniques were used in order to analyze the Yb3+–OH- and Yb3+-other impurity interactions. The role of OH- groups is also presented and discussed. The results show that in the low concentration region up to ∼3×1020 Yb ions/cm3, the interaction between Yb3+ and OH radicals is the dominant process reducing the lifetime, increasing the TL dioptric power, and then endangering the performance of the system. For the high Yb3+ concentration limit, the Yb3+-other impurity interaction, which is favored by energy migration, is comparable to the Yb3+–OH- energy transfer, even for high OH- concentration. The nonradiative decay rate due to hydroxyl groups follows Förster-Dexter theory [Ann. Phys. 2, 55 (1948); J. Chem. Phys. 21, 836 (1953)] except at low Yb3+ concentration, being more active at low OH- levels, where quenching rate is probably due to several impurities. [ABSTRACT FROM AUTHOR]

This work employs the Z-scan technique with 120 fs pulses to investigate the singlet excited state absorption spectra of tetrapyridyl and tetrasulfonatophenyl porphyrins. We have used a three-energy-level model to adjust Z-scan curves in order to obtain the singlet excited absorption cross section from 460 to 800 nm. Starting from these values, we determine the spectra of the ratio between excited and ground singlet state absorption cross sections, whose values are as good as the best found in the literature for reverse saturable absorbers. The results obtained point these porphyrins as good candidates for applications in optical limiting of ultrashort pulses. [ABSTRACT FROM AUTHOR]

The Nd–Nd and Nd–Yb energy transfer processes are studied in lead fluoroindogallate glasses with the following molar composition: 30PbF[sub 2]–20GaF[sub 3]–15InF[sub 3]–15ZnF[sub 2]–(20–X)CaF[sub 2]–XNdF[sub 3] (with X=0.1, 0.5, 1, 2, 4, and 5), 30PbF[sub 2]–20GaF[sub 3]–15InF[sub 3]–15ZnF[sub 2]–(20–X)CaF[sub 2]–XYbF[sub 3] (with X=0.1, 0.5, 1, 2, 3, and 5), and 30PbF[sub 2]–20GaF[sub 3]–15InF[sub 3]–15ZnF[sub 2]–(19–X)CaF[sub 2]–XYbF[sub 3]–1NdF[sub 3] (with X=0.1, 0.5, 1, 2, 3, and 5.5). The Dexter, Yokota–Tanimoto, and Holstein formalisms were used to treat the experimental data. The following microparameters of energy transfer were obtained: C[sub DD(Nd–Nd)]approx. 7×10[sup -40], C[sub DA(Nd–Nd)]=2.5×10[sup -40], and C[sub DA(Nd–Yb)]approx. 3×10[sup -40] cm[sup 6]/s. It was also shown that the energy migration between Nd ions depends on the third power of temperature (T[sup 3]) up to a saturation value of about 80 K. This behavior was attributed to the site to site energy migration. The Yb doped samples presented no nonradiative losses for the Yb[sup 3+] emission at 969 nm. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have used the white-light continuum (WLC) Z-scan technique to determine the degenerate two-photon absorption (2PA) cross-section spectrum of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene). The results obtained show good agreement with those employing a single-wavelength source, although much faster because of the wavelength multiplexing introduced by the use of a broadband source. The improved spectral resolution of the WLC Z-scan allowed the observation of a 2PA peak around 675 nm, a subtle feature in the nonlinear spectrum that could be fitted using a sum-over-states model. The WLC Z-scan technique has proved to be an important tool to determine the 2PA spectrum of polymers. [ABSTRACT FROM AUTHOR]

In this work, the thermal lens (TL) technique is used to determine the fluorescence quantum efficiency (η) of Yb3+-doped phosphate glasses. The role of nonradiative processes such as energy migration among Yb ions and the interaction with OH- radicals are presented and discussed. Two sets of samples with the same Yb concentrations were prepared, one at ambient conditions (set A) and the other in N2 atmosphere (set N). The TL technique was shown to be very sensitive to the amount of OH radicals. Moreover, the η values obtained from the TL method are in good agreement with the calculate ones (based on lifetime measurements). The results indicate that TL can be a valuable technique to evaluate the quantum efficiency and nonradiative rates in ion-doped materials. [ABSTRACT FROM AUTHOR]

Bistable black-body emission is reported from resonantly excited Er3+,Yb3+:Y2O3 nanopowders. A simple model based on thermo-optic nonlinear response in the strongly scattering random medium explains the observed behavior. [ABSTRACT FROM AUTHOR]

In this work, thermal lens spectrometry is performed to determine the fluorescence quantum efficiency (η) of the [sup 3]F[sub 4] level (1.8 μm emission) of 4 and 5 wt % Tm-doped water free low silica calcium aluminate glass. The η value obtained for both high Tm contents was approximately 30%, which is in agreement with the Judd–Ofelt calculation. It is also verified that heat generation under 0.79 μm pumping, which presents a high quantum defect, is minimized by a cross-relaxation mechanism. The high η values, and the excellent chemical and thermomechanical properties indicate this system is a potential host material for diode-pumped laser sources operating in the midinfrared spectral range. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]