Samuel L. Oliveira

Spectroscopy of Nd3+ and Yb3+ codoped fluoroindogallate glasses

The Nd–Nd and Nd–Yb energy transfer processes are studied in lead fluoroindogallate glasses with the following molar composition: 30PbF2–20GaF3–15InF3–15ZnF2–(20–X)CaF2–XNdF3 (with X=0.1, 0.5, 1, 2, 4, and 5), 30PbF2–20GaF3–15InF3–15ZnF2–(20–X)CaF2–XYbF3 (with X=0.1, 0.5, 1, 2, 3, and 5), and 30PbF2–20GaF3–15InF3–15ZnF2–(19–X)CaF2–XYbF3–1NdF3 (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: CDD(Nd–Nd)≈7×10−40, CDA(Nd–Nd)=2.5×10−40, and CDA(Nd–Yb)≈3×10−40 cm6/s. It was also shown that the energy migration between Nd ions depends on the third power of temperature (T3) 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 Yb3+ emission at 969 nm.

Cooperative luminescence in Yb3+-doped phosphate glasses

In this paper we present results on cooperative luminescence performed on Yb3+-doped metaphosphate glasses under 980 nm excitation. We have measured emission spectra and decay lifetimes in the visible and infrared regions as a function of Yb concentration. It was observed that, up to 10% of Yb concentration, cooperative emission increases while lifetime is observed to decrease. Such behaviour is attributed to the Yb interaction with OH− radicals and energy migration among Yb ions.

Thermal lens study of the OH− influence on the fluorescence efficiency of Yb3+-doped phosphate glasses

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.

Two-photon absorption cross-section spectrum of a π-conjugated polymer obtained using the white-light continuum Z-scan technique

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 675nm, 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.

High fluorescence quantum efficiency of 1.8 μm emission in Tm-doped low silica calcium aluminate glass determined by thermal lens spectrometry

In this work, thermal lens spectrometry is performed to determine the fluorescence quantum efficiency (η) of the 3F4 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.

Singlet excited state absorption of porphyrin molecules for pico- and femtosecond optical limiting application

This work employs the Z-scan technique with 120fs 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 460to800nm. 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.

Energy transfer processes and heat generation in Yb3+-doped phosphate glasses

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 ∼3×1020Ybions∕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 Forster-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 probabl...

Ultraviolet emission and Fano resonance in doped nano-alumina

Emission properties of Al2O3 nanopowders, synthesized by flame spray pyrolysis with Mg, Cr, and Sc dopants, are investigated, principally in the protein lysing range of 250–290nm (UV-C band). As expected, point defect densities depend on crystal phase and irradiation history and strongly influence emission properties at short wavelengths. Ultraviolet and visible emission intensities of aggregated point defect centers change upon electron beam exposure at high current densities, but ultraviolet emission from point defects is persistently enhanced over a narrow range of Mg-doped Al2O3 compositions slightly off spinel stoichiometry. At 40% Mg concentration, emission intensities at 320nm rise by over an order of magnitude after beam exposure. Quantum efficiency for cathodoluminescence in the 250–300nm range nevertheless remains low. Point defect ionization at high currents shifts the emission of Al2O3 nanopowders to the infrared and is shown to be correlated with a ubiquitous Fano resonance in ionized Cr-vaca...

Bistable emission of a black-body radiator

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.

Temperature dependence of fluorescence quantum efficiency of optical glasses determined by thermal lens spectrometry

Abstract In this work thermal lens spectrometry is applied to determine the fluorescence quantum efficiency, η, of Nd2O3-doped low silica calcium aluminosilicate glasses as a function of the temperature. The experiments were performed in the temperature range between 22 and 180 ° C. The results showed that the decrease in η observed with increase in the temperature of the samples was larger for the larger Nd2O3-doping concentrations. This observation indicates that the non-radiative relaxation processes induced by Nd–Nd ion interactions may be the dominant mechanism responsible for the η reduction. The results indicate that the thermal lensing technique is a useful tool to evaluate the effect of the active medium temperature in the performance of laser systems.