T. Catunda

Mode-mismatched thermal lens spectrometry for thermo-optical properties measurement in optical glasses: a review

Abstract In this work, the application of thermal lens spectrometry (TLS) to study thermo-optical and spectroscopic properties of optical glasses is described. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations of the method as compared with conventional measurements. The technique is applied to determine the thermal diffusivities, temperature coefficient of optical path length changes, d s/ d T , and the fluorescence quantum efficiencies of several glasses such as fluorides, chalcogenides, chalcohalides, soda lime and low silica calcium aluminosilicate. For some of these glasses, the effect of glass composition on thermo-optical properties was studied. For aluminosilicate glasses was observed an 8% decrease of the thermal diffusivity with the increase of Nd2O3 doping. Five kinds of fluoride glasses were studied and it was observed that fluorindate and fluoaluminate present better thermo-optical properties compared to fluorozirconate glasses: thermal diffusivity ∼ 20% higher and d s/ d T ∼ 50% lower (in modulus). All fluoride glasses had negative d s/ d T and all the other glasses a positive d s/ d T . For fluoride glasses, the TLS measurements were performed up to the glass transition temperature ( T g ). Near T g thermal diffusivity decreases and d s/ d T increases, both by one order of magnitude. The TLS was used to determine fluorescence quantum efficiency and concentration quenching in Nd3+-doped glasses (aluminosilicate and fluorozirconate). These results were compared with the Judd–Ofelt calculations.

Nd2O3 doped low silica calcium aluminosilicate glasses: Thermomechanical properties

The effects of Nd2O3 doping on the thermal and mechanical properties of vacuum melted, low silica, calcium aluminosilicate glasses are presented. For the doped glasses, the vitrification limit was found to correspond to a maximum load of 5 wt % Nd2O3. The influence of the rare earth doping on the thermal diffusivity, thermal conductivity, and Vickers hardness was such that all these physical parameters decreased by roughly the same amount, namely 8%, between the undoped and the 5 wt % doped sample. The dependence of these parameters, as a function of the Nd2O3 doping, strongly supports the idea that the Nd3+ act as network modifiers.

Multiwavelength thermal lens determination of fluorescence quantum efficiency of solids: Application to Nd3+-doped fluoride glass

A multiwavelength thermal lens (TL) method was developed to determine the fluorescence quantum efficiency of solids. The experiments have been performed in Nd3+-doped fluoride glasses, using five different excitation wavelengths: 514, 576, 741, 796, and 867 nm. In this work, the absolute value of fluorescence quantum efficiency can be obtained by the linear dependence of the TL signal with the excitation wavelength. Our results suggest that the method can be applied to study a wide range of fluorescent materials, with a special ability to be used for measurements of highly transparent materials with very low TL effect.

Optimizing and calibrating a mode-mismatched thermal lens experiment for low absorption measurement

We describe a calibrated two-beam mode-mismatched thermal lens experiment aimed at determination of the absorption coefficient and the photothermal parameters of a nearly transparent material. The use of a collimated probe beam in the presence of a focused excitation beam optimizes the thermal lens experiment. The signal becomes independent from the Rayleigh parameters and waist positions of the beams. We apply this method to determine the absolute value of the thermal diffusivity and absorption coefficient of distilled water at 533 nm.

Differential interferometric technique for the measurement of the nonlinear index of refraction of ruby and GdAlO3:Cr+3.

A differential interferometric method for direct measurements of the real part of the nonlinear index of refraction n2 has been developed. With this method we were able to measure n2 as low as 10−10 cm2/W. This method was used to measure n2 in both ruby and GdAlO3:Cr+3 for the first time. Our results show that the real part of n2 in ruby is an order of magnitude larger than the imaginary part. This result clarifies the controversy about the dispersive or absorptive origin of the phase conjugation in ruby- and cromium-doped crystals.

Upconversion effect on fluorescence quantum efficiency and heat generation in Nd 3+ -doped materials

The thermal lens technique was carried out to experimentally determine the influence of the energy transfer upconversion (ETU) processes on fluorescence quantum efficiency (eta) in Nd3+-doped materials. The samples with high Nd3+ concentration present a considerable reduction in eta?with the increasing excitation power due to the efficient ETU processes. Besides, the energy migration was identified as the mechanism responsible for the upconversion losses. In addition, it was verified that the critical inversion density is not concentration independent, as previously stated, but it decreases with the Nd concentration. Our results point out the approach based on TL technique as a valuable alternative because of its sensitivity allowing the measurements to be performed in a pump power regime that avoids damages in the investigated material.

Continuous-wave diode-pumped Yb:glass laser with near 90% slope efficiency

Room temperature highly efficient laser action from an ytterbium-doped phosphate glass is demonstrated when end pumped by a 975nm fiber-coupled laser diode. Laser slope efficiencies as high as 88% have been obtained, leading to pump-to-laser conversion efficiencies greater than 50%. From the analysis of pumping threshold and laser slope efficiencies the authors have determined both the optical losses and emission cross section at laser wavelength (1028nm).

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.

Thermal lens spectroscopy of Nd:YAG

In this work, the thermal lens (TL) spectrometry was performed as a spectroscopic experiment. The spectrum of heat generated by the Nd3+:YAG sample was obtained by continuously tuning a Ti:sapphire laser used as excitation beam. Besides the information about nonradiative efficiency, absolute values of fluorescence quantum efficiency and ds∕dT were determined through the multiwavelength TL method by using six discrete excitation wavelengths (514, 588, 739, 808, 817, and 869 nm). The pumping and fluorescence quantum efficiencies of Nd3+ metastable state (F3∕24) was observed to be constant throughout the range of excitation wavelengths used. For low doping levels (<0.75at.%ofNd3+), the effective quantum efficiency is over 93%.

Neodymium concentration dependence of thermo—optical properties in low silica calcium aluminate glasses

Abstract Thermal lens spectrometry of thermo-optical properties of low silica calcium aluminate glasses doped with different concentration of neodymium dioxide was conducted. Thermal lens signal amplitude presented a linear dependence for neodymium concentrations up to 2.0 wt%, indicating that there was no fluorescence quenching. The quantitative treatment for the thermal lens effect also allowed determination of the absolute value of quantum efficiency, 0.87, for all samples. Thermal diffusivity is around 5.5 × 10 −3 cm 2 /s with some dependence on Nd concentration.