Mode-mismatched dual-beam method to evaluate thermal and electronic laser-induced lensing effects in Cr3+- and Yb3+-doped crystals.

We report on a mode-mismatched thermal lens experiment performed to quantitatively evaluate thermal and electronic laser-induced lensing effects in ion-doped crystals Cr3+:LiSAF, Yb3+:KYW, and Yb3+:YAG. The large diameter of the probe beam resulted in a slow thermal effect (∼dozens of milliseconds) two orders of magnitude larger than the electronic one, improving the discrimination of both competitive effects. All thermal and electronic parameters are obtained from transient signals modeled by an analytical equation, valid for small phase shifts in the absence of upconversion effects.

Modeling population and thermal lenses in the presence of Auger Upconversion for Nd(3+) doped materials.

We present a theoretical model to thermal (TL) and population (PL) lenses effects in the presence of Auger upconversion (AU) for analysis of Nd(3+) doped materials. The model distinguishes and quantifies the contributions from TL and PL. From the experimental and theoretical results, the AU cannot be neglected because it plays an important role on the excited state population and therefore on the temperature and polarizability difference between excited and ground states. Considering the extensive use of these techniques, the model presented here could be useful for the investigation of materials and also to avoid misleading analysis of lenses transients.

Discriminating the role of sample length in thermal lensing of solids.

Thermal lens (TL) is a key effect in laser engineering and photothermal spectroscopy. The amplitude of the TL signal or its dioptric power is proportional to the optical path difference (OPD) between the center and border of the beam, which is proportional to the heat power (Ph). Due to thermally induced mechanical stress and bulging of end faces of the sample, OPD depends critically on the geometry of the sample. In this investigation, TL measurements were performed as a function of the sample length keeping the same Ph. It is experimentally demonstrated that for materials with positive ∂n/∂T OPD increases typically 30 to 50% with the decrease of sample length (from long rod to thin-disk geometry). For materials with negative ∂n/∂T, this variation is much larger due to the cancelation of the different contributions to OPD with opposite signs. Furthermore, the experimental investigation presented here validates a recently proposed unified theoretical description of the TL effect.

Resonant excited state absorption and relaxation mechanisms in Tb3+-doped calcium aluminosilicate glasses: an investigation by thermal mirror spectroscopy.

Resonant excited state absorption (ESA) and relaxation processes in Tb(3+)-doped aluminosilicate glasses are quantitatively evaluated. A model describing the excitation steps and upconversion emission is developed and applied to interpret the results from laser-induced surface deformation using thermal mirror spectroscopy. The fluorescence quantum efficiency of level (5)D(4) was found to be close to unity and concentration independent while, for the level (5)D(3), it decreases with Tb(3+) concentration. Emission spectroscopy measurements supported these results. ESA cross sections are found to be more than three orders of magnitude higher than the ground state absorption cross section.

Very low optical absorptions and analyte concentrations in water measured by Optimized Thermal Lens Spectrometry.

Thermal Lens Spectrometry has traditionally been carried out in the single-beam and the mode-mismatched dual-beam configurations. Recently, a much more sensitive dual-beam TL setup was developed, where the probe beam is expanded and collimated. This feature optimizes Thermal Lens (TL) signal and allows the use of thicker samples, further improving the sensitivity. In this paper, we have made comparisons between the conventional and optimized TL configurations, and presented applications such as measurements of very low absorptions and concentrations in water and Cr(III) aqueous solution in the UV-vis range. For pure water we found linear absorption coefficients as low as the Raman scattering one due to the stretching vibrational modes of OH group. The detection limit was estimated 1 × 10(-6)cm(-1) with a 180-mW excitation power using a 100-mm cell length. This sensitivity is very high, considering that water has a photothermal enhancement factor ∼ 33 times smaller than CCl(4), for example. For Cr(III) species in aqueous solution, the limit of detection (LOD) was estimated in ∼ 40 ng mL(-1) at 514 nm, or ∼ 10 ng mL(-1) at 405 nm, which is ∼ 30 times smaller than the LOD achieved with conventional transmission techniques. The more recent TL configuration is very attractive to obtain absorption spectra, since the result does not depend critically on the beam parameters, unlike the other configurations. The main drawbacks of this optimized TL configuration are the longer acquisition time and the need for larger samples.

Microstructuration induced differences in the thermo-optical and luminescence properties of Nd:YAG fine grain ceramics and crystals.

The temperature and compositional dependences of thermo-optical properties of neodymium doped yttrium aluminum garnet (YAG) crystals and fine grain ceramics have been systematically investigated by means of time-resolved thermal lens spectrometry. We have found that Nd:YAG ceramics show a reduced thermal diffusivity compared to Nd:YAG single crystals in the complete temperature range investigated (80-300 K). The analysis of the time-resolved luminescent properties of Nd(3+) has revealed that the reduction in the phonon mean free path taking place in Nd:YAG ceramics cannot be associated with an increment in the density of lattice defects, indicating that phonon scattering at grain boundaries is the origin of the observed reduction in the thermal diffusivity of Nd:YAG ceramics. Finally, our results showed the ability of the time-resolved thermal lens to determine and optimize the thermo-optical properties of Nd:YAG ceramic based lasers.

Thermal lens and interferometric method for glass transition and thermo physical properties measurements in Nd2O3 doped sodium zincborate glass.

In this work the time resolved thermal lens method is combined with interferometric technique, the thermal relaxation calorimetry, photoluminescence and lifetime measurements to determine the thermo physical properties of Nd(2)O(3) doped sodium zincborate glass as a function of temperature up to the glass transition region. Thermal diffusivity, thermal conductivity, fluorescence quantum efficiency, linear thermal expansion coefficient and thermal coefficient of electronic polarizability were determined. In conclusion, the results showed the ability of thermal lens and interferometric methods to perform measurements very close to the phase transition region. These techniques provide absolute values for the measured physical quantities and are advantageous when low scan rates are required.

Thermal lens and heat generation of Nd:YAG lasers operating at 1.064 and 1.34 microm.

We report on a simple and accurate method for determination of thermo-optical and spectroscopic parameters (thermal diffusivity, temperature coefficient of the optical path length change, pump and fluorescence quantum efficiencies, thermal loading, thermal lens focal length, etc) of relevance in the thermal lensing of end-pumped neodymium lasers operating at 1.06- and 1.3- microm channels. The comparison between thermal lensing observed in presence and absence of laser oscillation has been used to elucidate and evaluate the contribution of quantum efficiency and excited sate absorption processes to the thermal loading of Nd:YAG lasers.

Nonlinear electronic line shape determination in Yb3+-doped phosphate glass.

Nonlinear refraction spectroscopy has been performed in Yb3+-doped phosphate glass to determinate the line shape of real and imaginary parts of n2 (n2' and n2"). The n2' spectrum presented an asymmetric feature due to the interference of resonant and nonresonant contributions, where the nonresonant term arises from the polarizability difference between excited and ground states (delta alpha). The measurements were performed in the transient regime to determine population dynamics and the pump saturation intensity at 975 nm (peak of the absorption spectrum). Because of the small quantum defect of Yb3+, we estimated that the magnitude of the thermal lens effect is approximately 20 times smaller than the population lens effect, caused by n2.

Discrimination of resonant and nonresonant contributions to the nonlinear refraction spectroscopy of ion-doped solids.

Nonlinear refraction spectroscopy measurements were performed in resonance with absorption lines in Nd3+ and Cr3+ doped crystals. The observed line shapes can be explained by the interference of resonant and nonresonant contributions to the nonlinear refractive index. These effects were fully discriminated using a pump and probe configuration, in which a dispersive line shape was observed on the top of a plateau that was attributed to the polarizability difference between excited and ground states (Deltaalpha). A theoretical model was used to obtain Deltaalpha from the experimental data, and the results are in agreement with several previous experiments. Slow and fast light propagation are also discussed.

Thermal lens study of energy transfer in Yb(3+)/Tm(3+)-co-doped glasses.

Energy transfer (ET) and heat generation processes in Yb(3+)/Tm(3+)-co-doped low-silica calcium-aluminosilicate glasses were investigated using thermal lens and photoluminescence measurements. Stepwise ET processes from Yb(3+) to Tm(3+), with excitation at 0.976 mum, produced efficient emission in the mid-infrared range at around 1.8 mum, with high fluorescence quantum efficiency (~0.50) and relatively low thermal loading (

Nonlinear refraction spectroscopy in resonance with laser lines in solids.

We report a simple extension of the Z-scan technique that permits a spectral line-shape measurement of the real and the imaginary parts of n(2) . In this technique the sample is placed at the peak position of the usual Z-scan curve while the laser frequency is scanned. We employed this method to investigate the nonlinear susceptibility of the R lines of ruby and alexandrite, using a cw dye laser. This susceptibility can be explained by the resonant interaction and by a nonresonant contribution that is due to the difference in polarizability between Cr(3+) excited and ground states. For ruby, the nonresonant contribution to the technique is 1 order of magnitude larger than the resonant contribution. However, for alexandrite both contributions are comparable, and their interference leads to a shift between n(2)(?) and n(2)(??) spectra that is not observed in ruby.