A. Mendioroz

Anti-Stokes laser cooling in Yb 3+ -doped KPb 2 Cl 5 crystal

We report internal laser cooling in Yb(3+) -doped KPb(2)Cl(5) . From the quantum efficiency values measured in the heating and cooling regions by use of the photothermal deflection technique, we have obtained a room-temperature cooling efficiency of 0.2% in a sample doped with ~5x10(19)ions/cm(3) . Excitation spectra obtained under high irradiation fluences show an excess of fluorescence with regard to those obtained at low fluences, which agrees with the prediction of a model based on photon-ion-phonon interaction.

Simultaneous measurement of thermal diffusivity and optical absorption coefficient using photothermal radiometry. II Multilayered solids

coefficient using photothermal radiometry. II Multilayered solids Agustı́n Salazar, Raquel Fuente, Estibaliz Apiñaniz, Arantza Mendioroz, and R. Celorrio Departamento de Fı́sica Aplicada I, Escuela Técnica Superior de Ingenierı́a, Universidad del Paı́s Vasco, Alameda Urquijo s/n, Bilbao 48013, Spain Departamento de Matemática Aplicada, EINA/IUMA, Universidad de Zaragoza, Campus Rı́o Ebro, Edificio Torres Quevedo, Zaragoza 50018, Spain

Upconversion processes in Nd3+-doped fluorochloride glass

Abstract Infrared (IR) to visible (VIS) and ultraviolet (UV) upconversion in Nd3+-doped fluorochloride glass has been observed between 4.2 and 300 K under continuous wave (cw) and pulsed IR laser excitation in resonance with the 4 F 3/2 and 4 F 5/2 levels. The emission spectra have UV bands at 359 and 383 nm and VIS bands located around 416, 452, 527, 589, and 660 nm. The green, yellow, and red emissions are attributed to transitions from the 4 G 7/2 excited state. The excitation spectra of the upconverted emissions which are similar to the one-photon absorption spectrum, together with the features appearing in the decays of these emissions, indicate that an energy transfer upconversion (ETU) process seems to be responsible for the VIS luminescence observed, though other possible mechanisms cannot be disregarded. Referring to the blue and UV bands, the most probable origin of the upconverted fluorescence seems to be the 4 D 3/2 , 4 D 5/2 , 4 D 1/2 levels. In the last, the participation of three neodymium ions is necessary in the ETU process.

Extending the flash method to measure the thermal diffusivity of semitransparent solids

In this work, we extend the classical flash method to retrieve simultaneously the thermal diffusivity and the optical absorption coefficient of semitransparent plates. A complete theoretical model that allows calculating the rear surface temperature rise of the sample has been developed. It takes into consideration additional effects such as multiple reflections of the heating light beam inside the sample, heat losses by convection and radiation, transparency of the sample to infrared wavelengths and finite duration of the heating pulse. Measurements performed on calibrated solids, covering a wide range of absorption coefficients from transparent to opaque, validate the proposed method.

Characterization of vertical buried defects using lock-in vibrothermography: II. Inverse problem

An inversion procedure is presented to reconstruct buried heat sources (revealing defects) from surface temperature data obtained by multifrequency lock-in vibrothermography. The severe ill-posedness of the problem is overcome by regularizing the minimization of the squared differences between experimental and calculated data. Two regularization functionals, zero-order Tikhonov and total variation, have been tested by inverting synthetic data. For added uniform white noise levels as high as 20%, total variation has proven to give more accurate inversions. This procedure is applied to reconstruct heat sources from experimental lock-in vibrothermographic measurements performed on metallic samples containing calibrated inner heat sources. The results are very promising regarding the characterization of hidden defects using lock-in vibrothermography.

Accurate reconstruction of the thermal conductivity depth profile in case hardened steel

The problem of retrieving a nonhomogeneous thermal conductivity profile from photothermal radiometry data is addressed from the perspective of a stabilized least square fitting algorithm. We have implemented an inversion method with several improvements: (a) a renormalization of the experimental data which removes not only the instrumental factor, but the constants affecting the amplitude and the phase as well, (b) the introduction of a frequency weighting factor in order to balance the contribution of high and low frequencies in the inversion algorithm, (c) the simultaneous fitting of amplitude and phase data, balanced according to their experimental noises, (d) a modified Tikhonov regularization procedure has been introduced to stabilize the inversion, and (e) the Morozov discrepancy principle has been used to stop the iterative process automatically, according to the experimental noise, to avoid “overfitting” of the experimental data. We have tested this improved method by fitting theoretical data gene...

The strong influence of heat losses on the accurate measurement of thermal diffusivity using lock-in thermography

In modulated photothermal experiments the lateral thermal diffusivity can be obtained from the slope of the linear relation between the phase of the surface temperature and the distance to the heating spot. However, this slope is greatly affected by heat losses so that the measured thermal diffusivity is overestimated, especially for thin samples of poor thermal conducting materials. In this paper we definitely identify the physical mechanism responsible for the overestimation of the diffusivity as heat conduction to the surrounding gas. Accurate measurements of the thermal diffusivity using the “slope method” have been obtained by keeping the sample in vacuum.

Infrared to visible upconversion in Pr3+-doped KPb2Cl5 crystal

Abstract In this work we report the infrared to visible upconversion luminescence in the low phonon-energy host material KPb2Cl5 doped with Pr3+ ions. A strong orange luminescence from the 1D2 level, and less intense blue, green, and red emissions from the 3P0,1 levels have been observed under infrared excitation inside the 1G4 level and compared with those obtained under one photon excitation. The upconverted fluorescence from the 1D2 level shows a quadratic dependence on the pump power indicating a two photon process, whereas the pump power dependence of the blue fluorescence from level 3P0 is nearly cubic, which indicates a three photon mechanism. The possible excitation mechanisms responsible for these upconversion processes are discussed.

Laser spectroscopy and upconversion studies of Pr3+-doped halide modified sulfide glasses

The visible luminescence of Pr 3+ -doped halide modified sulfide glasses has been investigated for different halide modifiers and Pr 3+ concentrations by using steady-state and time-resolved laser spectroscopy. The measured luminescence lifetime of level 3 P 0 is nearly independent on Pr 3+ concentration up to 0.7 mol% whereas the fluorescence of the 1 D 2 level shows concentration quenching for Pr 3+ concentrations higher than 0.5 mol% even at low temperature. The IR-to-visible upconversion obtained under continuous wave infrared laser excitation has been investigated for all samples. The excitation wavelength and pumping power dependencies of the upconverted emission allow to point out two different upconversion mechanisms to populate the 3 P 0 and 1 D 2 levels.

On the origin of anti-Stokes laser-induced cooling of Yb3+-doped glass

Abstract Anti-Stokes cooling between room temperature and 77 K in a new fluorochloride glass (CNBZn) doped with 1 mol% of YbF 3 has been demonstrated by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency of CNBZn glass which is ∼2.0% relative to the absorbed laser power at 1010 nm and 300 K falls by about 20% at 77 K. A model accounting for the photon–ion–phonon interaction is in good agreement with the observed temperature dependence of the cooling process and shows its relation with the vibrational properties of the material.