A. Oleaga

Spectroscopy and frequency upconversion of Er3+ ions in lead niobium germanate glasses

Abstract In this work we report the optical properties and upconversion luminescence of Er 3+ ions in lead–niobium–germanate glass (GeO 2 –PbO–Nb 2 O 5 ) suitable for use in optical waveguide devices. The optical properties of Er 3+ ions have been established in terms of absorption and emission spectra and lifetime measurements in the visible and infrared regions. The 1.5 μm emission cross-section has been determined from the line shape of the emission spectrum and the calculated emission probability for the 4 I 13/2 level. Upconversion emissions at 529, 548, and 653 nm have been obtained under infrared excitation at 803 nm in the 4 I 9/2 level and compared with those obtained under one photon excitation. Pumping power measurements were also carried out. The possible excitation mechanisms responsible for this upconversion luminescence are discussed.

Nd3+ laser spectral dynamics in CaF2-YF3-NdF3 crystals

Abstract A spectral and dynamical study of the spontaneous and stimulated emissions of the yttrofluorite crystal CaF 2 –YF 3 (5%)–NdF 3 (2%) which proves the existence of two main sites for the rare earth ion is presented. Flash-lamp pumped laser experiments show the existence of two laser output lines centred at 1054 and 1063 nm which do not coincide with the spectral position of two of the resolved sites. The dependence of the time-resolved laser output energy and relative intensities of both lines on the pump energy suggests the existence of some kind of coupling among both emissions produced by the stimulated emission field itself.

Critical behavior near the Lifshitz point in Sn2P2(S1 − xSex)6 ferroelectric semiconductors from thermal diffusivity measurements

The thermal diffusivity of the ferroelectric family Sn(2)P(2)(Se(x)S(1 - x))(6) (0 ≤ x ≤ 1) has been measured by a high-resolution ac photopyroelectric technique, using single crystals, with the aim of studying the evolution of the ferroelectric transition with Se doping. Its change from second order character to first order while passing the Lifshitz point (x approximately 0.28) has been evaluated, as well as the splitting of the transition at high Se concentrations. The critical behavior of the ferroelectric transition in terms of the different universality classes and their underlying physical dominant effects (tricriticality, long-range dipole interactions, Lifshitz point) has been discussed using thermal diffusivity measurements in the very close vicinity of the critical temperature. This study reveals that for Se concentrations around the Lifshitz point, long-range dipole interactions do not play a significant role and that the critical parameters are close to those predicted for the Lifshitz universality class.

Critical behaviour of RMnO3 (R = La, Pr, Nd) by thermal diffusivity and specific heat measurements

An ac photopyroelectric calorimeter has been used to measure the thermal diffusivity and specific heat of the perovskite manganites RMnO3 (R = La, Pr, Nd) close to their magnetic transitions. Taking into account that the inverse of the thermal diffusivity has the same critical behaviour as the specific heat, the critical exponent α of the magnetic transition has been obtained by means of both magnitudes. The results in all cases are consistent with the Heisenberg model (α = −0.11), irrespective of the rare-earth ion.

Overcoming the influence of the coupling fluid in photopyroelectric measurements of solid samples

The thermal diffusivity of solid samples is systematically underestimated in a photopyroelectric technique used in the standard back configuration due to the presence of the coupling fluid between sample and detector. In this work, we propose a new method to overcome the undesired effect of the coupling fluid. It relies on the use of a transparent pyroelectric sensor and a transparent coupling fluid, together with a self-normalization procedure. In this way, we are able to measure accurately (a) the thermal diffusivity of opaque solid samples, and (b) the thermal diffusivity and the optical absorption coefficient of semitransparent solid samples.

Vertical cracks characterization using lock-in thermography: II finite cracks

The aim of this work is to characterize vertical cracks of finite size and arbitrary shape using optically excited lock-in thermography. In the first place, we have solved the direct problem, which consists of calculating the surface temperature distribution when the shape, size and width of the vertical crack are known. To do this we have developed a new method based on discontinuous finite elements, which allows one to deal even with very narrow cracks, for which classical finite element methods fail. The surface temperature of steel samples containing semi-infinite cracks and illuminated with a laser beam focused close to the crack has been measured using a lock-in thermography setup. A least square fit of the amplitude and phase of the surface temperature is used to retrieve the width and depth of the semi-infinite crack. A very good agreement between the nominal and retrieved values of both parameters is found, confirming the validity of the model.

Critical behavior of the thermal properties of KMnF3

An ac photopyroelectric calorimeter has been used to simultaneously measure the specific heat, thermal conductivity, and thermal diffusivity around the antiferromagnetic to paramagnetic phase transition in KMnF3. It has been found that the critical exponent and the amplitude ratio of the thermal diffusivity are the same as those of the specific heat. Both values agree with the predictions of the three-dimensional Heisenberg model for isotropic magnets, although this transition is not only magnetic but also structural as well. The thermal conductivity shows a broad peak at the Neel temperature that could be related to a reduction of the phonon scattering by the spin fluctuation as the transition temperature is approached.

A thermal paradox: which gets warmer?

In this paper we address a common misconception concerning the thermal behaviour of matter, namely that the front surface of a very thin plate, uniformly illuminated by a constant light beam, reaches a higher temperature than the front surface of a very thick slab made out of the same material. We present analytical solutions for the temperature rise above the ambient of thin and thick samples, after a heating lamp is switched on. It is shown that the temperature rise at the illuminated surface of the thick sample is twice that of the thin plate. However, the temperature rise of the thin plate is much faster (minutes) than the heating of the thick one (hours). This explains why our intuition, which learns from what happens at the very beginning of the process, leads us to the feeling that the front surface of a thin plate will get warmer than the illuminated surface of a thick slab, i.e. the above-mentioned misconception.

Thermal characterization of rods, tubes and spheres using pulsed infrared thermography

In this work we analyse the accuracy of an extension of the flash method to measure the thermal diffusivity of rods, tubes and spheres, which was recently proposed by the authors. We have performed measurements in a wide set of calibrated samples of different sizes and we have found that a lower limiting size of the radius can be established for the validity of the method. On the other hand, a procedure to retrieve the thermal conductivity of tubes, based on filling them with a contrast liquid (water), is proposed. Moreover, the thermal contact resistance between the two layers of coated cylinders is also obtained. Measurements on calibrated samples confirm the validity of the two latest methods.

The Aula EspaZio Gela and the Master of Space Science and Technology in the Universidad del País Vasco (University of the Basque Country)

We present the Aula EspaZio Gela, a facility dedicated to teaching Space Science and Technology at the master and doctorate level at the University of the Basque Country (Spain), and to promoting the development of this field in both public and private sectors. The one-year masters degree in Space Science and Technology (60 ECTS (European Credit Transfer and Accumulation System)) offers a group of compulsory courses which give way afterwards to a set of elective matters in which students choose one of two tracks: the scientific, primarily oriented to basic research at the University, or the technological, leading to the space industry and space agencies. After completion of the master thesis, our students have direct access to a PhD in both curricular lines. Here we detail the main features of the masters degree and the experience acquired in three years, including a comparative opinion survey to the students. We also describe the facilities at the Faculty of Engineering consisting of a specific classroom (Aula EspaZio Gela), an Astronomical Observatory, and different laboratories.