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The fluorescence properties of Nd3+in Nd : MgO : LiNbO3and Cr3+in Cr : MgO : LiNbO3systems have been systematically studied after site selective laser excitation. Three kinds of sites have been detected for the active (Nd3+or Cr3+) ions. Two sites correspond to the active ion located in Li+and Nb5+lattice positions, and also appear in the singly (without MgO) doped systems. Codoping with Mg2+ions induce the appearance of a new site for the active ion, together with a decreasing in the Li/Nb occupation ratio.

Laser excited site-selection spectroscopy of Nd3+ion in the Ca3Ga2Ge3O12laser garnet crystal has been investigated for the transitions [MATH] and [MATH] in order to understand the nature of the different Nd centers in terms of the possible charge compensating defects. Lifetime data of the [MATH] transition obtained as a function of Nd concentration have allowed to distinguish the Nd3+ions compensated by Ga3+ions (Nd3+(Ca2+)-Ga3+(Ge4+)) from other kind of Nd3+ions, which can be presumably associated with Nd3+(Ca2+)-Nd3+(Ge4+) pairs.

The dependence of the refractive indices, absorption edge and optical damage in congruent HfO2-doped LiNbO3 crystal are reported. A small amount of Cr3+ ions (0.1 mol%) was introduced in the doped crystals as optical probe. The experimental data indicates that the extraordinary and ordinary indices depend on the Hf4+ ions concentration showing a singularly for a dopant level of ~ 2.7 mol%. The dependence of the absorption edge also shows a transition region at this Hf4+concentration. The distortion of the transmitted light through samples doped with different concentrations of Hf4+ ions indicates that an inhibition of optical damages is achieved at above 3 mol% of dopant. A charge compensation mechanism is proposed to explain the role of the Hf4+ ions for the observed experimental result.

Luminescence of Ti2+: LiF crystals in the wavelength range 650–900 nm excited using laser radiation is reported. The values of 10Dq and the Racah B parameter obtained from the present results indicate that in LiF the 1E, 1T2, and 3T2 energy levels are almost at the “cross-over” point in the Tanabe-Sugano diagram. In consequence photoluminescence from all three energy levels to the ground state is observed. At low temperature (T ~ 14 K) the emission is mainly due to sharp emission lines with weak vibronic structure due to the 1E, 1T2?3T1 transitions of Ti2+ ions. The 3T2?3T1 broad-band emission is weak at low temperature, becoming stronger at 300 K due to a phonon-assisted tunnelling process, similar to that observed in Cr3+: garnets [1]. The excitation spectrum of these lines is a broad peak at 590 nm due to absorption in the 3T1?3T2 transition.

This paper reports on the luminescence spectrum, its excitation-wavelength dependence and decaytime of Cr3+:LiF crystals. In contrast to Al2O3 and MgO, where the Cr3+ luminescence at low temperature is due to the R-line and its vibronic sideband, luminescence from Cr3+:LiF is assigned to the broad-band 4T2?4A2 transition. A periodic structure in the vibronic sidebands of this transition occurs with energy separations of ca. 60 meV. The linear polarisation of the vibronic sidebands shows that their radiative transition takes place from the lowest excited level of the 4T2 state, i.e. 4B1, into the 4A2 ground state of Cr3+ ions in orthorhombic symmetry sites. The crystal-field parameters calculated from the experimental data, and luminescence decay-time of the two polarized configurations of the 4B1(4T2)?4A2 transition are also reported.

A parallel study of switching and the associated luminescence emission has been carried out in pure and Fe-doped BaTiO3 samples in the temperature range-100°C to 130°C that contains the four crystallographic phases of BaTiO3. The main result is the appearance of a second maximum in the switching and light pulses from the Fe-doped samples, that behaves with the applied electric field and temperature in a different way than those single pulses found in pure samples. The integrated light output peaks at the transition temperatures.

The optical absorption and luminescence of Cr3+ ions in ZnO doped LiNbO3:Cr3+ crystals have been studied. It has been found that ZnO produces the same effect as that observed in MgO co-doped samples: A strong reduction in the Cr3+ concentration together with a shift in the 4A2 → 4T1 vibronic transition to lower energy. As in the case of MgO co-doped samples a new Cr3+ centre is formed in ZnO co-doped samples.

A study is presented of solar concentrators based on total internal reflections. The concentrators are placed over photovoltaic cells. Experimental data obtained with different encapsulating materials and dependence on geometrical and physical variables are given. A theoretical model which includes the losses in the encapsulating material is proposed, giving good agreement with the experimental data. Gain factors as high as 30% have been obtained with these concentrators. The additional effect of organic dyes on absorption bands in the 350–360-nm range is also studied.

In this paper we present an outdoor evaluation of luminescent solar concentrators (LSC) throughout the whole day with different weather conditions. An electric gain of Gp= 25% was obtained with a LSC of PMMA doped with KF-241 dye. These results are compared with those previously reported.

The optical and Raman spectroscopy of Mn2+ ions in dipolar form and forming Suzuki-phase precipitates has been studied in NaCl. An analysis of the position of the crystal field transitions reveals that the impurity-ligand distance is slightly smaller for the Suzuki phase but the covalency is the same for both phases. Finally it is demonstrated that the intense band at 231 nm is related to the existence of the Suzuki phase and not to the presence of small aggregates. This band has been reasonably assigned to the double excitation 6A1(S) + 6A1(S)↪4T1(G) + [6A1(G), 4E(G)].

The heterogeneous interstitial nucleation model previously proposed to explain F-center growth curves in irradiated alkali halides has been extended to account for M-center kinetics. The interstitials produced during the primary irradiation event are assumed to be trapped at impurities and interstitial clusters or recombine with F and M centers. For M-center formation two cases have been considered: (a) diffusion and aggregation of F centers, and (b) statistical generation and pairing of F centers. Process (b) is the only one consistent with the quadratic relationship between M and F center concentrations. However, to account for the F/M ratios experimentally observed as well as for the role of dose-rate, a modified statistical model involving random creation and association of F+-F pairs has been shown to be adequate.

A luminescence band at 450 nm (2.7 eV) is observed during ion irradiation of fused silica at energies between 0.7 and 2.0 MeV. The intensity is a function of dose and ion species. In general, the intensity increases to a maximum at which it appears to remain for light ion excitation (H+, He+), but from which it subsequently decays for heavy ions (N+, O+). The energy deposition required to produce this maximum signal is of the same order as that reported for other saturable damage parameters in silica (such as the compaction associated with bond reorientation, and changes in the refractive index).