F. Lahoz

Ultraviolet and white photon avalanche upconversion in Ho3+-doped nanophase glass ceramics

Ho3+-doped fluoride nanophase glass ceramics have been synthesized from silica-based oxyfluoride glass. An intense white emission light is observed by the naked eye under near infrared excitation at 750nm. This visible upconversion is due to three strong emission bands in the primary color components, red, green, and blue. Besides, ultraviolet signals are also recorded upon the same excitation wavelength. The excitation mechanism of both the ultraviolet and the visible emissions is a photon avalanche process with a relatively low pump power threshold at about 20mW. The total upconverted emission intensity has been estimated to increase by about a factor of 20 in the glass ceramic compared to the precursor glass, in which an avalanche type mechanism is not generated.

Infrared-laser induced photon avalanche upconversion in Ho3+–Yb3+ codoped fluoroindate glasses

An intense green upconversion emission due to a photon avalanche mechanism has been observed in Ho3+–Yb3+ codoped fluroindate glasses, measured at 200 K, under excitation at around 750 nm. The influence of the Yb3+ concentration on the photon avalanche process has been studied. The pump power threshold of the avalanche mechanism decreases as the Yb3+ concentration is increased. A reduction by about a factor of 3 is observed from the 2.25 mol % Ho3+ single doped glass to the 2.25 mol % Ho3+–2.25 mol % Yb3+ codoped glass. This effect has been related to an extra feeding of the 5I7 intermediate level of Ho3+ ions via Ho3+→Yb3+ and Yb3+→Ho3+ energy transfer and back transfer processes, respectively.

Ho 3+ -doped nanophase glass ceramics for efficiency enhancement in silicon solar cells

Currently Er(3+)-doped fluorides are being used as upconversion phosphors to enhance the efficiency of Si solar cells, to our knowledge. However, this enhancement is strongly limited owing to the small solar spectral range around 1540 nm that is used. We demonstrate that Ho(3+)-doped oxyfluoride glass ceramics are adequate to enlarge the Si sub-bandgap region around 1170 nm that can be transformed into higher-energy photons, showing an upconversion efficiency 2 orders of magnitude higher than the precursor glass. As these materials are transparent at 1540 nm, they can be used complementarily with Er(3+)-doped phosphors for the same purpose.

Optical amplification in Ho3+-doped transparent oxyfluoride glass ceramics at 750nm

Positive transient optical gain has been demonstrated in Ho3+-doped transparent oxyfluoride glass-ceramics. A pump and probe experiment has been designed to show this result. High power laser pulses at 532nm were used as the pump source to strongly populate the Ho3+S25:F45 level due to nonresonant ground state absorption. Low power cw laser radiation at 750nm was used as the probe beam. The signal beam stimulates the emission associated with the Ho3+S25:F45→I75 electronic transition at 750nm. In addition to this, the high power pump pulses provide population inversion between the S25:F45 and I75, initial and final states of the transition, respectively, giving rise to the optical amplification of the signal beam. A gain coefficient of 3.7cm−1 (∼16dB∕cm) was obtained for a pump energy density of about 135mJ∕cm2 and a signal beam power density of 6μW∕cm2.

Novel erbium(III) complexes with 2,6-dimethyl-3,5-heptanedione and different N,N-donor ligands for ormosil and PMMA matrices doping

Three novel complexes, [Er(dmh)3(bipy)], [Er(dmh)3bath] and [Er(dmh)3(5NO2phen)], with 2,6-dimethyl-3,5-heptanedione (Hdmh) as the main sensitizer and either 2,2′-bipyridine (bipy), bathophenanthroline (bath) or 5-nitro-1,10-phenanthroline (5NO2phen) as synergistic ligands were synthesized. Upon excitation at the maximum absorption of the ligands, the complexes show the characteristic near-infrared (NIR) luminescence of the Er3+ ions, due to efficient energy transfer from the ligands to the central Er3+ ion via the antenna effect. Single crystals were grown and their structures were determined showing different Er–N distances. The compound with shorter Er–N distances, [Er(dmh)3(5NO2phen)], was found to be the best light harvester and the best for transferring the energy to the lanthanide among the three studied compounds. Finally, the novel complexes have been assessed for their application in sol–gel and polymer-based waveguides and optical amplifiers through their inclusion into ormosil and polymethylmethacrylate matrices. The dispersion was successful in the bipy and 5NO2phen cases, with the properties of the hybrid materials mimicking those of the pure complexes.

Structure and NIR-luminescence of ytterbium(III) beta-diketonate complexes with 5-nitro-1,10-phenanthroline ancillary ligand: assessment of chain length and fluorination impact

Seven new tris(β-diketonear-nate)ytterbium(III) complexes with the general formula [Yb(β-diketonate)3(5NO2phen)] (where the β-diketone is either 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione, 4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione, 1,1,1-trifluoro-2,4-pentanedione, 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione, 1,1,1,5,5,6,6,7,7,7-decafluoro-2,4-heptanedione, 2,4-hexanedione or 2,6-dimethyl-3,5-heptanedione, and 5NO2phen = 5-nitro-1,10-phenanthroline) were synthesized and characterized by elemental analysis, attenuated total reflectance Fourier transform infrared spectroscopy and photoluminescence spectroscopy. Single crystal X-ray structures have been determined for three fluorinated complexes and ground state geometries of the other four complexes have been predicted using the Sparkle/PM6 model. These experimental structures and those designed by semi-empirical models reveal octacoordination around the Yb(3+) ion. Photoluminescence studies and lifetime measurements show that the increase in the fluorinated β-diketonate chain length is associated with a decrease in Yb(3+) luminescence intensity of the (2)F5/2→(2)F7/2 transition at around 980 nm and the (2)F5/2 excited state lifetime, while the ligand lifetime value remains almost unaffected. Finally, fluorination of the ligands is only advised when the complexes are to be used for co-doping with isostructural Er(3+) complexes for optical amplifiers, since it leads to a slight decrease in luminescence intensity for the same β-diketonate chain length.

Dopant partitioning influence on the near-infrared emissions of Tm3+ in oxyfluoride glass ceramics

The doping distribution of Tm3+ ions in a transparent oxyfluoride glass ceramic has been investigated. Optical absorption, luminescence, and excitation measurements have been performed in order to determine the environment in which Tm3+ ions and the infrared emissions they give rise to are located. An interesting result has been found: the main contribution to the 1465 nm emission (S band) is due to Tm3+ ions in the crystalline phase for low doping level. However, when the Tm3+ concentration is high the S-band emission comes from the small portion of Tm3+ that remains in the vitreous phase. It has been concluded that cross relaxation (CR) processes are responsible for the quenching of the S-band emission in the crystalline phase for high doping concentration. Lifetime measurements of the H43 level have also been taken and the probability of CR processes deduced.

Temperature dependence of Nd3+↔Yb3+ energy transfer in the YAl3(BO3)4 nonlinear laser crystal

The temperature dependence of the Nd3+→Yb3+ energy-transfer rate in the YAl3(BO3)4 nonlinear laser crystal has been investigated from the analysis of fluorescence decay curves recorded in the 10–600 K range. Three different regimes, independent on the dopant concentration, have been observed in the thermal behavior of the Nd3+→Yb3+ energy-transfer rate. By comparing experimental results with theoretical predictions based on the Dexter model [J. Chem. Phys. 21, 836 (1953)], the origin of these different regimes has been explained. In addition, the influence of temperature and of both Nd3+ and Yb3+ concentrations on the Nd3+←Yb3+ energy back-transfer rate has been also investigated, concluding that it is a migration-assisted energy-transfer process. Finally, the populations of both Nd3+ and Yb3+ metastable states achieved after continuous-wave Nd3+ excitation have been calculated and measured and results have been explained in terms of the thermal behavior of both forward- and back-transfer rates.

Reduction of the amplified spontaneous emission threshold in semiconducting polymer waveguides on porous silica.

Hybrid organic-inorganic monomode waveguides of conjugated polymers on porous silicon (PS) substrates have been fabricated. Different low refractive index PS substrates, varying from 1.46 down to 1.18 have been studied. Amplified spontaneous emission (ASE) has been observed for all the samples and the ASE threshold has been monitored as a function of the PS refractive index. A decrease in the ASE threshold is detected when the PS refractive index decreases. These results have been analysed in the frame of a four level waveguide amplifier model and the theoretical predictions are in agreement with the experimental data.

Optical gain in dye-impregnated oxidized porous silicon waveguides

Positive optical gain under pulsed excitation in oxidized porous silicon planar waveguides impregnated with Nile blue (LC 6900) is reported. Amplified spontaneous emission measurements show a dramatic line narrowing when the pump energy is increased, together with a strong superlinear behavior. Variable stripe length measurements were performed to characterize quantitatively the amplification, and an unambiguous transition from losses to gain is observed with a threshold of ~3 mJ/cm (~40 dB/cm) is reported. Shifting excitation spot measurements confirm the reliability of our results. This system is interesting in view of an optically pumped silicon-based pulsed laser.