J. Méndez-Ramos

Dopant distribution in a Tm3+–Yb3+ codoped silica based glass ceramic: An infrared-laser induced upconversion study

The optically active dopant distribution in a Tm(3+)-Yb(3+) doped silica based glass ceramic sample has been investigated. A systematic analysis of the upconversion fluorescence of the Tm(3+)-Yb(3+) codoped glass and glass ceramic has been performed at room temperature. Tm(3+) and Yb(3+) single doped glass and glass ceramics have also been included in the study. Upon infrared excitation at 790 nm into the (3)H(4) level of the Tm(3+) ions a blue upconversion emission is observed, which is drastically increased in the Yb(3+) codoped samples. A rate equation model confirmed the energy transfer upconversion mechanism. Based on these results, the temporal dynamic curves of the levels involved in the upconversion process, (3)H(4), (2)F(5/2), and (1)G(4) were interpreted in the glass ceramic samples. The contribution of the optically active Tm(3+) and Yb(3+) ions in the crystalline and in the vitreous phase of the glass ceramic was distinguished and the ratio of Tm(3+) ions in the crystalline phase could be quantified for the 1 mol % Tm(3+)-2.5 mol % Yb(3+) glass ceramic. A surprising result was obtained for that concentration: the main contribution to the upconversion emission of the glass ceramic is due to Tm(3+)-Yb(3+) ions in the vitreous phase.

Optical properties of Nd3+ ions in oxyfluoride glasses and glass ceramics comparing different preparation methods

A study of optical properties of Nd3+ doped oxyfluoride glasses and glass ceramics prepared by three different methods has been carried out. These methods start from NdF3, Nd2O3, or a Nd3+ ion solution as doping agent. The alternative preparation method based on a preliminary dissolution of the Nd3+ ions is proposed in order to avoid nonhomogeneous dopant distribution and spontaneous devitrification during glass elaboration. In the frame of the Judd–Ofelt theory, main radiative parameters have been studied: transition probabilities, lifetimes, and stimulated emission cross sections. Fluorescence decay curves have been also analyzed in order to study the final distribution of the Nd3+ ions after the ceramming process, discerning between ions that reside in the fluoride nanocrystals precipitated during heat treatment and those remaining in the glassy phase. The NdF3 based glass ceramics present the best values for spectroscopic parameters as the stimulated cross section of the 4F3/2→4I11/2 laser transition.

Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses

Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix.

Increase of the 800 nm excited Tm3+ blue upconversion emission in fluoroindate glasses by codoping with Yb3+ ions

Intense Tm3+ blue upconversion emission has been observed in Tm3+–Yb3+ codoped fluoroindate glasses under direct excitation into Tm3+ ions with a diode laser at 796 nm. The dependence of the intensity of this upconversion emission on the Tm3+ and Yb3+ concentration has been studied in order to determine the optimum ion concentrations and the involved mechanisms. The blue upconversion emission is highly increased with the Yb3+ concentration. For fixed Yb3+ concentrations, the maximum upconversion efficiency was obtained with Tm3+ concentration of about 0.5 mol%. For this optimum Tm3+ concentration, the upconversion emission intensity is increased by a factor about 100 by codoping with 2.25 mol% of Yb3+. The results are well explained by a proposed rate equation model.

Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics

Optical properties of Eu3+ ions in oxyfluoride glasses and glass ceramics doped with two different concentrations, 0.1 and 1 mol %, have been analyzed and compared with previous results for higher concentrated samples, 2.5 mol %. The Eu3+ ions in the 0.1 mol % doped glass ceramics are diluted into like crystalline environments with higher symmetry and lower coupled phonon energy than in the precursor glasses; meanwhile in the 1 mol % doped glass ceramics the presence of EuF3 clusters has been observed in addition to diluted ions. Fluorescence line narrowing measurements indicate the presence of two main fluoride site distributions for the diluted Eu3+ ions in both glass ceramics.

Role of the Eu3+ ions in the formation of transparent oxyfluoride glass ceramics

Optical properties of Eu3+ ions in oxyfluoride glass ceramics and in the precursor glasses (30 SiO2, 15 Al2O3, 29 CdF2, 22 PbF2, 1.5 YF3, and 2.5 EuF3, in mol %) have been analyzed and compared. The Eu3+ ions in the glass ceramics are incorporated into a crystalline environment with higher symmetry and lower energy coupled phonons. Emission measurements indicate that this crystalline phase is EuF3, instead of PbxCd1−xF2 as generally considered.

Increase in the Tb3+ green emission in SiO2–LaF3 nano-glass-ceramics by codoping with Dy3+ ions

95SiO2–5LaF3 sol-gel derived nano-glass-ceramics single doped with 0.1Dy3+ or 0.1Tb3+ mol % and codoped with 0.1Dy3+ and xTb3+ (x=0.1,0.3) mol % were successfully obtained. XRD and HRTEM measurements confirm the precipitation of LaF3 nanocrystals during the ceramming process, with mean size ranging from 10 to 20 nm and increasing with the thermal treatment temperature. About 75% of lanthanide ions are partitioned into LaF3 nanocrystals, as calculated from luminescence decays. The effect of increasing the Tb3+ concentration and also of codoping with Dy3+ in the Tb3+ green emission from the D54 level have been studied. The energy transfer mechanisms between Tb3+ ions and also between Tb3+–Dy3+ ions, which favor the green emission, have been analyzed.

Ultraviolet and visible upconversion luminescence in Nd3+-doped oxyfluoride glasses and glass ceramics obtained by different preparation methods

Ultraviolet and visible upconversion emissions under infrared excitation have been obtained and compared in Nd3+-doped oxyfluoride glasses and glass ceramics prepared by different methods. The mechanisms involved in such processes have been analyzed. A spectroscopy study comparing absorption and upconversion excitation spectra has been carried out in order to discern in each case the mechanism involved in the origin of the upconversion emissions, i.e., two- or three-photon involved excited state absorption or energy transfer processes. The shape of the absorption and emission spectra and the relative upconversion efficiencies have been compared and explained taking into account the final distribution of the luminescent ions in the glassy and nanocrystalline phases.

Infrared-light induced curing of photosensitive resins through photon up-conversion for novel cost-effective luminescent 3D-printing technology

Here we explore the ground-breaking photonic approach for infrared-light induced photo-polymerization of organic resins by using photon up-conversion in rare-earth-doped luminescent materials. In particular, we show outstanding high-intense near-infrared to ultraviolet-blue up-conversion emissions from Tm3+-doped K2YbF5 crystals, energies of which perfectly match the activation energy of light-sensitive resins that contain photo-initiators for radical polymerisation, widely used in medical surgery, odontology or implants applications, as well as in microstereolithography three-dimensional (3D) printing technology. This innovative technique, in which a cost-effective and low-power continuous wave commercial near-infrared laser diode is used, establishes a significant step forward to the state-of-the-art, by offering lower nominal radiation power and exposure time requirements than those of simultaneous two-photon absorption techniques based on powerful femtosecond lasers. We also present quasi-instantaneous “laser-writing”-like printing of 3D-structures, which exhibit visible luminescence under infrared light, for novel applications in 3D security-ink stamps or fluorescent labels for visual identification of orthodontic adhesives.

Novel up-conversion luminescent rare-earth- doped organic resins for cost-effective applications in 3D photonic devices†

Up-conversion luminescent materials have emerged recently for the improvement of the photocatalytic activity of semiconductor electrodes, such as TiO2 and Fe2O3, used for the sustainable production of hydrogen via water-splitting. Here we present novel up-conversion luminescent organic resins doped with heavy rare-earth ions, which are used as constructive elements in the 3D technique, that open a fully unexplored path for the development of cost-effective, room-temperature and endlessly shaped 3D photonic structures, avoiding the technical difficulties of glass melting or crystal growth when used as rare-earth hosts. Moreover, these synthesized resins also present outstanding UV-Vis up-conversion luminescence of Er3+ and Tm3+ ions sensitized by Yb3+ ions under near-infrared excitation at 980 nm, and have been extensively analyzed as a function of the doping concentration. Furthermore, the increase in the Yb3+ to Er3+ and Tm3+ ratio results in a notable enhancement of the UV-blue high energetic emission bands, allowing the tailoring of the overall up-conversion luminescence to match the different band-gaps of selected photocatalysts.