J. Fernández

Anti-stokes laser cooling in bulk erbium-doped materials.

We report the first observation of anti-Stokes laser-induced cooling in the Er3+:KPb2Cl5 crystal and in the Er3+:CNBZn (CdF2-CdCl2-NaF-BaF2-BaCl2-ZnF2) glass. The internal cooling efficiencies have been calculated by using photothermal deflection spectroscopy. Thermal scans acquired with an infrared thermal camera proved the bulk cooling capability of the studied samples. The implications of these results are discussed.

Infrared-to-visible upconversion of Er 3+ ions in GeO 2 –PbO–Nb 2 O 5 glasses

We report the infrared-to-visible upconversion luminescence of Er3+-doped lead–niobium–germanate glasses (GeO2–PbO–Nb2O5) with different Er2O3 concentrations (0.5, 1, 2, and 3 wt.%) under continuous-wave and pulsed-laser excitation in the near-infrared region inside the 4I9/2 level. Intense green emission due to the (2H11/2, 4S3/2)→4I15/2 transitions was observed at room temperature together with a weak red emission corresponding to the 4F9/2→4I15/2 transition. These upconversion emissions are attributed to a two-photon process. The time evolution of the green emission from the 4S3/2 level indicates that energy-transfer upconversion and excited-state absorption are responsible for the upconversion luminescence. The increase of the weak red emission with increasing Er2O3 concentration, together with its temporal behavior under infrared excitation, suggests that for Er2O3 concentrations higher than 1 wt.%, the upconverted red emission is the result of multiphonon relaxation from the 4S3/2 level and energy-transfer processes.

Spectroscopy and frequency upconversion in Nd3+-doped TeO2–TiO2–Nb2O5 glass

In this work, we report the optical properties of Nd3+ ions in tellurite glasses (TeO2–TiO2–Nb2O5) for two different Nd3+ concentrations. Judd–Ofelt intensity parameters were derived from the absorption spectrum and used to calculate the radiative lifetime and stimulated emission cross section of the transition. The non-exponential character of the decays from the 4F3/2 level and the reduction of the lifetime even at low temperature as Nd3+ concentration increases reveal the presence of nonradiative processes. The time evolution of the decays from the 4F3/2 level is consistent with a dipole–dipole direct energy transfer mechanism. Upconversion processes that produce green, orange, and red emissions after infrared excitation in the 4F3/2 and 4F5/2 levels have been investigated by using steady-state and time-resolved laser spectroscopy. The characteristics of the excitation spectra together with the temporal behaviour of the decays from the upconverted luminescence suggest that energy transfer upconversion processes are the dominant mechanisms in such emissions.

Spectroscopy and optical characterization of thulium doped TZN glasses

This paper reports on the spectroscopic properties of the 3H4 and 3F4 Tm3+ levels in a TZN glass host with a concentration range from 0.82 to 22 × 1020 cm−3. Precise refractive index measurements are performed on five different wavelengths by the prism coupling method. Judd–Ofelt intensity parameters have been determined in order to obtain transition rate, branching ratio and radiative lifetime. Spectroscopic measurements show the most promising concentration for the1.8 µm short cavity laser emission at 6.84 × 1020 cm−3 Tm3+ with a 24% quantum efficiency and upper limit concentration of 11 × 1020 cm−3 Tm3+. Energy transfer microparameters and critical ion distance are determined for both emission levels within the framework of diffusion-limited regime and dipole–dipole interaction.

Infrared to visible upconversion of Nd 3+ ions in KPb 2 Br 5 low phonon crystal

In this work we present a detailed analysis of the infrared to visible upconversion in Nd(3+)-doped KPb(2)Br(5) low phonon crystal by using both steady-state and time-resolved luminescence spectroscopy. Efficient blue, green, orange, and red emissions have been observed under excitation into the (4)F(5/2) and (4)F(3/2) states. The low phonon energy of this crystal leads to a significant reduction of the multiphonon relaxation rates which allows most of the excited states to relax radiatively. To investigate the nature of the upconversion processes, emission spectroscopy and lifetime measurements for the visible fluorescence, performed by using one photon excitation, have been compared to the upconverted emissions and lifetimes obtained under infrared pulsed excitation. The analysis of the experimental results indicates that the upconverted emissions occur via energy-transfer upconversion.

Optical spectroscopic study of Eu 3+ crystal field sites in Na3La9O3(BO3)8 crystal

Time-resolved line-narrowed fluorescence spectroscopy of Eu(3+) ions in a new oxyborate Na(3)La(9)O(3)(BO(3))(8) crystal shows the existence of four independent symmetry crystal field sites for the rare-earth ion. A crystal field analysis and simulation of the experimental results have been performed in order to parametrize the crystal field at the Eu(3+) sites. A plausible argument about the crystallographic nature of these sites is given.

Site-selective laser spectroscopy of Nd 3+ ions in 0.8CaSiO 3-0.2Ca 3(PO 4) 2 biocompatible eutectic glass-ceramics

In this work we report the influence of the crystallization stage of the host matrix on the spectroscopic properties of Nd3+ ions in biocompatible glass-ceramic eutectic rods of composition 0.8CaSiO3-0.2Ca3(PO4)2 doped with 1 and 2 wt% of Nd2O3. The samples were obtained by the laser floating zone technique at different growth rates between 50 and 500 mm/h. The microstructural analysis shows that a growth rate increase or a rod diameter decrease leads the system to a structural arrangement from three (two crystalline and one amorphous) to two phases (one crystalline and one amorphous). Electron backscattering diffraction analysis shows the presence of Ca2SiO4 and apatite-like crystalline phases. Site-selective laser spectroscopy in the (4)I(9/2)→(4)F(3/2)/(4)F(5/2) transitions confirms that Nd(3+) ions are incorporated in crystalline and amorphous phases in these glass-ceramic samples. In particular, the presence of Ca(2)SiO(4) crystalline phase in the samples grown at low rates, which has an excellent in vitro bioactivity, can be unambiguously identified from the excitation spectra and lifetime measurements of the (4)F(3/2) state of Nd(3+) ions.

K5Nd(MoO4)4: a self-tunable laser crystal.

We report for the first time to our knowledge an experimental demonstration of wavelength self-tuning in a K5Nd(MoO4)4 broadband-emitting laser crystal (KNM), as well as a theoretical treatment of the system based on its birefringence properties. The self-frequency-tuning of the laser along the full spectral range of the KNM crystal was obtained by rotating a birefringent gain plate in its own plane. The gain plate was placed inside a resonator at Brewsters angle. The tuning characteristics of the spectral filter were obtained by use of the Jones vector formalism. The calculated wavelength-selective tuning precisely matches the experimental observations.

Time-resolved fluorescence line-narrowing of Eu3+ in biocompatible eutectic glass-ceramics

The spectroscopic properties of Eu(3+) in biocompatible glass and glass-ceramic eutectic rods of composition 0.8CaSiO(3)-0.2Ca(3)(PO(4))(2) doped with 0.5 wt% of Eu(2)O(3) are investigated to explore their potential applications as optical probes. The samples were obtained by the laser floating zone technique. Depending on the growth rate, they exhibit three (two crystalline and one amorphous) or two (one crystalline and one amorphous) phases. The crystalline phases correspond to Ca(2)SiO(4) and apatite-like structures. At high growth rates the system presents an amorphous arrangement which gives a glass phase. The results of time-resolved fluorescence line narrowing spectroscopy obtained under excitation within the inhomogeneous broadened (7)F(0)→(5)D(0) absorption band allow to isolate the emission from Eu(3+) ions in the crystalline and amorphous environments and to accurately correlate the spectroscopic properties with the microstructure of these eutectics.

Optical properties of a Cr3+-doped fluorophosphate glass investigated by steady state and time-resolved laser spectroscopy

The optical properties of trivalent-chromium-doped fluorophosphate glass (21.74 Al(PO3)3-57.8 BaF2-19.96 AlF3 (wt.%)) have been investigated in the 4.2-300 K temperature range using steady state and time-resolved laser spectroscopy. The luminescence of Cr3+ shows a strong thermal quenching which can be analysed in terms of the quantum mechanical single-configurational coordinate model. In order to establish a correlation between the glass matrix composition and the spectral and temperature behaviour of the Cr3+ photoluminescence, detailed time-resolved measurements have been performed. When compared with fluoride glasses, the results attained for the emission wavelength dependence of the lifetimes and excitation spectra show an enhancement of the site-dependent effects.