X-ray diffraction,optical absorption and emission studies on Er<Superscript>3+</Superscript>- and Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript>-doped Li<Subscript>3</Subscript>NbO<Subscript>4</Subscript> powder

Er³⁺(/Yb³⁺)-doped Li₃NbO₄ powder were prepared by thermally sintering mixtures of Er₂O₃ (0.5, 1.0 mol%), Yb₂O₃ (0, 0.5, 1.0 mol%), Li₂CO₃ (48–49 mol%) and Nb₂O₅ (50 mol%) at 1125, 1150 and 1450 °C over the durations of 8–22 h. The crystalline phases contained in these samples were determined by using X-ray diffraction and discussed in comparison with a vapor-transport-equilibration-treated (VTE-treated) Er(2.0 mol%):LiNbO₃ single crystal and ErNbO₄ powder previously reported. The results show that the X-ray patterns of the rare-earth-doped samples reveal little difference each other, but large differences with those of the VTE crystal and ErNbO₄ powder. The doped rare-earth ions Er³⁺ (and Yb³⁺) present in the powder as the ErNbO₄ (and YbNbO₄) phase(s). The possibility that the highly Er-doped LiNbO₃ crystal contains Li₃NbO₄ precipitates is small. Optical absorption and emission studies show that the only Er-doped Li₃NbO₄ powder shows similar absorption and emission characteristics with the pure ErNbO₄. The codopant Yb³⁺ ion enhances the 980-nm-upconversion emissions of Er³⁺ ions, results in remarkable spectral alterations at 0.98 μm region, and causes the alterations of relative absorbance and relative emission intensity of individual peaks or bands at 1.5 μm region. On the other hand, the Yb-codoping hardly affects the Er³⁺ energy structure and the lifetime of Er³⁺ ion at 1.5 μm. The measured lifetimes at 1.5 μm of Er³⁺ ions in the singly Er³⁺- and doubly Er³⁺/Yb³⁺-doped mixtures have a nearly same value of ∼ 1.5 ms. For the pure ErNbO₄ powder, the lifetime is prolonged to ∼2 ms perhaps due to radiation trapping effect.     

Effect of vapor transport equilibration and Mg doped on the luminescence of Er:LiNbO3

Luminescence properties of the congruent and vapor transport equilibration (VTE) treated Er:LiNbO₃ and Er:Mg:LiNbO₃ crystals were recorded at room temperature. It is observed that VTE treatment could enhance the emission intensity of Er³⁺ ions and doping with MgO would weaken it in the visible spectra. As a result, the luminescence intensity of Er³⁺ ions in the VTE treated Er:Mg:LiNbO₃ crystal increased up to 2.2 times than that in the congruent Er:LiNbO₃ crystal. In addition, both VTE treatment and doping with MgO result in some changes of the relative emission intensity of some peaks in the visible emission spectra. In the infrared emission spectra, the luminescence peak at 1540 nm of Er³⁺ ions shifts towards the larger wavelength when the Er:LiNbO₃ crystals were treated using VTE or doped with MgO. The changes in crystalline environment of Er³⁺ ions due to VTE treatment or doping with Mg²⁺ play a key role in these phenomena.     

Preparation and photoluminescence of Er-doped Al2O3 films by sol-gel method

The 0-1.5 mol% Er³⁺-doped Al₂O₃ films have been prepared on the thermally oxidized SiO₂/Si(100) substrate in the dip-coating process by the sol-gel method, using the aluminium isopropoxide [Al(OC₃H₇)₃]-derived γ-AlOOH sols with the addition of erbium nitrate [Er(NO₃)₃·5H₂O]. The continuous Er³⁺-doped Al₂O₃ films with the thickness of about 1.2 μm were obtained for nine coating cycles at a sintering temperature of 900 °C. The aggregate size for the Er³⁺-doped Al₂O₃ films increased with increasing the Er³⁺ doping concentration from 0 to 1.5 mol%. The root-mean-square roughness of the films was independent on the Er³⁺ doping, which was about 1.8 nm for the 0-1.5 mol% Er³⁺-doped Al₂O₃ films. The γ-Al₂O₃ phase with a (110) preferred orientation was produced for the Al₂O₃ film. The photoluminescence (PL) spectra of 0.1-1.5 mol% Er³⁺-doped Al₂O₃ films were observed at the measurement temperature of 10 K. There was no significant change for the PL peak intensity with the increase of Er³⁺ doping concentration from 0.1 to 1.5 mol%, and similar full width at half maximum of about 40 nm was detected for the 0.1-1.5 mol% Er³⁺-doped Al₂O₃ thin films. The Er³⁺-doped Al₂O₃ films possess the available PL properties for use in planar optical waveguides.     

Phase transition of Er3+-doped Al2O3 powders prepared by the non-aqueous sol–gel method

The Er³⁺-doped Al₂O₃ powders have been prepared by the non-aqueous sol–gel method using the aluminum isopropoxide as precursor, acetylacetone as chelating agent, nitric acid as catalyzer, and hydrated erbium nitrate, as dopant under isopropanol environment. The phase structure and phase transition of the Er³⁺-doped Al₂O₃ powders were investigated by using thermogravimetry/differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The phase contents diagram for the Er-doped Al–O system with the doping concentration up to 5 mol% was described at the sintering temperature from 550 to 1250 °C. There were the three crystalline types of Er³⁺-doped Al₂O₃ phases, γ-, θ- and α-(Al, Er)₂O₃, and the two relative stoichiometric compounds composed of Al, Er, and O, ErAlO₃ and Al₁₀Er₆O₂₄ phases in the Er–Al–O phase contents diagram. The Er³⁺ doping suppressed crystallization of the γ and θ phases and delayed phase transition of the γ → θ and θ → α. The increased Er³⁺ doping concentration and the elevated sintering temperature enhanced the precipitation of the ErAlO₃ and Al₁₀Er₆O₂₄ phases. The preparation procedure for the Er³⁺-doped Al₂O₃ powders in the non-aqueous sol–gel process, including chelating, hydrolysis, peptization, doping and gelation, has a significant effect on the phase formation and its transition for the Er³⁺-doped Al₂O₃ powders.     

Effect of Y3+ codoping on luminescence decays of the 4I13/2 level in Er3+-doped Al2O3 powders prepared by a sol-gel method

The luminescence lifetime of the 0.01 mol.%-0.1 mol.% Er³⁺- and 0–20 mol.% Y³⁺-codoped Al₂O₃ powders prepared at a sintering temperature of 900°C in a non-aqueous sol-gel method has been investigated to explore the enhanced mechanism of photoluminescence properties of the Er³⁺-doped Al₂O₃ by Y³⁺ codoping. For the 0.1 mol.% Er³⁺-Y³⁺-codoped Al₂O₃ powders, the measured lifetime of Er³⁺ gradually increases with increasing Y³⁺ concentration. Consequently, codoping with 20 mol.% Y³⁺ leads to an increase in the measured lifetime from 3.5 to 5.8 ms. By comparing the measured lifetime for different Er³⁺ concentrations in the Al₂O₃ powders, the radiative lifetime of both the Er³⁺-doped and the Er³⁺-Y³⁺-codoped Al₂O₃ powders is estimated to be about 7.5 ms. Infrared absorption spectra indicate that Y³⁺ codoping does not change the-OH content in the Er³⁺-Y³⁺-codoped Al₂O₃ powders. The prolonged luminescence lifetime of the ⁴I_13/2 level of Er³⁺ in Er³⁺-doped Al₂O₃ powders by Y³⁺ codoping is ascribed to the decrease in the energy transfer rate between the Er³⁺ ions and the Er³⁺ and -OH, respectively, due to the suppressed interaction between Er³⁺ ions.     

Modelling of high-power diode-pumped erbium 3 μm fibre lasers

Abstract

We present theoretical calculations that relate to the cw operation of a high-power Er³⁺, Pr³⁺:ZBLAN double-clad fibre laser. Using the measured energy-transfer, energy-transfer-upconversion and cross-relaxation parameters relevant to Er³⁺-doped and Er³⁺, Pr³⁺-codoped ZBLAN, we compare the theoretical calculations from the model with recent experimental measurements. The model is then used to analyse the important pump and interionic processes occurring in Er³⁺, Pr³⁺:ZBLAN fibre lasers. The theoretical results indicate that energy transfer from Er³⁺ to Pr³⁺ leads to a fast depletion of the lower laser level and, due to the cw threshold condition, to low population densities in both laser levels. Thus, ground-state bleaching, pump excited-state absorption and energy-transfer upconversion amongst the Er³⁺ ions are avoided.     

Synthesis and luminescence of ultrafine Er3+- and Yb3+-doped Gd11SiP3O26 and Gd14B6Ge2O34 particles for cancer diagnostics

In search of new contrast materials for NMR and fluorescence diagnostics and neutron capture therapy of cancer, we have synthesized ultrafine Er³⁺- and Yb³⁺-doped Gd₁₁SiP₃O₂₆ and Gd₁₄B₆Ge₂O₃₄ particles and studied their luminescence properties. We measured the Er³⁺ upconversion luminescence spectra of the gadolinium erbium ytterbium phosphosilicates and borate germanates in the visible range and evaluated the absolute quantum yield of their luminescence. The quantum yield of luminescence in the gadolinium phosphosilicate Gd₁₁SiP₃O₂₆ doped with 5.0 at % Yb and 2.5 at % Er is comparable to that in known Yb³⁺/Er³⁺ codoped fluorides. The nonradiative Yb³⁺ ?? Er³⁺ energy transfer efficiency is evaluated.     

Ridge waveguide laser in Nd:LiNbO3 by Zn-diffusion and femtosecond-laser structuring

Ridge waveguide lasers have been fabricated on Nd³⁺ doped LiNbO₃ crystals. The fs-laser writing technique was used to define ridge structures on a gradient-index planar waveguide fabricated by Zn-diffusion. This planar waveguide was formed in a z-cut LiNbO₃ substrate homogeneously doped with a 0.23% of Nd³⁺ ions. To obtain lateral light confinement, the surface was then micromachined using a multiplexed femtosecond laser writing beam, forming the ridge structures. By butting two mirrors at the channel waveguide end-facets, forming a waveguide laser cavity, TM-polarized laser action at 1085 nm was achieved by end-fire TM-pumping at 815 nm. The waveguide laser shows a threshold of 31 mW, with a 7% of slope efficiency.     

Broadband infrared fluorescence in Er-doped BaO–SiO2 glasses

Er³⁺-doped SiO₂ glasses with or without BaO were fabricated by containerless processing. Scanning electron microscope observations and the scan profiles of electron-probe microanalysis demonstrated that the Ba-silicate glass was homogeneous and no aggregation of Er³⁺ ions occurred. The infrared fluorescence at around 1.55 μm from Er³⁺ in the Ba-silicate glass excited by a 980-nm laser was broader and its lifetime was longer than that of the silica glass, indicating the difference in the local structures around Er³⁺ ions between the Ba-silicate and silica glasses; this was supported by Raman scattering measurements. These results demonstrated that the Ba-silicate-glass system might be a new candidate for a host glass for Er³⁺-doped fiber amplifiers.     

Luminescence in various classes of Er<Superscript>3+</Superscript>-doped inorganic compounds under IR excitation

We have studied the luminescence of various classes of Er³⁺-doped inorganic phosphors under IR excitation. The highest IR Stokes luminescence efficiency in the range 1.5–1.6 μm is offered by Yb_0.99Er_0.01PO₄ (xenotime structure). This efficient IR phosphor has been commercialized under the name AM-1500M-1. It is of considerable practical interest for security and authentication applications and also as a gain medium for mid-IR lasers operating in the spectral range 1.5–1.6 μm, safe to the human eye.     

Simulation of Tm 3+ -doped silica and Er 3+ -doped fluorozirconate fibre lasers

Computer simulation of the operation of diode-pumped Tm³⁺-doped silica and Er³⁺-doped fluorozirconate fibre lasers that use cladding-pumping models high-power continuous-wave output at wavelengths near 2 and 3 μm. The model has been used to examine the fibre laser performance for a variety of fibre and pump configurations. Good agreement has been found between the model calculations and published experimental measurements. The model has determined the slope effciency and relative performance of the fibre lasers when the pump wavelength was varied over the ³F₄ and ³H₄ absorption bands of Tm³⁺ and the ⁴⁺I_11/2 absorption band of Er³⁺. The calculated maximum slope effciencies were determined to be about 40, about 71 and about 31% respectively, for each absorption band pump scheme. The model was further used to analyse the fibre laser output when the Tm 3+ and Er 3+ dopant concentrations, absorption conditions and diffraction conditions are varied and the consequences on the operation of the fibre laser with these variations are discussed.     

Anti-Stokes laser-induced cooling in rare-earth doped low phonon materials

In this work we discuss the anti-Stokes laser-induced cooling of two matrices doped with Yb³⁺ and Er³⁺: a low phonon KPb₂Cl₅ crystal and a fluorochloride glass. In order to assess the presence of internal cooling in these systems we used photothermal deflection and conventional excitation spectroscopic techniques, whereas the bulk cooling in the Er³⁺-doped materials was detected by means of a calibrated thermal sensitive camera. Furthermore, we also consider some of our findings on cooling processes occurring in Yb³⁺-doped low phonon materials from a theoretical perspective. The experimental results are in good agreement with the predictions of a model based on the presence of a second order process in the cooling mechanism. The fluorescence excess shown by the excitation spectra of Yb³⁺-doped sample obtained at high fluences, by pumping at wavelengths in the cooling region, has been explained in the framework of the configurational coordinate model by considering that the frequencies of the vibrational modes in the ground and excited states change at high pumping intensities (quadratic coupling mode). In the case of Er³⁺ ion, it is worthwhile to mention that the cooling was observed in the spectral region where some upconversion processes that initiate at the pumped ⁴I_9/2 level occur. Together with the spectroscopic characterization, a short discussion on the experimental and theoretical background of the cooling process including the possible influence of upconversion processes is presented.     

Synthesis of BaMoO4:Er/Yb particles by an MAM method and their upconversion photoluminescence properties

Er³⁺-doped BaMoO₄ (BaMoO₄:Er³⁺) and Er³⁺/Yb³⁺ co-doped BaMoO₄ (BaMoO₄:Er³⁺/Yb³⁺) particles were successfully synthesized by a cyclic microwave-assisted metathetic (MAM) method, and show fine and homogeneous morphology with particle sizes of 0.5–1 μm. At 980-nm excitation, BaMoO₄:Er³⁺ and BaMoO₄:Er³⁺/Yb³⁺ particles exhibited a strong 525-nm emission band and a weak 550-nm emission band in the green region. The Raman spectrum of BaMoO₄:Er³⁺/Yb³⁺ particles indicated the appearance of additional peaks at higher frequencies (390 and 505 cm⁻¹) and at lower frequencies (218 and 255 cm⁻¹).     

Polarized spectral properties and 1.5–1.6 μm laser operation of Er:Sr3Yb2(BO3)4 crystal

Undoped and Er³⁺-doped Sr₃Yb₂(BO₃)₄ crystals were grown by the Czochralski method. Room temperature polarized spectral properties of the Er:Sr₃Yb₂(BO₃)₄ crystal were investigated. The efficiency of the energy transfer from Yb³⁺ to Er³⁺ ions in this crystal was calculated to be about 95%. End-pumped by a diode laser at 970 nm in a hemispherical cavity, a 0.75 W quasi-CW laser at 1.5–1.6 μm with a slope efficiency of 7% and an absorbed pump threshold of 3.8 W was achieved in a 0.5-mm-thick Z-cut crystal glued on a 5-mm-thick pure YAG crystal with UV-curable adhesive.     

Emission properties of highly doped Er3+ fluoroaluminate glass

Er³⁺-doped fluoroaluminate (AYF) glass was compared with fluorozirconate (ZBLAN) and tetraphosphate (PE) glass as a host material for 1.54-μm emission. Experimental results show that the Er³⁺:AYF glass has a smaller concentration quenching and much stronger intensity for the 1.54-μm emission. In high dopant, the 1.54-μm emission is two times stronger in Er³⁺/AYF glass than in ZBLAN glass, and 10 times stronger than in PE glass.     

Absorption and emission of ErNbO4 powder

Visible and near infrared absorption and emission (488 nm excitation) characteristics of ErNbO₄ powder, which were prepared by calcining the Er₂O₃ (50 mol%) and Nb₂O₅ (50 mol%) powder mixture at 1100 and 1600 °C for different durations, have been investigated at room temperature. The absorption and emission characteristics of these calcined ErNbO₄ powder were summarized and discussed in comparison with those of Er₂O₃. Weak emission of Er₂O₃ relative to the calcined ErNbO₄ is mainly conducted with absorption difference at the excitation wavelength 488 nm. The obvious spectral changes from Er₂O₃ to calcined ErNbO₄ samples are related to an elevated-temperature-assisted phase transformation according to the solid-state chemical reaction equation: Er₂O₃ + Nb₂O₅ 2ErNbO₄, which results in the changes of the ion environment of Er³⁺ and hence changes of the Stark levels of Er³⁺. The further spectral change as the strengthened calcination results from the improvement of ErNbO₄ purity in the calcined mixture. The borders between two green transitions and between two near infrared transitions in the emission spectra of both calcined samples and Er₂O₃ were tentatively identified by referencing earlier reported emission spectra of the precipitated Z-cut VTE Er(2.0 mol%):LiNbO₃ crystal and the match relation between absorption and emission spectra of the ErNbO₄ powder. A comparison was performed on the spectra of calcined ErNbO₄ powder and those of VTE Er(2.0 mol%):LiNbO₃ crystals. The results allow to preliminarily deem the contribution of ErNbO₄ precipitates, generated inside these crystals by the VTE procedure, to the spectra of these crystals.     

Luminescent properties of EuGa2S4:Er3+

We have studied the thermo-and photoluminescence of undoped and Er³⁺-doped EuGa₂S₄ crystals in the temperature range 77–450 K. In thermoluminescence studies, different heating rates and excitation intensities have been used. The results demonstrate that, with increasing heating rate, the intensity of thermoluminescence peaks grows, and the peaks shift to higher temperatures. EuGa₂S₄:Er³⁺ crystals exhibit both Stokes and anti-Stokes luminescence. The mechanisms of these processes are analyzed.     

Thermal and photoluminescence properties of Er2S3-doped (As2Se3)90(GaSe)5Ge5 glasses

Thermal and photoluminescence characteristics of (As₂Se₃)₉₀(GaSe)₅Ge₅ glasses doped with different amounts of Er₂S₃ (Er amount from 0.1 to 3 at %) have been studied. Temperature-modulated differential scanning calorimetry measurements were used to study the glass transformation and thermal stability of these glasses. A 980-nm laser diode was used to pump the Er³⁺-doped glass samples, inducing a 1550-nm photoluminescence output signal. By appropriately pulsing the excitation, we measured the photoluminescence lifetime of the Er³⁺ state in these chalcogenide glasses.     

Vapor transport equilibration-induced formation of micron-sized ErNbO4 precipitates in Zn/Er-codoped LiNbO3 crystals

Congruent LiNbO₃ crystals with Zn/Er-doping levels of 5.5/1, 6/0.15 and 7/0.8 mol%/mol% were thermally treated at 1100 °C over 101 h using vapor transport equilibration (VTE) technique. It is shown that the VTE treatment has brought these crystals closer to stoichiometric composition. X-ray powder diffraction and Er³⁺ spectroscopic characterizations show that the VTE treatment induced formation of micron-sized ErNbO₄ precipitates on the surface and in the bulk of all crystals studied. It was observed using an optical microscope that the precipitates grow preferably along X, Y and Z axes of the host matrix, and show different morphologies, densities and dimensions, depending on the Er³⁺ and Zn³⁺ doping level. The formation origin of the precipitate is qualitatively explained.