Influence of Zn2+ ions concentration on the optical properties of Zn/Er:LiNbO3 crystals

Congruent Er³⁺(3 mol%):LiNbO₃ crystals codoped with ZnO (X mol %, X=0, 3, 6 and 7) were grown by the Czochralski technique. The Er contents in the crystals were measured by an inductively coupled plasma atomic emission spectrometer (ICP‐AES). Under 800 nm excitation, the upconversion emission spectra reveal an enhancement of the green emission with respect to the red emission when the Zn²⁺ ions are introduced into Er:LiNbO₃ crystal. The effect of Zn²⁺ ions concentration on the intensity ratio of the green to red emission has been investigated. Two cross‐relaxation processes (²H_11/2 + ⁴I_13/2 → ⁴I_11/2 + ⁴F_9/2 and ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2) are involved in populating the ⁴F_9/2 state, which bypass the green‐emitting states. The OH^‐ absorption spectra indicate that the Zn²⁺ codoping leads to a decreased concentration of Er³⁺ cluster sites contributing to the enhancement of the green emission. The studies on UV‐vis absorption spectra show that the heavily codoped with Zn²⁺ results in the reformation of the Er³⁺ cluster sites in Er:LiNbO₃. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of and concentration quenching in Bi2O3-B2O3-Ga2O3 glasses

Er₂O₃-doped Bi₂O₃-B₂O₃-Ga₂O₃ glasses were prepared by the conventional melt-quenching method, and the Er³⁺:⁴I_13/2 → ⁴I_15/2 fluorescence properties are studied for different Er³⁺ concentrations. when the Er₂O₃ concentration increases from 0.03 to 3.0 mol%, the measured lifetime of Er³⁺:⁴I_13/2 level decrease from 2.24 to 0.9 m s, and from 0.25 to 0.20 m s for the Er³⁺:⁴I_11/2 level. The fast energy migration among Er³⁺ ions cause the reduction of lifetime of the ⁴I_13/2 level, whereas the change in the ⁴I_11/2 level is mainly due to a cooperative upconversion process (⁴I_11/2, ⁴I_11/2) → (⁴F_7/2, ⁴I_15/2). Based on the dipole-dipole interaction theory, the interaction parameter, C_Er,Er, for the migration rate of Er³⁺:⁴I_13/2 ↔ ⁴I_13/2 was calculated to be 32 × 10⁻⁴⁰ cm⁶ s⁻¹.     

Spectroscopic properties of La3Ga5SiO14:Er3+ (1%) crystals

Single crystals of Erbium (Er) doped La₃Ga₅SiO₁₄ (LGS) have been grown along c‐axis by using the Czochralski method. The absorption and fluorescence spectra of LGS: Er³⁺ single crystals have been measured and analyzed according to the Judd‐Ofelt theory. When applied, the following spectral parameters have been obtained: intensity parameters Ω_t, Ω₂= 2.741674×10^‐20cm², Ω₄= 0.66934×10^‐20 cm² and Ω₆= 0.592591×10^‐20 cm², radiative transition probabilities A_J,J”, P_J,J”. The radiative lifetime of levels ⁴I_13/2, ⁴H_9/2, ⁴S_3/2 are 11.333ms, 0.447ms and 0.704ms, respectively. The fluorescence branching ratios and the integrated emission cross sections are also calculated. The results suggest that LGS: Er crystals have potential applications as a laser material. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect structure and spectroscopic characteristics of codoped Hf: Er: LiNbO3 crystals

Codoped Hf: Er: LiNbO₃ crystals have been grown by the Czochralski technique. Defect structures of the crystals were analyzed by IR absorption spectra, and the compositions of the crystals were measured by X‐ray fluorescent spectrograph. A new OH^–‐associated vibrational peak at 3492 cm^–1 was revealed in 6 mol % Hf: 1 mol % Er: LiNbO₃ crystal. It was attributed to (Hf_Nb)^–‐OH^–‐(Er_Nb)^2– defect centers. The Er³⁺ concentrations in crystals gradually decreased with the increase of the codoped Hf⁴⁺ concentrations in the melts. The emission characteristics of the crystals were investigated by the fluorescence spectrum. It was found that the luminescent intensity in codoped 6 mol % Hf: 1 mol % Er: LiNbO₃ crystal was 3.5 times stronger than that in single doped 1 mol % Er: LiNbO₃ crystal. The luminescent enhancement effect was successfully explained on the basis of defect structure of the crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Energy transfer between Tm and Er ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in Tm³⁺/Er³⁺ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er³⁺ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm³⁺ ions. This is evidence of the relevant contribution of the route Er₁(⁴I_11/2) + Er₂(⁴I_13/2) → Er₁(⁴I_15/2) + Er₂(⁴F_9/2) to the process. Energy migration among pumped ⁴I_9/2 level reducing the efficiency of the upconversion emission rate (³H_11/2, ⁴S_3/2, and ⁴F_9/2) is observed for Er³⁺ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.     

Optical Properties of Er Doped Congruent and Stoichiometric LiNbO3 Single Crystals

Erbium doped LiNbO₃ (Er:LiNbO₃) single crystal fibers were grown free of cracks along c‐axis by the micro‐pulling down (μ‐PD) method. We have investigated the up‐conversion property with the change of doped Er₂O₃ concentration and the starting melt composition. An enhancement of green upconversion according host matrix is also observed the stoichiometric LiNbO₃. And, the dependence of the green emission according to Er³⁺ concentration is analyzed. The possible application of the Er³⁺ doped stoichiometric LiNbO₃ single crystal fiber for up‐conversion based optical devices is discussed.     

Effect of OH− content on emission properties in Er3+-doped tellurite glasses

A series of tellurite glasses of composition, 75TeO₂–20ZnO–(5 ? x)La₂O₃–xEr₂O₃ (x = 0.05, 0.1, 0.3, 0.6, 1.0, 2.0, and 3.0 mol%) with different hydroxl content were prepared. The effect of Er³⁺ and OH^? groups concentration on the emission properties of Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition in tellurite glasses was investigated. The constant K_OH–Er for Er³⁺ in tellurite glasses, which represents the strength of interaction between Er³⁺ and OH^? groups in the case of energy migration, was about 14 × 10^?19 cm⁴ s^?1. The interaction parameter C_Er,Er for the migration rate of Er³⁺: ⁴I_13/2 → ⁴I_13/2 transition in tellurite glass was 46 × 10^?40 cm², which indicates that concentration quenching in Er³⁺-doped modified tellurite glass for a given Er³⁺ concentration is much stronger than in silicate and phosphate glasses.     

Annealing effect on photoluminescence properties of Er doped Al2O3–SiO2 sol–gel films

Annealing effect on photoluminescence intensity of Er doped Al₂O₃–SiO₂ prepared from Er doped boehmite (AlOOH) and GPS (3-glycidoxypropyltrimethoxysilane) hybrid was investigated. The emission intensities peaked at 1.54 μm, which correspond to the ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ion, are greatly increased by about 8 times between 900 and 1000 °C, than that expected from TGA associated with the elimination of hydroxyl groups which is responsible for the fluorescence quenching. The residual hydroxyl groups for Er doped Al₂O₃–SiO₂ after annealing at high temperature was further analyzed by FT-IR. Finally, fluorescence intensities were compared with the variation of BET surface areas against the annealing temperature. It was found that photoluminescence intensity below 1000 °C was more dependent on surface hydroxyl groups re-adsorbed by a high specific surface area rather than internal hydroxyl groups remained in gel film.     

Optical spectroscopy and local structure of Er3+ luminescence centres in СaO–Ga2O3–GeO2 glasses

Optical absorption, luminescence excitation and emission spectra of Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂:Er glass with Er content of 1.46 wt% are presented and analysed. Luminescence kinetics for the main Er³⁺ transitions was satisfactorily described by single exponential decays with characteristic lifetimes. Oscillator strengths, phenomenological Judd–Ofelt intensity parameters, radiative decay rates (emission probabilities of transitions), branching ratios and radiative lifetimes for Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂:Er glass are calculated and compared with the corresponding parameters of the Ca₃Sc₂Ge₃O₁₂:Er³⁺ garnet and other crystals and glasses. Quantum efficiency, η, of the ⁴I_13/2 → ⁴I_15/2 Er³⁺ transition is determined. Incorporation peculiarities and local structure of Er³⁺ luminescence centres in Ca₃Ga₂Ge₃O₁₂:Er³⁺ glass are discussed in comparison with garnet crystals and oxide glasses. On the basis of the presented results and referenced EXAFS data for Er, Eu and Ho impurities (L₃-edge) it has been shown that Er³⁺ centres in Ca₃Ga₂Ge₃O₁₂ glass occupy network sites with the coordination number to oxygen of N = 6.     

Preparation and upconversion luminescence characterization of 12CaO·7Al2O3:Ho3+/Yb3+ polycrystals

The effect of Yb³⁺ concentration on the fluorescence of 12CaO·7 Al₂O₃:Ho³⁺/Yb³⁺ polycrystals is investigated. The Raman spectra of pure C12A7 under 633‐nm excitation show that the highest photon energy is 787.267 cm⁻¹, which is not much bigger than general fluorides, so it can realize high efficiency upconversion. The upconversion emission spectra suggest that the green upconversion emission centered at 548 nm and the red upconversion emission at 662 nm correspond to the ⁵F₄/⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transition of Ho³⁺ ions, respectively. The intensity of the upconversion luminescence and the ratio of red to green are changed with Yb³⁺ ion concentration. The pump dependence and luminescence decay dynamics spectra show the green and red upconversion emissions are populated by a two‐photon process, and the upconversion mechanisms are analyzed. The relative luminous efficiencies of green and red emissions are 2.035% and 0.7%, respectively. The normalized efficiency obtained for green emission of Ho³⁺ at RT when the sample is excited by 980‐nm light with an absorbed intensity of 7.5 W/cm² is 0.27 cm²/W. This result is comparable to the values obtained in YF₃ for the Yb³⁺, Er³⁺ green emission. The C12A7 with upconversion red and green light will be a promising luminous material.     

Influence of Mg/Er co‐doping on the principal laser parameters of LiNbO3 crystals

Mg: Er: LiNbO₃ crystals were grown by the Czochralski technique with various concentrations of MgO = 2 mol%, 4 mol%, 6 mol% and the fixed concentration of Er₂O₃= 1 mol% in the melt, and the 8 mol%Mg: 1 mol%Er: LiNbO₃ crystal was fabricated by the Czochralski technique with special technology process. The crystals were treated by polarization, reduction and oxidation. The segregation coefficients of Mg²⁺ and Er³⁺ in Mg: Er: LiNbO₃ crystals were measured by X‐ray fluorescence spectrograph, as well as the crystal's defect structure and optical properties were analyzed by the UV‐Vis, IR and fluorescent spectroscopy. The pump wavelength and the surge wavelength were determined. Using m‐line method tested optical damage resistance of those crystals, the results show that photodamage threshold of Mg: Er: LiNbO₃ crystals are higher than that of Er: LiNbO₃ crystal, and the oxidation treat could enhance the photodamage resistant ability of crystals while the reduction treat could depress the ability. The optical damage resistance of 8 mol%Mg: 1 mol%Er: LiNbO₃ crystal was the strongest among the samples, which was two orders magnitude higher than that of 1 mol%Er: LiNbO₃ crystal. The dependence of the optical properties on defect structure of Mg: Er: LiNbO₃ crystals was discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Er3+/Yb3+-codoped silica–germania sputtered films: structural and spectroscopic characterization

We report on the fabrication and spectroscopic characterization of highly photo-refractive Er³⁺/Yb³⁺ coactivated silica–germania slab waveguides, single mode at 1550 nm, deposited by radio-frequency-magnetron-sputtering technique. Details of the sputtering procedure are reported. The structural properties of the films were investigated by Raman spectroscopy. Propagation losses of guided modes were measured at 633 nm and 1320 nm. The emission of the ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ion was analyzed upon excitation of the TE₀ mode at 514 and 981 nm. Back energy transfer from Er³⁺ to Yb³⁺ was observed by measurement of Yb³⁺ emission upon Er³⁺ excitation at 514.5 nm. Photoluminescence excitation spectroscopy was used to obtain information about the Yb³⁺ to Er³⁺ energy transfer process.     

Growth and spectral properties of Er3+/Tm3+ co‐doped LiYF4 single crystal

High quality Er³⁺/Tm³⁺:LiYF₄ single crystals were grown by a Bridgman method. The absorption spectra and luminescent properties of the crystals were studied to characterize the effect of Tm³⁺ on the spectroscopic properties upon excitation of an 800 nm laser diode. The broaden 1.5 μm and the enhanced 2.7 μm emission were observed in the Er³⁺/Tm³⁺ co‐doped LiYF₄ single crystals. Meanwhile, the up‐conversion and 1.5 μm emission intensities from Er³⁺ decrease with increasing the ratio of Tm³⁺ to Er³⁺. The energy transfer processes between Tm³⁺ and Er³⁺ in the Er³⁺/Tm³⁺ co‐doped samples were analyzed. The energy transfer efficiency η_ETE from Er³⁺ to Tm³⁺ is calculated. The highest η_ETE of 65.30% for the sample with 0.296 mol% of Er³⁺, 0.496 mol% of Tm³⁺ concentration was obtained. The present work indicates that Er³⁺/Tm³⁺ co‐doped LiYF₄ single crystal can be a promising material for the potential application in infrared devices.     

Effects of yttrium codoping on fluorescence lifetimes of Er3+ ions in SiO2–Al2O3 sol–gel glasses

In order to find a new glass host and optimize erbium doping for IR glass optical amplifiers in photonic applications, a study on the optimization of the emission of erbium ions in the SiO₂–Al₂O₃ glass by codoping with Y₂O₃ is performed. It is first attempted to make a new sol–gel glass host based on SiO₂, Al₂O₃, and Y₂O₃ doped with Er³⁺ ions of the composition (1−x)SiO₂–xAl₂O₃–yY₂O₃:0.65Er₂O₃ (in mol%), x varies from 0 to 65, and y from 0 to 4. The optimal proportion in mol% of SiO₂ and Al₂O₃ for the Er³⁺ emission (at a fixed optimal concentration of 0.65) was 65 – 35. The effect of Y₂O₃ content on photoluminescence, decay curve profiles and lifetime of the ⁴I_13/2 level of Er³⁺ in SiO₂–Al₂O₃ glass is observed. The largest quantum efficiency and the higher emission intensity are observed in the sample with 65Al₂O₃ and 4Y₂O₃. The emission intensity at 1530 nm is two times higher than in glasses without Y₂O₃. A shift of 3 nm to shorter wavelengths is observed. The emission spectral profiles are flatter and broader for the glasses containing Al and Y (bandwidth of 59.5 nm). The decay curves show strong difference profiles for the different samples. The increase of the lifetime value τ (about ms) of the ⁴I_13/2 level of Er³⁺ in the SiO₂–Al₂O₃ with the Y₂O₃ is discussed.     

On the optical absorption and photoluminescence of Er-doped Ge–S–Ga glasses

We have studied the absorption and photoluminescence (PL) of (GeS₂)₈₀(Ga₂S₃)₂₀ glasses doped with 0.17, 0.35 and 1.05 at.% Er. The sharp bands centered at around 660, 810, 980 and 1540 nm in the absorption spectra can be associated with intra 4f-shell transitions in Er³⁺ ions from ⁴I_15/2 level to ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 levels, respectively. It has been observed that the absorption edge shifts towards lower energies with increasing Er concentration. A decrease in the absorption coefficient in the range of weak absorption, as well as the host luminescence in more heavily doped samples has been established, which may be associated with less native defects in the glassy structure. The role of excitation wavelength (λ_ex) on the PL emission band at 1540 nm using different Er³⁺-doping level has been evaluated. It has been found that the total PL band remains almost the same under direct excitation of Er³⁺ ions (at λ_ex = 644, 770 and 982 nm), while it becomes narrower under the host excitation (at λ_ex = 532 nm).     

Recent photoluminescence research on chalcogenide glasses for photonics applications

The optical properties of GeGaSe glasses doped with Er by the addition of Er₂S₃ have been investigated. Optically uniform glasses have been prepared using stoichiometric compositions with 9–12 at.% Ga and doped with 0.5–2 at.% Er. The radiative lifetime of the ⁴I_13/2 → ⁴I_15/2 transition has been estimated to be equal to 1.78 ms using the Judd–Ofelt analysis. The photoluminescence lifetime distribution has been investigated in optimized glasses using Quadrature Frequency-Resolved Spectroscopy at room and liquid helium temperatures and at different emission wavelengths. All lifetime distributions were found to be sharp peaks centered at ∼2 ms. A radiation diffusion model has been used to understand the discrepancy between measured photoluminescence spectra and those predicted by the McCumber theory. The model predicts a radiative lifetime of the ⁴I_13/2 → ⁴I_15/2 transition to be around 1.72 ms and a much longer non-radiative lifetime. These results assume quasi-uniform distribution of Er³⁺ ions with negligible concentration-self-quenching and negligible rate of non-radiative relaxation from ⁴I_13/2 to ⁴I_15/2.     

Er3+/Yb3+-activated silica-hafnia planar waveguides for photonics fabricated by rf-sputtering

95.8SiO₂–4.2HfO₂ planar waveguide activated by 0.2 mol% Er and 0.2 mol% Yb was fabricated by multi-target rf-sputtering technique. The optical parameters were measured by an m-line apparatus operating at 543.5, 632.8, 1319 and 1542 nm. The waveguide compositions were investigated by energy dispersive spectroscopy. The waveguide exhibits a single propagation mode at 1.3 and 1.5 μm with an attenuation coefficient of 0.2 dB/cm at 1.5 μm. The emission of ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ion, with a 42 nm bandwidth was observed upon TE₀ mode excitation at 980 and 514.5 nm. Photoluminescence excitation spectroscopy was used to obtain information about the effective excitation efficiency of Er³⁺ ions by co-doping with Yb³⁺ ions. Channel waveguide in rib configuration were fabricated by wet etching process in the active film.     

Crystal growth and spectroscopic properties of erbium doped Lu2SiO5

A high optical quality erbium doped Lu₂SiO₅ single crystal has been grown by the Czochralski method. The distribution coefficient of Er³⁺ was measured to be ∼0.926. The absorption and emission spectra as well as the fluorescence decay curve of the excited state ⁴I_13/2 were measured at room temperature. The spectroscopic parameters were calculated using the Judd–Ofelt theory, and the J–O parameters Ω₂, Ω₄ and Ω₆ were found to be 4.451×10^-20, 1.614×10^-20 and 1.158×10^-20 cm², respectively. The room-temperature fluorescence lifetime of the Er³⁺⁴I_13/2→⁴I_15/2 transition was measured to be 7.74 ms. The absorption and emission cross-section as well as the gain cross-section in the eye-safe regime of 1400–1700 nm were also determined and discussed.     

Analysis of spectral components in the 1.5 μm emission band of Er doped borosilicate glass

We measured the 1.5 μm emission spectra corresponding to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ in borosilicate glass within the temperature range from 11 to 300 K. The spectral components emitting from the lowest and upper Stark levels of ⁴I_13/2 state were distinguished by analyzing the spectra with normalized area. The effect of optical properties of the spectral components on the 1.5 μm emission bandwidth is investigated. The results indicate that to search for a host with higher spontaneous emission probability of the upper Stark levels of ⁴I_13/2 state for Er³⁺ ions is very important to broadening of the 1.5 μm emission band. An equivalent model of four-level system is presented and applied to explain the spectral shape and temperature characteristics of the 1.5 μm emission band.     

Broadband near-infrared emission from Bi–Er–Tm Co-doped germanate glasses

Bi–Er–Tm co-doped germanate glasses and Bi, Er, Tm singly doped glasses were prepared and characterized through absorption spectra, NIR emission spectra and decay lifetime. A super broadband near-infrared emission from 1000 nm to 1600 nm, covering the whole O, E, S, C, and L bands, was observed in the Bi–Er–Tm co-doped samples due to the result of the overlapping of the Bi related emission band (centered at 1300 nm), the emission from Er^{3+ 4}I_13/2 → ⁴I_15/2 transition (centered at 1534 nm) as well as the emission from Tm^{3+ 3}H₄ → ³F₄ transition (centered at 1440 nm), which is essential for broadly tunable laser sources and broadband optical amplifiers. The energy transfer process was also discussed at the end of the paper.