Preparation and characterization of stable aqueous suspensions of up-converting Er3+/Yb3+-doped LiNbO3 nanocrystals

The preparation of LiNbO₃:Er³⁺/Yb³⁺ nanocrystals and their up-conversion properties have been studied. It is demonstrated that polyethyleneimine- (PEI) assisted dispersion procedures allow obtaining stable aqueous LiNbO₃:Er³⁺/Yb³⁺ powder suspensions, with average size particles well below the micron range (100–200 nm) and the isoelectric point of the suspension reaching values well above pH 7. After excitation of Yb³⁺ ions at a wavelength of 980 nm, the suspensions exhibit efficient, and stable, IR-to-visible (green and red) up-conversion properties, easily observed by the naked eye, very similar to those of the starting crystalline bulk material.     

Luminescence kinetics of LiNbO3:Yb3+-Er3+, LiNbO3:Er3+, and LiNbO3:Yb3+ crystals under selective excitations in the impurity subsystem

The results of investigations of luminescent radiations’ kinetic characteristics for LiNbO₃:Yb³⁺-Er³⁺, LiNbO₃:Er³⁺, and LiNbO₃:Yb³⁺ crystals under optical excitations at 532 nm and 1064 nm wavelengths are presented. The shapes and times of rise and damping of luminescent signals at 550 nm, 980 nm and 1555 nm wavelengths under selective excitations in the impurity subsystem of the investigated materials are determined. Comparison of the temporal characteristics of luminescent responses of LiNbO₃ crystals doped separately with Yb³⁺ and Er³⁺ ions with those of the LiNbO₃:Yb³⁺-Er³⁺ crystal allows identifying the contributions from different energy transfer processes of optical excitation taking place in the impurity subsystem of the material.     

Spectral line broadening mechanism of Er3+ transitions in Er:Ti:LiNbO3 channel waveguides

In order to elucidate the interaction effects among the various defects present in a LiNbO₃-based integrated optical device, we investigated the change of the optical properties of Er³⁺ ions under the application of an external electric field and hydrostatic pressure. We obtained for stoichiometric bulk material a complete picture of the field-induced spectral shifts as a function of transition and site. As a first important application of these results we were able to clarify the mechanism of spectral broadening of the Er³⁺ transitions in Ti:Er:LiNbO₃ channel waveguides. By selecting different waveguide modes for excitation and using highly selective double-resonance excitation with two lasers, we found that the [Ti⁴⁺] concentration gradient caused by the indiffusion results in an internal E-field gradient. This translates, due to the averaging within the guided mode, into mode-dependent spectral line broadening. Received: 24 May 2001 / Published online: 23 October 2001     

Modelling of optical amplification in Er/Yb co-doped LiNbO3 waveguides

The optical amplification of LiNbO₃:Er³⁺/Yb³⁺ channel waveguides has been modelled in the small signal regime using the overlapping integrals method and the rate-equation formalism. It has been found that Yb³⁺ -sensitisation improves the pump efficiency at 980 nm and a higher gain is achievable in the high power-limit compared to singly-doped LiNbO₃:Er³⁺ amplifiers.     

Raman study on vapor-phase equilibrated Er:LiNbO3 and Er:Ti:LiNbO3 crystals

Raman spectra of Er:LiNbO₃ crystal and Ti-diffusedEr:LiNbO₃ strip waveguide, in which the Li/Nb ratio was altered using a vapor-phase equilibration (VPE) technique, were measured at room temperature in the wave-number range 50–3500 cm^-1. Both 488 and 514.5 nm radiations were used to excite Raman scattering, A₁(TO) and E(TO) modes were recorded at backward scattering geometry. The results indicated that the lattice vibrational spectra of the as-grown Er:LiNbO₃ are almost the same as those of pure LiNbO₃ except for the little shift of the peak position and the change of relative intensity of some peaks. In comparison with the spectra of as-grown Er:LiNbO₃ crystal the vapor-phase equilibrated Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals in the lattice vibrational region exhibit the following features: firstly, Raman peaks become narrow, indicating that the VPE process has brought Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals closer to a stoichiometric composition; secondly, relative intensity of some peaks varies with the VPE time; and finally, slight blue shifting in peak position was observed. Some of these features were correlated with the NbO₆ octahedra and with the site distribution of the doped Er ions. In addition, green fluorescence peaks and/or bands associated with the electron transitions ² H _11/2?⁴ I _15/2 and ⁴ S _3/2?⁴ I _15/2 of the doped Er³⁺ were also observed. For 488 nm excitation they appear in the wavenumber range of 1200–3000 cm^-1 and are well separated from lattice vibrational region; for 514.5 nm excitation, however, these fluorescence peaks shift towards the low wavenumber region and overlap partially with the lattice vibrational spectra. Received: 24 May 2000 / Accepted: 29 May 2000 / Published online: 13 September 2000     

Energy transfer in Er:Eu:Yb co-doped tellurite glasses: Yb as enhancer and quencher

Energy transfer has been studied from Er³⁺ to Eu³⁺ ions on excitation with NIR photons (796 and 980 nm) with and without Yb³⁺ ions. It is found that in one case the presence of Yb³⁺ enhances the fluorescence yield (980 nm excitation) whereas in the other case it quenches (796 nm excitation). Energy transfer from Er³⁺ ion's levels ⁴S_3/2 and ²H_11/2 is verified by decay curve analysis in both the cases. The nature of interaction between the donor (Er) and the acceptor (Eu) ions is found to be dipole-dipole. The energy transfer parameters viz. transfer probability, critical distance etc. have been calculated.     

A study of the quantum efficiency of multichannel relaxation in LiNbO<Subscript>3</Subscript>:Yb,Er crystals

Luminescence spectra of gradient-activated LiNbO₃:Yb, Er crystals with predefined concentration profiles of the optical centers are studied in different spectral regions. The process of electronic excitation energy transfer in the Yb³⁺–Er³⁺ system inside the LiNbO₃ matrix is calculated and dependences of the quantum efficiency of the up-conversion processes for the green and red luminescences of erbium ions on the time of excitation energy deactivation are obtained.     

Towards new lasers in Ti:Er:LiNbO3 waveguides: a study of the excited Er3+ states

Detailed excited state absorption measurements under pumping at 980 nm and 1.5 μm together with conventional absorption and emission spectroscopy is employed to investigate optical transitions of Er³⁺ in Ti:LiNbO₃ channel-waveguides. The experimental data were evaluated using the Judd–Ofelt method giving parameters close to those in the bulk. The good agreement between theoretical prediction and measurements allows us to calculate cross section, lifetimes, and branching ratios. Based on these results we developed and tested a model which is able to predict the conditions for which laser operation at 550 nm and 2.7 μm will be possible under 980 nm pumping. Received: 9 December 1998 / Revised version: 8 January 1999 / Published online: 24 March 1999     

Wide colour gamut generated in triply lanthanide doped sol–gel nano-glass–ceramics

The generation of a wide colour gamut, based on up-conversion of cheap near-infrared photons into the visible range, is of great importance for general lighting appliances and integrated optical devices. Here, we report for the first time on up-conversion luminescence under infrared excitation at 980 nm in Yb³⁺–Er³⁺–Tm³⁺ triply doped sol–gel derived SiO₂–LaF₃ based nano-glass–ceramics (SOL-YET), containing LaF₃ nanocrystals with a size about 13 nm. Efficient simultaneous up-conversion emission of the three primary colours (blue, green and red) gives rise to a balanced white overall emission. The ratio between up-conversion emission bands can be varied by changing pump power intensity resulting in colour tuneable up-conversion phosphor.     

水热法合成纳米晶NaYF4 : Er3+,Tm3+,Yb3+ 的上转换发光特性

以EDTA为络合剂,用水热法合成了Er³⁺,Tm³⁺和Yb³⁺共掺杂的NaYF₄纳米晶。XRD和TEM的结果表明:粒径约为30 nm,属于六方晶系。在980 nm半导体激光器激发下,研究了不同Er³⁺离子掺杂浓度对Tm³⁺和Er³⁺离子上转换发光性能的影响,光强与泵浦功率的双对数曲线表明,474,525,539,650 nm的发射均属于双光子过程,408 nm的发射属于三光子过程。讨论了样品的协作敏化和声子辅助共振能量传递的上转换发光机制。     

The UV,blue and green up-conversion luminescence of PbF2+GeO2:Er2O3 pumped with 650 nm

A detailed study of the spectroscopic properties of the PbF₂+GeO₂:Er₂O₃ vitroceramic sample upon 650 nm excitation was investigated. The absorption, emission, excitation spectra, and time-resolved spectra have been measured. The up-conversion of red radiation (650 nm) into UV (368 nm and 379 nm), blue (406.8 nm) and green (522 nm and 540 nm) emissions is observed for Er³⁺ ions in the sample. The up-conversion process involves a two-photon absorption for the violet, blue, and green emission bands. A three-photon process happens for another violet (379 nm) band.     

Energy transfer and frequency upconversion in Yb3+–Er3+-doped PbO-GeO2 glass containing silver nanoparticles

We report the infrared-to-visible frequency upconversion in Er³⁺–Yb³⁺-codoped PbO-GeO₂ glass containing silver nanoparticles (NPs). The optical excitation is made with a laser at 980 nm in resonance with the ²F_5/2→²F_7/2 transition of Yb³⁺ ions. Intense emission bands centered at 525, 550, and 662 nm were observed corresponding to Er³⁺ transitions. The simultaneous influence of the Yb³⁺→Er³⁺ energy transfer and the contribution of the intensified local field effect due to the silver NPs give origin to the enhancement of the whole frequency upconversion spectra.     

Dynamics of the Yb3+ to Er3+ energy transfer in LiNbO3

The energy transfer dynamics between Yb³⁺ and Er³⁺ ions in lithium niobate is investigated after ytterbium-pulsed excitation at 920 nm. The sensitisation of the LiNbO₃:Er³⁺ system with Yb³⁺ ions does not modify the lifetime of the ⁴I_13/2 erbium level (1.5-μm emission), whereas it induces a marked, concentration-dependent change in the lifetime of the ²F_5/2 (Yb³⁺) and ⁴S_3/2 (Er³⁺) multiplets (1060-nm and 550-nm emissions, respectively). The results are analysed by using the rate-equation formalism and cross-relaxation model for the energy transfer. Received: 15 October 1998 / Revised version: 24 November 1998 / Published online: 24 February 1999     

Synthesis, morphology and spectroscopy of cubic Y3NbO7:Er

Synthesis of Y₃NbO₇:Er powders with the aid of Li₂SO₄ flux is reported and spectroscopic properties of the resultant powders are presented. The dopant content varied in the range of 0.1-15 at%. The materials crystallized in the fluorite-type cubic structure in which all the metal ions—Y, Nb, and Er—randomly occupy the same site offered by the host lattice and the O-vacancy is also randomly distributed within the metal surrounding. Transmission electron microscopy images revealed that the agglomeration of particles is very low and the sizes of the grains are around 500 nm. Selected area electron diffraction patterns proved that each grain is monocrystalline. Absorption, excitation, and emission spectra are characterized by relatively broad structures related to the Er³⁺ ion. The broadening results from some inhomogeneity of the activator ion surroundings related to the specific structure of the host lattice. When the Er content is only 0.1% both photoluminescence and up-converted emission are dominated by a green luminescent band around 550 nm. However, the efficiency of up-conversion is very low . With increasing concentration of the dopant, a red band located around 665 nm appears and becomes systematically stronger. In up-converted emission, the intensity of the red band surpasses the green one when the Er concentration exceeds 5%. For low concentrations, the up-conversion occurs through a sequential absorption of two infrared (IR) (980 nm) photons from the excitation beam by Er³⁺ ion through excited-state absorption mechanism. For higher concentrations, the energy transfer between two neighboring excited Er ions plays dominant role. Surprisingly, the mechanism of up-converted low-intensity luminescence from ²H_11/2 state seems to diverge from the mechanism characteristic for the ⁴S_3/2 level, which conclusion comes from different slopes of the double-log relationships.     

The up-conversion properties of Yb and Er doped Y4Al2O9 phosphors

Er³⁺ doped and Yb³⁺/Er³⁺ co-doped Y₄Al₂O₉ phosphors are prepared by the sol-gel method. The effect of dopant concentration on the structure and up-conversion properties is investigated by X-ray diffraction (XRD) and photoluminescence, respectively. XRD pattern indicates that the sample structure belongs to monoclinic. Under 980 nm excitation, the green and red up-conversion emissions are observed and the emission intensities depended on the Yb³⁺ ion concentration. The green up-conversion emissions decrease with the increase of Yb³⁺ concentration, while red emission increases as Yb³⁺ concentration increases from 0 to 8 at% and then decreases at high Yb³⁺ concentration. The mechanisms of the up-conversion emissions are discussed and results shows that in Er³⁺ and Yb³⁺/Er³⁺ co-doped system, cross-relaxation (CR) and energy transfer (ET) processes play an important role for the green and red up-conversion emissions.     

Transient characteristics of green upconversion emission of Er3+ in MgO:LiNbO3 crystal: Mg threshold concentration effect

Transient characteristics of upconverted emission (560 nm) of Er³⁺ in LiNbO₃ crystals codoped with 0–7.4 mol% MgO were studied under pulse excitation at 800-nm wavelength. The results show that the transients display considerable Mg-doping-level-dependent nonexponential behavior and a clear Mg optical-damage-resistance threshold concentration effect. Below the Mg threshold concentration, the lifetime increases slightly with the increased Mg concentration. Above the threshold, however, the lifetime drops abruptly by 4–7 times and the nonexponential feature becomes more evident. It is found that each transient can be fitted by a double-exponential function contributed from isolated and clustered Er³⁺ sites. The fit parameters show that doping of MgO above the threshold concentration increases the clustered Er site concentration and the nonradiative cross relaxation probability. The Mg threshold concentration effect derived from the transients is in qualitative agreement with that from the fluorescence spectrum measured as a function of the Mg concentration. The effect of the Mg threshold concentration on the clustered Er site concentration is qualitatively explained on the basis of the microscopic defect model of MgO:LiNbO₃ and is conducted with the Mg site change around the threshold concentration.     

Photoluminescence of LiNbO3 crystals doped with Er3+ and Yb3+ ions

Results of cooperative phenomena investigations in the impurity subsystem of lithium niobate crystals doped with Er³⁺ and co-doped with Yb³⁺ impurity ions under continuous wave and pulsed excitation at 975 nm and 1064 nm wavelengths are presented. Dependences of some spectroscopic characteristics on the intensity of laser pumping are studied. Based on the pair centers model the analysis of the cooperative luminescence behavior in LiNbO₃:Yb³⁺+Er³⁺ crystals is performed.     

Comparative Studies of Er3☎ Ions in LiNbO3 Waveguides Produced by Different Methods

We compare the optical properties of Er^3? in LiNbO₃ waveguides produced by different techniques and find by confocal luminescence microscopy characteristic differences in the excitation and emission transitions. Besides a small redistribution among the regular defect sites, essentially no direct Er^3?—Ti^4? interaction can be observed in Ti^4? diffused waveguides. However a significant shift in transition energies is found, which increases with Ti^4?concentration. Based on earlier results we associate this shift with a decrease in the intrinsic electric field. In addition to a similar shift, we find in Zn^2? diffused waveguides new defect types which suggest a change in direct environment. Finally, in annealed proton exchange waveguides we find no interaction effects at all.     

Enhancement of up-conversion luminescence in Zn2SiO4:Yb3+, Er3+ by co-doping with Li+ or Bi3+

Yb³⁺/Er³⁺ co-doped Zn₂SiO₄ ceramics are rapidly synthesized by the microwave radiation method. Green and red up-conversion emissions are observed in Zn₂SiO₄: Yb³⁺, Er³⁺ ceramics under 980 nm excitation. The influence of co-doped Li⁺ or Bi³⁺ ion on luminescence intensity for the phosphors has been investigated. At Li⁺ or Bi³⁺ doping concentration of 1 mol%, up-converted green emission can be increased by 6 times and 20 times, respectively. It is believed that co-doped Li⁺ or Bi³⁺ ion results in the local distortion of Er³⁺ in Zn₂SiO₄, increasing the intra-4f transitions of Er³⁺ ions. The local distortion is proved by spectral probing method with Eu³⁺.     

Dual mode emission and harmonic generation in ZnO-CaO-Al2O3: Er nano-composite

Er³⁺ doped ZnO-CaO-Al₂O₃ nano-composite phosphor has been synthesized through combustion method and its emission and harmonic generation properties have been studied. The X-ray diffraction and thermal analysis techniques have been used to prove the dual phase (ZnO and CaO-Al₂O₃) nature of the phosphor. The phosphor has shown up-conversion emission on near-infra-red (976 nm) excitation and down-conversion emission on 355 nm excitation in presence of Er³⁺ and thus behaves as a dual mode phosphor. On excitation with 976 nm diode laser, material shows color tunability (calcination of composite material at different temperatures). Formation of ZnO nanocrystals on heat treatment of as-synthesized sample has shown its characteristic emission at 388 nm and also the energy transfer from ZnO to Er³⁺ ions. The low temperature emission measurements have been carried out and the results have been discussed. Phosphor has shown strong second harmonic generation (SHG) at 532 nm on 1064 nm and at 266 nm on 532 nm excitation.