Lifetime Of Er3 Research Articles

Enhancement of upconversion luminescence in NaBiF4: Er3+/Yb3+/Lu3+ for temperature detection

Optical thermometer has got lots of attention by virtue of non-contact, fast response speed, high sensitivity, anti-interference ability, which is less affected by the environment. However, most fluorescent thermometers still suffer from low sensitivity. Here, an optical thermometer based on NaBiF4: Er3+/Yb3+/Lu3+ with enhanced upconversion luminescence is reported. The sample emits strong green light of Er3+ under 980 nm laser excitation and with the increase of Lu3+, the whole luminescence intensity of NaBiF4: 2 % Er3+/20 % Yb3+/ 2.5 % Lu3+ is enhanced by about 3 times, which due to local crystal field distortion. Meanwhile, the lifetime of Er3+ is prolonged and the temperature detection performance is also improved with the substitution of Lu3+. The relative sensitivity reaches a maximum of 0.841 % K−1 at 301 K. The results suggest that it is an optical nanothermometer with great potential.

The influence of Pr3+ ions on the emission characteristics of Er3+-doped YSGG single crystal fibers in the 1–3 μm wavelength range

A series of 20 mol% Er3+, x mol% Pr3+: Y2.4-3xSc2Ga3O12 (Er3+, Pr3+: YSGG) single-crystal fibers (SCFs) were successfully grown by micro-pulling-down (μ-PD) method. Er3+, Pr3+: YSGG SCFs with varying concentrations of Pr3+ dopant exhibit consistent retention of the identical cubic structural space group. Under 980 nm laser diode (LD) pumping, Er3+, Pr3+: YSGG exhibits two emission bands centered at 1.6 and 2.7 μm, respectively. The incorporation of Pr3+ ions in Er3+, Pr3+: YSGG SCF materials has a negative impact on the emission peak intensity at 1.6 μm within a specific range of doping concentration, while it can relatively enhance the emission peak intensity at 2.7 μm. The lifetime of Er3+ ions at 4I13/2 is showing a significant decrease trend with increasing Pr3+ ions doping concentration. The results presented above indicate that the incorporation of Pr3+ ions increase the likelihood of particle number inversion, improving laser operation and demonstrating the desensitizing influence of Pr3+ ions. Quantitative analysis of the gain cross-section reveals a particle number inversion rate of 52.73% at approximately 2.7 μm. These findings provide valuable insights for developing applications aiming to achieve efficient and precise emission at 2.7 μm using Erbium–Praseodymium co-doped yttrium scandium gallium garnet single crystal fibers.

Exploring the Potential Applications of Lanthanide-Based Double Perovskite in Remote Pressure and Temperature Sensing.

Remote optical sensing with nondestructive, fast, and accurate detection capabilities is a powerful noncontact method widely used in natural, industrial, and biological fields. In this work, Cs2NaErCl6 double perovskite was synthesized via a hydrothermal method. The pressure-dependent photoluminescence (PL) lifetime of Er3+ in the range of 0-20 GPa was investigated, demonstrating its potential for pressure monitoring. The high-pressure relative sensitivity (SR) is ∼18.45% GPa-1. Temperature measurements were conducted using the fluorescence intensity ratio (FIR) of the thermal couple energy level (TCEL) and the nonthermal couple energy level (NTCEL) of Er3+ across a temperature range of 100-660 K, with a maximum SR of 5.36% K-1. By combining MXene with Cs2NaErCl6 and recording the FIR of Cs2NaErCl6 under 1550 nm excitation, the photothermal conversion temperature of MXene can be accurately determined. These findings highlight the potential of Cs2NaErCl6 for remote pressure and temperature sensing, particularly in the biomedical field.

Europium and erbium co-doped yttria-stabilized zirconia as a potential dual-sensor for simultaneous oxygen partial pressure and temperature measurements

In this study, the phosphorescence properties of Eu3+ and Er3+ co-doped yttria stabilized zirconia (YSZ: Eu3+, Er3+) as a function of oxygen partial pressure and temperature are investigated to explore the possibility of using this phosphor for pressure and temperature measurements at elevated temperature (up to 600 °C). The YSZ: Eu3+, Er3+ phosphors have a fixed Eu3+ concentration (1 mol%) and a varied Er3+ concentration (0.5–5 mol%). The phosphorescence intensity and lifetime of Eu3+ bands are sensitive to both oxygen partial pressure and temperature when temperature exceeds 400 °C. This oxygen/pressure sensitivity is related to the oxygen vacancy-induced non-radiative decay through charge transfer state (CTS), and is inversely correlated with Er3+ concentration. In contrast, the phosphorescence intensity and lifetime of Er3+ bands are temperature-sensitive but oxygen-insensitive, which is related to the dominant oxygen-independent 4S3/2 → 2H11/2 transition. Accordingly, two measurement strategies are proposed including an intensity ratio method and a lifetime method. The intensity ratio of the oxygen-sensitive Eu3+ bands and the oxygen-insensitive Er3+ reference bands is capable of 2D pressure measurement using a dual-camera system. But temperature-induced errors exist due to the temperature-sensitivity of intensity ratio. Simultaneous pressure and temperature measurements (point by point) can be achieved utilizing two PMTs that record the lifetimes of Eu3+ and Er3+, respectively. This method can resolve the issue of temperature sensitivity of Eu3+ and accurately correct the temperature-induced errors. The YSZ: Eu, Er phosphors show great potential for applications in high-temperature environment where the existing organic PSPs and TSPs cannot survive.

Towards highly efficient NIR II response up-conversion phosphor enabled by long lifetimes of Er3+

The second near-infrared (NIR II) response photon up-conversion (UC) materials show great application prospects in the fields of biology and optical communication. However, it is still an enormous challenge to obtain efficient NIR II response materials. Herein, we develop a series of Er3+ doped ternary sulfides phosphors with highly efficient UC emissions under 1532 nm irradiation. β-NaYS2:Er3+ achieves a visible UC efficiency as high as 2.6%, along with high brightness, spectral stability of lights illumination and temperature. Such efficient UC is dominated by excited state absorption, accompanied by the advantage of long lifetimes (4I9/2, 9.24 ms; 4I13/2, 30.27 ms) of excited state levels of Er3+, instead of the well-recognized energy transfer UC between sensitizer and activator. NaYS2:Er3+ phosphors are further developed for high-performance underwater communication and narrowband NIR photodetectors. Our findings suggest a novel approach for developing NIR II response UC materials, and simulate new applications, eg., simultaneous NIR and visible optical communication.

Fabrication and spectroscopic investigations on Er3+, Ho3+: SrF2 transparent ceramics for 2.7 μm emission

3 at.% Er3+, x at.% Ho3+: SrF2 (x = 0, 0.05, 0.1, 0.5, 1, 2) transparent ceramics, as the potential material for the 2.7 μm solid-state laser, were fabricated by hot-pressed sintering. XRD, TEM, SEM, and EDS measurements were used to investigate the phase composition, morphology, microstructure, and distribution of the elements of the nanoparticles and transparent ceramics. Results showed that the Er3+ ions and Ho3+ ions do not alter the SrF2 crystal structure, and they are distributed uniformly in the sample. With the increase of the Ho3+ doping concentration, the lattice parameter decreased from 5.799 Å to 5.784 Å, and the average grain size decreased gradually. The maximum transmittance of as-obtained ceramics is approximately 93 % which is close to the theoretical transmittance of SrF2. Moreover, the absorption spectra, emission spectra, and the lifetime of Er3+ and Ho3+ were investigated. The energy transfer processes between Er3+ and Ho3+ were discussed. After co-doping Ho3+, the lifetime difference between Er3+:4I11/2 and Er3+:4I13/2 levels was shortened from 8.50 ms to 1.12 ms. All the results show that the incorporation of Ho3+ with proper doping concentration is beneficial for achieving 2.7 μm laser output in Er3+: SrF2 transparent ceramics.

Re-study of Judd-Ofelt spectroscopic properties of Er3+-doped Gd3(Al, Ga)5O12 single crystal

A previous paper [J. Lumin. 177 (2016) 219] has studied Judd-Ofelt spectroscopic properties of Er3+ (0.5 at%)-doped Gd3(Al, Ga)5O12 garnet single-crystal, and reported the Judd-Ofelt intensity parameters Ω2 = 1.47 × 10−20 cm2, Ω4 = 1.38 × 10−20 cm2 and Ω6 = 1.19 × 10−20 cm2. We have performed a re-study on the Judd-Ofelt spectroscopic properties of a similar garnet single-crystal Er3+(20 at%)-doped Gd3(Al0.4Ga0.6)5O12, and obtained quite different values of Judd-Ofelt intensity parameters Ω2 = (0.34 ± 0.11) × 10−20 cm2, Ω4 = (0.48 ± 0.16) × 10−20 cm2 and Ω6 = (1.05 ± 0.17) × 10−20 cm2. The quite different Ωt values given by two studies are mainly due to remarkably different absorption characteristics of Er3+ in the crystals employed in two studies, and factors that lead to quite different absorption characteristics are discussed. Another reason is that the hypersensitive transition at 380 nm was excluded from the Judd-Ofelt calculation in that paper, resulting in deviation of their result. Instead, all of the Er3+ absorption bands were included in our calculation, allowing to obtain more accurate Ωt values. Quite different Ωt values lead to considerable differences of relevant spectroscopic parameters, such as radiative rates and lifetimes of typical Er3+ emissions, for which the difference from each other is as much as three times. In addition, visible, near-infrared and mid-infrared fluorescence spectra and lifetimes of Er3+ in the studied crystal were measured, and the emission characteristics are discussed.

Sellmeier dispersion and spectroscopic properties of Er3+-doped lutetium gallium garnet single crystal

The authors report growth, refractive index dispersion, optical absorption and Judd-Ofelt spectroscopic properties of Er3+-doped lutetium gallium garnet (Lu3Ga5O12) single-crystal. The crystal was grown by Czochralski technique. The density of the crystal was measured by Archimedes method. Er3+ concentration in the crystal was measured by inductively coupled plasma atomic emission spectroscopy. The measurement results allow to reach that Er3+ ion in the crystal has a desired segregation coefficient ∼1.0. Refractive index of the crystal was measured as a function of wavelength in the range of 245–1553 nm using an ellipsometer and a prism coupler, and a Sellmeier equation is established for the crystal. Optical absorption of Er3+ in the crystal was studied in the wavelength range of 300–1700 nm and Er3+ absorption cross section spectrum was obtained. Based on the absorption cross section spectrum and the Sellmeier equation, Judd-Ofelt analysis was done for the crystal. Visible, near-infrared and mid-infrared fluorescence spectra and lifetimes of Er3+ in the crystal were measured, and emission characteristics are demonstrated. Spectroscopic knowledge of peaking absorption cross section, oscillator strength, Judd-Ofelt intensity parameters, radiative rate, fluorescence branch ratio, radiative and fluorescence lifetime, and quantum efficiency of some typical Er3+ emissions is obtained.

Ferroelectric Properties and Spectroscopic Characterization of Pb(Mg1/3Nb2/3)O3-32PbTiO3:Er3+/Sc3+ Crystal

An Er3+/Sc3+ co-doped 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 ferroelectric single crystal was grown by high-temperature flux method. The remnant polarization Pr is 27.97 µC/cm2 and the coercive field Ec is 8.26 kV/cm for [100] oriented crystal. Green (524 and 551 nm) and red (654 nm) emission bands are generated at the 980 nm excitation, which corresponds to the 2H11/2→ 4I15/2, 4S3/2→ 4I15/2 and 4F9/2→ 4I15/2 transitions of Er3+, respectively. Judd–Ofelt theory has been applied to predict the spectroscopic characteristics of the as-grown crystals. The obtained J–O intensity parameters Ωt (t = 2, 4 and 6) are Ω2 = 0.76 × 10−20 cm2, Ω4 = 1.0 × 10−20 cm2, Ω6 = 0.55 × 10−20 cm2. Spectroscopic characteristics, including optical transition probabilities, branching ratio, and radiative lifetime of Er3+ in the crystal, are determined. The calculated radiative lifetimes of 4I13/2 and 4I11/2 energy levels are 2.82 ms and 2.61 ms, respectively. These investigations provide possibilities for the crystal Pb(Mg1/3Nb2/3)O3-0.32PbTiO3:Er3+/Sc3+ to be a new type of multifunctional crystal integrating electricity-luminescence.

Erbium-ytterbium co-doped aluminum oxide thin films: Co-sputtering deposition, photoluminescence, luminescent lifetime, energy transfer and quenching fraction

Abstract Erbium-ytterbium co-doped aluminium oxide (Al2O3:Er3+:Yb3+) thin films were deposited on thermally oxidized silicon wafers using reactive radio frequency co-sputtering deposition. The effects of deposition parameters (including oxygen flow rate, substrate temperature and substrate bias) were investigated to obtain low loss Al2O3 films (~0.15 dB/cm at 1550 nm). A set of singly- and co-doped Al2O3 films (surface roughness: ~144.8 p.m.) with various Er3+ concentrations (1.4, 2.0 and 2.5 × 1020 cm−3) and/or Yb3+ concentrations (1.3, 2.1, 3.2, 4.3 and 5.8 × 1020 cm−3) were deposited and investigated. The Er3+ photoluminescence (PL) spectrum around 1550 nm exhibits a pump power-independent full width at half maximum (FWHM) of ~40 nm with and without the presence of Yb3+ as a sensitizer. The luminescent lifetime of Er3+ (4I13/2, 6.52–7.14 m s) exhibits a power-dependent characteristic, while that of Yb3+ (2I5/2, 0.514–0.716 m s) is power-independent. A power-dependent role of energy transfer upconversion is concluded from both the power-dependent Er3+ PL intensity trend and its power-dependent PL decay rate. Yb3+-Er3+ ion energy transfer also exhibits power-dependent characteristics. The energy transfer coefficient (up to ~3.2 × 10−17 cm3s−1) and energy transfer efficiency (up to ~80%) depend on the Yb3+ and Er3+ concentrations. The PL intensity trend and power-dependent absorption of Er3+ and Er3+-Yb3+ under 1469-nm excitation can be well described by using a three-energy-level model. The derived Er3+ quenching fraction based on the fittings of the measured absorption and PL intensity, decreases with the presence of Yb3+ ions. In addition, the Yb3+ quenching fraction was derived based on a two-energy-level model for 973-nm absorption. This work provides parametric references and insights regarding deposition, characterization, and design of the evolving rare-earth-doped Al2O3 platform for on-chip near-infrared applications.

Spectroscopic analysis of Er:Y2O3 crystal at 2.7 μm mid-IR laser

High quality 7 at%Er:Y2O3 crystal was successfully grown by Fz method. Judd-Ofelt theory was applied to analyze spectroscopic properties at 2.7 μm. The larger value of Ω2 shows low local environment symmetry exists in Er:Y2O3 crystal, and the large Ω4/Ω6 indicates better spectral quality. The larger absorption cross-section and FWHM are beneficial to efficient absorption of pump source. The fluorescence spectra and lifetimes of Er:Y2O3 crystal were measured and studied at 2.7 μm. The emission cross-sections at 2723 nm was calculated to be 1.41 × 10−20cm2. Compared with other Er3+ doped crystals, the large spectral quality factors Q and low fluorescence lifetime ratio illustrate that Er:Y2O3 crystal has excellent laser properties at 2.7 μm. In addition, the low inversion ratio also indicates great potential of Er:Y2O3 crystal for laser generation at 2.7 μm.

Broadband and flat near-infrared emission from Er3+/Tm3+ codoped tellurite glass for amplifier applications

E r 3 + / T m 3 + codoped tellurite glasses have been synthesized using the melt-quenching technique; they are characterized by measurements of absorption spectrum, luminescence spectrum, fluorescence decay, Raman spectrum, and differential scanning calorimeter curve. Under the excitation of an 808 nm laser diode (LD), an extremely flat and broadband near-infrared emission with the full width at half-maximum of ∼ 160 n m has been obtained in tellurite glass doped with optimized E r 3 + − T m 3 + concentrations. This broadband and flat fluorescence emission is located in the region of 1350–1650 nm and covered the E, S, C, and L bands, which originate from the spectral overlapping of 1.53 µm band emission of E r 3 + : 4 I 13 / 2 → 4 I 15 / 2 transition and 1.47 µm band emission of T m 3 + : 3 H 4 → 3 F 4 transition. The Judd-Ofelt theory was applied to the absorption spectrum, and the radiative properties such as transition probability, branching ratio, and radiative lifetime of E r 3 + and T m 3 + ions have been determined to evaluate the potential luminescence characteristics. The time-resolved fluorescence decay of E r 3 + and T m 3 + ions demonstrated the existence of energy transfers between them, and the energy transfer micromechanism was further investigated quantitatively. The results indicate that tellurite glass codoped with appropriate amounts of E r 3 + − T m 3 + ions is promising for applications in broadband optical fibers and photonic devices.

Efficient Er3+:4I11/2→4I13/2 radiative transition regulated by optimizing the sensitization mechanism.

A series of fluorophosphates glass codoped with active Er3+ ions and sensitizing ions of different systems were prepared to systematically study their sensitization effect in order to obtain efficient MIR luminescence. Differential scanning calorimetry curve indicates the favorable thermal stability of the glass host. A comprehensive analysis of the sensitization mechanism is given based on the synthesis considering the position and intensity of fluorescence emissions together with the lifetime of Er3+:4I13/2 active level. The results show two positive sensitization effects: the eliminating effect to the lower laser level of Er3+ active ions represented by Pr3+ ions reducing the lifetime of 4I13/2 energy level to a great extent; and improving the absorption efficiency of pumping source sensitized by Yb3+ ions. The paper has provided a mental knowledge for sensitization mechanism in rare earth multi-doped materials together with the aiming of promoting the MIR luminescence of Er3+ ions.

Investigation of new color-tunable up-conversion phosphors and their long-persistent luminescence properties for potential biomedical applications

Long-persistent phosphor and up-conversion particles used for optical imaging continue to interest researchers. The present work identifies an Er3+ doped micro-sized CSAO-CSO mixed material that exhibits both up-conversion and persistent luminescence. The cross-relaxation that occurs among Er3+ ions contributes to enhancing the up-conversion intensity. The Tm3+/Ho3+ co-doped strategy was used to induce tunable up-conversion colors. The cross-relaxation process corresponding to the energy transfer mechanism was investigated via the luminescence lifetimes of Er3+. Moreover, the persistent luminescence and thermo-luminescence properties of CSAO-CSO: Er3+ were also investigated systematically. Our results combine the fields of up-conversion and persistent luminescence, which may support other explorations of new kinds of long-persistent and up-conversion materials.

Diode end-pumped dual-wavelength Er,Pr:GSAG laser operating at 2696 and 2828 nm

We demonstrated the 962 nm laser diode end-pumped Er3+, Pr3+ co-doped GSAG laser operated in dual-wavelength of 2696 and 2828 nm. The absorption and fluorescence spectra of Er,Pr:GSAG crystal were investigated to determine the potential of laser generation in mid-infrared wavelength regions. The lifetimes of Er3+ in GSAG crystal for the upper laser level 4I11/2 and lower level 4I13/2 are 0.18 and 0.47 ms, respectively, for co-doped Pr3+ ion. The maximum average output power of 344 mW is achieved. In addition, a series of investigations for Er,Pr:GSAG laser performance were launched on the pump pulse width, repetition frequency and resonant cavity length. The results indicate that the small pump pulse width and low repetition frequency are beneficial to decrease the thermal lensing effect and improve the laser efficiency and beam quality.

Investigation of Y2.1Er0.9(ScxGa1−x)5O12 Matrix Components on the Spectral Properties around 3.0 μm by Micro-Pulling-Down Method

Single crystal fibers of 30% Er3+-doped compound of Y3(ScxGa1−x)5O12 have been grown by using the micro-pulling down (μ-PD) technique successfully. Our main purpose is to tune the fluorescence properties by adjusting the ratios of Sc3+ and Ga3+ ions inside the matrix crystals. The crystal structures of the series compounds were measured and analyzed through X-ray diffraction (XRD) measurements. The components and doping elements distributions were measured by the X-ray Fluorescence spectrometry and electron-probe microanalyzer. The absorption and mid-infrared fluorescence spectra, including the fluorescent lifetime of Er3+:4I13/2 and 4I11/2 levels were measured and compared systematically at room temperature. Spectral analysis indicated that the fluorescent lifetime of Er3+:4I13/2 tended to shorten and the emission spectra began to show a red shift when the proportion of YSG increased in the compound. Furthermore, the Raman spectra were measured to reveal the variations of lattice vibration and phonon energy.

Comment on "A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the β-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation" by B. Chen, S. Xu et al., Phys. Chem. Chem. Phys., 2018, 20, 15876.

A recent paper [Y. Q. Zhang, B. J. Chen, S. Xu, X. P. Li, J. S. Zhang, J. S. Sun, X. Q. Zhang, H. P. Xia and R. N. Hua, A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the β-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation, Phys. Chem. Chem. Phys., 2018, 20, 15876-15883] used the Er3+/Yb3+:β-NaYF4 phosphor as an example to demonstrate a method used for evaluating Judd-Ofelt parameters of rare-earth-ion-doped powder. The method is actually established on the basis of a measured diffuse reflectance spectrum and fluorescence lifetime of Er3+ emission at 1.5 μm. The paper compared the Judd-Ofelt parameters obtained from two least-squares fitting processes with and without inclusion of two strong Er3+ absorption transitions from 4I15/2 to 2H11/2 and 4G11/2, and erroneously concluded that the values of the Judd-Ofelt parameters depend mainly on the majority of transitions with even weak intensities rather than on a few strong transitions. We have repeated the two fitting processes and the reverse conclusion has been made. Moreover, we have studied the effects of the transitions selected for the analysis and comprehensive explanations are given for the effects. In addition, the previously reported methods, including the aforementioned one, are reviewed, compared and discussed in detail.

Czochralski growth and spectral investigations of Er:GSAG laser crystal

Er3+ doped GSAG laser crystal with high optical quality was grown successfully by the Czochralski method for the first time. The structural parameters and the high crystalline quality of this crystal were determined. Judd-ofelt (J-O) parameterization scheme was applied to analyze the absorption spectra at room temperature. Accordingly, the lines strengths, oscillator strengths, transition probabilities, fluorescence branching ratios, and radiative lifetimes were calculated to characterize the laser properties of Er:GSAG crystal. Additionally, the fluorescence spectra and lifetimes of Er:GSAG crystal were measured and studied. The stimulated emission cross-section spectra and gain cross-section spectra were evaluated for 4I13/2 →4I15/2 (1.5–1.6 µm) transitions which indicate high potential of 1 at% Er:GSAG for laser emissions at 1567 and 1605 nm. Furthermore, the possibility of Er:GSAG crystal to realize the visible laser output pumped by the GaN LDs were initially studied and analyzed.

X-rays, γ-rays, electrons and protons radiation-induced changes on the lifetimes of Er3+ and Yb3+ ions in silica-based optical fibers

The evolution of Ytterbium 2F5/2 and Erbium 4I13/2 energy level lifetimes versus doses of various radiation types (40keV X-rays, 480MeV protons, 1.2MeV γ-rays and 6MeV electrons) were investigated on samples of silica-based Rare-Earth Doped optical Fibers (REDFs). For each studied sample (Er-doped and Er/Yb-doped), a strong dependence of the lifetime value on the irradiation dose (for equivalent doses ranging from 10Gy(SiO2) to 10MGy(SiO2)) is observed regardless of the radiation nature. For both fiber types and luminescent ions, complex dose dependences are observed: a limited decrease of the lifetime at the lower doses, whereas a large reduction is reported at doses exceeding 100kGy(SiO2). The results highlighted the vulnerability of REDF based systems, such as optical amplifiers and sources, for space and nuclear applications. In such harsh environments, Radiation Induced Attenuation (RIA) is also a key issue. The positive effect of Ce co-doping on both the RIA and the lifetimes is reported even at the highest dose of 10MGy(SiO2). The basic mechanisms involved during the interaction between radiation and fiber material were also investigated through low temperature spectroscopic measurements that revealed the fundamental role of radiation-induced point defects.

The luminescence parameters of Yb3+:Er3+-doped LiLa(WO4)2 single crystal grown in the form of fiber for up-conversion green emission

This report details the first study of the luminescence properties of a single crystal grown in the form of fiber for prospective application as the gain medium for fiber laser emission at 552nm. The excited state decay processes related with the 4S3/2→4I15/2 transition in double Yb3+:Er3+-doped LiLa(WO4)2 crystal have been investigated using time-resolved fluorescence spectroscopy with a Er3+ concentration of 0.5mol% and Yb3+ with 2, 5, 7, 10 and 15mol%. Selective laser excitation of the 2F5/2 energy level of Yb3+(972nm) and selective laser excitations of the 4I11/2 and 4I13/2 energy levels of Er3+(972 and 1550nm), respectively has established that in a similar way to other optical materials, a strong energy-transfer up-conversion by way of a dipole-dipole interactions between an Yb3+ excited and Er3+ ions, the 4F5/2 level (Yb3+) populates the 4S3/2 upper laser level of the 550nm transition. The 4S3/2 energy level emits luminescence with peaks having the wavelength center at 550nm with luminescence efficiency increasing from 7% for Er3+ singly doped to 36% for Yb3+(15mol%) co-doped crystals. The 4S3/2 lifetime of Er3+ is observed to increase due to the saturation of the multiphonon relaxation rate at high excited-state density of Yb3+ ions. At high excited-state density, Yb3+ ions saturates the accepting modes inside of a critical volume of RC=39.4Å centered at an excited Er3+(4S3/2) ion, by the high-energy phonons generated from emission sideband of Yb3+ ions in Yb(x%):Er(0.5%) crystals. It is established that the green (552nm) up-conversion luminescence of Er3+ is optimized using an Yb3+ concentration of 11.5mol% for Er(0.5%):LiLa(WO4)2 crystal.