Multiphonon Relaxation Rates Research Articles

Effects of GeO2 concentration on the absorption and fluorescence behaviors of Yb3+ in tellurite glasses

Yb3+-doped tellurite glasses exhibited largest absorption and emission cross sections. However, it is more valuable to achieve larger Stark splitting of Yb3+ than improve the absorption and emission cross sections. In this work, we introduced a part of GeO2 into tellurite glass matrix to enlarge the Stark splitting of Yb3+. The effects of GeO2 concentration on the spectroscopic characteristic of Yb3+ were systematically investigated. The stimulated absorption and emission cross sections increased to be 1.62 pm2 and 0.88 pm2 when the GeO2 concentration up to 30 mol%, then decreased with the GeO2 concentration further increased. The Stark splitting of Yb3+ do not change until the GeO2 concentration up to 30 mol%. The NIR fluorescence lifetimes of Yb3+ drastically decreased from 850 μs to 385 μs with the GeO2 concentration increasing, which was due to the increasing multi-phonon relaxation rate and reabsorption. Intense NIR fluorescence were observed when excited by the ultraviolet light, which can be ascribed to the energy transfer from the CTB of Yb3+-O2- couple to Yb3+. The CTB showed a slight blue-shift from 338 to 313 nm with GeO2 concentration increasing, which was ascribed to the size of the cation site would increase.

Silica-modified luminescent LaPO4 :Eu@LaPO4 @SiO2 core/shell nanorods: Synthesis, structural and luminescent properties.

Monoclinic-type tetragonal LaPO4 :Eu (core) and LaPO4 :Eu@LaPO4 (core/shell) nanorods (NRs) were successfully prepared using a urea-based co-precipitation process under ambient conditions. An amorphous silica layer was coated around the luminescent core/shell NRs via the sol-gel process to improve their solubility and colloidal stability in aqueous and non-aqueous media. The prepared nano-products were systematically characterized by X-ray diffraction pattern, transmission electron microscopy, energy dispersive X-ray analysis, and FTIR, UV/Vis, and photoluminescence spectroscopy to examine their phase purity, crystal phase, surface chemistry, solubility and luminescence characteristics. The length and diameter of the nano-products were in the range 80-120nm and 10-15nm, respectively. High solubility of the silica-modified core/shell/Si NRs was found for the aqueous medium. The luminescent core NRs exhibited characteristic excitation and emission transitions in the visible region that were greatly affected by surface growth of insulating LaPO4 and silica layers due to the multiphonon relaxation rate. Our luminescence spectral results clearly show a distinct difference in intensities for core, core/shell, and core/shell/Si NRs. Highly luminescent NRs with good solubility could be useful candidates for a variety of photonic-based biomedical applications.

Effect of the heat treatment on the spectroscopic properties of Er3+-Yb3+-doped transparent oxyfluoride nano-glass-ceramics

Er3+-doped and Er3+-Yb3+-co-doped transparent oxyfluoride glass-ceramics (GCs) containing LaF3 nano-crystals (NCs) have been prepared by the melting-quenching method and adequate thermal treatments. A detailed structural characterization, performed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM), confirms the precipitation of LaF3 NCs with a size in the 10–15nm range. The crystal growth is fast in the very beginning, from a few min up to 3–5h of treatment at 620°C and then it slows down, up to 20h. A SiO2 enriched viscous barrier is formed around the NCs, thus preventing further crystal growth. The incorporation of Er3+ and Yb3+ ions into the NCs is proved by HR-TEM and energy dispersive X-ray spectroscopy (EDXS) analysis. Photoluminescence measurements have been performed upon visible and near infrared excitation. The energy transfer between Yb3+ and Er3+ ions is evidenced by the enhancement of the near infrared and Up-Conversion (UC) emissions of Er3+ ions in the co-doped samples under 980nm excitation. A moderate back transfer from Er3+ to Yb3+ ions has been also observed under near infrared excitation of Er3+ ions at 800nm. Upconverted green ((2H11/2,4S3/2) → 4I15/2) and red (4F9/2 → 4I15/2) emissions were observed in all samples and attributed to a two photon process. The strong enhancement of the UC emission for the co-doped glass-ceramics samples further confirms the incorporation of the rare-earth ions in the LaF3 NCs with a lower maximum phonon energy environment which reduces multi-phonon relaxation rates. Different red-to-green emission ratios are obtained depending on the heat treatment conditions for the glass-ceramic samples.

Photochemical studies of monodispersed YPO 4 :Eu microspheres: The role of surface modification on structural and luminescence properties

Abstract Well-aqueous soluble, monodispersed, luminescent YPO 4 :Eu (core), YPO 4 :Eu@LaPO 4 (core/shell) and YPO 4 :Eu@LaPO 4 @SiO 2 (core/shell/Si) microspheres (MSs) were fabricated at low temperature with a facile synthesis process. TEM micrographs demonstrate the monodispersed, spherical shaped MSs were effectively covered with rough, porous silica surface, which is average grain size around 223 nm. Energy dispersive X-ray analysis and FTIR spectral studies verified the silica surface modification. High absorbance of silica modified core/shell/Si MSs in the ultraviolet region suggests the good solubility and colloidal stability in an aqueous environment. Photoluminescence intensity of the electric-dipole ( 5 D 0 → 7 F 2 ) transition was dramatically decreased and magnetic-dipole ( 5 D 0 → 7 F 1 ) transition was increased in core/shell MSs regardless core MSs, it could be due to the formation of the undoped LaPO 4 shell surrounding the luminescent seed core-MSs. The formation of silica layer over the core/shell MSs not only increase the particle size but also improve the solubility character, however silica layer greatly quenched the luminescence intensity because of enhancement in multiphonon relaxation rate. Our finding suggests that monodispersed, spherical shaped luminescent MSs could be suitable materials for future use in photonic based bioapplications.

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.

Impact of surface coating on physical properties of europium-doped gadolinium fluoride microspheres

A facile polyol with co-precipitation method was utilized for preparation of GdF3:Eu microspheres (core-MSs) at low temperature. The as-prepared core-MSs were successfully coated with lanthanum fluoride and silica shells to enhance the luminescence and solubility characters of the nanomaterials. The as-prepared core and GdF3:Eu@LaF3 (core/shell) MSs possess pure orthorhombic structure, which was spherically shaped with the average crystalline size of about 400nm. Energy dispersive x-ray (EDX) analysis confirms the composition and doping of the europium ion into the nanomaterial lattice. On comparing the luminescence properties under same experimental conditions, the excitation and emission intensity of core/shell MSs was remarkably enhanced due to a reduction in nonradiative transition decay. However, the luminescence intensity in silica surface modified GdF3:Eu@LaF3@SiO2 (core/shell/SiO2) MSs was quenched in respect to core and core/shell MSs, causing the presence of abundant surface SiOH molecules, which are responsible for enhancing the multiphonon relaxation rate. However, these silanol groups greatly improve the solubility character in an aqueous and non-aqueous solvent. The observed results indicate that the significant improvement in solubility of as-prepared silica modified core/shell/SiO2 MSs can be used more desirable luminescent lanthanides nanocrystals for photonic based bio-applications.

Tuning upconversion fluorescence emission of -NaLuF4:Yb3+/Ho3+ nanocrystals through codoping Ce3+ ions

Rare-earth-doped up-conversion (UC) fluoride materials have been widely used in phosphors, color displays, optical storages, solid-state lasers, solar cells and biomedical imaging, due to the fact that their low phonon energy can effectively suppress the nonradiative multiphonon relaxation process. In this work, the NaLuF4:Yb3+/Ho3+ nanocrystals are successfully synthesized by a facile solvothermal method. The crystal structure and morphology of the NaLuF4 nanocrystals are characterized by the X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) respectively. The diffraction peaks are well consistent with those of high-purity hexagonal NaLuF4 (JCPDS No. 77-2042, P63/m space group). The TEM image reveals that the product is composed of monodisperse hexagonal rods with an average length of about 170 nm and an average diameter of 30 nm. The crystal structure and morphology do not present obvious change with the increasing Ce3+ ion concentration, which is due to the similarity in ion radius between Ce3+ and Lu3+. Under 980 nm excitation, the UC emissions of -NaLuF4:Yb3+/Ho3+ nanocrystals with different Ce3+ codoping concentrations are carefully studied. The strong green and red UC emissions of Ho3+ ions are observed in -NaLuF4 nanocrystals. It can be found that the UC emission of Ho3+ ions is tuned from green to red in -NaLuF4 nanocrystals through increasing Ce3+ ion concentrations from 0 to 12%, and the red-to-green (R/G) ratio is enhanced from 0.34 to 8.44. According to the level structure of Ho3+ ions, the red UC emission originates from the excited state 5F5. However, the population of the 5F5 excited state mainly depends on the two nonradiative relaxation processes of 5S2/5F45F5 and 5I65I7 transitions. In fact, the two nonradiative relaxation processes are very difficult to occur according to multiphonon nonradiative relaxation rate. When Ce3+ ion is introduced into the system, the red UC emission intensity and R/G ratio of Ho3+ are increased, because the energy gap from the excited state 5F7/2 to the ground state 2F5/2 is about 3000 cm-1 for Ce3+ ions, which is similar to the gaps of 5S2/5F45F5 and 5I65I7 transitions of Ho3+ ions. According to the energy conservation law, the two inefficient nonradiative processes from the 5S2/5F4 and 5I6 states of Ho3+ ions are substituted in order by resonant cross relaxation (CR) processes 5S2 (5F4) (Ho3+) + 2F5/2 (Ce3+5F5 (Ho3+) + 2F7/2 (Ce3+) and 5I6 (Ho3+) + 2F5/2 (Ce3+)5I7 (Ho3+) +2F7/2 (Ce3+) between Ho3+ and Ce3+ ions. These two resonant CR processes can transfer populations from the 5S2/5F4 state and 5I6 state to the 5F5 state and its intermediate 5I7 state, respectively. The resonant modality and the strong interaction between Ho3+ and Ce3+ ions are employed to enhance the red emission and suppress the green emission. The occurrence of CR process between Ho3+ and Ce3+ ions is further proved by the down-conversion emission spectra of Ho3+ ions under 532 and 980 nm laser excitation, respectively. We demonstrate that the highly efficient red UC emission of -NaLuF4:Yb3+/Ho3+/Ce3+ nanocrystals offers opportunities as desired optical materials for color displays, anticounterfeiting techniques and multiplexed labeling applications.

Luminescence and phonon side band analysis of Eu3+-doped lead fluorosilicate glasses

Lead fluorosilicate (SPbKNLFEu) glasses doped with different concentrations of Eu3+ ions have been prepared by the melt quenching technique. The structural and spectroscopic analysis have been carried out by Raman, absorption, excitation, emission, phonon side band (PSB) spectra and decay time measurements. The Judd-Ofelt theory has been used to predict the radiative properties for the emission levels of Eu3+ ions. Local structure around the Eu3+ ions and the phonon energy of SPbKNLFEu glasses have been confirmed on the basis of PSB associated with the 7F0 → 5D2 transition. The decay curves of the 5D0 and 5D1 levels exhibit single exponential nature with a lifetime of 2240 μs and 20 μs, respectively. The multiphonon relaxation rates (Wmp) from the excited levels to the next lower level of Eu3+ ions have been calculated. The higher stimulated emission cross-section and the strong red emission at 613 nm corresponding to the 5D0 → 7F2 transition suggests that the present lead fluorosilicate glasses could be useful for the optical display devices.

The Temperature-Dependence of Multiphonon Relaxation of Rare-Earth Ions in Solid-State Hosts.

Rare-earth ions are used in a wide range of emissive devices - ranging from lasers to displays - where high optical efficiency and narrow-linewidth are important. While their radiative properties are important, nonradiative properties are also critical since they can reduce optical efficiency and generate heat. In this paper, theories for multiphonon relaxation rate are reviewed for rare-earth excited states in solid-state dielectric hosts. A range of various approaches are used to simplify the mathematical form of the rate equations. The 2H9/2 excited state of Er3+, responsible for a technologically significant green emission, is modeled to show how the various theories manifest an order-of-magnitude variation in the thermal dependence of the multiphonon relaxation rate, as well as anomalous local minima in phonon scattering for temperatures above 0 K. This work proposes a corrective term of two quanta (Δν= + 2) of the mediating phonon energy to energy gap, so the calculated and the experimentally determined relaxation rates are equal. Radiative quantum efficiencies of both the 1G4→3H5 ~1.3 μm and 3F3→3F2 ~7 μm of Pr3+ are calculated to show the importance of both proper measurement of phonon energy and application of the multiphonon relaxation rate theory.

Spectral tuning via multi-phonon-assisted stokes and anti-stokes excitations in LaF3: Tm3+ nanoparticles

We present a facile and highly effective method to tailor upconversion (UC) emission from LaF3: Tm3+ nanoparticles (NPs) by adjusting ambient temperature from 20 K to 400 K accompanied with the pulse laser excitation. Spectral tuning mechanism controlled by ambient temperature at pulse laser excitation is revealed, and a mechanism based on the modification on multi-phonon relaxation rates for the rapid population of intermediate level 3H4 and multi-phonon-assisted excited state absorption is proposed. Based on multi-phonon relaxation theory and time-resolved photoluminescence studies, it is reasonable that UC luminescence under short-pulse laser excitation mainly originates from the ions at/near the surface of NPs. These exciting findings in ambient temperature accompanied with the short-pulse excitation dependent UC selectivity offer a general approach to tailoring lanthanide related UC emissions, which will benefit multicolor displays and imaging.

Compositional investigation of ∼2 μm luminescence of Ho3+-doped lead silicate glass

Lead silicate glass samples with varying lead oxide content were prepared in this study, and their luminescent properties were examined and analyzed. It was found that with increasing lead oxide content, the maximum phonon energies of the glass samples decreased, while their spontaneous transition probabilities first increased and then decreased. The influence of the spontaneous transition rate, A10, and the multi-phonon relaxation rate, W10, on the sample luminescent properties was analyzed using rate equations. As a result, it was found that with increasing lead oxide content, W10/A10 decreased, while the quantum efficiency increased. Thus, the luminescent intensity at ∼2μm increased in the glass samples with increased lead oxide content. The high luminescent intensity and long lifetime indicate that silicate glasses containing high levels of lead oxide could potentially be used in ∼2μm lasers.

Radiative transition, local field enhancement and energy transfer microcosmic mechanism of tellurite glasses containing Er3+, Yb3+ ions and Ag nanoparticles

Er3+-doped, Er3+/Yb3+ co-doped tellurite glass with and without Ag NPs were synthesized by melt-quenching method. The high resolution transmission electron microscopy (HR-TEM) and selected area electron diffractions (SAED) manifest growth of Ag NPs. The UV–vis–NIR absorption spectroscopy and fluorescence spectroscopy were measured. The optical band gap and multiphonon relaxation rate constants were calculated. The electronic band structure and local density of state (DOS) of Ag NPs are calculated. The fluorescence emission and enhancement mechanism including localized surface plasmon resonance (LSPR) and energy transfer (ET) microcosmic mechanism were discussed. The electric field distributions of Ag NPs are emulated by FDTD solutions software. Local field enhancement (LFE) induced by LSPR and lightning rod effect was found to be responsible for the fluorescence enhancement while energy transfer from Ag NPs to rare-earth was considered ignorable in the samples without photoluminescent emission.

Judd–Ofelt analysis and radiative properties of LiLa(1−x)Eux(PO3)4

Judd–Ofelt (J–O) theory has been applied to a series of LiLa1−xEux(PO3)4 (x=5%; 10%; 15%; 20% and 30%) polycrystalline powders based on their emission spectra. J–O intensity parameters Ωλ (λ=2, 4) and various radiative properties such as the radiative transition probability (A), radiative life time (τR), branching ratio (βR) and stimulated emission cross-section (σe) have been determined. The observed trend for J–O parameters (Ω2<Ω4) reveals the ionic character of La(Eu)–O bond as well as the symmetry environment around the Eu3+ ion in LiLa(PO3)4. The experimental decay curves of the 5D0 level in LiLa1−xEux(PO3)4 have a single exponential profile. The registered long life time, low multiphonon relaxation rates (WNR), high quantum efficiency (η) and the best optical gain parameter (σexτexp) have been discussed as function of Eu3+ concentration, chemical composition and crystal structure of the host matrix and particle-size distribution. A comparative study reveals that LiLa1−xEux(PO3)4 is promising for use in optical devices as well as for developing visible red lasers to emit at 613nm. The optical band gap energies of LiLa1−xEux(PO3)4 have been calculated from the diffuse reflection spectral measurements. The results were discussed as a function of Eu3+ concentration and compared to reported theoretical results.

Judd–Ofelt analysis of spectroscopic properties of Eu3+ doped KLa(PO3)4

Based on Judd–Ofelt (J–O) theory, the intensity parameters (Ω2 and Ω4) have been evaluated from the emission spectra of KLa(1−x)Eux(PO3)4 (where x=2, 5, 10, 15, 20 and 30mol%) polycrystalline powders as a function of Eu3+ ion concentration. The registered trend (Ω2<Ω4) reveals the ionic character of the La(Eu)–O bonds as well as a relatively high symmetry environment around the rare earth ion (RE) site in KLa(PO3)4 as compared to other host matrix. From these intensity parameters various radiative properties like spontaneous emission probability (A), branching ratio (βR), stimulated emission cross-section (σ) and the radiative lifetime (τR) have been calculated. The decay profiles for the 5D0 excited level of Eu3+ in KLa(PO3)4 have been found to be single exponential and independent of Eu3+ ion concentration even at 30mol%. In comparison with other Eu3+-doped materials, KLa(PO3)4:Eu3+ has a long lifetime, low multiphonon relaxation rates (WNR), high quantum efficiency (η) and better optical gain parameter (σxτexp). The determined radiative properties reveal the usefulness of KLa(1−x)Eux(PO3)4 in developing visible red lasers as well as optical display devices at around 613nm. Our results were discussed as a function of Eu3+ concentration, crystal structure of KLa(PO3)4 and particle-size distributions.

Enhanced 2.7 μm emission from Er3+ doped oxyfluoride tellurite glasses for a diode-pump mid-infrared laser

The influence of fluoride and shielding gas (O2 or Ar) on the physical and spectroscopic properties of Er3+ doped TeO2-ZnO-ZnF2 glass system is investigated. The larger electronegativity of F than O accounts for the gradual decrease of refractive index, density, and J-O parameters with increasing ZnF2. An analysis on Fourier transform infrared transmission spectra reveals that the absorption coefficient of OH− around 3 μm as low as 0.247 cm−1 can be achieved when 30 mol% ZnF2 containing sample is treated with Ar gas during glass melting process. The reduction of OH− groups combined with the low multiphonon relaxation rate (207 s−1) contributes to the enhanced emissions at 1.5 and 2.7 μm, along with prolonged lifetimes of 4I11/2 and 4I13/2 levels. A high branching ratio (17.95%) corresponding to the Er3+: 4I11/2 → 4I13/2 transition, the large absorption and emission cross section (0.44 × 10−20 cm2 and 0.45 × 10−20 cm2), and good gain cross section demonstrate that oxyfluoride tellurite glass could be a promising material for a diode-pump 2.7 μm fiber laser.

~2 µm Luminescence properties and nonradiative processes of Tm3+ in silicate glass

Two novel Tm3+-doped silicate glasses (one with fluoride addition) with good thermal stability were prepared in this work. Luminescence properties of these two samples were compared. After the addition of fluoride, the luminescence intensity of 1.8µm was much improved whereas that of 1.47µm was reduced. The rates of nonradiative processes populating and depopulating 3F4 were calculated quantitatively and a new method of calculating the quenching rate of hydroxyls was proposed. It was found that multiphonon relaxation rate of 3F4→3H6 was really low and could be neglected in both samples. The addition of fluoride in glass could reduce the quenching rate of hydroxyls but raised the cross-relaxation (3H6+3H4→3F4+3F4) rate. Thus the quantum efficiency of 1.8µm luminescence was improved after the introduction of fluoride.

Multiphonon relaxation in rare-earth doped fluorozirconate-based glasses containing BaCl2 nanocrystals

Nd3+-doped fluorozirconate-based glasses which contain hexagonal BaCl2 nanocrystals are analyzed for their photoluminescence and multiphonon relaxation (MPR) properties. The MPR rates of various Nd3+ levels are obtained from time-resolved spectroscopy using selective laser pulse excitation. The nonradiative decay rates are estimated from the difference between measured and calculated radiative decay rates as well as from the analysis of luminescence rise times. The MPR rates display an exponential dependence on the energy gap. Temperature-dependent studies of the decay indicate that phonons of the BaCl2 nanocrystals are involved in the MPR processes leading to extremely low MPR rates which are orders of magnitude lower than in conventional oxide and halide glasses. Photoluminescence emissions, which are usually quenched by MPR, and enhanced radiative quantum efficiencies are found.

Use of thulium-sensitized rare earth-doped low phonon energy crystalline hosts for IR sources

Crystalline hosts with low phonon energies enable novel energy transfer processes when doped with rare earth ions. Two applications of energy transfer for rare earth ions in thulium-sensitized low phonon energy crystals that result in infrared luminescence are discussed. One application is an endothermic, phonon-assisted cross-relaxation process in thulium-doped yttrium chloride that converts lattice phonons to infrared emission, which raises the possibility of a fundamentally new method for achieving solid-state optical cooling. The other application is an optically pumped mid-IR phosphor using thulium-praseodymium-doped potassium lead chloride that converts 805-nm diode light to broadband emission from 4,000 to 5,500 nm. These two applications in chloride crystals are discussed in terms of critical radii calculated from Forster-Dexter energy transfer theory. It is found that the critical radii for electric dipole-dipole interactions in low phonon energy chloride crystals are comparable to those in conventional oxide and fluoride crystals. It is the reduction in multi-phonon relaxation rates in chloride crystals that enable these additional energy transfer processes and infrared luminescence.

Studies on preparation and property evaluation of some Tm3+-ion doped oxyfluorophosphate glasses

The present paper reports the preparation of Tm3+ doped fluoride-phosphate (FP) glasses in the system of (100-18n) Ba(PO3)2 × n (2.0 MgF2 + 9.6 CaF2 + 6.4 AlF3), where n = 0.00, 2.00, 4.00 and 5.17. Various physical properties namely density, refractive index with special emphasis on the infrared (IR), ultraviolet–visible (UV–vis) absorption and fluorescence spectra of the prepared glasses has been systematically studied. The spectral studies reveal that upon addition of fluoride, the structure of the glass changes from metaphosphate (PO3−)n to P2(O,F)7 and P(O,F)4 in combination with the formation of Al(O,F)6, [AlF6]3− and [AlF4]−. The position of UV–vis absorption band has been correlated with quantitative calculation of nephelauxetic parameter and covalent bonding characteristics of the host. Appearance of two emission bands at ∼650 nm and ∼790 nm corresponds to 1G4 → 3F4 and 3H4 → 3H6 transitions, respectively. It is evident from the IR study that the fluorescence phenomena are expedited by the low multiphonon relaxation rate in oxyfluorophosphate glasses owing to their high intense low phonon energy (∼595 cm−1), which is very close to that of the fluoride glasses (500–600 cm−1).

Upconversion fluorescence studies of sol–gel-derived Er-doped KNN ceramics

Er-doped K0.5Na0.5NbO3 luminescent ceramics have been prepared using a sol–gel method. The results of X-ray diffraction show that Er3+ diffuses into the K0.5Na0.5NbO3 lattices, forming an orthorhombic-structured perovskite. Under an excitation of 980 nm, the ceramics exhibit luminescent emission bands at 527 nm (green), 548 nm (green), 660 nm (red) as well as 487 nm (blue), which are attributed to the transitions 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, 4F9/2 → 4I15/2 and 4F7/2 → 4I15/2, respectively. Our Raman results also reveal that the ceramics have low phonon energy (∼860 cm−1) and thus slow multi-phonon relaxation rate. The sol–gel process and Er3+-doping can produce ceramics with better compositional homogeneity as well as finer and uniform grains. All these are beneficial for improving both the upconversion and luminescent efficiencies of the ceramics, and thus leading to very strong green emissions even at a low quenching concentration of 2 mol%. These suggest that even without using sensitizers, the sol–gel-derived Er-doped KNN ceramics are promising candidates for multifunctional optoelectronic applications.