Radiative Lifetimes τR Research Articles

Spectroscopic investigations on Tb3+ doped lead fluoroborate glasses

This article presents the optical properties of Tb3+ in lead fluoroborate glasses of the type X PbF2·(89–X)B2O3·10 Al2O3·1Tb2O3 (where X=8, 12, 16, 20, 24, 28, 34 and 36). The standard Judd–Ofelt model was applied to the room temperature absorption intensities of Tb3+ (4f8) to determine the phenomenological intensity parameters Ω2, Ω4 and Ω6. These parameters have been used to calculate radiative transition probabilities (Arad), lifetimes (τR) and branching ratios (βR) for the excited level 5D4. The predicted values of τR are compared with the measured values for 5D4 level for eight glass compositions (Glass (A–H)). Among the eight-terbium glasses Glass A with 8mol% of PbF2 (as the optimum content) has revealed an intense green emission with maximum life time and higher quantum efficiency. The stimulated emission cross section σ(λP) is also evaluated for the 5D4→7FJ (J=6, 5, 4 and 3) transitions.

Structural and optical investigations of Eu3+ ions in lead containing alkali fluoroborate glasses

Lead containing alkali fluoroborate glasses (LAFB) with molar composition of 20PbO+5CaO+5ZnO+10AF+59B2O3+1Eu2O3 (where A=Li, Na and K) were prepared and investigated by the TG-DTA, FT-Raman, optical absorption, fluorescence and decay curve analysis. The influence of alkali content on the structure of borate glasses was investigated by FT-Raman spectroscopy. The thermal properties of the glasses have been studied by TG-DTA analysis. Judd–Ofelt intensity parameters are derived from the absorption spectra and also from the emission spectra under various constraints. The effect of thermalization on the oscillator strengths of the absorption transitions originating from the ground (7F0) and the first excited (7F1) states of Eu3+ ions have been discussed. The J–O intensity parameters obtained by applying thermal correction to 7F0→5D2 and 7F6 absorption oscillator strengths were used to calculate the various spectroscopic properties. The predicted values of radiative lifetime (τR) and luminescence intensity branching ratio (βR) are compared with the measured values for 5D0 level. The decay profiles were found to be single exponential in all the three glasses. The spectroscopic properties confirm the potentiality of present LAFB glasses doped with Eu3+ ions as laser host materials to produce an intense red luminescence at 612nm corresponding to 5D0→7F2 emission level and have significant importance in the development of emission rich optical systems.

Exact and Closed Form Solutions for the Quantum Yield, Exciton Diffusion Length, and Lifetime To Reveal the Universal Behaviors of the Photoluminescence of Defective Single-Walled Carbon Nanotubes

The exact and closed form solutions for the photoluminescence quantum yield (PL QY), effective exciton lifetime, and effective exciton diffusion length were derived for defective single-walled carbon nanotubes (SWCNTs) with randomly and uniformly distributed defects or quenching sites. Asymptotic analysis of the exact and closed form solution not only recovered but also corrected the analytical QY for defective SWCNTs recently derived by Hertel et al. (ACS Nano2010, 4, 7161–7168). Using the exaction solutions, we reanalyzed the length-dependent QYs and exciton lifetimes of fractionated (6, 5) tubes and determined the intrinsic QY η∞0 = 0.106, exciton diffusion length L∞0 = 272 nm, internal quenching site density c = 0.0039 nm–1, exciton diffusion coefficient D = 2.2 cm2/s, radiative lifetime τr = 1.6 ns, and intrinsic exciton lifetime τ = 168 ps. The effects of F4TCNQ doping on the SWCNT PL intensity were quantitatively studied as well. Through numerical studies of the PL QYs of defective SWCNTs, we eluci...

Collateral evidence for an excellent radiative performance of AlxGa1−xN alloy films of high AlN mole fractions

Recombination dynamics for the deep-ultraviolet (DUV) near-band-edge emission of AlxGa1−xN epilayers of high AlN mole fractions (x) are studied using time-resolved spectroscopy. Their low-temperature radiative lifetime (τR) is longer than that for the epilayers of low-x AlxGa1−xN, AlN, or GaN due to the contribution of bound and localized tail-states. However, τR shows little change with temperature rise, and the value is a few ns at 300 K. The results essentially indicate an excellent radiative performance of AlxGa1−xN alloys of high x, although the luminescence efficiency of AlxGa1−xN DUV light-emitting-diodes reported so far is limited by the short nonradiative lifetime.

Optical properties of Nd3+-doped La2CaB10O19 crystals with different Nd3+ concentrations

La2CaB10O19 (LCB) is a new NLO crystal, in which Nd3+ can be doped with different concentrations to be self-frequency doubling crystals. We obtained seven LCB crystals with different Nd3+ concentrations grown by flux method. Refractive indices, absorption spectra and fluorescence spectra of the seven LCB crystals with various Nd3+ doping concentrations have been measured and evaluated. It has been found that the 5mol% Nd doped LCB crystal is most appropriate to be a laser material, by fitting the absorption spectrum with the Judd–Ofelt theory, radiative lifetime τr of Nd3+ in the 5mol% Nd doped LCB crystal is determined to be 237.8μs. For the 5mol% Nd doped LCB crystal, the absorption cross section at 799nm and the emission cross section at 1050nm are 4.52×10−20 and 1.29×10−19cm2, respectively. The measured fluorescence lifetime τf of the 5mol% Nd doped LCB crystal is 85μs and its quantum efficiency is 35.7%.

Major impacts of point defects and impurities on the carrier recombination dynamics in AlN

Impacts of point defects and impurities on the carrier recombination dynamics in AlN are revealed by time-resolved spectroscopy and positron annihilation measurements. Intrinsically short low-temperature excitonic radiative lifetime (τR∼10 ps) was elongated with the increase in Al-vacancy concentration up to 530 ps, irrespective of threading dislocation density. A continuous decrease in τR with temperature rise up to 200 K for heavily doped samples revealed the carrier release from the band-tail formed due to impurities and point defects. Because room-temperature nonradiative lifetime was equally short for all samples, high temperature growth with appropriate defect management is necessary in extracting radiative nature of AlN.

Fluorescence features of Sm3+ ions in Na2SO4–MO–P2O5 glass system—Influence of modifier oxide

Abstract Na2SO4–P2O5:Sm3+ glasses mixed with three different alkaline earth modifier oxides viz., MgO, CaO and BaO were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature; the Judd–Ofelt theory was successfully applied to characterize these spectra. From this theory, various radiative properties like transition probability A, branching ratio βr, the radiative lifetime τr, for various emission levels in the spectra of these glasses have been determined and reported. An attempt has also been made to throw some light on the relationship between the structural modifications and luminescence efficiencies in the change of scenario for modifying oxides in the glass network.

Fluorescence Properties of Pr<sup>3+</sup> Doped Calcium Fluoroborate Glasses

Calcium fluoroborate glasses doped with trivalent praseodymium ions have been prepared using melt quenching technique with the molar composition (42-x) B2O3 + 20 CaF2 + 15 CaO + 15 BaO + 8 Al2O3 + x PrF3 (x = 0.05, 0.1, 0.5, 1.0, 2.0 and 4.0). Absorption, fluorescence and lifetime spectra were recorded for the title glasses. Judd-Ofelt intensity parameters Ωt (t = 2, 4 and 6) have been determined for the measured oscillator strengths of the absorption bands by including and excluding the 3H4 → 3P2 hypersensitive transition. From the intensity parameters spontaneous emission probabilities (AR), excited state radiative lifetimes (τR) and branching ratios (βR) were calculated. Fluorescence quenching is observed in all the peaks and is explained by energy transfer mechanisms. Using the fluorescence spectra stimulated emission cross section (σe) for all emission transitions have been estimated. Energy transfer processes were also explained to emphasize the quenching of fluorescence intensity of certain emission transition. Based on the results, the utility of Pr3+ doped calcium fluoroborate glasses as laser active materials as well as wave guides in the visible region is discussed.

Influence of Crystallization on the Luminescence Characteristics of Pr3+ Ions in PbO–Sb2O3–B2O3 Glass System

PbO–Sb2O3–B2O3 glasses mixed with different concentrations of Pr2O3 (ranging from 0 to 0.6 mol%) were crystallized. The samples were characterized by X‐ray diffraction (XRD) and transmission electron microscopy techniques. The XRD studies have revealed the presence of Pb5Sb2O8, Pb3(SbO4)2, PbB4O7, β‐PrSbO4, Pr3SbO7, and Pr3Sb5O12 crystalline phases. Optical absorption (in the visible and NIR regions) and fluorescence spectra of these samples have been recorded at room temperature. The Judd–Ofelt theory could successfully be applied to characterize the absorption and luminescence spectra of Pr3+ ions in these samples. From the luminescence spectra, various radiative properties like transition probability A, branching ratio βr, and the radiative lifetime τr for various emission levels originating from the 3P0 level of Pr3+ ion in the glasses glass ceramic samples have been evaluated. The radiative lifetime of the 3P0→3H6 transition is measured and the quantum efficiency has been estimated. The crystallization causes to improve the luminescence efficiency to a large extent. The changes observed in the radiative properties due to crystallization have been discussed in the light of varying co‐ordinations of antimony ions in the glass network.

Role of Al coordination in barium phosphate glasses on the emission features of Ho3+ ion in the visible and IR spectral ranges

Abstract The glasses of the composition (39−x)BaO–xAl2O3–60P2O5:1.0Ho2O3 (in mol%) with x value ranging from 1.0 to 4.0 have been synthesized. The IR spectral studies of these glasses have indicated that there is a gradual transformation of Al3+ ions from tetrahedral to octahedral with increase in the concentration of Al2O3 up to 3.0 mol%. Optical absorption and fluorescence spectra (in the visible and NIR regions) of these glasses have been recorded at room temperature. The Judd–Ofelt theory could successfully be applied to characterize the absorption and luminescence spectra of Ho3+ ions in these glasses. From the luminescence spectra, various radiative properties like transition probability A, branching ratio βr, the radiative lifetime τr and emission cross-section σE for various emission levels of these glasses have been evaluated. The radiative lifetime of the 5S2→5I8 (green emission) transition has also been measured. The variations observed in these parameters have been discussed in the light of varying co-ordinations (tetrahedral and octahedral positions) of Al3+ ions in the glass network. The influence of hydroxyl groups on the luminescence efficiency of the transition 5S2→5I8 has also been discussed. Finally the optimum concentration of Al2O3 for getting maximum luminescence output has also been identified and reported.

Photoluminescence properties of Er3+-doped alkaline earth titanium phosphate glasses

Abstract Er3+-doped alkaline earth titanium phosphate (RTP) glasses with molar composition of 24 (NaPO3)6 + 30 KH2PO4 + 25 TiO2 + 20 RCl2 + 1 Er2O3 were prepared by melt quenching technique. Judd–Ofelt intensity parameters (Ω2,4,6) were determined from the experimental oscillator strengths (fexp) of absorption bands. From these parameters spontaneous emission probabilities (AR), luminescence branching ratios (βR) and radiative lifetimes (τR) have been calculated. Visible and near infrared photoluminescence spectra has been recorded by exciting the samples at 380 and 970 nm respectively. An intense broad emission band at 1.53 μm was observed corresponding to 4I13/2 → 4I15/2 transition. McCumber theory has been applied to determine the emission cross-sections (σe) of the 4I13/2 → 4I15/2 transition using the absorption cross-sections (σa). The lifetimes of 4S3/2 level were measured for the glasses by exciting the samples at 540 nm wavelength and the quantum efficiencies were also determined.

Proceedings of the International Conference on Strongly Correlated Electron Systems

Li2O–MO (Nb2O5, MoO3 and WO3)–B2O3 glasses doped with four rare earth ions, viz., Pr3+, Nd3+, Sm3+ and Er3+ (of 1.0 mol% each) were prepared. The glasses were characterized by X-ray diffraction, differential scanning calorimetry, ESR, optical absorption and photoluminescence spectra. From the measured intensities of various absorption bands of these glasses, the Judd–Ofelt parameters Ω2, Ω4 and Ω6 have been evaluated. The Judd–Ofelt theory could successfully be applied to characterize the absorption and luminescence spectra of these glasses. From this theory, various radiative properties like transition probability A, branching ratio βr, the radiative life time τr and the emission cross-section σE for various emission levels of these glasses have been determined and reported. An attempt has also been made to throw some light on the relationship between the structural modifications and luminescence efficiencies of all the three glasses. The analysis of the data indicated high non-radiative losses in Nb2O5 mixed glasses.

Optical absorption and fluorescence spectral studies of Sm3+ ion in BaO-M2O3 (M = Ga, Al, In)-P2O5 glass systems

BaO-P2O5 glasses mixed with three IIIA group metal oxides viz., Al2O3, Ga2O3 and In2O3 doped with Sm2O3 were prepared. Optical absorption and photoluminescence spectra of these glasses have been studied. The absorption spectra of these glasses have revealed the absorption bands due to 6H5/2 → 4I11/2, 4I13/2, 4G9/2, 6P5/2, 4F7/2, 6P7/2, 4D5/2, 4K17/2 transitions. By performing least square fit analysis, the Judd-Ofelt intensity parameters Ωλ for these glasses were evaluated; these parameters have exhibited the following trend: Ω2 > Ω4 >Ω6. The photoluminescence spectra with the excitation wavelength 400 nm recorded at room temperature of these glasses have exhibited the following emission transitions: 4G5/2 → 6H5/2, 6H7/2, 6H9/2. From luminescence spectra, various radiative properties like transition probability A, branching ratio βr, the radiative lifetime τr and the emission cross-section σE for various emission levels of these glasses have been determined and reported. Out of the three J-O parameters (Ωλ), the value of Ω2 which is related to the structural changes in the vicinity of the Sm3+ ion indicated the highest covalent environment of Sm3+ ion in indium mixed glasses. The luminescence spectra of all the three glasses suggested the highest of values of transition probability 4G4 → 6H9/2 in BGPSm glasses. The analysis of these results further indicted comparatively high non-radiative losses in In2O3 mixed glasses.

Spectroscopic properties and Judd-Ofelt analaysis of Sm3+ and Dy3+doped chlorophosphate glasses

Spectroscopic properties of Sm3+ and Dy3+ (1 mol%) doped chlorophosphate glasses of the type 69P2O5. 20Na2HPO4. 10RCl (R=Na, K, Mg and Ca) and 69P2O5. 20Na2HPO4. 5NaCl. 5RCl (R=K, Mg and Ca) are studied. The spectroscopic parameters such as oscillator strengths (f), Judd-Ofelt intensity parameters (Ωλ), radiative transition probabilities (AT), radiative lifetimes (τR), branching ratios (βR) and absorption cross-sections (Σ) have been determined and are reported. The hypersensitive nature has been discussed in the present work. The magnitudes of branching ratios (βR) are higher for 6F7/2→6H5/2 transition of Sm3+ ion (except Na-Ca) and 6F5/2→6H15/2 transition of Dy3+ ion among various transitions. From the fluorescence spectra, emission cross-sections (σP) are obtained for the three transitions, 4G5/2→6H5/2, 4G5/2→6H7/2 and 4G5/2→6H9/2 of Sm3+ ion and two transitions, 4F9/2→6H9/2 and 4F9/2→6H13/2 of Dy3+ ion. From these studies certain transitions have been selected for laser excitation.

Spectroscopic properties and Judd–Ofelt analysis of Sm3+ doped lead–germanate–tellurite glasses

Spectroscopic properties of Sm3+ (1 mol%) ions in 49 PbO–30 GeO2–20 TeO2 glass have been characterized through optical absorption and temperature dependent fluorescence. The spectroscopic parameters such as oscillator strengths (f), Judd–Ofelt (J–O) intensity parameters (Ωλ), spontaneous emission probability (AR), branching ratios (βR) and radiative lifetimes (τR) of various excited levels have been determined from the absorption spectrum by using J–O analysis. From the fluorescence spectra, four emission transitions have been observed from the 4 G5/2 state to the lower lying states 6H5/2, 6H7/2, 6H9/2 and 6H11/2 upon exciting the sample with a 488 nm line of an argon ion laser. The stimulated emission cross-sections (σe) and branching ratios (βmeas) were estimated from the emission spectra for all emission transitions and decay curve analysis for the 4G5/2 level has also been carried out for different concentrations. Based on these results, the utility of Sm3+ doped lead–germanate–tellurite glasses as laser active materials in the visible region is discussed.

Synthesis, characterization and optical properties of LaAlO3:Ho3+ phosphor

Abstract Holmium-doped lanthanum aluminate (LaAlO3) powder phosphors have been prepared at furnace temperatures as low as 500 °C by using the combustion route without further calcining treatment. Powder X-ray diffraction and Fourier-transform infrared spectrometry measurements were used to characterize the prepared combustion products, while the optical properties were studied using UV–vis–NIR and photoluminescence spectroscopy. The Judd–Ofelt (J-O) model was applied to obtain the oscillator strengths (f) and three phenomenological intensity parameters. Using J-O parameters (Ω2, Ω4 and Ω6) the radiative transition probabilities (Aab), radiative lifetimes (τR) and branching ratios have been calculated for certain excited states of Ho3+. Using the Fuchtbauer–Ladenberg formula, the stimulated emission cross-sections (σemi) for some interesting transitions, such as 5S2→5I8 and 5F5→5I8 of Ho3+ in LaAlO3, were determined and discussed.

Optical properties of alkali and alkaline-earth lead borate glasses doped with Nd3+ Ions

Spectroscopic and physical properties of Nd3+-doped alkali lead borate glasses of type 20R2O · 30PbO · 49.5B2O3 · 0.5Nd2O3 (R = Li and K) and alkaline-earth lead borate glasses 20RO · 30PbO · 49.5B2O3 · 0.5Nd2O3 (R = Ca, Ba, and Pb) have been investigated. Optical absorption spectra have been used to determine the Slater-Condon (F2, F4, and F6), spin orbit ξ4f, and Racah parameters (E1, E2, and E3). The oscillator strengths and the intensity parameters Ω2, Ω4, and Ω6 have been determined by the Judd-Ofelt theory, which, in turn, provide the radiative transition probability (A), total transition probability (AT), radiative lifetime (τR), and branching ratio (βR, %) for the fluorescent levels. The lasing efficiency of the prepared glasses has been characterized by the spectroscopic quality factor (Ω4/Ω6), the value of which is in the range 0.2–1.5, typical of Nd3+ in different laser hosts. A red shift of the peak wavelength is observed upon addition of alkali or alkaline-earth oxides to the lead borate glass. A higher value of the W2 parameter for potassium-doped glass indicates a higher covalency for this glass matrix. The relative intensity of the peaks 4I9/2 → 4F7/2, 4S3/2 has also been studied.

Radiative and nonradiative lifetimes in nonpolar m-plane InxGa1−xN∕GaN multiple quantum wells grown on GaN templates prepared by lateral epitaxial overgrowth

Different from the case for polar (0001) InxGa1−xN multiple quantum wells (MQWs), the effective radiative lifetimes (τR,eff) at 8K of violet (V: 3.15eV), purple (P: 3.00eV), and blue (B: 2.83eV) emission peaks in nonpolar (11¯00) InxGa1−xN∕GaN MQWs fabricated on various GaN templates were found to be nearly independent of InN molar fraction x being approximately 1ns. The result indicates the absence of polarization fields parallel to the MQW normal. For each luminescence peak, the effective nonradiative lifetime (τNR,eff) at room temperature of the MQWs grown on “Ga-polar” wings of the GaN template prepared by lateral epitaxial overgrowth (LEO) was longer than that for the MQWs grown on “N-polar” wings, windows, or on conventional GaN templates, which had high density basal plane stacking faults and threading dislocations. Since τR,eff was little affected by the presence of defects, the increase in τNR,eff brought fivefold improvement in the equivalent internal quantum efficiency (ηinteq) of V peak. Because B peak was mostly generated from defective areas, the increase in ηinteq was not so remarkable (29%–36%). However, these values are approximately twice that reported for (112¯0) InGaN∕GaN MQWs grown on LEO GaN templates [Onuma et al., Appl. Phys. Lett. 86, 151918 (2005)].

Emission analysis of Eu3+: CaO–La2O3–B2O3 glass

This paper reports on the analysis of the emission spectra of Eu3+: CaO–La2O3–B2O3 glass. From the measurement of its optical absorption spectrum, three phenomenological Judd–Ofelt intensity parameters have been computed and used to parameterize the radiative properties such as spontaneous emission probability (A), radiative rate (AT), radiative lifetime (τR), branching ratios (β) and stimulated emission cross sections (σPE) of the measured emission transitions. A significant change has been observed in the fluorescence features of Eu3+ glass due to the occurrence of crystallization from the controlled heat treatment schedules. Local environment structure in the glass around the rare earth ion has also been understood. From the XRD spectral profiles of the ceramized Eu3+: calcium lanthanum borate glass, the crystalline phases have been analyzed in correlation with the measurement of FT-IR spectra of this red luminescent glass.

Optical properties of Nd3+:NaLa (WO4)2 single crystal

Nd3+-doped NaLa(WO4)2 single crystal with a dimension of ∅20mm×40mm and a good optical quality was grown by Czochralski method. The polarized absorption spectra and emission spectra were measured at room temperature. The absorption cross-section and emission cross-section were presented. The Judd–Ofelt theory, extended to anisotropic system, has been applied to evaluate the intensity parameters Ωt (t=2,4,6), radiative transition rates A, radiative lifetimes τR and fluorescent branching ratios β. The calculated radiative lifetime was compared with the experiment data for the 4F3/2 emitting level. All spectral features are strongly affected by an inhomogeneous broadening connected with the ‘disordered crystal’ character of the title compound.