Author's Accepted Manuscript
Spectroscopic properties of Er3 þ ions in multicomponent tellurite glasses SajnaM.S. , Sunil Thomas, K.A. Ann Mary, Cyriac Joseph, P.R. Biju, N.V. Unnikrishnan
PII: DOI: Reference:
S0022-2313(14)00633-4 http://dx.doi.org/10.1016/j.jlumin.2014.10.062 LUMIN12992
To appear in:
Journal of Luminescence
Received date: 8 May 2014 Revised date: 10 October 2014 Accepted date: 26 October 2014 Cite this article as: SajnaM.S. , Sunil Thomas, K.A. Ann Mary, Cyriac Joseph, P.R. Biju, N.V. Unnikrishnan, Spectroscopic properties of Er3 þ ions in multicomponent tellurite glasses, Journal of Luminescence, http://dx.doi.org/10.1016/j. jlumin.2014.10.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Spectroscopic properties of Er3+ ions in multicomponent tellurite glasses Sajna M.S., Sunil Thomas, Ann Mary K.A., Cyriac Joseph, P.R. Biju, N.V. Unnikrishnan School of Pure & Applied Physics, Mahatma Gandhi University, Kottayam - 686 560, India *Tel: +91 9745047850, Email: [email protected]
Abstract In the present work, multicomponent tellurite glasses were elaborated by the melt quench technique with different concentrations of Er3+ ions. Amorphous nature of all the glasses was confirmed using X-ray diffraction patterns. The thermal parameters, such as glass transition temperature ( ) and the onset of crystallization temperature ( 3+
differential scanning calorimetry. Judd-Ofelt parameters were derived for 0.5 mol% Er 4
) were determined by the
-doped glass from the absorption
measurements, and in turn, used to find the radiative properties of S3/2, H11/2 and I11/2 levels of Er3+ ion. A green emission corresponding to 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 transitions of Er3+ ions was observed in the glasses under investigation. Efficient green upconversion luminescence was observed under 976 nm excitation. The emission bands centered at 529 and 543 nm confirmed that two photons contribute to the upconversion processes.
We have also analyzed the dependence of
downconversion as well as upconversion as a function of Er3+ ion concentration, which shows quenching of photoluminescence intensity above 0.5mol % doping. From the emission spectra, CIE color coordinates of 0.5 mol% Er3+ -doped glass was examined. Fluorescence decay curves for the 4S3/2 → 4I15/2 transition for all the glasses have been measured and analysed. Absorption cross-section and calculated emission cross-section, using the McCumber method, for the 4I13/2 ↔ 4I15/2 transitions, were evaluated and discussed. Keywords: Erbium, Tellurite glasses, Fluorescence properties, Judd-Ofelt analysis, Upconversion, McCumber theory
1. Introduction Optical properties of trivalent rare earth (RE3+) ions doped glassy materials have better prospects to be exploited as good candidates for employment in laser technology . It also plays a vital role in fabricating IR-visible up-converters, optical fibers, waveguides and fiber amplifiers for optical transmission network. Compositional changes carried out systematically in the host matrices may well dictate the optical properties by modifying its covalency and local structure . Erbium has proven its role as a dopant for infrared pumped visible luminescence and laser emission. In particular, lasing at the green transition 4S3/2 → 4I15/2 of Er3+ leads to excellent results while doped in host matrices . Er3+ doped glasses also significant, since they have very good applicability in the development of infrared lasers and optical amplifiers . They are widely studied for possible laser applications, especially in the spectral range 1.53 μm (4I13/2 → 4I15/2), since they have a low loss in optical waveguides. For the past few years, Er3+ doped tellurite glasses and fiber amplifiers based on tellurites have gained attention, because of the demand for high transmission capacity of Wavelength- Division- Multiplexing (WDM) networks. Tellurium based erbium doped fiber amplifier (EDFA) have a wide gain spectrum and thus provides a number of channels for carrying large information. It is also known to be a valid candidate for 1.5 μm broad band optical amplifier. Comparing with other oxide glasses, tellurite based glass systems have peculiarities like effective high refractive index, low phonon energy, good rare earth ion solubility, etc. 1
Properties like high dielectric constant and very good stability of tellurium based glasses make it suitable at application levels. Marjanovic et. al suggest that, the tellurite glasses are more suitable for optical applications compared to other glasses, because of its strong crystal field, effective re-emission and reabsorption processes . In the present work, the composition of Er3+ -doped multicomponent tellurite glasses was chosen after a systematic search expecting to evolve glasses having desirable properties. Even though B2O3 causes a rise in phonon energy of the lattice, addition of it is justified by the fact that it can enhance the population accumulation in the 4I13/2 level and 980 nm pumping efficiency as reported earlier . The incorporation of network modifiers like potassium, will lead to the formation of stable glasses and the P2O5 can enhance the local symmetry around RE3+ ion. The Zn component can help the formation of a more connected network in the present glass like systems, by the modification of the network with the introduction of chain like structural units with more non-bridging oxygens (NBO’s) in (Te-O)- bonds as well as Te-O-Te linkages as reported earlier . Fluoride glasses are known to have efficient IR and upconversion emissions even though they have poor stability and chemical durability. Even if the phonon energy of tellurite glasses is very low (~750 cm-1), other glass formers like P2O5 and B2O3 have comparatively very high phonon energies. In the current investigation, we carried out a systematic evaluation of the spectroscopic properties of Er3+ -doped borophospho-tellurite glasses.
The influence of erbium ion concentration on the
spectroscopic properties of glasses was investigated by analysing luminescence spectra. 4
emission spectra of the I11/2 → I15/2, S3/2 → I15/2 and H11/2 → I15/2 transitions, the luminescence properties were drawn out. Decay curves of the 4S3/2 level of all the glasses were measured and the lifetimes were estimated. The theoretically predicted results were compared with the experimental observations to recognize the potential of the current glass system as a laser material. Lasing at the 4S3/2 → 4I15/2 (green emission) has led to useful outcomes in this particular glass structure. Therefore, for laser applications as well as in optical communication systems, the upconversion luminescence of Er3+ -doped materials can be exploited.
It is also worth to design the particular composition of glasses according to
the requirement and optimize them for the production of more significant and useful optical devices. 2. Experimental 2.1
Fabrication of the glasses Optically transparent multicomponent tellurite glasses with the composition (mol%) of (60-
x)TeO2+10K2O+10P2O5+10B2O3+10ZnF2+xEr2O3, where x = 0.0, 0.05, 0.1, 0.3, 0.5 and 1.0 (hereafter referred as TPBKZFEr) were prepared using TeO2, KH2PO4, H3BO3, ZnF2 and Er2O3 (high purity reagent grade) as starting materials by standard melt quench method. In these glasses, an increase of Er2O3 doping content is accompanied at the expense of TeO2.
The ingredient chemicals of appropriate
quantities were weighed according to the stoichiometry to derive 15 g batches of tellurite glasses with 2
varying concentration of Er3+ ions. The dry mixture of the precursors prepared were placed in a platinum crucible and melted at a temperature range 850 – 950˚C for 2 h in an electric furnace in air atmosphere to achieve complete melting. For ensuring homogeneity, the melt was swirled and then quenched to produce vitreous samples by pouring the melts between a couple of 250˚C preheated brass plates. It was then annealed at a temperature of 250˚C for 15 h and allowed to cool slowly inside the furnace itself until an ambient temperature was reached, in order to release the thermal and mechanical stress. Different compositions of tellurite glasses with varying concentration of Er3+ ions were prepared and had yellowish to pink color with the addition of Er2O3 content. The bulk glasses were polished by a lapping machine to make the thickness around 5 mm. The glasses were now checked for optical homogeneity, in order to meet the requirements for the measurements of optical properties. The batch compositions and the corresponding labels of the fabricated glasses are listed in Table 1. The glass labelled TPBKZFEr05 showed the best optical quality and better luminescence among all the prepared glasses was chosen for further studies and theoretical predictions. 2.2
Measurements The calorimetric measurement was carried out using a TA-Q20-2047 Differential Scanning
Calorimeter (DSC) in nitrogen purge system at a heating rate of 10˚C/min.
From the tangent
intersections of the endothermic peaks in the DSC curve, the glass transition temperature crystallization temperature
) were determined. X-ray diffraction (XRD) measurements were carried
out using PANalytical X’pert PRO X-ray Diffractometer with Cu Kα radiation (λ = 1.54 Ǻ) at room temperature (RT) in order to confirm the amorphous nature of the glass samples. The refractive indices ( ) of the glasses were measured at 632.8 nm using an ellipsometer (J.A. Woollam Co., Inc EC-400). Optical absorption of the TPBKZFEr05 glasses was measured with a UV-Vis-NIR Spectrophotometer (Perkin Elmer Lambda-950) in 200-2500 nm wavelength range with 1 nm resolution. The emission spectra of all doped glasses having different Er3+ ion concentration were also taken at excitation wavelengths of 520 and 378 nm line of Xe flash lamp using Horiba Scientific fluromax-4 Spectrofluorimeter. The upconversion emission spectra were obtained by the same Spectrofluorimeter using an excitation wavelength of 976 nm. The band intensities of the emissions were also measured. For comparing the luminescence intensity of varying Er3+ ion doped glass samples accurately, the samples were set at the same place in the experimental set-up and the position, power of the pumping beam and the width of the slits (2 nm) were fixed, to collect the luminescence signal. The luminescence decay curves for 4S3/2 level of Er3+ ions in the prepared glasses were measured under 378 nm excitation using the Edinburg instruments F900 Fluorimeter with a time resolution of 0.2 μs.
measurements were performed at RT. Random errors were evaluated through statistical analysis and were reduced by averaging over a number of observations.
Systematic errors in spectroscopic
measurements usually come from the measuring instruments and were obtained from the instruments details. 3. Results and Discussion 3.1
Some basic physical parameters of the Er3+-doped borophospho-tellurite glasses, which depend on the glass composition and doping percentages, such as electronic polarizability (
), interionic distance ( ),
field strength ( ), etc. have been calculated using relevant expressions. Archimedes’ principle was employed with distilled water as immersion medium at RT for density measurements of the glasses using the formula ,
are the weight of the sample in air and water, respectively.
is the density of water.
Optical path lengths of the glasses were measured using screw gauge. The concentration of Er3+ ions inside the host matrix is given by, % .
The obtained density ( ), concentration ( ) and refractive index ( ) have been used to calculate various physical properties of the glasses using relevant expressions. The polaron radius ( ), interionic distance ( ) and field strength ( ) can be estimated by taking the RE3+ ion concentration into account. The polaron radius can be calculated as (3) Electronic polarizability (
) is given by the Lorentz-Lorenz equation,
is the molar volume. It is also evident from Eq. (4) that the linear refractive index is related to
electronic polarizability the
and molar volume
in such a way that the refractive index increases as
Molar refractivity (
) is determined as
is the average molecular weight of the glass. Dielectric constant ( ) is given by (6)
Electric susceptibility is expressed as (7) The interionic distance ( ) for RE3+ doped glasses is given by (8) The average distance between acceptors and donors (
) can be calculated using the formula
Field strength ( ) is expressed as (10) where
is the charge of the ion.
Reflection loss ( ) is given by 100%
The aforesaid physical properties for all the glasses are collected in Table 1. It can be seen that the increment in density value of Er2O3 containing multicomponent tellurite glasses was due to the increase in molecular mass of oxide ion as the replacement of TeO2 (M.W = 159.6 gmol-1) by Er2O3 (M.W = 382.56 gmol-1). As a result, the refractive index also increases. Due to high polarity, the Er3+ ions capture one electron from the oxygen, thus forming NBO’s and the molecules become denser . It is noted that the refractive indices of tellurite glasses are high, which is beneficial to obtain efficient radiative transition. The refractive indices are quite large for these erbium doped multicomponent tellurite glasses in relation with other erbium doped glasses  and is actually inherent from the nature of the major component (tellurite) in the present glass matrix. Refractive index is also a relevant factor in governing the performance of an optical amplifier by controlling the mode profile . The polaron radius decreases with the Er2O3 content as reported previously . Eq. (5) gives the average molar 5
refraction for isotropic substances such as liquids, glasses and cubic crystals. It is interesting to note that the increase of Er2O3 content in the glass systems enhances various optical parameters such as molar refractivity, optical dielectric constant, electric susceptibility, field strength, and reflection loss. On the other hand, as expected interionic distance and donor-acceptor distance values show a decreasing trend with the increase of Er2O3 content. 3.2
Differential Scanning Calorimetry analysis The DSC trace of the TPBKZFEr05 glass is shown in Fig. 1. Corresponding to the glass
transition temperature ( ), there can be seen a broad endothermic hump. It can also be seen that this transition is followed by an exothermic peak which characterizes the crystallization temperature and represents the temperature of crystallization onset. Based on Fig. 1, the obtained thermal parameters were
= 471˚C (peak of crystallization). The ∆ value (∆
) is a
suitable measure for the thermal stability against crystallization and it represents the interval during which the nucleation takes place. The difference between
is 105˚C for the present glass
system. The lower ∆ value, compared to some other tellurite glasses [12, 13] implies lesser thermal rigidity and lower stability against crystallization. 3.2.2
XRD analysis From the XRD patterns of the prepared glasses, it is clear that they are vitreous samples and there
was no crystallinity observed for any of the glasses (Fig. 2). The broad hump in all the glasses is indicative of the amorphous nature of the glass. 3.3
Optical Absorption studies
Absorption spectra and energy band gap analysis Figs. 3(a) and 3(b) respectively present the absorption spectra of TPBKZFEr05 glass, as a
representative case, in the UV-Vis and NIR spectral regions. It is analogous with the soda-lime-silicate (SL) and aluminosilicate glasses (AS) . The spectra exhibit inhomogenously broadened absorption bands in the Vis-NIR region which can be attributed to 4f-4f transition of Er3+ from the ground state 4I15/2 to different excited states. Twelve distinct bands can be labelled corresponding to ground state (4I15/2) absorption to the 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F5/2, 4F3/2, (2G, 4F, 2H)9/2, 4G11/2 and 4G9/2 upper levels centered at 1532, 976, 793, 651, 545, 521, 488, 451, 443, 407, 378 and 365 nm, respectively by comparing the previously reported Er3+ energy bands [14, 15]. The profile and peak positions remain unchanged, with only a slight variation in the intensity of the bands in accordance with chemical composition and the transitions are irrespective of the chemical composition. 6
The optical band gap energy (
) is an important parameter for describing solid-state materials.
The optical absorption coefficient,
for TPBKZFEr05 glass was calculated using the following
is the absorbance and is the optical path length.
From Mott and Davis model, the quantity,
can be expressed by the relation 
can be equal to 3, 2, 1/2 and 3/2 depending on the nature of the electronic transitions leading
to light absorption. Absorption spectra have been used to analyze the optical transition and electronic band structure. The measured absorption fits well to the Eq. (13) for direct allowed transitions, for TPBKZFEr05 glass.
Inset of Fig. 3(a) shows the variation of α
versus photon energy for
TPBKZFEr05 glass in which a considerable part of the curve is observed to be linear. The value of optical band gap ( 3.3.2
) was evaluated from the extrapolation of this curve and is obtained as 3.32 eV.
Nephelauxetic ratio and bonding parameter In order to find the nature of Er3+- ligand bond in the glass, nephelauxetic ratio and bonding
parameter have been evaluated. The nephelauxetic ratio ( ) can be calculated by  (14) Here,
refer the energies (cm-1) for a particular transition in the host under investigation and the
aqua-ion respectively . The bonding parameter ( ) has been evaluated using the equation 100 where
is the average value of nephelauxetic ratios for all observed transitions in the absorption
spectrum and is given by
refers to the number of aquo-ion levels used to compute
values. The nephelauxetic ratio ( ) and bonding parameter ( ) were calculated and presented inTable 2, for the TPBKZFEr05 glass under study. The RE3+- ligand bond may be covalent or ionic depending on the positive or negative sign obtained for the bonding parameter. The negative value of 3+
indicates that the Er ion-ligand bond for the present glass is ionic in nature.
Judd-Ofelt (JO)intensity analysis The radiative transition within the 4fn configuration of a rare earth ion can be analysed by the
widely accepted JO theory. From the measured absorption spectrum, experimental oscillator strengths (
) of the transitions were calculated by measuring integrated absorption area of each band using the
following equation [19, 20], 4.32 where
(16) (cm-1) of the
is the molar absorptivity for the transition corresponding to the energy
transition. According to JO theory, the oscillator strength of an absorption transition from the initial state / ,
to the final state /
6 and is given by the
equation [21, 22] ∑ where
is the refractive index of the glass,
denotes the mass of the electron,
(17) is the wavenumber of
is the doubly reduced matrix elements of the unit tensor operator of rank
the transition and
6, which are independent of the ligand species surrounding the RE3+ ions but depend on
the concerned RE3+ ion. Carnall et. al have already calculated the values of the squares of the reduced matrix elements for the transitions of RE3+ ions .
For the present study also, the same matrix
elements were employed for the calculation. The three intensity parameters
contributions from the crystalline electric field, the interconfigurational radial integrals and so on. The parameters, which provide information on the efficiency and performance of the luminescent material, have been calculated by the least square fit method by applying JO theory to the experimental oscillator strengths (
. The quality of the fit is generally described by the root mean square deviation of the
observed and calculated oscillator strengths ( ), which has been evaluated from the results obtained for the experimental and calculated oscillator strengths (
) based on the following relationship:
represents the total number of experimentally observed energy levels included in the least
square fit. The experimental and calculated oscillator strengths of each transition of TPBKZFEr05 glass under investigation were found and is presented in Table 3. The value of root mean square (rms) deviation
is found to be 0.394×10-6, which implies that the error in this calculation is comparatively
lower and the calculation process is reliable. The local structure and bonding in the vicinity of RE3+ ions can be assumed, according to the value of JO intensity parameters. The JO intensity parameters ( 8
4 and 6) derived for 0.5 mol % Er3+ -doped TPBKZFEr05 glass from the least square fitting of the experimental and calculated oscillator strengths are summarized in Table 4 along with other Er3+ -doped tellurium based glasses reported by researchers [7, 9, 23-26]. It is observed from Table 4 that, the JO intensity parameters follow the trend as around Er3+ions. The
value confirms the lower site symmetry
value obtained for the present glass is found to be higher compared with the
earlier reported tellurite glasses mentioned. The spectroscopic quality factor /
, which is defined as the
has due importance in predicting the nature of various lasing actions in the matrix.
Lower the value of
, higher is the laser emission intensity . So, a low
is desirable for using the
glass in laser applications. The currently investigated glass system has a small value of
other systems already reported, which is also mentioned in Table 4 to compare the lasing action among these glasses. As the interaction between the Er3+and oxygen ions increases, the gives an indication of covalent character of bonding, whereas
value also increases and
related to the rigidity of the host . The covalent surrounding of the glass network has a significant contribution towards the high value of Judd-Ofelt intensity parameters. The RE3+ ions in oxide and chalcogenide glasses have greater values of Here, in the tellurite glass also
parameter compared to crystals and fluoride glasses .
parameter is obtained to be larger and it can be attributed to the fact
that chalcogenide glasses have highest covalency among other host glasses . The low magnitude of in the present glass system establishes lesser rigidity of the glass matrix. The higher value of indicates, the covalent nature and this result is incompatible with the results obtained from the bonding parameter calculation. These types of contrary results were reported earlier too [25, 30, 31]. 3.4
Radiative properties The JO parameters obtained from the absorption spectra can be used to find the radiative
transition probabilities, branching ratio and lifetime of radiative transitions. The radiative transition probability for a transition from an excited state and magnetic , where
to a lower state
′, ′ ′ is the sum of electric
dipole transition probabilities and is given by :
′ ′ ′
are the electric and magnetic dipole contributions, respectively.
are the line strengths of the electric and magnetic-dipole transitions
respectively, and are given by the expressions ∑
′, ′ ′
′, ′ ′
The magnetic line strength is independent of the host matrix, but depends on the RE3+ ion only. The 2 ⁄9 and
are the local field corrections for electric and magnetic-dipole transitions,
respectively adopted in Eq. (19). The dominant processes in crystals and glasses doped with lanthanide (Ln3+) ions are the electric dipole transitions . And also, magnetic dipole transitions which obey the 1,
0, play a substantial role only in a few transitions of Ln3+ ions.
The values of the Judd-Ofelt parameters of tellurite glasses are usually found to be larger, and then the values are also comparatively greater, which in turn results in higher spontaneous emission ).
The fluorescence level relaxation involves transitions to their lower levels; the total radiative transition probability
can be defined as  ∑
′ ′ ′
The predicted radiative lifetime (τ ) of an emitting state is related with the total spontaneous emission probability for all transitions from the state and is given by τ
and the branching ratio,
for a transition from the level
to the lower lying
′ ′ level is given by ,
The calculated electric dipole line strengths of different absorption bands obtained are collected in Table 5. Employing
values and some phenomenological parameters, the important radiative properties such
as, spontaneous transition probabilities, branching ratio and radiative lifetime of the transition were calculated by Eqs. (19)- (24), in particular from 2I11/2 ,4S3/2 and 2H11/2 to the lower levels of Er3+ ions in the tellurite glass. These predicted values, with the Judd-Ofelt model were listed in Table 5. High radiative probabilities, characteristic to the eletctric dipole transitions are obtained for the 4I11/2 , 4S3/2 and 2
H11/2 manifold to the 4I15/2 ground state. The branching ratios for 4I11/2→4I15/2 , 4S3/2→ 4I15/2 and 2H11/2→
I15/2 transitions are 86.2, 67.7 and 95.8 % respectively. The luminescence branching ratio is an
important factor which characterises the prospect of accomplishing stimulated emission from any certain transition. Thus, it is evident that under suitable excitation conditions, it is possible to get efficient green emission from the TPBKZFEr05 glass. The predicted total spontaneous emission probability for 4I11/2 state is comparable to the previously reported gallium tellurite glasses . Since the spontaneous emission probability depends intensively on the refractive index, the large predicted spontaneous emission probabilities of 4S3/2 and 2H11/2 levels are quite reasonable in this case. This value is very 10
beneficial for obtaining intense green emission. It is found that the radiative lifetime of 4I11/2 (2480 μs) is very large compared to 4S3/2 (269 μs) and 2H11/2 (59 μs) emitting levels. The lifetime of the 4I11/2 level of Er3+ ion in multicomponent tellurite glasses has been reported earlier and the present value is greater than boro-tellurire glasses  and comparable to TZNP glassy materials  and it is beneficial to the efficient Excited State Absorption (ESA) process. And also the predicted radiative lifetime of 2H11/2 and 4
S3/2 levels are comparable with the previous studies on erbium doped glasses [38, 39].
Er3+Luminescence properties and Intensity analysis Fig. 4 shows the photoluminescence (PL) spectra relative to the 4I11/2 → 4I15/2 transition at the
wavelength 976 nm of all doped glasses having different Er3+ ion concentration at an excitation of 520 nm. The inset of Fig. 4 gives the dependence of infrared luminescence as a function of the erbium concentration. Fig. 5 depicts the high resolution (0.2 nm) luminescence spectra of all the Er3+-doped borophospho-tellurite glasses recorded at 378 nm excitation. The obtained peaks can be identified as Er3+ transitions and marked in the corresponding figure. There are no shifts in the emission maxima noticed with the increase of erbium content. The inhomogeneous broadening of the emission bands was caused by the site dependent variations in the local ligand field. The two emission peaks located at wavelengths 529 and 543 nm belong to the transitions 2H11/2 →4I15/2 and 4S3/2 → 4I15/2 of Er3+ ions, respectively. The emission band at 543 nm is prominent in all doped glasses in the present study. The appearance of shoulders in addition to Stark splitting points out that there must be multi-site incorporation of Er3+ ions . The frequency upconversion was studied for the Er3+ ions in multicomponent tellurite glasses and upconversion fluorescence color of near pure-green is observed under the excitation wavelength of 976 nm. The upconverted spectra, measured under similar conditions, for different concentrations of Er3+ ions were analyzed and is plotted in Fig. 6. As observed, the spectra exhibit intense green band at around 543 nm with a weak green band at around 529 nm, identified as 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 transitions, respectively of Er3+ ions. It is clear from the result that, the erbium content variation will contribute to the whole upconverted spectra. Fig. 7 shows variation of the PL intensity as a function of Er3+ concentration (ions/cc) relative to photoluminescence emission at the excitation wavelengths 378 and 976 nm. In these two cases, there exists quenching behaviour beyond a critical Er2O3 concentration of 0.5 mol%.
At 1.0 mol% concentration of Er2O3, quenching is observed in both up and down
conversion fluorescence intensity. At a higher concentration, RE3+ ions became close enough and the interaction between RE3+ increases, the cross relaxation process occurs followed by a non radiative decay due to the phenomenon of self-quenching. As the dopant content increases, the erbium ion population at the excited level increases and the fluorescence intensity is increased due to spontaneous radiation transition. At the same time, the emission was weakened due to the quenching effect above 0.5 mol% of Er3+ ions. The Er3+ions from the excited state relax via multiphonon relaxations. The same luminescence quenching effect was found in 4I11/2 → 4I15/2 transition (Inset of Fig.4).
A simplified energy level diagram for the Er3+ ions with the indication of excitation pathways and the PL transitions are depicted in Fig. 8. The possible upconversion mechanism for the green emission upon excitation at 976 nm is also portrayed in the simplified energy level diagram (Fig. 8). The lifetime of 4I11/2 level is estimated to be 2480 μs (Table 5). This comparatively larger lifetime makes the ESA possible from the 4I11/2 to the 4F7/2 level. The incident pump photons at 976 nm, due to multiphoton absorption is promoted to 4F7/2 level and this level is also populated, but its contribution depends on the concentration of Er3+ ions. The excited Er3+ ions nonradiatively decayed to 2H11/2 and 4S3/2 levels and from there the ion relaxes radiatively to the ground level generating the recorded emission signals shown in Fig. 6 (upconversion emission spectra). Cross relaxation (CR) energy transfer (nonradiative) process grounds for the discrepancy in the emission intensity. So, it can be said that the upconversion process occurs because there are phonon assisted intermediate steps. By the color coordinates of the chromaticity diagram, emitted color of the glass can be represented. The emission from the material was used to determine the color points in analogy with those calculated by human eye detection spectrum of the primary color using the 1931 CIE (Commission International de l’Eclairage, France) system. The CIE chromaticity coordinates of the TPBKZFEr05 glass (the best doping concentration, 0.5 mol%) were calculated and represented in Fig. 9, in order to evaluate glass performance on color luminescence. For 378 nm excitation the (x, y) values of CIE chromaticity coordinates were obtained as (0.262, 0.723) and with 976 nm excitation it was found to be (0.225, 0.752), respectively and this result is represented in Fig. 9. The color purity is a measure of the saturation of a color defined as the ratio of the space separation between white point (0.333, 0.333) and a given coordinate (x, y) and the distance from white point to shaped locus intersection . Purity of color for the present glass is found to be 97.7 and 98.5 % respectively for down and upconversion. The stimulated emission cross-section of a transition is found from the emission spectra and it is expressed as : ,
is the peak wavelength of emission band and
is the effective linewidth and
is the light
velocity. The value of stimulated emission cross-section indicates the rate of energy extraction from the lasing material. The figure-of merit (FOM) for bandwidth is defined by the effective width of the emission peak and the stimulated emission cross-section and is given as follows (26) The various observed emissions with stimulated emission cross-section and wavelengths 378, 520 and 976 nm are listed in Table 6. The 12
for different excitation
(х10 cm ) for the TPBKZFEr05 glass
for the 543 nm green emission is found to be 4.338, which is more than for TZLFEr, sodalime silicate and NaTFP glasses . This large value of
is necessary for continous wave laser applications with
low threshold and high gain . 3.6
Decay curve analysis The decay curves of the intense emission transition (4S3/2 → 4I15/2) of Er3+ ions located near 543
nm using an excitation pulse at 378 nm in borophospho-tellurite glasses have been measured and shown in Fig. 10 and the lifetime variation with Er3+ ion concentration is given as inset. From which it is evident that for 0.05 and 0.1 mol% doped glasses, the curves could be described by a single exponential function and is the indication of the absence of energy transfer among the Er3+ ions. Whereas the decay curves of the other Er3+ doped glasses at higher concentration levels, the emission decay curves did not follow a single exponential function, but has a non-exponential behavior because of the cross relaxation through multipolar interactions between the Er3+ ions where, part of the energy of an ion in the 4S3/2 level is transferred to another ion in the ground state . In single exponential decay, the experimental lifetime
) was estimated by fitting the following
function to the decay curve. /
where ‘ ’ denotes the time after excitation. While for the non exponential decays, the lifetime
evaluated using the expression , (28) where
is the intensity of emission at time ‘ ’
Figure-of merit for amplifier gain is given by  (29)
The non-radiative decay rate can be expressed as 
The luminescence quantum efficiency ( ) is defined as the ratio of the number of photons emitted to the number of photons absorbed. For a luminescence level in the case of RE3+ ions, it is equal to the ratio of measured experimental lifetime to the predicted radiative lifetime obtained from the JO theory 13
As the concentration of the Er3+ doped glasses increased from 0.05 to 1.0 mol%, the lifetime values were obtained as 11.36, 10.42, 9.87, 8.98 and 6.52 μs (inset of Fig. 10). The lifetime values obtained for the present glasses are comparable with the previously reported Er3+ -doped systems . For the green transition (4S3/2 → 4Il5/2), figure of merit (FOM) gain bandwidth of TPBKZFEr05 glass were obtained for 378 and 976 nm excitations and are 3.90 and 14.41 (x10-26 cm2s), respectively. The calculated value of the non-radiative decay rate (
) and the quantum efficiency
of 4S3/2 level in TPBKZFEr05 glass
are found to be 0.108 s-1 and 3.34 %, respectively. This quantum efficiency value of the 4S3/2 level is comparable with that of the other erbium doped oxyfluorotellurite glasses reported earlier . The explanation for the lower efficiency of the 4S3/2 level in that of oxyfluorotellurite glass was quite reasonable and is also applicable for the present glass system too. The comparatively small value of the efficiency may be due to non-radiative relaxation or due to multiphonon relaxation.
interaction between the closely spaced 4S3/2 and 2H11/2 levels can also cause reduction in the quantum efficiency. 3.7
Absorption cross-section and McCumber spectral evaluation at 1.53μm band The ability to absorb and emit light was quantified by the factor of cross-section. Absorption
cross-section was determined from the absorption spectrum.
From the experimentally measured
absorption coefficient and the Er concentration ( ) in the glass, the absorption cross-section was calculated using the following equation  .
: is the thickness of the glass,
(32) is the concentration of the Er3+ ions and
is the optical
absorbance at the wavenumber . Fig. 11 depicts the typical absorption cross-section for the 4I15/2→4I13/2 transition of the TPBKZFEr05 glass and the stimulated emission cross-section at 1.53 μm band was calculated by McCumber theory. This particular band is very important for Er3+-doped fiber amplifier operating at 1.5 μm. From the previous reports, it is evident that the emission bandwidth of Er3+at 1.5 μm band is very much dependent on the host glass . It is obtained from the predicted radiative properties through JO theory, that the values of transition probability and radiative lifetime of 4I13/2 level of TPBKZFEr05 glass are 308.34 s-1 and 3243 μs, respectively. Inset of Fig. 11 shows the possible energy level transitions of TPBKZFEr05 glass in the NIR region. The stimulated emission cross-section ions calculated by measured absorption cross-section
of the 4I13/2→4I15/2 transition of Er3+
by the following relation 
is the photon frequency,
is the Plancks constant, 3+
is the Boltzman’s constant and
represents the free energy needed to excite one Er ion from I15/2 ground state to 4I13/2 state at RT which is found by the method proposed by Miniscalco et. al . The integrated values of the absorption crosssection was found to be
231x10-21cm2, which is higher than that of the Er3+ -doped Li2O-2SiO2
glass . The peak absorption cross-section
(x10-21cm2) and peak emission cross-section
cm2) for the analysed glass are obtained as 4.42 and 4.45, respectively. The integrated emission cross-
section is also found to be
4. Conclusions Physical properties and refractive indices of all the investigated glasses were found and presented. XRD measurements have confirmed the amorphous nature of the prepared glasses. The crystallization behaviour and thermal properties of the multicomponent tellurite glass have been studied using DSC. In conclusion, the optical transitions of Er3+ in tellurite glasses are investigated and it is clear that the optical properties are sensitive to erbium ion concentration and composition of glasses and it alters and modifies the physical properties too. From the integrated absorption spectrum, the nature of Judd-Ofelt intensity parameters for f-f transitions are obtained as
and are used to calculate the radiative
properties of fluorescence levels of Er ions in TPBKZFEr05 glass. The effect of erbium doping on the up and down conversion fluorescence process was examined. It has been found through the present work that, the optimal doping content of Er3+ ion was 0.5 mol%. From the results of infrared to visible upconversion, we emphasize that the upconversion pumping mechanism was accomplished with the involvement of two pump photons at 976 nm for populating the excited state emitting level of Er3+ active ions. Effective visible upconversion of Er3+ ions points out the fact that multicomponent tellurite glass is a promising laser host. The CIE chromaticity diagram indicates that the green emission exists in the TPBKZFEr05 glass. The present glass system is useful for developing visible lasers, since the stimulated emission cross-section has large values in the visible region. By making use of McCumber theory, we found the absorption and emission cross-section in the 1.53 μm region of Er3+ in the host matrix.
Acknowledgments This research work has been supported by the Council of Scientific and Industrial Research through project No.03 (1241)/12/EMR II dated 16/4/2012. The authors are also thankful to UGC (Govt. of India) and DST (Govt. of India) for the financial assistance through SAP-DRS and DST-PURSE programs, respectively. The authors are also thankful to Dr. Gin Jose (Institute for Materials Research, University of Leeds, UK) for lifetime measurements of the glasses. Two of the authors, Sunil Thomas
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Table 1 Glass Labels, composition in mol % and physical properties of the Er3+ doped borophospho-tellurite glasses
60TeO2,10P2O5, 10K2O,10B2O3, 10ZnF2
59.95TeO2,10P2O5, 10K2O,10B2O3, 10ZnF2,0.05Er2O3
59.9TeO2,10P2O5, 10K2O, 10B2O3, 10ZnF2,0.1Er2O3
59.7TeO2,10P2O5, 10K2O, 10B2O3, 10ZnF2,0.3Er2O3
59.5TeO2,10P2O5, 10K2O, 10B2O3, 10ZnF2,0.5Er2O3
59TeO2,10P2O5, 10K2O, 10B2O3, 10ZnF2,1Er2O3
Optical path length (mm) Refractive Index Concentration of Er3+ (×1020 ions/cc) Polaron radius (nm) Electronic polarizability (×10-24) ⁄ (×10-25) Molar refractivity (cm3) Dielectric constant Electric susceptibility Interionic distance (nm) Donoracceptor distance (×10-8 cm) Field strength (×1015 cm2 )
Reflection Loss 9.72 9.72 9.73 9.78 (%) Random error: Density = ± 2.2%, optical path length = ± 1.3%, refractive index = ± 0.06%
Table 2 Energies for Er3+ ions in TPBKZFEr05 glass (ν c ) and aquo-ion (ν a )
SI S’L’J’ 4 No. I15/2 → (cm-1) (cm-1) 4 1 I13/2 6527 6600 0.9889 4 2 I11/2 10246 10250 0.9996 4 3 I9/2 12610 12400 1.0169 4 4 F9/2 15361 15250 1.0073 4 5 S3/2 18349 18350 0.9999 2 6 H11/2 19194 19150 1.0023 4 7 F7/2 20492 20450 1.0021 4 8 F5/2 22173 22100 1.0033 4 9 F3/2 22573 22500 1.0032 10 (2G,4F,2H)9/2 24570 24550 1.0008 4 11 G11/2 26455 26400 1.0021 4 12 G9/2 27397 27400 0.9999 = 1.0022 = -0.2195 (ionic bonding) Systematic error: absorbance = ± 0.0001 wavenumber = ±5 cm−1 (UV/Vis), ±19 cm−1 (NIR)
Table 3 Experimental and calculated oscillator strengths for Er3+ -doped TPBKZFEr05 glass
SI No. 1 2 3 4 5 6 7 8 9 10 11 12
S’L’J’ I15/2 → 4 G9/2 4 G11/2 (2G,4F,2H)9/2 4 F3/2 4 F5/2 4 F7/2 2 H11/2 4 S3/2 4 F9/2 4 I9/2 4 I11/2 4 I13/2 4
(cm-1) 27397 26455 24570 22573 22173 20492 19194 18349 15361 12610 10246 6527
(cm-1) 27478 26496 24505 22422 22074 20422 19256 18462 15245 12378 10219 6610
(cm-1) -81 -41 65 151 99 70 -62 -113 116 232 27 -83
matrix elements 0.0000 0.9183 0.0000 0.0000 0.0000 0.0000 0.7125 0.0000 0.0000 0.0000 0.0282 0.0195
0.2416 0.5262 0.0189 0.0000 0.0000 0.1469 0.4125 0.0000 0.5354 0.1733 0.0003 0.1173
0.1235 0.1172 0.2256 0.1272 0.2232 0.6266 0.0925 0.2211 0.4618 0.0099 0.3953 1.4316
(×10-6) 1.889 23.991 0.571 0.118 0.429 2.114 14.224 0.555 2.404 0.024 0.761 2.118
(×10-6) 1.469 24.458 1.081 0.529 0.912 2.750 13.784 0.748 2.358 0.302 1.010 1.935
(×10-6) 0.420 -0.467 -0.510 -0.411 -0.483 -0.636 0.440 -0.193 0.046 -0.278 -0.249 0.183
Table 4 Judd-Ofelt parameters,
(×10-20cm2) and spectroscopic quality factor ( Q ) for 0.5 mol% Er3+ doped TPBKZF
glass and for some reported Er3+: tellurite-based glasses
TPBKZFEr05 75TeO2-14ZnO-10Na2O-1Er2O3 Soda-lime silicate Aluminosilicate 80TeO2-20BaO(BT) 75TeO2–20ZnO–4Na2CO3–1Er2O3(TZNE1) 30B2O3-10ZnO-59.5TeO2-0.5Er2O3 (BTZ) 70TeO2–25ZnO–5La2O3–0.01Er2O3 (TZL)
7.23 5.98 4.32 5.46 5.88 4.93 5.73 5.34
1.02 1.32 0.83 1.61 1.15 1.30 2.01 1.75
1.46 1.47 0.41 0.85 1.56 1.31 2.37 0.94
0.699 0.890 2.024 1.894 0.737 0.990 0.850 1.861
Present work       
Table 5 Calculated radiative transition rates S’L’J ’
Energ y (cm-1)
(s-1), branching ratios
and the radiative life time
(μs) of Er3+ ions in
0.000 0 0.351 2 0.207 0 0.029 9 0.022 4 0.708 4
0.196 3 0.019 8 0.086 2 0.176 6 0.058 9 0.410 8
0.010 2 0.004 0 0.312 0 0.043 3 0.057 6 0.094 9
0.000 0 0.000 0 0.000 0 0.000 0 0.000 0
0.000 2 0.075 0 0.005 0 0.000 0 0.000 0
0.024 8 0.254 2 0.079 2 0.342 8 0.225 2
0.033 4 0.028 8
0.171 2 0.000 3
0.087 3 0.396 2
(×1022 ) 0.00
21.420 256.56 2 204.11 7
0.00 0.00 0.00
30.610 567.86 6
50.186 32.969 200.71 2 78.861
0.000 77.40 1 0.00
0.09 71.78 255.08 131.25 252.54 16116.4 3 1.43 111.77 79.59 1005.12 2508.93 39.94 347.33
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0 15.8 8 0.00
0.00 0 0.00 4 0.01 5 0.00 8 0.01 5 0.95 8
0.00 0 0.03 0 0.02 2 0.27 1 0.67 7
0.13 8 0.86 2
0.09 71.78 255.08 131.25
252.54 16116.4 3 1.43 111.77 79.59 1005.12 2508.93 55.82 347.33
Table 6 Observed stimulated emission cross-section and gain band width for different transitions for TPBKZFEr05 glass
ΔG (×10-28 cm3)
H11/2 →4I15/2 S3/2 →4I15/2
H11/2 → I15/2 18904 5.398 85.045 S3/2 →4I15/2 18416 4.945 16.044 Systematic error: wavenumber = ± 16 cm-1
Fig. 1. Differential scanning calorimetry trace of TPBKZFEr05 glass
Fig. 2. XRD spectra of TPBKZFEr glasses for different erbium concentrations
Fig. 3. Room temperature absorption spectra of Er3+ ions in TPBKZFEr05 glass in the (a) UV-Vis (Variation of α2 versus hv as inset) and (b) NIR spectral regions (The final states of the 4I15/2→ 2S’+1L’J’ transitions are labelled). 23
Fig. 4. Luminescence spectra of glasses at room temperature under an excitation of 520 nm. The inset figure shows erbium ion concentration vs. luminescence intensity.
Fig. 5. Photoluminescence emission spectra of Er3+ in TPBKZFEr05 glasses (λex= 378 nm). 24
Fig. 6. Frequency upconversion spectra of Er3+ doped tellurite glasses under 976 nm excitation.
Fig. 7. Variation of emission intensity with erbium concentration (ions/cc).
Fig. 8. Simplified energy level diagram of Er3+ ions with the indication of radiative transitions observed and illustrating the possible pathways by solid straight lines with up and down arrows.
Fig. 9. The CIE chromaticity diagram showing the x,y emission colour coordinates for TPBKZFEr05 glass.
Fig. 10. Decay curves of the 4S3/2 emitting level of Er3+ ions in TPBKZFEr glasses. Inset shows the variation of lifetime with Er3+ ion concentration.
Fig. 11. Absorption cross section and the stimulated emission cross section of Er3+ ions at 1.53 μm in the case of TPBKZFEr05 glass. 27
Highlights • • • • •
Multicomponent tellurite glasses were fabricated for laser applications. Spectroscopic parameters were evaluated using Judd–Ofelt theory. Effects of Er3+ concentration on luminescence of the glasses were studied. Efficient green upconversion mechanism was discussed. CIE color coordinates of the glass was examined.