Structural and spectroscopic properties of γ-ray irradiated Er3+-doped lead phosphate glasses

Structural and spectroscopic properties of γ-ray irradiated Er3+-doped lead phosphate glasses

Journal of Luminescence 203 (2018) 322–330 Contents lists available at ScienceDirect Journal of Luminescence journal homepage:

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Journal of Luminescence 203 (2018) 322–330

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage:

Structural and spectroscopic properties of γ-ray irradiated Er3+-doped lead phosphate glasses T. Maheswaria, Ch. Basavapoornimaa, K. Lingannab,c, S. Jub, W.-T. Hanb, C.K. Jayasankara,



Department of Physics, Sri Venkateswara University, Tirupati 517 502, India School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology, 123 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, Republic of Korea c Optical Lens Research Center, Korea Photonics Technology institute, Cheomdan venchure-ro, 108 beon-gil 9, Buk-gu, Gwangju 61007, South Korea b



Keywords: Er3+:Phosphate based lead glasses γ-ray irradiation, Judd-Ofelt parameters Optical gain band width, Upconversion luminescence

Structural and spectroscopic properties of varying concentrations of Er3+ ion in P2O5- PbO-K2O-Al2O3-Na2O (PPbKANEr) glasses were studied before and after consecutive γ-ray irradiation. FTIR spectra of all the contemplated glasses confirm the presence of characteristic vibrational bands mainly because of phosphate groups. The optical absorption spectra of Er2O3-doped glasses are more stable and do not exhibit any variations after γ-ray irradiation particularly when Er3+ ions are present in higher concentrations. Judd-Ofelt (JO) analysis was carried out for γ-ray irradiated glasses and the resultant three phenomenological intensity parameters were found to be Ω2 = 1.98, Ω4 = 0.78 and Ω6 = 1.24 (×10–20 cm2). A bright emission corresponding to 4 S3/2 → 4I15/2 transition in the green region was observed with the excitation of 379 nm. Broad luminescence peak at 1535 nm (4I13/2 → 4I15/2) are observed in the region of 1400–1700 nm with the excitation of 980 nm. The important spectroscopic properties such as absorption (σabs(λ)) and emission cross-sections (σemi(λ)), gain band width (FWHM × σemi(λ)) for 4I13/2 → 4I15/2 transition at 1.53 µm were assessed for γ-ray irradiated PPbKANEr1.0 glass and the values found to be 6.73 × 10–21 cm2, 7.33 × 10–21 cm2 and 264 × 10–28 (cm3), respectively. The decay curves for the 4I13/2 → 4I15/2 transition of the γ-ray irradiated PPbKANEr glasses exhibit exponential nature and the lifetimes found to be decreased (3.72–0.62 ms) with the increasing Er2O3 concentration (0.05–4.0 mol%). The upconversion luminescence spectra exhibit two emissions at 525 nm (2H11/2 → 4I15/2) and 543 nm (4S3/2 → 4I15/2) in the green region along with a fairly feeble red emission at around 650 nm (4F9/2 → 4I15/2).

1. Introduction As of now, lanthanide (Ln3+)-doped glasses are broadly examined due to their exceptional spectroscopic properties and photonic applications as luminescence upconversion, laser amplification and emission, solid state lasers and optical memory devices [1–4]. Among various host matrices, glasses are one of the promising materials as a host environment for Ln3+ ions due to their unique properties like inhomogeneously broadened linewidths, simplistic compositional variation, easy mass production, flexibility of shape and size and high transparency [4]. To indentify new optical devices for particular utility or devices with improved performance, an appropriate host must be identified. Phosphate glasses has an extensive variety of applications in many fields which includes detection, laser technology and optical data transmission due to their numerous properties such as low melting temperatures, high gain density, low optical dispersion and high

Corresponding author. E-mail address: [email protected] (C.K. Jayasankar). Received 7 February 2018; Received in revised form 8 June 2018; Accepted 15 June 2018

Available online 18 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.

transparency [5–8]. In general, the doping of Ln3+ ions into the phosphate glass is aimed to improve the physico-chemical properties like chemical resistance against atmospheric moisture [9]. Based on these properties, phosphate glasses are better than other oxide based glasses like silicates, borates and tellurites. However, the properties such as high hygroscopic, poor chemical durability, and volatile nature not permitted them from substituting the conventional glasses in a wide variety of applications [10]. With the addition of modifiers like PbO, Al2O3, Na2O, TeO2, B2O3, etc., some of the properties can be improved and makes competitive to other glass systems. In view of the available literature/reported data, the spectroscopic investigations of Ln3+ ions in lead phosphate glasses are less renowned. The PbO-P2O5 glasses containing Ln3+ ions are utilized for the fabrication of optical fibers which have a great importance in optical data communication, in the field of opto-electronics, sensing, detection, etc. By considering the thermal and optical parameters obtained for

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2. Experimental details

lanthanides, it can be confirmed that PbO-P2O5 glasses show interesting thermo-optical and non-linear properties. Er3+ ion is one of the well known ion and plays an important role in communication with the transmission in the infrared region around 1550 nm (4I13/2 → 4I15/2) and 3000 nm (4I11/2 → 4I13/2) in medical applications as well as in the visible region around 550 nm (4S3/2 → 4 I15/2) [11] for optical storage and green emission applications. The 4 I13/2 → 4I15/2 transition of Er3+ ion has been useful in erbium doped fiber amplifiers which is applied in wavelength division multiplexing transmission systems. In order to develop the most proficient optical amplifiers with broad flat gain profiles, a number of active host materials doped with the Er3+ ion have been investigated [12–15]. The host matrices strongly influence the luminescence properties of Er3+ ions. Moreover, the optical and luminescence properties of the Er3+ ion in various systems are well documented by various research groups but the structural and spectroscopic properties of the γ-ray irradiated Er3+ ions are less treatises. The γ-ray irradiation and other ionizing radiations with glasses are expected to induce different types of defects. Radiation energy higher than the band gap of the material potentially ionizes the resulting positive holes which are captured by the already present intrinsic defects. The induced defects due to irradiation are expected to depend on the composition of the glass, irradiation dose, type of the irradiation source and possible formation of electron/hole pairs. Radiation durability, as extensively recognized one represents an outstanding condition for the photonic devices mainly for those applied in space or in extreme environment. The defects induced by irradiation usually absorb visible light and therefore exacerbate the optical properties of the materials and influence their radiation efficiency and sensitivity. During the last two decades, there has been an increasing interest and several research groups are working in the field of radiation dosimetry and study the spectroscopic investigations on γ-ray or other ionizing radiations irradiated RE/transition metal ion doped glasses for photonic applications [16–27]. Sudhakar et al. [16] highlighted the influence of oxide modifier on spectroscopic and thermo luminescence characteristics of Sm3+ ion in antimony borate glass system, ElBatal et al. [17] reported the effect of γ-ray irradiation on NdF3-doped borophosphate glasses. Sharma et al. [18] presented spectroscopic investigation on γ-ray irradiated Eu3+ and Dy3+-doped oxyfluoride glasses. Polina Ebeling and Doris Ehrt [19] studied the radiation-induced defects in fluoride-phosphate glasses by means of optical absorption and EPR spectroscopy. Influence of induced structural changes on thermo luminescence characteristics of γ-ray irradiated PbO-Al2O3SiO2-Dy3+ glasses are described by Sundara Rao et al. [20]. Hari Babu et al. [21] studied the warm white light generation in γ-irradiated Dy3+, Eu3+ co-doped sodium aluminoborate glasses. Temperature effect during the γ-irradiation on CaSO4: Dy3+ was explained by Hernandez-Medina et al. [22]. The effect of gamma irradiation on some electrical properties and optical band gap of bulk Se92Sn8 chalocogenide glass was studied by Al-Ewaisi et al. [23]. Balboul [24] studied the optical effects induced by γ-ray and UV irradiation in chalcogenide glass. Fatma H. Elbatal et al. [25] described the UV–Visible, Raman and ESR studies of γ-ray irriadiated NiO-doped sodium metaphosphate glasses. Effect of γ-ray irradiation on optical properties of Nd-doped phosphate glass is discussed by Rai et al. [26]. Pukhkaya et al. [27] studied the impact of rare earth element clusters on the excited state lifetime evolution under irradiation in oxide glasses. Rui-xian Xing et al. [28] investigated the radiation resistance of Er/Ce co-doped silicate glasses under 5 kGy γ-ray irradiation. Marzouk [29] highlighted the optical characterization of some rare earth ions doped bismuth borate glasses and effect of γ-ray irradiation. In sight of above significance/facts, the present study focuses attention on the effect of γ-ray irradiation on optical and luminescence as well as pump power dependent upconversion properties of the Er3+doped P2O5-PbO -K2O-Al2O3-Na2O glasses.

Phosphate based glasses with molar composition of 44 P2O5 + (24-x) PbO + 17 K2O+ 9 Al2O3 + 6Na2O+xEr2O3, where x = 0.05, 0.1, 0.5, 1.0, 2.0 and 4.0 mol% referred as PPbKANEr0.05, PPbKANEr0.1, PPbKANEr0.5, PPbKANEr1.0, PPbKANEr2.0 and PPbKANEr4.0, respectively, were prepared by a melt quenching technique. The details of the preparation and spectroscopic measurements of the PPbKANEr glasses are explained in the ref. [30]. The PPbKANEr glasses were irradiated with a total dose of 48.56 kGy using γ-rays from a Co60 radiation source (MSD Nordion, pencil type/C-188 sealed). Structure of the PPbKANEr glasses before and after γ-ray irradiation were characterized by X-ray diffraction (XRD) pattern using a XPERTPRO analytical diffractometer with Cu Kα radiatioion. The Fourier transform infrared spectra of the present PPbKANEr glasses were recorded with JASCO FTIR 460 plus in the 400–4000 cm−1 region with a spectral resolution of ± 1.0 cm−1. 3. Results and discussion X-ray diffraction measurements were carried out to determine the structure of the Er3+-doped PPbKAN glass before and after γ-ray irradiation and the obtained XRD patterns are displayed in Fig. 1. As can be seen from the Fig. 1, the glass samples reveal the similar XRD patterns and do not have any discrete diffraction peaks, but having broad humps between 20° to 40°. It confirmed that the glass samples after γ-ray irradiation also maintain the amorphous nature. The Fourier transform infrared (FTIR) spectrum of the PPbKAN glass is having vibrational bands within the region of 450–4050 cm−1 are presented in Fig. 2 and the inset of Fig. 2 represents the FTIR spectrum in shortest wavenumber range (350–600 cm−1). The FTIR spectra of all glasses before and after γ-ray irradiation reveal that there were no absolute changes in their number of vibrational mode positions. The observed change is neither the increase nor prominence of the vibrational bands. The stability of such glasses towards γ-ray irradiation is due to the presence of heavy metal cations and also to the ability of Pb2+ ions to participate in the network structure by forming additional PbO4 or PbO3 structural groups. This comparative study indicates that the phosphate glasses containing PbO is the most efficient glass candidate for shielding effect towards successive irradiation. The evaluation and assessment of the IR spectra before and after γ-ray irradiation can be summarized as follows [29,31,32]. The peak at 364 cm−1 is attributed to skeletal deformation vibration of phosphate chain and PO3 deformation vibrations of pyrophosphate segments. The band noticed around 382 cm−1 is due to Pb-O bonds of PbO4 and PbO3 units. The band at 392 cm−1 is consigned to the

Fig. 1. XRD patterns of the Er3+-doped lead phosphate glasses before and after γ-ray irradiation. 323

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significantly shifted to shorter wavelengths (∼ 310 nm) in comparison to lead borate system (∼ 400 nm) belonging to the same heavy metal glass family as reported by Pisarski et al. [36]. The absorption bands are clearly resolved and are attributed to the electronic transitions from the ground state 4I15/2 to the higher excited levels. The absorption bands corresponding to the 4I15/2 → 2H11/2 transition are called hypersensitive transition which is sensitive to small changes of the glass environment around rare earth ions. From the Fig. 3(a), it was observed that the intensity of the absorption band increases with increasing concentration. The Fig. 3(b) represents the before and after γ-ray irradiated 1.0 mol% Er3+-doped lead phosphate glasses. The induced absorption bands after γ-ray irradiation can be explained as follows. When glasses are irradiated with ionized γ-rays generated electrons and positive holes [37–39]. Glassy materials are known to be amorphous in nature consisting of non-periodic arrangement having actual defects like vacancies, non-bridging oxygens and trace metals present as impurities. These sites are pretentious by γ-ray irradiation and are assumed to bring into being induced defects through some progression including ionization and photochemical reactions. From the Fig. 3(a) and (b), it was observed that, with the successive γray irradiation, the absorption bands remain very similar except small variations in the intensities. It was suggested that the Er3+ ions capture the released pairs of holes and electrons and exhibit no visual induced changes with γ-ray irradiation. This indicates that the PPbKANEr glasses show obvious resistance to the effect of γ-ray irradiation. To study the effect of irradiation and the addition of Er3+ ions on the energy levels involved in the optical transitions, optical band gap (Eg) was calculated and shown in Fig. 4 as well as the values are reported in Table 1. The optical band gap could be tailored by the configuration of color center defects that lead to the introduction of new energy levels between the original bands. The irradiated samples depict a relatively comparable Eg value (3.75 eV) in comparison to their unirradiated counterparts (3.71 eV) as an evidence of the slight modifications induced in the energy levels due to γ-ray irradiation. The important feature of the absorption edge of amorphous materials is the exponential increase in the absorption coefficient with photon energy [40] and the absorption edge here is known as the Urbach energy. The Urbach energy and the exponential absorption tails are associated with the relation [41]

Fig. 2. Fourier transform infrared spectrum of the Er3+-doped lead phosphate glasses before and after γ-ray irradiation. The inset of the Fig. 2 represents the shorter wavenumber range.

symmetric stretching of the P-O bonds and P-O-P bending modes of the orthophosphate PO43− units. The vibrational bands around 405–475 cm−1 can be associated to bending vibration of O-P-O and δ(PO2) vibrations and some distribution of Pb-O vibrations. In the case of γ-ray irradiation samples, the band from 405 to 475 cm−1 exhibit several kinks at 428, 437, 449 and 461 cm−1. The observed band at 524 cm−1 is to be shifted to 532 cm−1 with distinct higher intensity and corresponds to harmonics of P-O-P vibration/deformation mode of PO-. The peak at 758 cm−1 found to be shifted to 750 cm−1 with increasing intensity and the band assigned to symmetric stretching vibrations of PO-P to Q2 units. The peak at 914 cm−1 found to be shifted to 901 cm−1 with increasing intensity and the band assigned to asymmetric stretching of P-O-P chains (νas (P-O-P)). The peak at 1124 cm−1 related to asymmetrical and symmetrical stretching of PO32− (Q−1). The band at 1640 cm−1 can be related to bending modes of P-OH. The band at 2920 cm−1 is related to stretching vibration of P-O-H. The main absorption band around 3442 cm−1 can be related to molecular water, OH stretching during the preparation of KBr discs from powdered glassy materials. The limited variations of the intensities or slight shift of band positions can be inferred as follows. The effect of γ-ray irradiation causes breaking of some connecting bonds between the structural building groups and succeed by reforming and concerning some changes in the bond lengths which cause the observed shifting/increasing intensities of some vibrational bands. Hobbs et al. [33] implicated that radiation process originates the generation of the so-called Frenkel pairs which successfully breaks the connectivity of the network. They added that accumulation of such broken linkages evidently results in local structural collapse and stochastic rebinding. Primak [34] has well described the sequences of measures upon the irradiation process leading to the changes in bond lengths and/or bond angles within the building groups. Piao et al. [35] highlighted the mechanism of radiation induced defects by assuming that during irradiation, the ionization process generates free electron/hole pairs, providing lane for bond rearrangement, reducing the constraints on structural relaxation. The relaxation processes release some of the excess energy stored in the structure, accompanied by a decrease of the average building bond angle. Due to the nonexistence of normal periodic structure, the relaxation involves longrange effects and essentially the entire structure participates. Absorption spectra for γ-ray irradiated Er3+-doped lead phosphate glasses in the UV–VIS–NIR region are shown in Fig. 3(a). The UV–VIS cut off wavelength defined as the intersection between the zero base line and the extrapolation of absorption edge which is located in UV range near 310 nm. The absorption edge for lead phosphate glass is

hν ⎞ α (ν ) = α 0 exp ⎛ ⎝ ΔE ⎠


where α 0 is the constant and ΔE is the Urbach energy corresponds to the optical transition between the localized tail states adjacent to the valence band and the conduction band which extend into the band gap. The Urbach energy value was found to be 0.40 (0.33) before (after) γray irradiation. The Urbach energy specifies the disorderness in the material and alters in the values of ΔE are due to the creation of defects in the glasses. These imperfections produce localized states in the glasses causing the reduction in the width of the localized states in the band gap and in turn increase the band gap values. In order to attain additional information about the local structural properties and symmetry around Ln ions as well as to determine the important spectroscopic parameters of Ln3+-doped glasses, Judd–Ofelt (JO) theory [42,43] has been commonly applied. The JO intensity parameters have been evaluated with the help of oscillator strengths of the observed absorption peaks and doubly reduced matrix elements by least square fit. These doubly reduced matrix elements are independent of host matrix. The γ-irradiated glasses yield the JO intensity parameters as Ω2 = 1.98, Ω4 = 1.81 and Ω6 = 1.24 × 10–20 cm2 and follow the trend as Ω2 > Ω4 > Ω6 (see Table 1). Large Ω2 indicates the higher asymmetry and strong covalent bonding between Er3+ and ligands. As mentioned earlier, the intensity parameters (Ω2, Ω4 and Ω6) depend on the local structure in the vicinity of the Ln3+ ion site and the covalency of the metal-ligand bond. If the Ln-doped systems have higher value of Ω2, it indicates strong covalent nature. From the observation of the JO 324

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Fig. 3. (a). γ-ray irradiated concentration dependent optical absorption spectra of Er3+-doped lead phosphate glasses in UV–Vis and NIR regions. (b) optical absorption spectra of 1.0 mol% Er3+-doped PPbKAN glass in UV–Vis and NIR regions before and after γ-ray irradiation.

intensity parameters listed in Table 1, it was found that the covalent nature was reduced after γ-ray irradiation. Overall, the variation in JO intensity parameters in present glasses is attributed to the site-to-site variation giving rise to the modification in asymmetry around erbium ions. The excitation spectra for PPbKANEr1.0 glass is measured by monitoring the emission at 550 nm and is shown in Fig. 5. The excitation spectra cover the spectral region from UV to VIS (300–500 nm). The excitation spectra exhibit several excitation bands at 356 nm (4I15/2 → 2K15/2), 365 nm (4I15/2 → 4G9/2), 379 nm (4I15/2 → 4G11/2), 407 nm (4I15/2 → 2G9/2), 443 nm (4I15/2 → 4F3/2), 451 nm (4I15/2 → 4F5/2) and 488 nm (4I15/2 → 2H11/2). Among these bands, the peak at 379 nm (4I15/ 4 2 → G11/2) is more intense and this peak is used as excitation wavelength for Er3+ visible luminescence. The room temperature luminescence spectra in the visible under the excitation of 379 nm are presented in Fig. 6. Three prominent visible emission bands centered at 525 nm, 550 nm and 660 nm are assigned to 2 H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively, are observed. The luminescence spectra before irradiation were

Table 1 Refractive index (n), Judd-Ofelt parameters (Ωλ= 2,4 and 6, × 10–20 cm2), radiative lifetime (τR, ms) for the 4I13/2 level, bandgap (eV) and Urbach energy (eV) of Er3+ ion in PPbKANEr1.0 glass before and after γ-ray irradiation. n

Before irradiation After irradiation



Band gap

Urbach energy


















comparable with the spectra obtained after irradiation. There was no change in peak positions and shapes with increasing concentration excluding minor change in intensity of the emission peaks. The 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions are splitted into two Stark levels and the magnitude of these Stark splittings are increased with increasing concentration. The luminescence intensity increases with

Fig. 4. Tauc's plots of the (αhν)2 and (αhν)1/2 as a function of hν for the γ-ray irradiation PPbKANEr1.0 glass. 325

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Fig. 5. Excitation spectrum of the γ-ray irradiated PPbKANEr1.0 glass.

Fig. 7. The experimental decays of the luminescence from the level of 4S3/2 → 4 I15/2 of PPbKANEr1.0 glass upon 379 nm excitation.

Fig. 6. Concentration dependent visible emission spectra of γ-ray irradiated PPbKANEr glasses under the excitation of 379 nm.

increasing concentration and reaches the highest value when the concentration of Er3+ is 2.0 mol%, thereafter it decreases for further increase in Er3+ concentration. Quenching of luminescence with increasing Er3+ concentration is a typical property of rare earth doped systems where the distance between the neighboring Er3+ ions decreases with the increase of Er3+ dopant concentration. This decrease in the distance between the ions leads to the cross-relaxation among them and thereby the probability of radiative transition is reduced. Among these three transitions, the luminescence peak at 550 nm corresponding to 4S3/2 → 4I15/2 (green) transition has more intensity and confirms that the present γ-ray irradiated PPbKANEr glasses are useful for green emitting applications. The luminescence decay curve of the 4S3/2 → 4I15/2 level under 379 nm excitation have been measured and are shown in Fig. 7. The decay curve exhibits single exponential for all the concentrations with more or less same lifetime (104 μs, 99 μs, 96 μs, 100 μs, 106 μs and 107 μs). Therefore, the decay curves are presented only for 1.0 mol% Er2O3 doped PPbKANEr glass. The fluorescence spectra of the γ-ray irradiated PPbKANEr glasses under the 980 nm excitation are presented in Fig. 8(a). The near-infrared emission at 1.53 µm observed for all the concentrations is associated to main 4I13/2 → 4I15/2 transition of Er3+ which is demanded for optical amplifiers operating in the third telecommunication window. These broad NIR emissions at 1.53 µm in the range of 1440 ∼1650 nm cover the three bands of S, C and L which are the low loss communication window. From Fig. 8(a), it can be seen that the NIR emission peak position (λp) located at ~1531 nm is shifted from 1531 to

Fig. 8. (a). Concentration dependent NIR emission spectra of γ-ray irradiated Er3+-doped PPbKANEr glasses. The inset of the figure represents the absorbance of the Er3+-doped PPbKANEr glasses. (b). NIR emission spectra of before and after γ-ray irradiated Er3+-doped PPbKANEr1.0 glass.

1571 nm with increasing of Er3+ ion concentration. On the other side, FWHM varies from 28 to 52 nm with increasing of Er2O3 content from 0.05 mol% to 4.0 mol%, respectively. Generally, the linewidth broadening of Er3+ ions is mainly inhomogeneous broadening which is caused by differences in the local ligand field from Er3+ site to site. And also due to other radiation trapping effects in which the emission of the excited Er3+ ion is re-absorbed by the non-excited Er3+ ion, considering the 4I13/2 → 4I15/2 transition and the subsequent emission [44,45]. This phenomenon always occurs in a typical 3-level system where the absorption and emission spectrum overlap with each other. Fig. 8(b) represents the photoluminescence spectra of the PPbKANEr1.0 glass before and after γ-ray irradiation. From the figure, it was observed that there is a significant change in the relative intensity of the emission bands after the γ-ray irradiation. When the glass sample is irradiated it produces secondary electrons from the sites where they are in a stable state, which now have excess energy and also 326

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Fig. 9. The absorption and emission cross-section spectra of 4I13/2 → 4I15/2 transition of Er3+: PPbKANEr1.0 glass.

Fig. 10. The gain cross-section for 4I13/2 → 4I15/2 transition of γ-ray irradiated Er3+-doped PPbKANEr1.0 glass.

non-bridging oxygens (NBOs). The electrons may traverse through the glass lattice depending upon their energy and the composition of the glass network, but they are finally trapped, forming color centers [46,47]. All types of γ-ray irradiated defects (O2-, change in valency of ions and other color centers) overlap and show increase in intensity of the luminescence depending on the dose of irradiation. And also the defects released due to irradiation recombines with previously ionized Er3+ ions to form an excited Er3+ ions that subsequently de-excites. Hence, the photoluminescence intensity increases after γ-ray irradiation in the present work. It is commonly known that the spectroscopic properties of optically active ions are important for many practical applications. According to the measured absorption spectra shown in Fig. 3(a), the absorption (σabs(λ)) and peak stimulated emission cross-sections (σemi(λ)) in the studied glass as a function of wavelength have been calculated using the McCumber theory [48] and are shown in Fig. 9. The σabs(λ) and σemi(λ) of the 4I13/2 → 4I15/2 transition in the studied glasses are found to be 6.73 × 10–21 cm2 and 7.33 × 10–21 cm2, respectively. On the basis of the σabs(λ) and σemi(λ), the wavelength dependence of net gain as a function of population inversion for the Er3+:4I13/2 level is calculated in order to determine the gain property qualitatively. Assuming that the population of Er3+ ions is distributed only between the 4I15/2 ground state and the 4I13/2 first excited state, the optical gain properties are directly associated with the absorption and emission cross-sections,

the development of these devices, the τexp × σemi(λ) and the σemi(λ) × FWHM products are commonly used as figure of merit, since high values of these parameters are required for amplification and can also be used to compare them with those obtained for other glass compositions. All these parameters are experimentally accessible. The amplification parameters for all the concentrations of the present γ-ray irradiated PPbKANEr glasses are presented in Table 2. Subsequently, one of the most significant spectroscopic parameter is measured lifetime for excited state of the lanthanide ions. The longer lifetime of the metastable state required for the high population inversion is an important factor for the success of EDFA that is useful in the optical communications. The decay curve for the 4I13/2 state of Er3+ ions were measured for all the PPbKANEr glasses and are shown in Fig. 11. All the luminescence decay curves for the 4I13/2 → 4I15/2 exhibited single exponential nature in spite of increase of the Er2O3 concentration and the experimental lifetimes were determined by the single exponential fitting. It was found that the lifetimes of the 4I13/2 level decreased with the increase of Er2O3 concentration. The lifetime values are found to be 3.72, 3.43, 2.00, 1.69, 1.26 and 0.62 ms for 0.05, 0.1, 0.5, 1.0, 2.0 and 4.0 mol% Er2O3 concentrations, respectively. The shortening of lifetime with increasing concentration is due to enrichment of non-radiative energy transfer process such as energy migration among Er3+ ions followed by transfer to recombination centers and interaction between Er3+ ions and the glassy host defects [49]. The decay curves for the 4I13/2 state of Er3+ ions were measured before and after the γ-ray irradiation and found to be single exponential nature irrespective of the Er2O3 concentration or irradiation. Before (after) γ-ray irradiation the lifetimes were determined to be 4.5 (3.70), 4.57 (3.43), 2.68 (2.00), 2.06 (1.69), 1.48 (1.26) and 1.04 (0.62) ms. From the analysis of the lifetime, it was confirmed that the lifetime was found to decrease after successive application of the γ-ray irradiation due to radiation induced defects. Furthermore, reducing the measured

G(λ, P) = Pσemi (λ) − (1 − P) σabs (λ)


where P represents the population of the upper laser level. The calculated gain cross-sections as a function of the emission wavelength for several p values are shown in Fig. 10 for the PPbKANEr1.0 glass and the gain will be positive when the population inversion is larger than 0.4. It suggested that a flat gain bandwidth in the range from 1460 to 1585 nm could be obtained for a normal population inversion above 40%. This gain covers all the S (1460 −1530 nm), C (1530 −1565 nm) and part of the L (1565 – 1625 nm) bands in the optical communication window, which specifies that more channels may be acceptable in the wavelength division multiplexing (WDM) network in the PPbKANEr1.0 glass. The full-width at half-maximum (FWHM) and σemi(λ) are the most important spectral parameters for the optical gain medium of erbium doped fiber amplifiers (EDFA) to achieve high gain amplification and broadband. As already mentioned that FWHM is very important in telecommunication since it allows to cover two of the optical communication bands, the C and L bands. The experimental lifetime (τexp) and the σemi(λ) are also crucial parameters for the optical amplification process. Amplification characteristics of the EDFA gain medium are described mainly by the gain (τexp × σemi(λ)) and bandwidth quality (FWHM × σemi(λ)). In order to test the availability of these glasses for

Table 2 Concentration dependent absorption cross-section (σabs (λ), × 10–21 cm2), emission cross-section (σemi(λ), × 10–21 cm2), lifetime (τexp, ms), FWHM (nm), gain bandwidth (FWHM × σemi(λ), × 10–28 cm3) and gain quality factor (σemi (λ) × τexp, 10–21cm2 ms) for the 4I13/2 level of Er3+ ions after γ-ray irradiation.


Glass label

σabs (λ)

σemi (λ)



FWHM × σemi(λ)

σemi (λ) × τexp

PPbKANEr0.05 PPbKANEr0.1 PPbKANEr0.5 PPbKANEr1.0 PPbKANEr2.0 PPbKANEr4.0

7.86 6.40 6.48 6.73 6.29 5.98

8.92 7.64 7.57 7.33 7.13 6.78

3.72 3.43 2.00 1.69 1.26 0.62

28 29 35 36 43 52

249.76 221.56 264.5 264.00 306.59 352.56

33.18 26.21 15.14 12.39 8.98 4.20

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Fig. 11. Decay curves for the 4I13/2 metastable state of Er3+:PPbKANEr glasses after γ-ray irradiation. Fig. 13. Partial energy level diagram of the Er3+:PPbKAN glasses along with visible, NIR emissions and possible upconversion mechanisms.

Table 3 Comparison of spectroscopic parameters of the Er3+ ions in PPbKANEr1.0 glass before and after γ-ray irradiation. Spectroscopic parameters

Before irradiation

After irradiation

σabs (λ), × 10–21 cm2 σemi (λ), × 10–21 cm2 τexp (ms) FWHM (nm) FWHM × σemi(λ), × 10–28 cm3 σemi (λ) × τexp, 10–21cm2 ms

5.85 6.73 2.06 46 310 13.86

6.73 7.33 1.69 36 264 12.39

mechanisms for populating the 4S3/2 and 4F9/2 levels after 980 nm excitation, the evolution of the upconverted emission intensities at 550 and 660 nm for different pumping powers have been obtained. The dependence of the intensity on the pump power is quadratic which indicates that a two photon (TP) upconversion process populates 4S3/2 and 4F9/2 levels. This in turn may be associated to excited state absorption (ESA) and/or to energy transfer upconversion (ETU). Fig. 13 displays the energy level diagram and energy transfer channels of Er3+ along with 980 nm LD pumping. Firstly, ions of the Er3+: 4I15/2 level is excited to 4I11/2 state by ground state absorption (GSA) when radiated by 980 nm. In addition, ions in 4I13/2 level can transfer radiatively to ground level along with 1.53 µm emissions. On the other hand, excited state absorption (4I11/2 + a photon→ 4F7/2) and energy transfer upconversion processes (4I11/2 + 4I11/2 → 4F7/2 + 4I15/ 4 2) making ions in F7/2 level populated. According to the non-radiative relaxation processes (NR), ions in 4F7/2 state depopulated to 2H11/2, 4S3/ 4 2 and F9/2 levels. Thus, 530 nm, 550 nm and 660 nm light emissions occur via Er3+: 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions. Besides, ions in 4I13/2 state might undergo the excited state absorption (ESA2) process: 4I13/2 + a photon → 4F9/2, which is beneficial for 660 nm emissions. Furthermore, ETU2 (4I13/2 + 4I13/2 → 4I9/2 + 4I15/2) process may take place and benefit the accumulation of ions in 4I11/2 level and reduction of ions in 4I13/2 level. The upconverted red emission can be the result of multiphonon relaxation from the 4S3/2 level and ETU processes. Energy transfer can take place via transitions (4I9/2 → 4I13/2) and (4I11/2 → 4F9/2) and/or (4I11/2 → 4I15/2) and (4I13/2 → 4F9/2). There exists another possible process to populate the 4F9/2 level in which two Er3+ions, one of them in the 4I11/2 level and the other one in the 4F7/2 level, interact and go both to level 4F9/2 (See Fig. 13). In an UC mechanism the UC emission intensity (IUP) is proportional to the nth power of the IR excitation intensity (IIR), i.e., IUP ∝ Inth IR , where n is the number of IR photons absorbed per visible photon emitted. A plot of ln(IUP) versus ln(IIR) yields a straight line with slope n. The inset of Fig. 12 shows such a plot for the 548 nm (4S3/2 → 4I15/2) emissions, and the value of ‘n′ was obtained as 1.49. This result confirms that two photons contribute to the UC of the three visible emission bands in Er2O3-doped PPbKANEr glass. The radiative decay through ESA process occurs during the excitation pulse width, and leads to an immediate decay of the upconversion luminescence after excitation of 980 nm. Upconversion by energy transfer leads to a decay curve for the upconversion luminecence which proves a rise time after the laser pulse, followed by decay with a lifetime higher than the one after direct excitation (Fig. 14 (a)). This

lifetime, can occur if energy is transferred from excited Er3+ ions to acceptor states in the host such as defects introduced by the ion beam. All the spectroscopic parameters of the PPbKANEr glass before and after γ-ray irradiation are presented in the Table 3. The variation in the values of the PPbKANEr glass is due to the defects and color centers generated in the glass material after the γ-ray irradiation. The upconverted emission spectra obtained after γ-ray irradiation of PPbKANEr glasses under 980 nm excitation are shown in Fig. 12. The profile of the emission spectra is similar to the one obtained under direct excitation (385 nm). The room temperature upconversion luminescence spectra in the range of 500–700 nm show the green (530 and 548 nm) and red (660 nm) visible light emissions. The 530 nm, 550 nm and 660 nm emissions correspond to the 2H11/2→ 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transition, respectively. To investigate the excitation

Fig. 12. Pump power dependent upconversion luminescence of Er3+-doped PPbKANEr1.0 glass after γ-ray irradiation. 328

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Er3+-doped PPbKANEr glasses having exponential nature for all the Er3+ ion concentrations. The decay analysis revealed that the lifetime for the 4I13/2 level of Er3+ ion decreases with increasing concentration is due to energy migration among Er3+ ions followed by transfer to recombination centers and interaction between Er3+ ions and the glassy host defects. Strong green visible upconversion luminescence was observed and two photons contributed to the UC in Er2O3-doped PPbKANEr glass with the excitation of 980 nm laser diode. The mechanism of upconversion involves mainly excited-state absorption and partly energy-transfer upconversion. The decay obtained after 980 nm excitation shows a rise and an exponential decay with a lifetime longer than that of 4S3/2 level under direct excitation. The results confirm that the Er3+-doped lead phosphate glasses are useful for the development of optical fiber amplifiers as well as green emission applications in harsh environment. Acknowledgements Dr. Ch. Basavapoornima is thankful to University Grants Commission, New Delhi, for the award of Post Doctoral Fellowship for Women for the year of 2011-12 (F.15–1/2011-12/PDFWM-2011-12OB-AND-9964 (SA-II), dt. 1–11-2013). The author (CKJ) would like to thank DAE-BRNS, Govt. of India for the sanction of major research project (No.2009/ 34/36/BRNS/3174, dt.12–02-2010) under MoU between RRCAT, Indore, BARC, Mumbai and S.V. University, Tirupati. This work was partially supported by the KEPCO Research Institute (KEPRI) and managed by KESRI (Project Number: KEPRI-16–23), South Korea.

Fig. 14. (a). The fluorescence decay of the 4S3/2 level for PPbKAN glass doped with 1.0 mol% of Er2O3 obtained after excitation at 980 nm. (b). Comparison of the fluorescence decay of the 4S3/2 level for γ-ray irradiated leadphosphate glass doped with 1.0 mol% of Er2O3.

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distinction is possible when the pulse width is much shorter than the time constant of the relevant energy transfer step. The upconversion decay lifetimes are found to be more or less similar with small variations with increasing concentration of the Er3+ ions. Hence, the decay curve presents only for one concentration (PPbKANEr1.0) and was shown in Fig. 14 (a). Fig. 14 (b) represents the time evolution of the 4 S3/2 emission in PPbKANEr1.0 glass obtained under direct excitation at 379 nm and under infrared excitation at 980 nm. The decay obtained after 980 nm excitation shows a rise and an exponential decay with a lifetime longer (250 μs) than that of 4S3/2 level under direct excitation (100 μs). The same behavior is observed for the other concentrations of the PPbKANEr glass samples. The time evolution of the upconverted green luminescence from the 4S3/2 level point out that an ETU process is responsible for the upconversion luminescence from this level although the presence of an ESA mechanism cannot be disregarded. 4. Conclusions The structural and spectroscopic properties by varying Er3+ ions concentration in lead phosphate glasses were successfully studied after successive γ-ray irradiation. FTIR spectra show characteristic vibrational bands of the metaphosphate groups and γ-ray irradiation causes some limited variations in the intensity of the vibration bands. The absorption spectra are found to be similar and no additional peaks are observed after γ-ray irradiation due to the shielding effect of the heavy metal oxide (Pb2O3). Bright green emission (4S3/2 → 4I15/2) in the visible region (500–700 nm) and broad NIR luminescence (4I13/2 → 4I15/2) were observed for the present PPbKANEr glasses. With the help of McCumber theory, the absorption and emission cross-sections were evaluated and the emission cross-section for the present glass was found to be higher after γ-ray irradiation. The gain will be positive for a population inversion above 40%, showing a flat gain bandwidth in the 1475–1625 nm range and covers both the C and L bands in the optical communication window. The decay curves for the 4S3/2 level of 329

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