Fluorescence features of Tm3+-doped multicomponent borosilicate and borotellurite glasses for blue laser and S-band optical amplifier applications

Optical Materials 96 (2019) 109354

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Fluorescence features of Tm3+-doped multicomponent borosilicate and borotellurite glasses for blue laser and S-band optical amplifier applications

T

G. Lakshminarayanaa,∗, U. Caldiñob, A.N. Meza-Rochac, A. Lirad, P. Venkateswara Raoe, Vijay Singhf, A. Dahshang,h, I.V. Kityki, Dong-Eun Leej,∗∗, Jonghun Yoonk,∗∗∗, Taejoon Parkl,∗∗∗∗ a

Intelligent Construction Automation Center, Kyungpook National University, 80, Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, P.O. Box 55-534, México, D.F, 09340, Mexico c CONACYT-Benemérita Universidad Autónoma de Puebla, Postgrado en Física Aplicada, Facultad de Ciencias Físico-Matemáticas, Av. San Claudio y Av. 18 sur, Col. San Manuel Ciudad Universitaria, Puebla, Pue, 72570, Mexico d Departamento de Física, Facultad de Ciencias, Universidad Autónoma del Estado de México, C.P. 50000, Toluca, Mexico e Department of Physics, The University of the West Indies, Mona Campus, Jamaica f Department of Chemical Engineering, Konkuk University, Seoul, 05029, South Korea g Department of Physics - Faculty of Science - King Khalid University, P.O. Box 9004, Abha, Saudi Arabia h Department of Physics, Faculty of Science, Port Said University, Port Said, Egypt i Institute of Optoelectronics and Measuring Systems, Faculty of Electrical Engineering, Czestochowa University of Technology, 17 Armii Krajowej Str., 42-200, Czestochowa, Poland j School of Architecture and Civil Engineering, Kyungpook National University, 80, Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea k Department of Mechanical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Ansan, Gyeonggi-do, 15588, Republic of Korea l Department of Robotics Engineering, Hanyang University, 55 Hanyangdaehak-ro, Ansan, Gyeonggi-do, 15588, Republic of Korea b

ARTICLE INFO

ABSTRACT

Keywords: Multicomponent borosilicate glass Multicomponent borotellurite glass Tm3+: 1458 nm fluorescence 453 nm blue laser S-band optical amplifier

Novel B2O3–SiO2–Al2O3–ZnO–Li2O/MgO and B2O3–TeO2–PbO–ZnO–Li2O–Na2O glasses with different concentrations of Tm2O3 were synthesized by using the melt-quench method. All the fabricated samples have been characterized by visible emission spectra and decay times, including near-infrared (NIR) luminescence measurements. For 0.5 mol% Tm3+-doped borotellurite glass, several radiative parameters are evaluated using the Judd-Ofelt parameters. The intensity of all the visible emission bands increased with the increase of Tm2O3 concentration up to 0.5 mol%, and beyond this doping content, luminescence concentration quenching takes place. The luminescence intensity quenching is attributed to energy transfer (ET) processes through cross-relaxation (CR) channels. The visible luminescence decay curves were well fit with a single exponential (for Tm3+: 1 D2 level) and double exponential (for Tm3+: 1G4 level) functions for the multicomponent borosilicate samples, while Inokuti-Hirayama model was used for the multicomponent borotellurite glass 1D2 level decay time fit. The derived decay lifetimes of the 1D2 level are found to be much shorter than that of the 1G4 level. In Li2O (alkali) or MgO (alkaline) containing borosilicate samples, pumped under 808 nm laser diode, the 3H4→3F4 (1.458 μm) emission intensity increased from 0.1 to 2.0 mol% Tm3+ ion concentration, indicating negligible CR processes. The computed Full-Width at Half-Maximum (FWHM) values for the 1458 nm emission in 2.0 mol% Tm3+-doped Li and Mg series borosilicate samples are 117 and 125 nm, respectively, while the FWHM value for 0.5 mol% Tm2O3 content doped borotellurite glass is 118 nm. Following the analyzed visible and NIR optical results, the fabricated Tm3+ glasses could be useful for blue laser and S-band optical amplifier applications.

1. Introduction It is well known that compared with inorganic phosphors, ceramics

or single crystals, optical glasses as thriving materials show several advantages like wide transparent spectral window, ease of synthesis connected with a relatively cheap manufacturing cost in mass

Corresponding author. Corresponding author. ∗∗∗ Corresponding author. ∗∗∗∗ Corresponding author. E-mail addresses: [email protected] (G. Lakshminarayana), [email protected] (D.-E. Lee), [email protected] (J. Yoon), [email protected] (T. Park). ∗

∗∗

https://doi.org/10.1016/j.optmat.2019.109354 Received 4 August 2019; Received in revised form 27 August 2019; Accepted 28 August 2019 Available online 02 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.

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production, and ease of making in thin film or optical fiber (i.e., fiber lasers and fiber amplifiers) form, following the desired application. Further, compared with fluoride and sulfide or chalcogenide glasses (non-oxide glasses), oxide glasses such as silicate, borate, phosphate, germanate, tellurite, gallate and heavy metal oxide (Bi2O3, PbO etc.) glasses possess better mechanical strength, good chemical durability and thermal stability for their practical photonic applications [1‒8]. Commercially, hydroxyl (OH)-free silica optical fibers are available for 1.3 μm and 1.55 μm low-loss transmission windows at ∼ 1200–1700 nm wavelength region. Optical amplifiers functioning in this region are required for telecommunication systems [9]. Though silica glass shows high mechanical strength, high thermal shock resistance and good optical (waveguide) features for optical fiber application, it is difficult to incorporate various rare-earth (RE) ions in a high doping concentration into the silica glass due to a well-defined nature of SiO2 glass network structure. However, for a multicomponent silicate glass, recently, Lee et al. [2] reported a heavy (up to 7 wt%) Tm3+ ion doping for fiber amplifier application. On the other hand, B2O3 is a superior glass former and borate glasses are good hosts for moderate to high RE ion doping. Thus, a multicomponent borosilicate glass, with a combination of silicate and borate glass physical, structural and optical features, can be considered for achieving a higher solubility of RE ions and moderate phonon energy (< 1100 cm−1) [3]. Moreover, borosilicate glasses are considered low-cost materials, which are available commercially in large-scale quantities by several vendors. Besides that, these glasses can be easily produced in different compositions following the desired photonic and optical applications. Further, borosilicate glasses possess refractive indices close to that of silica (nD of Borosilicate = 1.473 and nD of silica = 1.458). This decreases the losses in connecting borosilicate and optical fiber-based silica (easy to splice with silicate based fiber optic components). Silica is the most commonly used base material for optical fibers in existing telecommunication networks. Recently, we have reported Nd3+-doped heavy metal oxide based multicomponent borate glasses for 1.06 μm solid-state NIR laser and O-band optical amplification applications [10], as well structural and optical studies of Er3+-doped alkali/alkaline oxide containing zinc boro-aluminosilicate glasses for 1.5 μm optical amplifier applications [11]. Currently, commercially available silica-based erbium (Er)-doped fiber amplifiers (EDFAs) cover the C-band (1530–1560 nm) region for optical communication systems. In recent times, for the short wavelength side of the EDFA, numerous researches are focussed on extending further the wavelength division multiplexing/dense wavelength division multiplexing (WDM/DWDM) network transmission capacity at 1450–1530 nm wavelength range (S+ (1450–1480 nm) and S-bands (1480–1530 nm)). In this wavelength range silica fiber exhibits low attenuation loss (∼0.25 dB/km) and minimal bending loss, using thulium (Tm3+)-doped fiber amplifier (TDFA), as Tm3+ ion shows a promising near-infrared (NIR) emission band centered at ∼1.45 μm wavelength (corresponding to 3H4→3F4 transition) [4,5,12–14]. Additionally, singly or when co-doped with other transition metal ion (such as Cr3+) or RE ion (such as Yb3+, e.g. Thulium–Ytterbium codoped fiber (TYDF)), Tm3+ exhibits an efficient NIR laser luminescence at ∼1.8–2.1 μm (3F4→3H6 transition) wide spectral region under relatively inexpensive and commercially available 808 nm and/or 980 nm laser diode (LD) pumping. Such NIR luminescence region is the socalled “eye-safe” region for remote sensing, laser medical surgery, atmospheric pollution monitoring and military applications [6,7,15–17]. Moreover, meta-stable excited levels of Tm3+ ion are suitable for emitting blue (∼450 nm (1D2→3F4) and ∼480 nm (1G4→3H6)), red (∼650 nm (1G4→3F4)) and near-infrared (∼800 nm (1G4→3H5/3H4 →3H6)) up-conversion (UC) luminescence. Therefore, glasses singly doped with Tm3+ or Tm3+/Yb3+-co-doped are useful for visible lasers, color displays, under sea-water communication and solar cell applications [3,18–21]. The inclusion of Al2O3 (phonon energy ∼780 cm−1) into silica fibers and silicate glasses increases the refractive index and mechanical

strength of the host matrix. It enhances also the RE ion solubility by minimizing the RE-clustering between adjacent RE ions (RE‒O‒RE bonds) with Al–O‒RE bonds, which causes a decrement of energy transfer (ET) process losses among RE ions increasing the laser performance or luminescence quantum efficiency [22,23]. Based on the added Al2O3 content in the glass network structure, Al3+ ions can form tetrahedral (AlO4) coordination sites as a network former or octahedral (AlO6) coordination sites as a network modifier. Here, the four coordinated Al shares non-bridging oxygens (NBOs) with RE ions and plays a crucial role in the RE ion de-clustering. Recently, Ashok et al. [24] have studied various structural, thermal, and optical features of Au2O3-doped sodium antimonate glasses and found that gold metallic particles concentration is gradually increased with an increase in Au2O3 content. Further, Kalpana et al. [25] have reported that with the Al2O3 content increment from 0 to 5.0 mol% in steps of 1.0 mol%, the green emission intensity of Tb3+ ion was enhanced for 30BaO–30B2O3-(39-x) P2O5-xAl2O3:1Tb2O3 glasses due to de-clustering of Tb3+ ions by Al3+ ions. In another work, Suresh et al. [26] have demonstrated that visible and NIR luminescence of the Pr3+ -doped lead bismuth silicate glasses was enhanced nearly four times with the addition of 5.0 mol% of Bi2O3 due to a possible de-clusterization of Pr3+ ions by Bi3+ ions. It is well known that at a lower added content in the glass matrix, ZnO acts as a network modifier, and possesses the ability to break the bonds between oxygen and glass former (e.g. B, Si) ions, forming NBOs in the glass network. ZnO also improves the UV transparency, non-hygroscopicity and thermal stability of the glass [27–29]. In glass network, the addition of alkali (Li2O) or alkaline (MgO) oxides causes to enhance the glass forming ability as network modifiers and create NBOs by breaking the glass network former (e.g. B–O–B and Si–O–Si bridges) ion bonds [29,30]. In our earlier work, we have reported on the gamma-ray shielding features (using both XCOM software and MCNP5 simulation code) and optical absorption spectra analysis of 0.1–2.0 mol% Tm3+-doped alkali (Li2O) or alkaline (MgO) oxides containing zinc alumino borosilicate glasses [31]. In the present study, our main aim is to investigate the visible and NIR luminescence features of Tm3+-doped Li and Mg series glasses as well as a 0.5 mol% Tm3+-doped borotellurite glass for their potential application in blue lasers and S-band fiber amplifiers. Photoluminescence excitation (PLE), photoluminescence (PL), including NIR emissions, and visible emission decay lifetimes are measured and analyzed for all the samples. The Ωλ (λ = 2, 4 and 6) Judd–Ofelt (J‒O) intensity parameters and radiative features, such as radiative emission transition probability (AR), radiative lifetime (τR) and branching ratio (βR) of the excited levels of the Tm3+ ion are evaluated from the absorption and emission spectra analysis of 0.5 mol% Tm3+-doped borotellurite glass. Several PL bands corresponding to Tm3+ ion located in the visible spectral region are identified for all the glasses. The Commission Internationale de I'Eclairage (CIE) chromaticity (x, y) coordinates are derived from the visible luminescence spectra and they are represented in CIE 1931 diagram. 2. Experimental Tm3+-doped Li2O or MgO containing zinc alumino borosilicate glasses, nominal composition (in mol%), studied in this work, are given below and denoted as “Li1”, “Li2”, “Li3”, “Li4”, “Li5”, “Mg1”, “Mg2”, “Mg3”, “Mg4” and “Mg5”, respectively. Li1: 39.9 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 Li2O−0.1 Tm2O3 Li2: 39.75 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 Li2O−0.25 Tm2O3 Li3: 39.5 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 Li2O−0.5 Tm2O3

2

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Li4: 39 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 Li2O−1.0 Tm2O3 Li5: 38 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 Li2O−2.0 Tm2O3 Mg1: 39.9 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 MgO−0.1 Tm2O3 Mg2: 39.75 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 MgO−0.25 Tm2O3 Mg3: 39.5 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 MgO−0.5 Tm2O3 Mg4: 39 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 MgO−1.0 Tm2O3 Mg5: 38 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 MgO−2.0 Tm2O3 Fig. 1. Partial energy-level diagram of the Tm3+ ion for the studied glasses, depicting the UV and NIR excitations, and visible and NIR emission transitions, including CR channels for 1D2 and 3H4 excited levels.

Additionally, for the comparison of the luminescence results, a borotellurite glass with 0.5 mol% Tm3+-doping was also fabricated in the 49.5 B2O3‒10 TeO2‒10 PbO‒10 ZnO‒10 Li2O‒10 Na2O‒0.5 Tm2O3 (mol%) chemical composition, and is labeled as “G1”. For all the glasses, Tm2O3 is added as an increment within the glass matrix composition. The standard melt-quenching technique was used for the preparation of all the above-mentioned glasses. B2O3 (99.98%), SiO2 (99.99%), TeO2 (99.995%), Al2O3 (98%), PbO (≥99%), ZnO (99%), Li2CO3 (99.99%), MgO (99%), Na2CO3 (99.5%) and Tm2O3 (99.99%) chemicals (inclusive of their purity) were used as starting materials for the glass fabrication. The details of the synthesis procedure for the Li and Mg series glasses were reported elsewhere [31]. In short, following the respective glass compositions, all the samples were fabricated in 20 g batch each separately, using an appropriate amount of chemicals. High purity alumina crucibles were used for the well-mixed raw materials melting at 1400 °C for 1 h for Li and Mg series glass synthesis, and for 30 min at 975 °C the “G1” chemical powder mixture was melted, in an air atmosphere, using an electric furnace. The glass melt was subsequently poured onto a stainless steel mould. Using a muffle furnace, the obtained 0.6 cm thickness Li and Mg series glasses were annealed for 5 h at 450 °C, and “G1” for 5 h at 300 °C, in air, to relinquish the internal thermal stress, after which they were allowed to cool slowly inside the furnace. Finally, all the glasses were double-sized polished carefully to a thickness of 0.3 cm for the optical measurements. The physical properties like thickness, density, and refractive index of the ‘G1’ sample have been determined by using a sliding caliper gauge, using the simple Archimedes method with toluene as an immersion liquid, and by an Abbe refractometer at nd (589.3 nm) wavelength employing a sodium lamp, respectively. The X-ray Diffraction (XRD) profiles were recorded using Ital Structure APD 2000 diffractometer with CuKα (λ = 1.542 Å) radiation by applying a voltage of 40 kV and 20 mA anode current. The scan range was from 10° up to 80° with a scan rate of 2°/min. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of the glass powders were recorded over the 300-1600 cm−1 range by a PerkinElmer Spectrum 100 FTIR spectrometer with a spectral resolution around 4 cm−1. The finely ground glass powder was pressed directly onto the ATR diamond crystal in the FTIR measurements. The Raman spectra of the glasses were recorded with a WITec alpha 300R Confocal Raman system equipped with cw doubled frequency Nd:YAG laser at 532 nm as excitation source, being the incident power of 10 mW. All the Raman spectra were recorded into the spectral range of 0–3800 cm−1 for the Raman shift and with an integration time of 5 s. Ultraviolet–visible–near-infrared (UV-VIS-NIR) optical absorption, UV–visible excitation and emission spectra, and luminescence decay times, including NIR fluorescence spectra upon 808 nm pulsed laser diode (LD) excitation, were recorded by the same equipment and

spectral parameters given in our earlier works [10,11]. The cut-off wavelength in the NIR range of the scanning spectrometer was 1600 nm. For all the fabricated Tm3+ ion doped fully transparent glasses, the experimental conditions (i.e.; sample position and incident angle related to the excitation light) for every sample was kept the same in order to compare the obtained results. All the indicated optical characterization techniques were carried out at ambient temperature. The chromaticity coordinates (x, y) were calculated following the respective visible emission spectra using a relevant software. 3. Results and discussion 3.1. Tm3+ ion energy level scheme for the studied glasses Fig. 1 illustrates the Tm3+ ion partial energy-level scheme for the studied glasses. The UV and NIR excitation transitions and observed all visible and NIR emission transitions, including CR channels for 1D2 and 3 H4 excited levels, are indicated in the energy level diagram. Under 358 nm excitation, Tm3+ ions in the ground state 3H6 will reach to the 1 D2 level. Then, most of the Tm3+ ions in the 1D2 level will rapidly relax to the 3F4 level through a radiative transition. Thus, Tm3+ ions exhibits a strong blue emission centered at 453 nm and a weak green band at 513 nm (3H5 terminal level), as the decay lifetime of the 1D2 level is much shorter than the next lower level, 1G4 (see Table 1). The remaining Tm3+ ions in the 1D2 state depopulate to the 1G4 level, then finally they relax to the 3H6 and 3F4 levels giving rise to weak blue and red emissions. Under suitable pumping, due to the large energy gap, ΔE (∼6000 cm−1) between the 3F4 level and ground state 3H6, one can expect radiative emission transition between these levels. Similarly, Table 1 Decay times (ms) of 1D2 and 1G4 excited levels for the studied Tm3+-doped Li and Mg series glasses under 358 nm excitation wavelength.

3

Sample code

Level 1D2

Li1 Li2 Li3 Li4 Li5 Mg1 Mg2 Mg3 Mg4 Mg5

0.0146 0.0160 0.0153 0.0117 0.0071 0.0144 0.0146 0.0141 0.0112 0.0069

± ± ± ± ± ± ± ± ± ±

1

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

G4

0.166 0.122 0.076 0.045 0.034 0.163 0.115 0.096 0.053 0.020

± ± ± ± ± ± ± ± ± ±

0.101 0.076 0.042 0.018 0.023 0.107 0.042 0.028 0.038 0.019

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upon 808 nm LD pumping, Tm3+ ions initially in the ground state are excited to the 3H4 level. Afterward, a significant fraction of the Tm3+ ions will relax to the 3F4 level, contributing to the 1.458 μm NIR emission. Usually, in higher phonon energy glasses like silica (phonon energy ∼1100 cm−1), due to the ΔE between the 3H4 level and next lower level 3H5 (∼4300 cm−1, see Fig. 1), the MPR becomes an influential process between these levels and it is hard to achieve population inversion between the 3H4 and 3F4 levels [32]. Here, it should be mentioned that, generally, the lifetime of higher 3H4 level for 1.4 μm emission is shorter than that of the 3F4 terminal level, so low phonon energy glasses are suitable hosts to prevent the depopulation of ions by MPR to 3H5 level and to acquire an efficient laser action. On the other hand, when the Tm3+ ion concentration is sufficiently high in a selected host matrix, most of the Tm3+ ions in the 3H4 state decay to the 3 F4 metastable level via CR channel, as shown in Fig. 1, which eventually results in a weaker 1.4 μm luminescence and more intense 1.8 μm emission (3F4→3H6 transition). Generally, this CR process (promotion of two Tm3+ ions into the higher 3F4 level with a single 808 LD pump photon/“two-for-one” excitation) depends on the distance between adjacent Tm3+ ions. However, at Tm3+ ion high doping concentrations, Tm3+ ions might form clusters with each other, causing to adverse energy losses through up-conversion (UC) and re-absorption processes. 3.2. Tm3+-doped borosilicate glasses 3.2.1. Luminescence features and time decay profile analysis Fig. 2 (a) depicts the photoluminescence excitation (PLE) spectra for all the Tm3+-doped Li series glasses within the wavelength range of 240–400 nm, measured by monitoring emission wavelength at 453 nm. From Fig. 2 (a), one can notice three weak excitation peaks (which are magnified × 40 times) centered at 262 nm, 274 nm, and 290 nm along with an intense excitation peak at 358 nm. These excitation peaks are due to the electronic transitions of Tm3+: 3H6 → 3P2 (262 nm), 3H6 → 3 P1 (274 nm), 3H6 → 3P0 (290 nm) and 3H6 → 1D2 (358 nm), respectively [33]. Generally, for Tm3+ ion, at 250–300 nm wavelength range, ground state absorption of five distinct energy states can be observed [33]. Here, the 0.5 mol% Tm3+-doped Li glass (Li3) possesses the highest excitation intensity for all the identified excitation bands at the same position compared with all other glasses. Due to its’ highest intensity, 358 nm excitation peak was selected for the photoluminescence (PL) spectral measurement of Li1 to Li5 glasses at 420–720 nm spectral region, and they are presented in Fig. 2 (b). The PL spectra exhibit mainly four emission bands: two blue emission bands centered at 453 and 476 nm are assigned to Tm3+: 1D2 → 3F4 and 1G4 → 3H6 transitions, green emission band situated at 513 nm is due to 1D2→ 3H5 transition and a red emission band located at 649 nm could be attributed to 1G4 → 3F4 transition [33,34]. Usually, for Tm3+ ion, the higher 1 D2 and 1G4 excited levels cause emissions in the visible spectral region and the lower 3F4 and 3H4 excited states emit in the NIR region. The intensity of the blue emission band at 453 nm is much more intense compared with that of remaining all other emission bands and in fact, green and red emissions are fairly weak, so they are zoomed in 50 times in the PL spectra. From all PL bands it is clear that the emission intensity increases with increasing concentration of Tm3+ in the glasses up to 0.5 mol%, and beyond 0.5 mol% Tm2O3 content, the Tm3+ ion PL band intensities are decreased due to the luminescence concentration quenching. Generally, in RE-doped optical materials, at higher RE ion doping levels, as in our case, beyond 0.5 mol% Tm3+, due to a reduction of inter-ionic distance between adjacent/neighbouring Tm3+ ions, nonradiative (NR) energy transfer among Tm3+ ions (mutual Tm3+–Tm3+ interactions between excited state Tm3+ ions and Tm3+ ions in 3H6 ground state) occurs. Such ET processes (e.g. cross-relaxation (CR), resonant energy transfer (RET) etc.) in turn weakens the visible emission intensity, and thereby decreases the fluorescence quantum efficiency. In fact, the blue emission band intensity accounts for more than

Fig. 2. (a) Photoluminescence excitation (PLE) spectra of Tm3+-doped Li series glasses by monitoring emission at 453 nm wavelength and (b) Photoluminescence spectra (PL) of Tm3+-doped Li series glasses under 358 nm excitation wavelength.

90% of the total identified visible emission intensity in the Li1→Li5 glasses. For Tm3+-doped Mg series glasses, the measured PLE and PL spectra are shown in Fig. 3 (a) and (b), respectively. From Fig. 3 (a) and (b), one can see that features or positions of the identified four excitation peaks and four emission bands for Mg1–Mg5 glasses are the same as those for the Li1–Li5 samples, which are assigned to the Tm3+ ion optical transitions, similarly, as described in Fig. 2. For Tm3+-doped Mg series samples also Mg3 sample exhibits the highest intensity for all the PLE and PL bands compared with Mg4 and Mg5, due to the occurrence of luminescence concentration quenching. However, Mg3 sample displays the only 3/4th value of intensity for 453 nm PL peak, when compared with Li3 sample 1D2 → 3F4 transition intensity. Fig. 3 (c) illustrates the comparison of PL band (453 nm) intensity for all Tm3+-doped Li and Mg series glasses upon 358 nm excitation wavelength. It is clear that apart from Li3 sample, Li2 and Li4 samples also possess higher intensity for 453 nm emission band when compared with Mg2 and Mg4 samples (see Fig. 3 (c)). Similarly, upon 358 nm excitation wavelength, the comparison of 476 nm, 513 nm, and 649 nm PL bands intensity for all the Li1→Li5 and Mg1→Mg5 samples was also shown in Fig. 3(d–f), respectively. From Fig. 3(d–f) one can notice that Li3 glass possesses relatively higher intensity than the Mg3 sample for all the 1G4→3H6, 1D2→3H5, and 1G4→3F4 emission transitions. Further, 1 D2 → 3F4 and 1D2→3H5 optical transitions show a similar trend for the observed luminescence intensity enhancement for Tm3+-doped Li series and Mg series samples as Li1 < Li5 < Li2 < Li4 < Li3 and 4

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Fig. 3. (a) Photoluminescence excitation (PLE) spectra of Tm3+-doped Mg series glasses by monitoring emission at 453 nm wavelength, (b) Photoluminescence (PL) spectra of Tm3+-doped Mg series glasses under 358 nm excitation wavelength, and Comparison of emission intensity for Tm3+-doped Li and Mg series glasses at (c) λex = 358 nm and λem = 453 nm (d) λex = 358 nm and λem = 476 nm (e) λex = 358 nm, and λem = 513 nm, and (f) λex = 358 nm and λem = 649 nm.

Mg1 < Mg5 < Mg2 < Mg4 < Mg3 while both 1G4→3H6 and 1G4→3F4 PL transitions for Li1→Li5 and Mg1→Mg5 glasses exhibit a similar intensity increasing trend as Li5 < Li1 < Li4 < Li2 < Li3 and Mg5 < Mg1 < Mg4 < Mg2 < Mg3. Fig. 4(a–e) and (f-j) shows the measured time decay profiles of the 1 D2 and 1G4 excited levels for all the Tm3+-doped Li series glasses upon 358 nm excitation wavelength, respectively. The best fit was obtained by using a single exponential function as It = I0 × (e-t/τ), where I0 and It are the emission intensities at time t = 0 and at t = ‘t’, respectively, for all the 1D2 level decay profiles. Generally, decay curves exhibit single exponential behavior for low concentration of RE ions due to an absence of ET between RE3+- RE3+ ions. However, the 1G4 level decay profiles exhibit a non-exponential nature, which can be well fitted by a two-exponential function through the relation I (t ) = Aexp ( t / 1) + B exp( t / 2 ) , where 1 and 2 are short-and longdecay components, respectively, and parameters A and B are fitting constants. An average lifetime τ was then obtained from the relation = (A 12 + B 22)/(A 1 + B 2) . The derived fluorescent decay lifetimes (ms) of 1D2 and 1G4 excited states are listed in Table 1. Similarly, for all the Tm3+-doped Mg series glasses, measured fluorescent lifetime (τexp.) values of the 1D2 and 1G4 states under 358 nm excitation, for which all the decay profiles (see Fig. 5(a–e) and (f-j)) follow single exponential and double exponential functions, respectively, are also listed in Table 1. One can notice from Table 1 that with an increase in Tm3+ ion concentration in the Li and Mg series glasses, the τexp. values for 1G4 level are gradually decreased up to the lowest value for Li5 and Mg5 samples. For 1D2 level, Li2 sample possesses a higher τexp. value than Li1 sample, whereas within the experimental error, τexp. values of Mg1 and Mg2 samples match very closely. However, for the 1D2 excited state decay time values decrease upon Tm2O3 content increment in the Li and Mg series glasses (Li2→Li5 and Mg2→Mg5). This decay time decrement with an increase in Tm3+ concentration reveals the typical nature of concentration quenching. These results are also in agreement with the PL spectral studies (Figs. 2 and 3). It is interesting to note that for 1D2

upper energy level, Li1–Li5 samples exhibit longer lifetimes comparing with those of Mg1–Mg5 samples while for Li3 and Li4 samples, the 1G4 level decay time values are lower than the Mg3 and Mg4 samples 1G4 level decay times. Here, for Li and Mg series glasses, the gradual decrement of decay times with increasing Tm3+ ion concentration could be mainly due to the increased NR transitions through CR channels and ET between acceptor ions (ground state 3H6: Tm3+ ions) and donor ions (upper energy states 1D2 and 1G4: Tm3+ ions). Usually, for Tm3+-doped glasses, the intensity of blue or green emissions from 1D2 level is quenched through CR channels like (1D2, 3H6)→(3F2,3H4) and (1D2, 3 H6)→(3F3, 3F3), respectively, due to ET. Further, the blue or red emission intensity from 1G4 excited state could be depleted by the (1G4, 3 H6)→(3F4, 3F2), (1G4, 3H6)→(3H5, 3H4), (1G4, 3H6)→(3H4, 3H5) and 1 ( G4, 3H6)→(3F2, 3F4) CR channels [35]. 3.3. Tm3+-doped borotellurite glass 3.3.1. Optical features and decay time curve analysis including CIE color coordinates The XRD profile, which indicates the amorphous nature of the G1 glass, FTIR and Raman spectroscopy plots, and identified IR and Raman band assignment data of the G1 sample, are presented in the supplementary material as Fig. S1, and Table S1 and Table S2, respectively. Fig. 6 (a) shows the both PLE (left side) and PL (right side) spectra of G1 glass within the wavelength regions 300–445 nm and 375–700 nm under monitored emission wavelength at 453 nm, and 358 nm excitation, respectively. Here, only one strong excitation peak centered at 358 nm (3H6 → 1D2) is identified from the excitation spectrum. Similarly, following the PL spectrum, an intense blue emission band situated at 453 nm (1D2 → 3F4), and a weak emission band at 477 nm (1G4 → 3 H6) along with two additional fairly weak PL bands at 510 nm (1D2→ 3 H5) and 657 nm (1G4 → 3F4) are observed [33]. Comparing with PLE and PL spectral results of Li3 and Mg3 samples (Figs. 2 and 3), one can notice that G1 glass exhibits a lower PLE and/or PL intensity. Fig. 6 (a) 5

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Fig. 4. 0.5 mol% Tm3+-doped Li series glass (Li3) decay time profile and decay profile fit using a single exponential function for the (a) 453 nm emission (1D2 level) and a double exponential function for the (b) 476 nm emission (1G4 level) under 358 nm excitation wavelength.

Fig. 5. 0.5 mol% Tm3+-doped Mg series glass (Mg3) decay time profile and decay profile fit using a single exponential function for the (a) 453 nm emission (1D2 level) and a double exponential function for the (b) 476 nm emission (1G4 level) under 358 nm excitation wavelength.

inset illustrates the UV–Vis–NIR absorption spectrum for G1 glass within 350–1850 nm wavelength region. The absorption spectrum shows six bands centered at 467 nm, 656 nm, 685 nm, 790 nm, 1210 nm and 1686 nm, which are assigned to the transitions from 3H6 ground level to the excited states 1G4, 3F2, 3F3, 3H4, 3H5 and 3F4, respectively [31,33]. The identified absorption bands at ∼685 and ∼790 nm hint that G1 sample can be pumped effectively by using commercial 685 and 808 nm LDs. From G1 sample absorption spectrum, the evaluated experimental oscillator strengths (fexp.) were used to calculate the JuddOfelt [36,37] intensity parameters Ωt (t = 2,4,6) of Tm3+ ions. The parameters Ωt, determined by using a least-squares fitting approach, are included in Table 2. Here, the trend of Ω2 > Ω6 > Ω4 is obtained for G1 glass. The more asymmetry (higher Tm–O covalency) at the Tm3+ ion site, the larger the Ω2 value. By applying the J‒O parameter values (Ω2 = 3.69 × 10−20 cm2, Ω4 = 1.27 × 10−20 cm2 and Ω6 = 2.97 × 10−20 cm2), theoretical oscillator strengths (fcal.) and spectroscopic quality factor (χ = Ω4/Ω6) value are computed and presented in Table 2. Further, radiative parameters like electric-dipole (Aed), magneticdipole (Amd) and total radiative emission transition probabilities (AR), branching ratios (βcal) and radiative lifetimes (τrad) for various excited states of Tm3+ ions are evaluated for G1 sample using the evaluated J‒O parameters. All these parameters are listed in Table 3. The relevant formulae for the calculation of these radiative parameters can be found elsewhere [38,39]. For the G1 sample blue emission transitions 1 D2→3F4 and 1G4→3H6, the derived AR and βcal values are 2103 s−1 and 29%, and 479 s−1 and 20%, respectively. For the G1 glass, the weak emission at 477 nm might be due to the non-radiative decay through

multiphonon relaxation (MPR) from the 1D2 to 1G4 level, as the AR and βcal values for 1D2 → 1G4 transition are very low, 46 s−1 and 1%, respectively (Table 3). Fig. 6 (b) depicts the decay time profile for the 1D2 level of Tm3+ ions upon 358 nm excitation wavelength in the G1 sample. Here, the fit of the G1 glass decay curve is performed using the Inokuti-Hirayama (IH) model [40], where the related equations were reported in our recent work [10], to reveal the dominant mechanism of interaction. In this model, the 14.8 μs lifetime was taken as the time at which the luminescent intensity has decayed to (1/e) of its initial value. For Li3 and Mg3 samples, the 1D2 level decay times are 15.3 μs and 14.1 μs, respectively (see Table 1). For G1 sample, the best fit is attained with a dipole-dipole (d-d) interaction, though dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) interactions are also matched well for the decay fit. From Fig. 6 (b) inset curve fit data, one can see that the energy transfer parameter (G = 0.0358) value is very small, and therefore there exists a quite small energy migration between Tm3+ ions. For G1 sample, the evaluated quantum efficiency ƞ is 10.92%, as listed in exp Table 3 ƞ = × 100 was derived from 1D2 excited state calculated cal decay time (τcal. = 135.53 μs) (see Table 3) and measured decay time (14.8 μs). The content of Al2O3 10 mol% in the studied glasses resulted to be appropriate for a good solubility of Tm3+ ions into the glasses to attain such quantum efficiency. Under 358 nm excitation wavelength, the Commission Internationale de I'Eclairage (CIE) chromaticity (x, y) coordinates are computed for the overall emission of G1 glass (0.154, 0.015) from the 6

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doped Li and Mg series glasses upon 358 nm excitation wavelength are presented in Table 4 and corresponding CIE diagrams including color purity values are depicted in Fig. S2(a-j) (see supplementary material). The detailed procedure for the CP calculation of the dominant emission color in all the studied Tm3+-doped glasses can be found elsewhere [42]. 3.4. NIR luminescence Fig. 7(a–c) shows the NIR luminescence spectra of all the Li1–Li5, Mg1–Mg5, and G1 samples within the wavelength range of 1.35–1.6 μm upon 808 nm LD excitation. A broad asymmetric emission band with a maximum peak position located at 1458 nm, which corresponds to the Tm3+: 3H4→3F4 transition [35], is identified from all the spectra where this main peak remains practically at the same position. Due to the detector sensitivity higher limit, the NIR emissions were recorded only up to 1.6 μm wavelength. From Fig. 7 (a) and (b), one can notice that the NIR fluorescence intensity for Li1–Li5 and Mg1–Mg5 samples increased with an increase in Tm2O3 concentration from 0.1 → 2.0 mol% without any luminescence concentration quenching effect. This indicates that the CR processes play an insignificant role for the observed 3 H4→3F4 emission transition, for the selected borosilicate glasses composition. Here, the evaluated Full-Width at Half-Maximum (FWHM) of the 1458 nm band, for the Li5, Mg5 and G1 samples, are 117 nm, 125 nm and 118 nm, respectively, which is much higher than ZBLAN glass (76 nm) [43], and slightly larger than the Tm3+-doped calcium fluoroborate glass (112 nm) [35]. Thus, Li5, Mg5 and G1 samples show potentiality for broadband optical amplifier application, mainly in the spectral region that coincides with the EDFAs transmission region. Moreover, another NIR emission at ∼1.8 μm (3F4→3H6 transition) strongly depends on the ET (via CR channel (3H4, 3H6→3F4, 3 F4), see Fig. 1) to expand the transmission capability into the 1.6–1.9 μm region, when the Tm3+ ions are directly pumped to 3H4 level with 794 nm excitation or 808 nm LD [44]. Fig. 7 (d) depicts the comparison of 1458 nm NIR fluorescence band intensity for all Tm3+-doped Li and Mg series glasses upon 808 nm LD excitation. Here, it is noticed that Li5 sample possesses relatively higher luminescence intensity for the 3H4→3F4 band when compared to Mg5 glass. Based on the above presented visible luminescence characteristics and NIR emission spectral features of the investigated Tm3+-doped multicomponent borosilicate and borotellurite glasses, we suggest that these glasses can have potential applications in blue lasers and S-band optical amplification. Moreover, for the studied borosilicate samples, Tm3+ ion exhibits broadband and shows no luminescence quenching effect up to 2 mol% doping concentration for 1.458 μm emission, which is promising for practical application in optoelectronics and photonics where suitable higher RE ions like Dy3+, Er3+, Yb3+, Pr3+, Tb3+, etc doping is desirable in white-light-emitting diodes (W-LEDs), NIR fiber lasers, and upconverter and downconverter luminescent materials for photovoltaic solar cells application. Fig. 6. (a) Excitation (λem. = 453 nm) and emission (λex. = 358 nm) spectra of G1 glass (inset shows UV–Vis–NIR absorption spectrum of G1 glass), (b) Decay time profile and decay profile fit results using the IH model for glass G1 under λex. = 358 nm and λem. = 453 nm and (c) CIE chromaticity diagram for glass G1 under 358 nm excitation wavelength including color purity (CP). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4. Conclusions Tm3+-doped multicomponent borosilicate and borotellurite glasses were synthesized by the melt-quench technique, and visible fluorescence, decay times and NIR optical features have been studied. From the energy level positions and the intensity analysis of absorption bands, the J‒O intensity parameters (Ω2 = 3.69 × 10−20 cm2, Ω4 = 1.27 × 10−20 cm2, and Ω6 = 2.97 × 10−20 cm2) have been evaluated for 0.5 mol% Tm3+-doped borotellurite sample. The value of Ω2 is found to be larger than the Ω4 or Ω6 value, indicating a larger asymmetry (higher covalence) at the Tm3+ ion site into the borotellurite glass. Using the derived J–O parameters, for various upper energy levels of the Tm3+ ion, several radiative parameters such as Aed and Amd dipole transition probabilities, AR, radiative transition

emission spectrum (Fig. 6 (a), and illustrated in a CIE diagram (see Fig. 6 (c)). The (x, y) coordinates computation method in detail can be found elsewhere [41]. It is identified that such CIE color coordinates represent a blue tonality of 455 nm as dominant wavelength and color purity of 97.6%. Similarly, CIE color coordinates derived from the visible emission spectra (Figs. 2 (b) and Fig. 3 (b)) of all the Tm3+7

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Table 2 Absorption band assignments (from the ground state, 3H6), energy (cm−1), experimental (fexp × 10−6) and calculated (fcal × 10−6) oscillator strengths, residual quantities ( × 10−6) of “G1” glass along with J‒O parameters. Wavelengths (nm) correspond to average transition energies. Levels

Wavelength

Energy

fexp

fcal

Residuals

3

1685 1200

5936 8333

2.832 2.908

0.093 0.052

798 685 659 468

12529 14603 15184 21371

2.739 3.448 ed 2.960 md 0.488a 4.865 5.452 0.415 2.108 0.696

4.864 5.719 1.573 0.890

0.001 0.266 1.158 1.218

F4 H5

3 3

H4 F3 3 F2 1 G4 3

RMS deviation ( ×10 6)

O parameters (× 10

J

20

cm2)

Table 4 CIE Color coordinates (x,y) derived from the visible emission spectra (Figs. 2 (b) and Fig. 3 (b)) of all the Tm3+-doped Li, and Mg series glasses. Sample code

Li1 Li2 Li3 Li4 Li5 Mg1 Mg2 Mg3 Mg4 Mg5

3.69 ± 0.02 1.27 ± 0.02 2.97 ± 0.03 0.427

2 4 6

Spectroscopic quality factor (χ)

Table 3 Emission transitions (SLJ → S′L′J'), wavelengths (nm), electric (Aed, s−1), magnetic (Amd, s−1) and total radiative transition probabilities (AR, s−1), calculated branching ratios (βcal, %), and radiative (τrad, μs) lifetimes of “G1” glass. Measured lifetime (τmeas, μs) and quantum yield (η, %) of 1D2 higher energy level are also included. λemi

Aed

Amd

AR

βcal

τrad

126

100

7958.086

2 293

1 99

3386.474

55 62 962

5 6 89

926.972

1 116 215 1536

0 6 12 82

535.564

0 4 115 200 457

0 1 15 26 59

1288.551

10 55 29 713 1061 479

0 2 1 30 45 20

426.095

τmeas

3

F4

3

H6

1685

126

3

F4 H6

4171 1200

2 254

H5 F4 H6

2383 1517 798

40 62 962

H4 H5 3 F4 3 H6

4822 1595 1154 685

1 116 168 1536

3

F3 H4 H5 3 F4 3 H6

17195 3766 1460 1081 659

0 4 115 200 457

3

1616 1478 1131 767 657a 477a

10 55 29 713 1061 479

3

H5

3 3

H4

3 3 3

3

F3

3 3

3

F2

3 3

1

G4

F2 F3 H4 3 H5 3 F4 3 H6 3 3

1

D2

1

G4 F2 F3 3 H4 3 H5 3 F4 3 H6 10.920 3

3

Η a

1506 780 746 646 510a 453a 358a

46 352 887 1428 73 2103 2489

39 15

47 0

46 352 887 1428 73 2103 2489

1 5 12 19 1 29 34

x

y

0.147 0.148 0.149 0.148 0.149 0.145 0.147 0.146 0.148 0.148

0.026 0.024 0.022 0.021 0.022 0.025 0.024 0.023 0.021 0.021

transition with respect to all other samples, exhibiting their potentiality for blue laser application at 453 nm wavelength. The possible ET through CR mechanisms: (1D2, 3H6)→(3F2,3H4) and (1D2, 3H6)→(3F3, 3 F3) is explained for the blue luminescence concentration quenching observed in borosilicate glasses. For Li3 and Mg3 samples, the evaluated 1D2 level decay times were 15.3 μs and 14.1 μs, respectively. For G1 sample, the decay curve was better fit to the IH model for S = 6, indicating that the ET between Tm3+ ions could be dominated by an electric dipole-dipole interaction. However, the ET parameter (G = 0.0358) value is very small. The CIE chromaticity coordinates and color purity for all the fabricated glasses were evaluated from their corresponding visible emission spectra (λex. = 358 nm), and these coordinates and color purity values represent a blue tonality in the CIE diagram. Among Li3, Mg3 and G1 glasses, for which each sample possesses an equal amount of Tm3+ ion mol% concentration, Li3 sample (39.5 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 Li2O−0.5 Tm2O3 mol%) is best suited for blue laser application due to its’ relatively higher emission intensity and luminescence decay lifetime value for 453 nm emission peak under 358 nm excitation. Upon 808 nm LD excitation, the NIR fluorescence spectra of all the Tm3+-doped samples displayed a peak centered at 1.458 μm, corresponding to the 3H4→3F4 transition. Moreover, for all Tm3+-doped Li and Mg series borosilicate samples, the NIR emission band intensity gradually increased from 0.1 to 2.0 mol% Tm2O3 content without any luminescence quenching with concentration, indicating that there are no effective CR mechanisms (e.g. 3H4, 3H6→3F4, 3F4)) involved in this NIR emission. The evaluated NIR optical features suggest that the studied glasses might be suitable for 1.458 μm optical fiber amplifiers. Particularly, Li5 sample (38 B2O3−10 SiO2−10 Al2O3−30 ZnO−10 Li2O−2.0 Tm2O3 mol%) exhibits relatively higher NIR luminescence intensity at 1.458 μm compared to Mg5 and G1 glasses, suggesting Li5 sample potentiality for Sband optical amplifier application.

a Magnetic dipole (md) oscillator strength (0.488× 10−6) was subtracted from experimental oscillator strength (3.448 × 10−6) for JO analysis.

Transition

Color coordinates (x,y)

Declaration of competing interest 135.530

a

14.8

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF2018R1A5A1025137). The author G.L.N would like to acknowledge the use of the facilities of Universiti Putra Malaysia (UPM), Malaysia. The author (A. Dahshan) gratefully thank the Deanship of Scientific Research at King Khalid University for financial support through research groups program under grant number (R.G.P.2/34/40).

Experimental data.

probabilities, βR, calculated branching ratios and τR, radiative lifetimes were calculated. For the studied borosilicate glasses, following the visible emission spectra, it is identified that the 0.5 mol% Tm2O3 doped samples (Li3 and Mg3) possess higher intensity for 1D2 → 3F4 emission 8

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Fig. 7. NIR fluorescence spectra for all the Tm3+-doped (a) Li series and (b) Mg series glasses, and (c) G1 glass, upon 808 nm excitation and (d) Comparison of fluorescence intensity for Tm3+-doped Li and Mg series glasses at λex = 808 nm LD excitation and λem = 1458 nm.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.109354.

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