Enhanced broadband near-infrared luminescence from Pr3+-doped tellurite glass with silver nanoparticles

Optical Materials 73 (2017) 102e110

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Enhanced broadband near-infrared luminescence from Pr3þ-doped tellurite glass with silver nanoparticles Pan Cheng, Yaxun Zhou*, Minghan Zhou, Xiue Su, Zizhong Zhou, Gaobo Yang College of Information Science and Engineering, Ningbo University, Zhejiang 315211, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 April 2017 Received in revised form 5 June 2017 Accepted 21 July 2017

Pr3þ-doped tellurite glasses containing metallic silver NPs were synthesized by the conventional meltquenching technique. Structural, thermal and optical properties of the synthesized glass samples were characterized by X-Ray diffraction (XRD) curves, Raman spectra, differential scanning calorimeter (DSC) curves, transmission electron microscopy (TEM) images, UV/Vis/NIR absorption and near-infrared fluorescence emission spectra. The XRD curves confirmed the amorphous structural nature of the synthesized glasses, the Raman spectra identified the presence of different vibrational groups, the DSC curves verified the good thermal stability, and the TEM images revealed the nucleated silver NPs with average diameter about 10 nm dispersed in the glass matrix and its surface Plasmon resonance (SPR) absorption band was located at around 510 nm. Besides, Judd-Ofelt intensity parameters Ut (t ¼ 2, 4, 6) and other important spectroscopic parameters like transition probability, radiative lifetime, branching ratio were calculated to evaluate the radiative properties of Pr3þ levels from the measured optical absorption spectra. It was found that Pr3þ-doped tellurite glasses could emit an ultra-broadband fluorescence extending from 1250 to 1650 nm under the 488 nm excitation, and this fluorescence emission increased further with the introduction of silver NPs. The enhanced fluorescence was mainly attributed to the increased local electric field around Pr3þ induced by silver NPs. The present results demonstrate that Pr3þ-Ag codoped tellurite glass is a promising candidate for the near-infrared band ultra-broadband fiber amplifiers covering the expanded low-loss communication window. © 2017 Elsevier B.V. All rights reserved.

Keywords: Tellurite glass Pr3þ doping Broadband near-infrared fluorescence Silver NPs

1. Introduction Nowadays, the extensive multimedia data transmissions on the internet, such as high definition TV, ultrahigh-definition video, video-on-demand (VOD) and digital cinema, brought an explosive growth for the demand in network traffic, which stimulates the rapid development of optical communication systems with ultrabroad bandwidth [1,2]. Accordingly, developing ultra-broadband optical fiber amplifiers that can cover the low-loss wavelength region from 1200 to 1700 nm of silica transmission fiber to establish more efficient wavelength-division-multiplexing (WDM) communication networks has become very urgent [3,4]. Many previous researches have demonstrated that rare-earth ions such as Er3þ, Tm3þ, Ho3þ, and Pr3þ are the effective dopants, which can be used as active ions in optical fiber amplifiers operating at C-, L-, S-, and O-bands in the communication window [5e8]. Meanwhile, some

* Corresponding author. E-mail address: [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.optmat.2017.07.044 0925-3467/© 2017 Elsevier B.V. All rights reserved.

transition metal ions such as Biþ, Niþ, Crþ, and Pbþ have also been investigated largely for the potential broadband near-infrared luminescence [9e12]. Unfortunately, the complex preparation technique and the still controversial luminescent mechanism for the transition metal ions have hindered their applications. Compared with other rare-earth ions, trivalent Pr3þ exhibits attractive multichannel emission characteristics in the visible and near-infrared band regions due to the 4f2 configuration with 91 degenerate energy levels [13]. Particularly, its ultra-broadband near-infrared luminescence covering the wavelength range from 1.2 to 1.7 mm is very desirable for optical amplifiers operating at O-, E-, S-, C-, and L-band simultaneously. At present, the fluorescence emissions of Pr3þ located at 1.3 mm (Pr3þ: 1G4/3H5), 1.47 mm (Pr3þ: 1 D2/1G4) and 1.6 mm (Pr3þ: 3F4,3 / 3H4) have been observed in a variety of glass hosts [14e16]. However, due to the concentration quenching effect, the doped concentration of Pr3þ in the glass hosts must be kept at a low level [17], which leads to the fluorescence emission is weak. Recently, coupling rare-earth ions with metallic nanoparticles (NPs) has been developed as a valuable strategy to

P. Cheng et al. / Optical Materials 73 (2017) 102e110

modify the structural properties and in turn improve the emission efficiency of doped ions due to the local field enhanced effect induced by surface Plasmon resonance (SPR) of metallic NPs [18e20]. When the frequency of the excitation beam and/or the luminescence frequency of doped rare-earth ions are close to the frequency of local SPR band of metallic NPs, the induced increased local electric field can contribute to the enhancement of luminescent efficiency of the ions in the vicinity. For example, Y.Y. Du et al. [18] have synthesized germanium-tellurite glasses incorporated with Pr3þ and metallic silver NPs by melt-quenching method, and an about 36% enhancement in seven visible-band emissions was found in comparison with the glass without silver NPs. G. Lakshminarayana et al. [21] have reported the Plasmon induced fluorescence emissions in Pr3þ-doped tellurite glasses, and an obvious enhancement in visible-band emissions (530, 594, 612 and 645 nm) was demonstrated with the incorporation of silver NPs. However, there are few reports up to now about the enhancement of nearinfrared band fluorescence emissions of Pr3þ ions induced by metallic NPs. It is known that the photo-physical properties of doped rareearth ions are sensitive to the chemical environment of glass host, therefore, the choice of host material is very important. Compared with other glass hosts, tellurite glass possesses many excellent properties, such as [22e24]: (1) has a fairly wide transmission range (0.35e5 mm) compared with silicate glass with only 0.2e3 mm, (2) has good glass stability, rare-earth ion solubility and corrosion resistance compared with fluoride and sulfide glasses, (3) has high refractive index n (~1.8e2.3) and non-linear refractive index coefficient n2 (~2.5  1019 m2/W) compared with fluoride glass (n~1.5, n2~1021 m2/W) and silica glass (n~1.46, n2~1020 m2/ W), (4) has a low phonon energy among the oxide glasses, and (5) has a low melting point (~800  C) and is easy to be prepared. In the present work, the Pr3þ-doped tellurite glass containing metallic silver NPs were synthesized using melt-quenching technique for the potential applications in broadband amplifiers and tunable lasers, and the structural, thermal and spectroscopic properties were investigated with focus on the improved effect of photo-luminescence of Pr3þ induced by metallic silver NPs. The silver NPs were nucleated and grown by subjecting the synthesized glasses to the controlled heat-treatments above the glass transition temperature. The photo-luminescence of Pr3þ was studied in the near-infrared band range extending from 1.3 to 1.6 mm under the excitation at 488 nm.

2. Experimental procedures The Pr3þ-doped tellurite glasses with molar compositions 75TeO2-15ZnO-(9.75-x) Na2O-0.25Pr2O3-xAgNO3 (x ¼ 0, 0.1, 0.25, 0.5 and 1.0 mol%) were prepared using conventional meltquenching technique, in which component Na2O was introduced in Na2CO3 compound due to its stability. Hereafter, the prepared

103

glass samples were respectively named as PA0, PA1, PA2, PA3 and PA4 with AgNO3 amount for a better readability. Batches of 10.0 g ingredient chemicals were weighed and ground thoroughly in an agate mortar to obtain homogeneous mixture. The well-mixed powder was placed in a platinum crucible and melted in an electric furnace at temperature of 900  C for 1 h. The obtained glass melt was poured onto a brass mold preheated at 320  C to avoid thermal shock and annealed for 2 h below the glass transition temperature to release mechanical stress and then cooled slowly down to the ambient temperature. To thermally reduce Agþ ions to Ag0 atoms and then nucleate and grow into the metallic silver NPs, all the prepared glass samples were polished and heat-treated again at 360  C for 6 h in the muffle furnace. The prepared glass samples were characterized by the physical and spectroscopic measurements which were carried out at room temperature, and the obtained physical parameters are listed in Table 1. In which, the glass sample density was measured based on the Archimede's principle using pure water as an immersion liquid. The refractive index was measured using a prism coupler (Sairon Tech-SPA4000 TM) at a wavelength 632.8 nm. The thermal stability was evaluated using a differential scanning calorimeter (DSC) of TA Instrument Q2000 at a heating rate of 10 K/min. The powder X-ray diffraction (XRD) spectrum was recorded using a power diffractometer with Cu Ka radiation (40 kV  25 mA) and a graphite monochromator. The image of nucleated silver NPs in glass matrix was captured by a transmission electron microscopy (TEM 2100, JEOL) with an acceleration voltage of 200 kV. The Raman spectrum was measured with a Renishaw Micro-Raman instrument. The UV/ Vis/NIR absorption spectrum was recorded by a Perkin-ElmerLambda 950 spectrophotometer. The near-infrared fluorescence emission spectrum was collected by Jobin Yvon Fluorolog spectrophotometer equipped with a photomultiplier tube (PMT) detector. The excitation spectrum was recorded using the same setup and the excitation source was tuned from a continuous xenon lamp by a monochromator.

3. Results and discussion 3.1. Structural behavior Fig. 1 displays the measured X-ray diffraction (XRD) spectra of Pr3þ-doped and Pr3þ-Ag codoped glass samples. The diffractograms of the synthesized glasses are similar in shape with the absence of any discrete or sharp diffraction peaks but with a broad hump centered at about 2q ¼ 28 , indicating the lack of long-range atom periodicity arrangement, or the glass structure possesses amorphous structural nature [25]. In addition, the characteristic peak of silver NPs is not observed probably due to the extremely small volume fraction [19]. Transmission electron microscopy (TEM) image can reveal the presence of silver NPs precipitated in the glass matrix. Fig. 2(a) and

Table 1 The average molecular mass (Mav, g/mol), refractive index (n), thickness (d, mm), density (r, g/cm3), concentration (N,  1020/cm3), glass transition temperature (Tg ), crystallization onset temperature (Tx ) and the difference ðDT ¼ Tx  Tg Þ of the studied glass samples. Glass

Mav

n

d

r

N (Pr3þ)

N (Ag)

Tg

Tx

DT

Reference

PA0 PA1 PA2 PA3 PA4 NPZ TWP ZBP

144.79 144.86 144.95 145.11 145.43 e e e

2.010 2.012 2.015 2.019 2.020 e e e

1.80 1.66 1.70 1.80 1.84 e e e

4.97 4.75 4.69 4.62 4.49 e e e

1.03 0.98 0.97 0.95 0.93 e e e

e 0.41 0.99 1.92 3.72 e e e

340.6 339.5 338.8 337.4 336.5 275.0 397.0 536.9

454.1 456.7 458.0 459.8 461.4 378.0 518.0 627.0

113.5 117.2 119.2 122.4 124.9 103.0 121.0 90.1

Present Present Present Present Present [37] [39] [40]

work work work work work

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P. Cheng et al. / Optical Materials 73 (2017) 102e110

Fig. 1. The XRD patterns of PA0, PA1, PA2, PA3 and PA4 glass samples.

(b) present the TEM images of Pr3þ-doped PA3 glass sample incorporating 0.5 mol% amount of AgNO3 with different resolutions of 50 and 20 nm, respectively. The black particles in the TEM image clearly display the existence of near-spherical silver NPs with different sizes, which disperse uniformly in the glass matrix. Fig. 2(c) gives the selected area electron diffraction (SAED) pattern, in which the small bright circular spots also indicate the presence of silver NPs, which corresponds to (111), (200), and (220) plane reflections of face-centered cubic structure of silver NPs [26]. Fig. 2(d)

presents the size histogram of silver NPs in the PA3 glass sample and it is shown that the size distribution of silver NPs exhibits Gaussian characteristics with average diameter of about 10 nm. The silver NPs in the glass matrix originates from the thermo-chemical reduction reactions [27]. The Ag0 atoms first occur during the glass melting process through the reduction pathways, and next different sizes of metallic silver NPs generate through the nucleation of Ag0 atoms and growth of Ag particles via the mechanisms of Ostwald ripening phenomenon, in which large particles are grown through the feeding by smaller particles and in turn the smaller particles start to vanish [28]. Apart from the silver NPs, it is worthy to mention that the existence of different species of silver such as silver ions, dimers and trimers is also possible [29e31] since Agþ ions in glass matrix are movable and possess a strong trend to aggregation. Fig. 3 displays the measured Raman spectra of PA0 and PA3 glass samples with and without silver NPs. The Raman spectra are very similar in shape with three major broad peaks in two glass samples but with obvious distinctive vibration intensities. The broad bands of Raman spectra are attributed to the disorderliness of glass structure. Among them, the band located at 440 cm1 indicates the presence of symmetric stretching (bending) vibrations of Te-O-Te linkages between the various Te-based structural units, which are formed by vertex sharing of TeO4, TeO3þ1 and TeO3 polyhedra in the studied glasses [32]. The band located at 670 cm1 is attributed to the asymmetric stretching vibrations of the continuous network composed of TeO4 trigonal bipyromids (tbps). The TeO4 tbps is featured by a 5s2 electron lone pair in one of the five sp3d hybrid orbits, and therefore likely to possess higher linear and nonlinear optical properties [33]. The band located at 760 cm1 corresponds

Fig. 2. (a) The TEM image with resolution of 50 nm, (b) the TEM image with resolution of 20 nm, (c) the SAED pattern of silver NPs and (d) the histogram of silver NPs size distribution of the PA3 glass sample.

P. Cheng et al. / Optical Materials 73 (2017) 102e110

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the table that the DT value in the studied glasses is larger than that of other glass systems [39,40], indicating the synthesized tellurite glasses possess good thermal stability. It is also noted that the difference DT increases slightly with the introduction of metallic silver NPs in the glass matrix, implying the thermal stability of glass materials is somewhat improved. The increased DT comes from the combined contributions of the decreased glass transition temperature Tg due to the reduced viscosity of glass matrix as well as the increased glass crystallization onset temperature Tx with the addition of silver NPs. 3.3. Optical absorption spectrum and Judd-Ofelt analysis

Fig. 3. The Raman spectra of PA0 and PA3 glass samples.

to the asymmetric vibrations of linkages associated to the TeO3 structural units [34]. According to the literature [35], the decrease in the vibration intensity of 440 cm1 Raman band indicates a cleavage of Te-O-Te linkage and a formation of non-bridging oxygen (NBO) atom, which is consistent with the conversion of TeO4 tbps into TeO3 polyhedra having one NBO atom. Thus, the introduction of silver NPs to tellurite glass establish more Te-O-Te linkages, leading to an increase in the Te coordination number and the vibration intensity of 440 cm1 Raman band in Pr3þ-Ag codoped tellurite glass. Similarly, the vibration intensities of other two Raman bands in the spectrum also get a significant enhancement in the Pr3þ-Ag codoped glass. This enhancement in Raman spectrum is mainly attributed to the excitation of surface Plasmons of silver NPs, which originates from the incident field action together with the strong localized secondary field of Raman scatterers (molecules) [36]. 3.2. Thermal stability Thermal stability is defined as the resistance ability to any change in properties caused by heat effect. When applications for glass materials such as fiber drawing, it will undergo a reheating process, in this period the fiber preform is subjected to various heating cycles, then crystallization could occur in the glass during the heating cycles. The formed crystals will attenuate rather than amplify the input optical signal, and thus requires the prepared glass should possess good thermal stability. The thermal stability is usually characterized by the physical parameter DT ð¼ Tx  Tg Þ, i.e. the difference between the glass transition (Tg ) and onset crystallization (Tx ) temperatures [37]. These two temperatures Tg and Tx are identified at the baseline inflexion in the DSC curves related with heat capacity change between the glass and viscoelastic state [38]. Obviously, a large difference DT is beneficial to suppress the crystallizing action as a wide range of working temperature can be allowed. Therefore, it is desirable for a glass host to have DT as large as possible. Generally speaking, more than 100  C for DT can meet the requirement of conventional fiber drawing process. As representative, the measured differential scanning calorimeter (DSC) curves of Pr3þ-doped (PA0) and Pr3þ-Ag codoped (PA3) glass samples are displayed in Fig. 4, and the obtained glass transition temperature Tg , crystallization onset temperature Tx and the difference DT of the studied glass samples together with the some reported glass hosts are summarized in Table 1. As can be seen from

The absorption properties of doped rare-earth ions depend on their surrounding environments and the interactions with ligand field in the glass matrix. The recorded absorption spectra of Pr3þdoped (PA0) and Pr3þ-Ag codoped (PA3) glass samples are displayed in Fig. 5. In the measured 400e2000 nm wavelength range, the absorption spectra consist of eight inhomogeneously broadening absorption bands centered at 446, 473, 485, 594, 1013, 1445, 1533 and 1943 nm, which correspond to the transitions from 3H4 ground state to the various excited states 3P2, [3P1,1I6], 3P0, 1D2, 1G4, 3 F4, 3F3 and 3F2, respectively. These transitions come mainly from the electric-dipole transition contribution and only the 3H4/1G4, 3 F4,3 transitions have a negligible magnetic-dipole contribution. It is worth noting that the 3H4/3P2 transition is hypersensitive in nature as it depends strongly on the neighbouring ligand field and is governed by the selection rules [41]. Meanwhile, there is no obvious change in the position of absorption bands for different glasses owing to the shielding nature of 4f electrons by outermost orbital's and only the intensities are somewhat affected by the surrounding ligand environment due to the composition variation [42]. The surface Plasmon resonance (SPR) absorption related to the metallic silver NPs is not identified in the Pr3þ-Ag codoped glass sample probably due to the small amount of silver NPs which is not large enough to generate a noticeable SPR signal [19] and thus is obscured by the intense absorption bands of Pr3þ. Therefore, in order to monitor the SPR band, one Pr3þ free glass sample containing 1.0 mol% AgNO3 with composition 75TeO2-15ZnO-9Na2O1AgNO3 (named as TZNA) was synthesized with the same prepared technique. As it can be seen in the inset of Fig. 5, a weak SPR absorption band appears at about 510 nm, verifying again the

Fig. 4. The DSC curves of PA0 and PA3 glass samples.

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P. Cheng et al. / Optical Materials 73 (2017) 102e110

(~1500 cm1) leads to the negative value [52], which causes a breakdown in the energy difference approximation used to resolve the emission and absorption coefficients in the Judd-Ofelt model [53]. Thus, in order to improve the calculated reliability of JuddOfelt parameters, two chief measures have been proposed. In one case, the standard Judd-Ofelt theory is used to obtain positive parameters with reduced number of transitions excluding the hypersensitive 3H4/3P2 transition from the calculations [54]. In other one case, a modified Judd-Ofelt theory is applied to solve the energy difference problem and succeed in obtaining the positive parameters of Pr3þ ions [55]. According to the modified Judd-Ofelt theory [51,56], the experimental oscillator strength (fexp ) for the different absorption bands can be expressed as:

fexp ¼

Fig. 5. The absorption spectra of PA0 and PA3 glass samples. The inset displays the SPR band (510 nm) of TZNA glass sample.

existence of metallic silver NPs in the glass matrix. This absorption band originates from the resonant effect of incident photons with the collective oscillations of conduction electrons [43]. According to the Mie theory [44], the peak wavelength (lpeak ) of SPR absorption can be estimated by the following expression:

l2peak ¼ ð2pcÞ2 mNe2


 ε∞ þ 2n2 ε0

(1)

where ε∞ is the dielectric function of metallic NPs, ε0 is the permeability of free space, n is the refractive index of glass host, c is the speed of light, m is the effective mass of conduction electron and N is the concentration of free electron. Obviously, the lpeak of SPR absorption band is strongly dependent on the refractive index (n) of glass host, and by increasing the n of the surrounding medium, the SPR band will red-shift to a longer wavelength region. In the case of sodalime silicate glass with n ~1.5, the SPR band of silver NPs is reported to be located at about 410 nm [45]. Therefore, in the present work for the tellurite glass with n ~2, the peak wavelength of SPR absorption band should have a larger value (~510 nm) as observed above. With the aid of Judd-Ofelt theory [46], the absorption spectra can be further used to predict the spectroscopic properties of Pr3þ ions in the studied glasses. By fitting the experimental and theoretical oscillator strengths using the least-square procedure provided in Ref. [47], the three Judd-Ofelt intensity parameters Ut (t ¼ 2, 4, 6) of Pr3þ can be determined. The Judd-Ofelt intensity parameters provide information about the local structure and chemical bonding between the rare-earth ions and ligand anions [20]. The ligand can significantly affect the inter-configurational transitions in terms of number of lines and the ratio of transition intensities [48]. Among the three intensity parameters, U2 is sensitive to the degree of covalency between the rare-earth ions and ligand anions along with the asymmetry of local environment near rare-earth ion site, while U4 and U6 are related to the long-range effects and the rigidity of the glass system [49]. However, it has been reported that the parameter U2 value of Pr3þ ions in some glass hosts is negative by using the standard Judd-Ofelt theory [50,51], which is incompatible with the fact that the Ut (t ¼ 2, 4, 6) parameters should be numerically positive. It has been argued that the 4f2 ground configuration lying very close to the first opposite parity excited configuration (4f15d1) of Pr3þ ions

2:303mc2

pe2 Ndl

Z

2

ODðlÞdl

(2)

where ODðlÞ is the measured absorption optical density, l is the mean wavelength of absorption band, N is the concentration of doped rare-earth ions, d is the sample thickness, and m, e, and c have their usual meaning. The theoretical oscillator strength (fcal ) for an electric-dipole allowed transition from an initial state jS; L; J〉 to the final state jS0 ; L0 ; J 0 i is expressed by:

fcal ¼

8p2 mc 3hlð2J þ 1Þ



2
2 n þ2  Sed 9n

(3)

where J is the total angular momentum for the lower state, n is the refractive index, ðn2 þ 2Þ2 =9n is the local field correction for electric-dipole transition, and Sed is the spectral line strength:

Sed ¼

X





Ut 1 þ 2a EJ þ EJ 0  2Ef0



 j〈S; L; JjjU l jjS0 L0 J 0 ij

2

(4)

t¼2;4;6

where Ut are the modified Judd-Ofelt parameters, and a is a parameter defined in Ref. [55] that has a value about 105 cm1 for Pr3þ ions. EJ and EJ 0 are the energy of initial and final manifolds involved in the transition. The energy Ef0 is the center of gravity of     the 4f configuration and equals 9940 cm-l for Pr3þ ions. U l  is the reduced matrix element corresponding to absorption transition and its value is almost insensitive to the host environment. Thus, the above modified theoretical oscillator strength is used to fit the experimental oscillator strength for the different transitions (except for magnetic-dipole transitions that satisfy the selection rules as DS ¼ DL ¼ 0, DJ ¼ 0, ±1), and the obtained Judd-Ofelt parameters for all glass samples (PA0-PA4) are listed in Table 2.

Table 2 Judd-Ofelt parameters of Pr3þ ions for the PA0-PA4 and the other reported glass hosts. Glass system

PA0 PA1 PA2 PA3 PA4 Fluorotellurite Lead telluroborate Fluorozirconate Phosphate

Judd-Ofelt parameter (1020 cm2)

Reference

U2

U4

U6

3.09 3.13 3.25 3.32 3.69 3.57 3.07 6.2 6.86

5.87 5.95 6.10 6.24 6.61 6.60 3.36 3.7 1.92

4.53 4.71 4.89 5.01 5.26 5.18 8.62 8.4 2.93

Present Present Present Present Present [17] [51] [54] [56]

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P. Cheng et al. / Optical Materials 73 (2017) 102e110

It is found that the three Judd-Ofelt parameters follow a trend

U2 < U6 < U4 in this work, and the value of U2 parameter is com-

parable to the values in the fluorotellurite [17] and lead telluroborate [51] glasses, but smaller than in the fluorozirconate [54] and phosphate [56] glasses. The smaller the value of U2 , the more centro-symmetric of the ion site and the more ionic of its chemical bond with the ligand [44]. In addition, the value of U2 increases slightly with the concentration of silver NPs in the glass matrix, indicating the decreased symmetry and the increased covalency around Pr3þ ions. With the three Judd-Ofelt parameters, some important spectroscopic parameters such as the spontaneous radiative transition probabilities (Arad ), radiative lifetimes (trad ) and fluorescence branching ratios (b) from one level to other levels can be calculated. The radiative transition probability Arad from an excited state (J) to a lower lying state (J 0 ) is given by:

Arad ðJ/J 0 Þ ¼

"
# 2 n n2 þ 2 64p4 n3 3 S þ n S ed md 9 3ð2J þ 1Þhc3

Table 3 The radiative transition probability Arad (s1), fluorescence branching ratio b (%) and radiative lifetime trad (ms) of the PA3 glass sample.

A

J0

ðJ/J 0 Þ Arad ðJ/J 0 Þ

(5)

(6)

The radiative lifetime trad of an emitting level is inversely proportional to its total transition probabilities, as given by:

trad ¼ P

1

J 0 Arad ðJ/J

0Þ

Transitions

Energy (cm1)

Arad (s1)

b (%)

trad (ms)

3

3925 10805 13727 14121 15478 16378 18545 20619 6880 7650 9802 10196 11553 12453 14620 16694 2922 3316 4673 5573 7740 9814 394 1751 2651 4818 6892 1357 2257 4424 6498

152.01 10622.51 36791.25 0 9225.03 38805.89 0 156703.66 2090.69 2874.15 3836.74 1920.23 4360.44 4984.93 1505.73 12557.86 103.49 41.97 78.55 798.62 1735.75 757.90 0.12 7.70 154.30 642.47 1276.86 4.41 87.37 510.55 2239.16

0.06 4.21 14.58 0 3.66 15.38 0 62.12 6.13 8.42 11.24 5.63 12.78 14.61 4.41 36.79 2.94 1.19 2.23 22.71 49.36 21.55 0.01 0.37 7.41 30.87 61.34 0.16 3.07 17.97 78.80

3.96

P0/1D2 3 P0/1G4 3 P0/3F4 3 P0/3F3 3 P0/3F2 3 P0/3H6 3 P0/3H5 3 P0/3H4 1 D2/1G4 1 D2/1G14 1 D2/3F4 1 D2/3F3 1 D2/3F2 1 D2/3H6 1 D2/3H5 1 D2/3H4 1 G4/3F4 1 G4/3F3 1 G4/3F2 1 G4/3H6 1 G4/3H5 1 G4/3H4 3 F4/3F3 3 F4/3F2 3 F4/3H6 3 F4/3H5 3 F4/3H4 3 F3/3F2 3 F3/3H6 3 F3/3H5 3 F3/3H4

where Sed and Smd are the electric- and magnetic-dipole transition line strengths, respectively. The fluorescence branching ratio b is defined as:

b ¼ P rad

107

29.30

284.39

480.43

351.93

(7)

The calculated radiative transition probabilities, fluorescence branching ratios and radiative lifetimes of 3P0, 1D2, 1G4, 3F4 and 3F3 levels are listed in Table 3 for the PA3 glass sample as representative. It is seen that the transitions of 3P0/3H4, 1D2/3H4, 1G4/3H5, 3 F4/3H4 and 3F3/3H4 have the greatest transition probabilities and branching ratios among the 3P0, 1D2, 1G4, 3F4 and 3F3 levels, respectively, indicating the intense fluorescence emissions from these radiative transitions can be expected. In which, the latter three fluorescence emissions are located at near-infrared band region.

G4/3H5, 1D2/1G14, 1D2/1G4, 3F4/3H4 and 3F3/3H4 transitions, respectively. It can be seen that the broadband near-infrared emission of Pr3þ ions with effective spectral width R (Dleff ¼ IðlÞdl=Imax ) about 234.8 nm covers two most important communication windows, i.e. the 1.3 mm second and 1.5 mm third communication bands, which is promising for the realization of ultra-broadband fiber amplifiers. Here, IðlÞ is the fluorescence intensity at wavelength l and Imax is the fluorescence maximum. The multichannel emission mechanism of Pr3þ ions in the glass sample under the 488 nm excitation is illustrated in Fig. 8. Furthermore, the fluorescence emission intensity increases with 1

3.4. Near-infrared fluorescence emission In order to determine the appropriate excitation wavelength for the Pr3þ ions, the excitation spectrum was first measured with the emission wavelength monitored at 1460 nm and the obtained excitation spectra of PA0 and PA3 glass samples are displayed in Fig. 6. Four obvious excitation bands located at about 446, 470, 485 and 590 nm, in agreement with the Pr3þ absorption transitions 3 H4/3P2, 3H4 /3P1, 3H4/3P0 and 3H4/1D2 respectively shown in the inset of Fig. 6, are observed, indicating the pump lights with the above wavelengths can be selected as the excitation sources. It is noting that the excitation intensity of PA3 glass sample with silver NPs gets an obvious enhancement compared with the PA0 glass sample. Fig. 7 displays the measured near-infrared fluorescence emission spectra covering a wavelength range from 1250 to 1650 nm under the excitation of 488 nm pump of the prepared PA0-PA4 glass samples, which shows the multichannel transition emission characteristics from the excited states (1D2, 1G4 and 3F4,3) of Pr3þ ions to its ground sate. Among them, the emission bands located at about 1300, 1381, 1460, 1536 and 1603 nm are originated from the

Fig. 6. The excitation spectra of PA0 and PA3 glass samples monitored at 1460 nm. The inset is the absorption spectra in the range of 400e700 nm of PA3 glass sample.

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P. Cheng et al. / Optical Materials 73 (2017) 102e110

Fig. 7. The near-infrared fluorescence spectra of PA0-PA4 glass samples under the 488 nm excitation.

the introduction of silver NPs in the glass matrix. With respect to the PA0 glass sample without silver NPs, the fluorescence intensity of Pr3þ ions in the PA3 glass sample with 0.5 mol% amount of AgNO3 compound increases by about 30%. The enhanced nearinfrared fluorescence emission could be attributed to the increased local electric field that acts on the Pr3þ ions located in the proximity of silver NPs and the possible energy transfer (ET) from silver NPs to Pr3þ ions [57]. The interaction of incident light with metallic NPs results in the oscillation of conduction band free electrons and such oscillation induces a confined electric field in the vicinity of NPs due to the difference in the values of relative permittivity of the metal and surrounding host glass [58,59]. This induced local electric field around the rare earth ions, lying within the proximity of metallic NPs, is then increased due to the concentration of light in sub-wavelength structure and the metallic screening effect [60]. Meanwhile, the 488 nm excitation light is close to the SPR absorption band (~510 nm), which will contribute the further increase of local electric field owing to the SPR effect of silver NPs. Consequently, the increased local electric field around silver NPs promotes the radiative transition rates as well as the excitation rates of Pr3þ ions (see the excitation spectrum of Fig. 6),

and all these factors stimulate the enhancement of fluorescence emission intensity of Pr3þ ions. The energy transfer (ET) from silver NPs to Pr3þ ions is the second possible factor for the luminescence enhancement. The silver NPs can increase the density of photons around the Pr3þ ions situated in the vicinity and thereby enhance the number of photons captured by the Pr3þ ions [30]. Therefore, the population of Pr3þ excited states increases and accordingly the radiative transition rate increases. However, the quenching of near-infrared band fluorescence of Pr3þ ions is also observed in the PA4 glass sample with 1.0 mol% amount of AgNO3 compound, which is attributed to the back energy transfer from Pr3þ ions to silver NPs. With the increased concentration of silver NPs in the glass matrix, the decreased distance between the rare-earth ions and metallic NPs favors energy transfer from the excited ions to the NPs because the multipole interactions between them is strong for small distances [61,62]. When the distance between the rare-earth ions and metallic NPs is smaller than 5 nm, the energy transfer from rareearth ions to metallic NPs will become more primary [63], which brings about an unfavorable luminescence quenching. In the present experiment, however, the fluorescence intensity in the glass sample with 1.0 mol% amount of AgNO3 compound is still larger than that of the glass sample without silver NPs, indicating that the introduction of a certain amount of silver NPs can be favorable for the near-infrared band fluorescence enhancement of doped Pr3þ ions.

3.5. Gain coefficient Optical gain coefficient is an important factor to evaluate the emission ability of doped rare earth ions. What's more, the gain coefficient spectrum for a certain energy level determines its signal gain spectral shape and amplification characteristic when rare earth doped glass is applied as a laser gain medium. The gain coefficient GðlÞ can be calculated by the following equation [64]:

GðlÞ ¼ N½P sem ðlÞ  ð1  PÞsabs ðlÞ

(8)

where N is the doping concentration of rare earth ions, P is the population inversion ratio between the upper and lower levels, sem ðlÞ and sabs ðlÞ represent the emission and absorption crosssections. The emission cross-section can be calculated using Füchtbauer-Ladenburg equation [65]:

sem ðlÞ ¼

l4 Arad Z IðlÞ 8pcn2 IðlÞdl

(9)

where n is the refractive index, Arad is the radiative transition probability and IðlÞ is the fluorescence emission intensity at wavelength l. The absorption cross-section is related to the emission cross-section by the following expression [66,67]:

Z E  hcl1 sem ðlÞ ¼ sabs ðlÞ 1 exp Zl Zu kT

Fig. 8. The schematic diagram of Pr3þ multichannel emission transitions under the 488 nm excitation.

! (10)

where k is the Boltzmann constant and T is the temperature. Zu and Z1 denote the partition functions of upper and lower levels, respectively, and the EZl (‘zero-line’ energy) is defined as the energy separation between the lowest crystal field levels of the upper and lower manifolds. The obtained emission and absorption crosssections for Pr3þ:1D241G4 transitions in the studied PA0-PA4 glass samples are listed in Table 4. Based on the cross-section data, the gain coefficient for Pr3þ:1D2/1G4 radiative transition in

P. Cheng et al. / Optical Materials 73 (2017) 102e110

109

Table 4 The emission cross-section (sem ), absorption cross-section (sabs ), FWHM and FOM as the product of sem  FWHM related to the Pr3þ:1D2/1G4 transition for the studied glass samples. Glass

sem (  1020 cm2)

sabs (  1020 cm2)

FWHM (nm)

FOM (  1027 cm3)

PA0 PA1 PA2 PA3 PA4

1.62 1.98 2.13 2.40 2.25

0.81 0.99 1.07 1.17 1.13

217.0 217.9 218.5 225.8 222.1

351.54 431.44 465.41 541.92 499.73

transition rates of Pr3þ, was mainly responsible for the observed fluorescence enhancement. The good thermal stability and strong broadband fluorescence emission indicate that the Pr3þ-doped tellurite glass with a certain amount of silver NPs is a promising luminescent material applied for ultra-broadband fiber amplifiers as well as tunable solid-state lasers. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (61178063), the Natural Science Foundation of Zhejiang Province (Q16F050001), the Natural Science Foundation of Ningbo City (2016A610061), and was sponsored by K. C. Wong Magna Fund and Hu Lan Outstanding Doctoral Fund in Ningbo University. References Fig. 9. The gain spectrum corresponding to Pr3þ:1D2/1G4 radiative transition of PA3 glass sample.

the PA3 glass sample, as representative, is calculated and the obtained gain coefficient spectrum with different P (¼0, 0.2, 0.4, 0.6, 0.8, 1) is displayed in Fig. 9. From Fig. 9, it can be seen that the positive gain coefficient can be obtained under the lower population inversion distribution (P ¼ 0.4) in the present tellurite glass. Apart from the cross-section data, the full-width at halfmaximum (FWHM) is also an important spectroscopic parameter. The bandwidth quality factor, i.e. the product of FWHM  sem , is generally introduced as a figure of merit (FOM) to characterize the broadband amplifying ability when the rare earth doped glass is applied for optical active devices. As shown in Table 4, the value of FOM for Pr3þ:1D2/1G4 radiative transition in the PA3 glass sample is the largest (541.92  1027 cm3), indicating the Pr3þ-doped tellurite glass with 0.5 mol% amount of silver NPs is a favorable gain medium applied for broadband amplifiers.

4. Conclusions The tellurite glasses containing Pr3þ ions with and without silver NPs has been synthesized by melt-quenching technique and its structural, thermal and spectroscopic properties were investigated. All glass samples were amorphous in structure and possessed good thermal stability, TEM images revealed near-spherical shape silver NPs dispersed uniformly in the glass matrix with average diameter about 10 nm. The Judd-Ofelt parameters, spontaneous transition probabilities, fluorescence branching ratios and the radiative lifetimes were calculated and analyzed. Under the 488 nm excitation, the ultra-broadband near-infrared band fluorescence of Pr3þ ions was observed in the present tellurite glasses and enhanced significantly with the introduction of silver NPs. The increased local field induced by silver NPs, which promoted the excitation and radiative

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